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+Project Gutenberg's A Study of Recent Earthquakes, by Charles Davison
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Study of Recent Earthquakes
+
+Author: Charles Davison
+
+Release Date: April 12, 2008 [EBook #25062]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A STUDY OF RECENT EARTHQUAKES ***
+
+
+
+
+Produced by Jeannie Howse, Henry Craig and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
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+
+       *       *       *       *       *
+
+    +-----------------------------------------------------------+
+    | Transcriber's Note:                                       |
+    |                                                           |
+    | Inconsistent hyphenation in the original document has     |
+    | been preserved.                                           |
+    |                                                           |
+    | In the equations, superscripted characters are marked     |
+    | with ^'s, and subscripted characters are marked with      |
+    | _ (underscore) in front.                                  |
+    |                                                           |
+    | Obvious typographical errors have been corrected. For     |
+    | a complete list, please see the end of this document.     |
+    |                                                           |
+    +-----------------------------------------------------------+
+
+       *       *       *       *       *
+
+
+
+
+A STUDY OF
+RECENT EARTHQUAKES.
+
+BY
+CHARLES DAVISON, Sc.D., F.G.S.
+
+AUTHOR OF
+"THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896."
+
+
+WITH 80 ILLUSTRATIONS
+
+
+London and Newcastle-on-Tyne:
+THE WALTER SCOTT PUBLISHING CO., LTD.
+1905
+
+
+
+
+PREFACE.
+
+
+The present volume differs from a text-book of seismology in giving
+brief, though detailed, accounts of individual earthquakes rather than
+a discussion of the phenomena and distribution of earthquakes in
+general. At the close of his _Les Tremblements de Terre_, Professor
+Fouqué has devoted a few chapters to some of the principal earthquakes
+between 1854 and 1887; and there are also the well-known chapters in
+Lyell's _Principles of Geology_ dealing with earthquakes of a still
+earlier date. With these exceptions, there is no other work covering
+the same ground; and he who wishes to study any particular earthquake
+can only do so by reading long reports or series of papers written
+perhaps in several different languages. The object of this volume is
+to save him this trouble, and to present to him the facts that seem
+most worthy of his attention.
+
+The chapter on the Japanese earthquake is reprinted, with a few slight
+additions, from a paper published in the _Geographical Journal_, and I
+am indebted to the editor, not only for the necessary permission, but
+also for his courtesy in furnishing me with _clichés_ of the blocks
+which illustrated the original paper. The editor of _Knowledge_ has
+also allowed me to use a paper which appeared four years ago as the
+foundation of the ninth chapter in this book.
+
+                                            CHARLES DAVISON.
+
+  BIRMINGHAM,
+      _January, 1905._
+
+
+
+
+CONTENTS.
+
+
+CHAPTER I.                                                   PAGE
+
+INTRODUCTION                                                    1
+
+
+CHAPTER II.
+
+THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857                7
+
+
+CHAPTER III.
+
+THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY
+28TH, 1883                                                     45
+
+
+CHAPTER IV.
+
+THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884               75
+
+
+CHAPTER V.
+
+THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886                102
+
+
+CHAPTER VI.
+
+THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887                 138
+
+
+CHAPTER VII.
+
+THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891                 177
+
+
+CHAPTER VIII.
+
+THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND
+THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901              215
+
+
+CHAPTER IX.
+
+THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897                      262
+
+
+CHAPTER X.
+
+CONCLUSION                                                    321
+
+
+INDEX                                                         349
+
+
+
+
+LIST OF ILLUSTRATIONS.
+
+
+FIG.                                                              PAGE
+
+1. Diagram to illustrate Simple Harmonic Motion                      4
+
+2. Isoseismal Lines of the Neapolitan Earthquake                     9
+
+3. Diagram to illustrate Wave-path and Angle of Emergence           12
+
+4. Diagram to illustrate Mallet's Method of determining
+   Position of Epicentre                                            14
+
+5. Plan of Cathedral Church at Potenza                              16
+
+6. Fallen Gate-pillars near Saponara                                17
+
+7. Model to illustrate the Motion of an Earth-particle
+   during an Earthquake                                             19
+
+8. Plan of Directions of Fall of Overturned Stone-lamps
+   at Tokio during the Earthquake of 1894                           19
+
+9. Meizoseismal Area of Neapolitan Earthquake                       22
+
+10. Distribution of Death-rate within Meizoseismal Area
+    of Neapolitan Earthquake                                        24
+
+11. Diagram to illustrate Mallet's Method of determining
+    Depth of Seismic Focus                                          26
+
+12. Vertical Section of Cathedral Church at Potenza                 27
+
+13. Diagram of Wave-paths at Seismic Vertical of Neapolitan
+    Earthquake                                                      29
+
+14. Geological Sketch-map of Ischia                                 47
+
+15. Isoseismal Lines of Ischian Earthquake of 1881                  51
+
+16. Isoseismal Lines of Ischian Earthquake of 1883                  59
+
+17. Diagram of Wave-paths at Seismic Vertical of Ischian
+    Earthquake of 1883                                              62
+
+18. Diagram showing connection between Depth of Focus
+    and Rate of Decline in Intensity                                68
+
+19. Isoseismal Lines of Andalusian Earthquake according
+    to Taramelli and Mercalli                                       79
+
+20. Isoseismal Lines of Andalusian Earthquake according
+    to Fouqué, etc.                                                 81
+
+21. Magnetograph Records of Andalusian Earthquake at Lisbon         83
+
+22. Nature of Shock of Andalusian Earthquake                        88
+
+23. Diagram to illustrate Variation in Nature of Shock              89
+
+24. Structure of Meizoseismal Area of Andalusian Earthquake        100
+
+25. Isoseismal Lines of Charleston Earthquake                      106
+
+26. Curve of Intensity at Charleston                               110
+
+27. Flexure of Rails at Jedburgh                                   113
+
+28. Epicentral Isoseismal Lines of Charleston Earthquake
+    according to Sloan                                             118
+
+29. Epicentral Isoseismal Lines of Charleston Earthquake
+    according to Dutton                                            119
+
+30. Planes of Oscillation of Stopped Pendulum Clocks at
+    Charleston                                                     121
+
+31. Diagram to illustrate Dutton's Method of determining
+    Depth of Seismic Focus                                         124
+
+32. Diagram to explain Origin of Regions of Defective
+    Intensity                                                      136
+
+33. Isoseismal Lines of Riviera Earthquake                         144
+
+34. Meizoseismal Area of Riviera Earthquake                        148
+
+35. Nature of Shock of Riviera Earthquake                          152
+
+36. Seismographic Record at Moncalieri                             155
+
+37. Distribution of Observatories at which Magnetographs
+    were disturbed by Riviera Earthquake                           158
+
+38. Record of Tide-gauge at Nice                                   163
+
+39. Record of Tide-gauge at Genoa                                  164
+
+40. Distribution of Seismic Activity in the Riviera                172
+
+41. Isoseismal Lines of Japanese Earthquake                        178
+
+42. Structure of Meizoseismal Area of Japanese Earthquake          180
+
+43. Plan of Directions of Fall of Overturned Bodies at
+    Nagoya                                                         187
+
+44. Map of Mean Directions of Shock and Isoseismal Lines
+    in Central District of Japanese Earthquake                     188
+
+45. Meizoseismal Area of Japanese Earthquake                       190
+
+46. Fault-scarp near Fujitani                                      191
+
+47. Fault-scarp at Midori                                          192
+
+48. Displacement of Field-divisions by the Fault near
+    Nishi-Katabira                                                 193
+
+49. Map of Swamp formed by Stoppage of River Toba by
+    Fault-scarp                                                    194
+
+50. Shifting of Trees by Fault at Uméhara                          195
+
+51. Daily Frequency of After-shocks at Gifu and Nagoya             196
+
+52. Monthly Frequency of After-shocks at Gifu                      197
+
+53. Distribution of Preliminary Shocks in Space                    202
+
+54. Distribution of After-shocks in Space, Nov.-Dec. 1891          203
+
+55. Distribution of After-shocks in Space, Jan.-Feb. 1892          204
+
+56. Distribution of After-shocks in Space, March-April 1892        205
+
+57. Distribution of After-shocks in Space, May-June 1892           206
+
+58. Distribution of Audible After-shocks in Space, Nov.
+    1891-Dec. 1892                                                 208
+
+59. Map of Adjoining Regions in which Seismic Activity
+    was affected by Japanese Earthquake                            210
+
+60. Isoseismal and Isacoustic Lines of Hereford Earthquake         216
+
+61. Nature of Shock of Hereford Earthquake                         222
+
+62. Coseismal Lines of Hereford Earthquake                         228
+
+63. Map of Minor Shocks of Hereford Earthquake                     239
+
+64. Geology of Meizoseismal Area of Hereford Earthquake            241
+
+65. Isoseismal Lines of Inverness Earthquake                       248
+
+66. Diagram to illustrate supposed Fault-displacement
+    causing Inverness Earthquake                                   256
+
+67. Map of Epicentres of After-shocks of Inverness
+    Earthquake                                                     258
+
+68. Isoseismal Lines of Indian Earthquake                          263
+
+69. Section of Tombs in Cemetery at Cherrapunji                    270
+
+70. Time-curve of Indian Earthquake                                278
+
+71. Seismographic Record of Indian Earthquake at Rocca
+    di Papa                                                        282
+
+72. Seismographic Record of Indian Earthquake at Edinburgh         283
+
+73. Displacement of Alluvium at Foot of a Hill                     287
+
+74. Twisting of Monument at Chhatak                                294
+
+75. Epicentral Area of Indian Earthquake                           303
+
+76. Plan of Chedrang Fault                                         305
+
+77. Re-triangulation of Khasi Hills                                313
+
+78. Diagram of Thrust-planes                                       318
+
+79. Seismographic Record of Tokio Earthquake of 1894               329
+
+80. Time-curves of Principal Epochs of Earthquake-waves of
+    Distant Origin                                                 338
+
+
+
+
+A STUDY OF RECENT EARTHQUAKES.
+
+
+CHAPTER I.
+
+INTRODUCTION.
+
+
+I propose in this book to describe a few of the more important
+earthquakes that have occurred during the last half century. In
+judging of importance, the standard which I have adopted is not that
+of intensity only, but rather of the scientific value of the results
+that have been achieved by the study of the shocks. Even with this
+reservation, the number of earthquakes that might be included is
+considerable; and I have therefore selected those which seem to
+illustrate best the different methods of investigation employed by
+seismologists, or which are of special interest owing to the unusual
+character of their phenomena or to the light cast by them on the
+nature and origin of earthquakes in general.
+
+Thus, the Neapolitan earthquake possesses interest from a historical
+point of view; it is the first earthquake in the study of which modern
+scientific methods were employed. The Ischian earthquakes are
+described as examples of those connected with volcanic action; the
+Andalusian earthquake is chiefly remarkable for the recognition of the
+unfelt earth-waves; that of Charleston for the detection of the double
+epicentre and the calculation of the velocity with which the
+vibrations travelled. In the Riviera earthquake are combined the
+principal features of the last two shocks with several phenomena of
+miscellaneous interest, especially those connected with its submarine
+foci. The Japanese earthquake is distinguished from others by its
+extraordinary fault-scarp and the very numerous shocks that followed
+it. The Hereford earthquake is a typical example of a twin earthquake,
+and provided many observations on the sound phenomena; while the
+Inverness earthquakes are important on account of their connection
+with the growth of a well-known fault. The great Indian earthquake
+owns few, if any, rivals within historical times, whether we consider
+the intensity of the disturbance or the diversity and interest of the
+phenomena displayed by it--the widespread changes in the earth's
+crust, both superficial and deep-seated, and the tracking of the
+unfelt pulsations completely round the globe.
+
+
+TERMS AND DEFINITIONS.
+
+Some terms are of such frequent use in describing earthquakes that it
+will be convenient to group them here for reference, others more
+rarely employed being introduced as they are required.
+
+An earthquake is caused by a sudden displacement of the material which
+composes the earth's interior. The displacement gives rise to series
+of waves, which are propagated outwards in all directions, and which,
+when they reach the surface, produce the sensations known to us as
+those of an earthquake.
+
+The region within which the displacement occurs is sometimes called
+the _hypocentre_, but more frequently the _seismic focus_ or simply
+the _focus_. The portion of the earth's surface which is vertically
+above the seismic focus is called the _epicentre_. The focus and
+epicentre are often spoken of for convenience as if they were points,
+and they may then be regarded as the centres of the region and area in
+which the intensity was greatest. This is not quite accurate, but to
+attempt a more exact definition would at present be out of place.
+
+An _isoseismal line_ is a curve which passes through all points at
+which the intensity of the shock was the same. It is but rarely that
+the absolute intensity at any point of an isoseismal line can be
+ascertained, and only one example is given in this volume. As a rule,
+the intensity of a shock is determined by reference to the degrees of
+different arbitrary scales. These will be quoted when required.
+
+In every strong earthquake there is a central district which differs
+in a marked manner from that outside in the far greater strength and
+complexity of the phenomena. As this district includes the epicentre,
+it is sometimes referred to as the _epicentral area_, but the term
+_meizoseismal area_ is more appropriate, and will be employed
+accordingly.
+
+The district over which an earthquake is perceptible to human beings
+without instrumental aid is its _disturbed area_. In like manner, that
+over which the earthquake-sound is heard is the _sound-area_.
+
+A great earthquake never occurs alone. It is merely the most prominent
+member of a group of shocks of greater or less intensity, and is
+known as the _principal shock_ or _earthquake_, while the others are
+called _minor_ or _accessory shocks_, and _fore-shocks_ or
+_after-shocks_ according as they occur before or after the principal
+earthquake. When the sound only is heard, without an accompanying
+tremor being anywhere perceptible, it is more accurately called an
+_earth-sound_, but is frequently for convenience numbered among the
+minor shocks.
+
+   [Illustration: FIG. 1.--Diagram to illustrate simple harmonic
+   motion.]
+
+The movement of the ground during a vibration of the simplest
+character (known as simple harmonic motion) is represented in Fig. 1.
+The pointer of the recording seismograph is here supposed to oscillate
+along a line at right angles to AB, and the smoked paper or glass on
+which the record is made to travel to the left. The distance MP of the
+crest P of any wave from the line AB represents the _amplitude_ of the
+vibration, the sum of the distances MP and NQ its _range_, and the
+length AB the _period_ of the vibration. From the amplitude and period
+we can calculate, in the case of simple harmonic motion, both the
+_maximum velocity_ and _maximum acceleration_ of the vibrating
+particles of the ground.[1]
+
+A few terms describing the nature of the shock are also in common use
+among Italians and Spaniards. An _undulatory_ shock consists of one or
+several waves, the movement to and fro being along a nearly horizontal
+line; a _subsultory_ shock of movements in a nearly vertical
+direction; while a _vorticose_ shock consists of undulatory or
+subsultory movements crossing one another in different directions.
+
+
+ORIGIN OF EARTHQUAKES.
+
+Earthquakes are grouped, according to their origin, into three
+classes. The first consists of slight local shocks, caused by the fall
+of rock in underground passages; the second of _volcanic_ earthquakes,
+also local in character, but often of considerable intensity near the
+centre of the disturbed area; while in the third class we have
+_tectonic_ earthquakes, or those directly connected with the shaping
+of the earth's crust, which vary in strength from the weakest
+perceptible tremor to the most destructive and widely felt shock. Of
+the earthquakes described in this volume, the Ischian earthquakes
+belong to the second class, and all the others to the third.
+
+That tectonic earthquakes are closely connected with the formation of
+faults seems now established beyond doubt. They occur far from all
+traces of recent volcanic action. Their isoseismal lines are elongated
+in directions parallel to known faults, and this is sometimes the case
+in one and the same district with faults that occur at right angles to
+one another. Indeed, when several isoseismals are carefully drawn, it
+is possible from their form and relative position to predict the
+position of the originating fault.[2] The initial formation and
+further spreading of the rent may be the cause of a few earthquakes,
+but by far the larger number are due to the subsequent growth of the
+fault. The relative displacement of the rocks adjoining the fault,
+which may amount to thousands of feet, occasionally even to miles, is
+the result, not of one great movement, but of innumerable slips taking
+place in different parts of the fault and spread over vast ages of
+time. With every fault-slip, intense friction is suddenly brought into
+action by the rubbing of one mass of rock against the other; and,
+according to the modern view, it is this friction that gives rise to
+the earthquake waves.
+
+In most earthquakes, the slip takes place at a considerable depth,
+perhaps not less than one or several miles, and the vertical slip is
+so small that it dies out before reaching the surface. But, in a few
+violent earthquakes, such as the Japanese and Indian earthquakes
+described in this volume, the slip is continued up to the surface and
+is left visible there as a small cliff or fault-scarp. In these cases,
+the sudden spring of the crust may increase and complicate the effects
+of the vibratory shock.
+
+FOOTNOTES:
+
+[1] If _a_ is the amplitude of the vibration and T its period, the
+maximum velocity is 2*pi*a/T and the maximum acceleration 4*pi^2a/T^2
+
+[2] See Chapter VIII., on the Hereford and Inverness earthquakes.
+
+
+
+
+CHAPTER II.
+
+THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857.
+
+
+Half a century ago, seismology was in its infancy. On the Continent,
+Alexis Perrey of Dijon was compiling his earthquake catalogues with
+unfailing enthusiasm and industry. In 1846, Robert Mallet applied the
+laws of wave-motion in solids, as they were then known, to the
+phenomena of earthquakes; and his memoir on the Dynamics of
+Earthquakes[3] may be regarded as the foundation-stone of the new
+science. During the next twelve years he contributed his well-known
+Reports to the British Association,[4] and prepared a series of
+instructions for the observation and study of earthquake-shocks.[5]
+The latter, it is worth noting, contains an outline, but hardly more
+than an outline, of the methods of investigation which he developed
+and employed eight years afterwards in studying the Neapolitan
+earthquake.
+
+The history of Mallet's preparation for his great work is somewhat
+strange. No one else at that time possessed so full a knowledge of
+earthquake phenomena. It was, however, a knowledge that had little,
+if any, foundation in actual experience; for, when he was awakened by
+the British earthquake of November 9th, 1852, he failed to recognise
+its seismic character. Although this shock disturbed an area of about
+75,000 square miles and was felt in all four parts of the kingdom, the
+paucity of observations and the absence of durable records combined in
+preventing the successful application of his new modes of study.[6]
+Nevertheless, with confidence unshaken in their power, he awaited the
+occurrence of a more violent shock, but five years had to pass before
+his opportunity came towards the close of 1857.
+
+So destructive was the Neapolitan earthquake of this year (Mallet
+ranks it third among European earthquakes in extent and severity),
+that nearly a week elapsed before any news of it reached the outer
+world. Without further loss of time, he applied for and obtained a
+grant of money from the Council of the Royal Society, and proceeded
+early in the following February to what was then the kingdom of
+Naples. Armed with letters of authority to different officials, he
+visited the chief towns and villages in the meizoseismal area; and, in
+spite of unfavourable weather and the difficulties of travelling in a
+country so recently devastated, he completed his examination in little
+more than two months. It was a task, surely, that would have baffled
+any but the most enthusiastic investigator or one unspurred by the
+feeling that he possessed the key to one of the most obscure of
+Nature's problems.
+
+Mallet's confidence in the accuracy of his methods was almost
+unbounded. His great report was published four years later; but he
+seems to have regarded it almost as a text-book of "observational
+seismology" and the results of his Neapolitan work as mere
+illustrations. His successors, however, have transposed the order of
+importance, and rank his two large volumes as the model, if not the
+inspirer, of many of our more recent earthquake monographs.
+
+   [Illustration: FIG. 2.--Isoseismal Lines of the Neapolitan
+   Earthquake of 1857. (_Mallet._)]
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+The position of the meizoseismal area, to which Mallet devoted most of
+his time, is indicated by the small oval area marked 1 in Fig. 2,
+represented on a larger scale in Fig. 9. It is 40 miles long and 23
+miles wide,[7] and contains 950 square miles. Within this area, the
+loss of life was great and most of the towns were absolutely
+prostrated.
+
+The next isoseismal, No. 2, which is also shown more clearly in Fig.
+9, bounds the area in which the loss of life was still great and many
+persons were wounded, while large portions of the towns within it were
+thrown down. Its length is 65 miles, width 47 miles, and area 2,240
+square miles. The third isoseismal includes a district in which
+buildings were only occasionally thrown down, though none escaped some
+slight damage, and in which practically no loss of life occurred. This
+curve is 103 miles long, 82 miles wide, and includes 6,615 square
+miles. Lastly, the fourth isoseismal marks the boundary of the
+disturbed area, which is 250 miles long, 210 miles wide, and contains
+not more than 39,200 square miles; an amount that must be regarded as
+strangely small, and hardly justifying Mallet's estimate of the
+Neapolitan earthquake as the third among European earthquakes in
+extent as well as in seventy.
+
+
+DAMAGE CAUSED BY THE EARTHQUAKE.
+
+As regards destruction to life and property, however, the Neapolitan
+earthquake owns but few European rivals. Less favourable conditions
+for withstanding a great shock are seldom, indeed, to be found than
+those possessed by the mediæval towns and villages of the meizoseismal
+area. In buildings of every class, the walls are very thick and
+consist as a rule of a coarse, short-bedded, ill-laid rubble masonry,
+without thorough bonding and connected by mortar of slender cohesion.
+The floors are made of planks coated with a layer of concrete from six
+to eight inches thick, the whole weighing from sixty to a hundred
+pounds per square foot. Only a little less heavy are the roofs, which
+are covered with thick tiles secured, except at the ridges, by their
+own weight alone. Thus, for the most part, the walls, floors, and
+roofs are extremely massive, while the connections of all to
+themselves and to each other are loose and imperfect.
+
+Again, the towns, for greater security from attacks in early times,
+are generally perched upon the summits and steep flanks of hills,
+especially of the lower spurs that skirt the great mountain ranges;
+and the rocking of the hill-sites, in Mallet's opinion, greatly
+aggravated the natural effects of the shock. The streets, moreover,
+are steep and narrow, sometimes only five feet, and not often more
+than fifteen feet, in width; and the houses, when shaken down, fell
+against one another and upon those beneath them. As Dolomieu said of
+the great earthquake in 1783, "the ground was shaken down like ashes
+or sand laid upon a table."
+
+Of the total amount of damage, not even the roughest estimate can be
+made. The official returns are clearly, and no doubt purposely,
+deficient, and obstacles were placed in Mallet's way when he
+endeavoured to ascertain the numbers of persons killed and wounded.
+Taking only the towns into account, he calculated that, out of a total
+population of 207,000, the number of persons killed was 9,589, and of
+wounded 1,343.[8] A few towns were marked by an excessively high
+death-rate. Thus, at Montemurro, 5000 out of 7002 persons were killed
+and 500 wounded; at Saponara, 2000 out of 4010 were killed; and, at
+Polla, more than 2000 out of a population of less than 7000.
+
+
+GENERAL OBJECTS OF INVESTIGATION.
+
+The principal objects of Mallet's investigation were to determine the
+position of the epicentre and the depth of the seismic focus. If, in
+Fig. 3, F represents the seismic focus (here, for convenience,
+supposed to be a point), the vertical line FE will cut the surface of
+the earth in the epicentre E.[9] The dotted lines represent circles
+drawn on the surface of the earth with E as centre and passing through
+the places P and Q.
+
+   [Illustration: FIG. 3.--Diagram to illustrate wave-path and angle
+   of emergence.]
+
+When the impulse causing the earthquake takes place at the focus, two
+elastic waves spread outwards from it in all directions through the
+earth's crust. The first wave which reaches a point P consists of
+longitudinal vibrations, that is, the particle of rock at P moves in a
+closed curve with its longer axis in the direction FP. Mallet supposes
+this curve to be so elongated that it is practically a straight line
+coincident in direction with FP. In the second or transversal wave,
+the vibration of the particle at P takes place in a plane at right
+angles to FP. These vibrations Mallet, for his main purpose, neglects.
+
+Returning to the longitudinal wave, Mallet calls the line FP the
+_wave-path_ at P. The direction EP gives the azimuth of the wave-path,
+or its direction along the surface of the earth. The angle LPA, or
+EPF, he defines as the _angle of emergence_ at the point P. If Q be
+farther from E than P, the angle EQF is less than the angle EPF, or
+the angle of emergence diminishes as the distance from the epicentre
+increases. At the epicentre, the angle of emergence is a right-angle;
+at a great distance from the epicentre, it is nearly zero.
+
+Mallet argued that the direction of the wave-path FPA, or its
+equivalents, the horizontal direction EPL and the angle of emergence
+EPF, should be discoverable from the effects of the shock at P. The
+cracks in damaged buildings, he urged, would be at right angles to the
+wave-path FPA; overturned monuments or gate-pillars should fall along
+the line EPL, either towards or from the epicentre according to their
+conditions of support; loose or slightly attached bodies, such as the
+stone balls surmounting gate-pillars, should be projected nearly in
+the direction of the wave-path FPA, and their subsequent positions,
+supposing the balls not to have rolled, should give the horizontal
+direction EPL of the wave-path, and might, in some circumstances,
+determine the angle of emergence and the velocity with which they were
+projected. I shall return to details later on. For the present, it is
+clear that, in the destruction wrought by the earthquake, Mallet
+expected to find the materials most valuable for his purpose. Indeed,
+so obvious did this mode of examination appear to him, that he could
+not conceal his surprise at the blindness of his predecessors. They
+seem, he says, "to have been perfectly unconscious that in the
+fractured walls and overthrown objects scattered in all directions
+beneath their eyes, they had the most precious data for determining
+the velocities and directions of the shocks that produced them."
+
+
+POSITION OF THE EPICENTRE.
+
+_Mallet's Method of Determining the Position of the Epicentre._--In
+many cases the examination of a damaged building or of an overthrown
+body served more than one purpose, providing materials for
+ascertaining the depth of the seismic focus as well as the position of
+the epicentre. For the present, however, it will be convenient to
+consider alone the method by which the latter object was to be
+attained.
+
+   [Illustration: FIG. 4.--Diagram to illustrate Mallet's method of
+   determining position of epicentre.]
+
+Nothing could be simpler than the principle of the method proposed.
+The horizontal direction PL of the wave-path at any place P (Fig. 4),
+when produced backwards, must pass through the epicentre E; and the
+intersection of the directions at two places, P and Q, must therefore
+give the position of the epicentre. In practice, it is of course
+impossible to determine the direction with very great accuracy, and
+Mallet therefore found it necessary to make several measurements in
+every place, and to visit all the more important towns within and near
+the meizoseismal area.
+
+In a ruined town there are many objects from which the direction may
+be ascertained, the most important of all, according to Mallet, being
+fissures in walls that are fractured but not overthrown. He regarded
+such fissures, indeed, as "the sheet-anchor, as respects direction of
+wave-path, to the seismologist in the field," and at least three out
+of every four of his determinations of the direction were made by
+their means. If the buildings are detached and large, simple and
+symmetrical in form, well built and not too much injured, the fissures
+in the walls should, he argued, occur along lines at right angles to
+the wave-path, whether that path be parallel or inclined to the
+principal axis of the building. Cracks in the floors and ceilings
+should also be similarly directed, and provide evidence which Mallet
+regarded as only second in value to that given by the walls.
+
+   [Illustration: FIG. 5.--Plan of Cathedral Church at Potenza.
+   (_Mallet._)]
+
+No building showed the different kinds of evidence on which Mallet
+relied as clearly as the cathedral church at Potenza, the plan of
+which is given in Fig. 5, and the vertical section along its axis in
+Fig. 12. This is a modern work, nearly 200 feet long, with its axis
+directed east and west. The walls are composed of fairly good rubble
+masonry and brick; and the arches in the nave and transepts, the
+semi-cylindrical roof and the central dome are made of brick. The
+fissures represented in both diagrams were drawn to scale by the
+cathedral architect before Mallet's arrival, and, as the work of an
+unbiassed observer, are of special value. Most of those in the roof,
+it will be seen, were transverse to the axial line of the church; but
+there were others parallel to this line, one in particular running
+right along the soffit of the nave and chancel. There were also
+numerous small fissures in the dome, due to local structural causes
+and therefore of varying direction, and a large portion of the dome
+slipped westward, leaving open fissures of seven to eight inches in
+width. The mean direction of the wave-path, as deduced from nine sets
+of fissures, none of which differs more than four degrees from the
+mean, is W. 2-1/2° S. and E. 2-1/2° N., which corresponds precisely
+with the direction of throw on the displaced portion of the dome. The
+great east and west fissures in the arch of the nave and chancel
+Mallet attributed to a second shock, of the existence of which there
+is ample evidence.
+
+   [Illustration: FIG. 6.--Fallen gate-pillars near Saponara.
+   (_Mallet._)]
+
+Next to fissures, Mallet made most use of overthrown objects, such as
+the two gate piers near Saponara, represented in Fig. 6. They were
+made of rubble ashlar masonry, three feet square and seven feet in
+height. Both were fractured clean off at the level of the ground, the
+mortar being poor, and fell in directions that were accurately
+parallel, indicating a wave-path towards S. 39-1/2°E. A few
+observations were also made on projected stones, fissures in nearly
+level ground, and the swinging of lamps and chandeliers; but their
+value was small, except as corroboration of the more important
+evidence afforded by fissures in the walls and roofs of buildings.
+
+_Remarks on Mallets Method._--It would have been more difficult in
+Mallet's day than it is now, to offer objections to his method of
+determining the position of the epicentre. The focus, as he was well
+aware, could not be a point, and, at places near the epicentre (the
+very places where most of his observations were made), there must be
+rapid changes of direction due to the arrival of vibrations from
+different parts of the focus. He records the occurrence of the
+so-called vorticose shocks at several places, though he attributes
+them to another cause. Perhaps the best known example of such a shock
+is that which has been so well illustrated by the late Professor
+Sekiya's model of the motion of an earth-particle during the Japanese
+earthquake of January 15th, 1887. The motion in this case was so
+complicated that the model was, for simplicity, made in three parts,
+the first of which alone is represented in Fig. 7.[10] It is clear
+that in such an earthquake, Mallet's method would utterly fail in
+giving definite results.
+
+While this shock was one of great complexity, another Japanese
+earthquake, that of June 20th, 1894, was unusually simple in
+character. The movement at Tokio consisted of one very prominent
+oscillation with a total range of 73 mm. or 2.9 inches in the
+direction S. 70° W.; the vibrations which preceded and followed it
+being comparatively small. Most, if not all, of the damage caused by
+the earthquake must have been due to this great oscillation; and yet
+the cylindrical stone-lamps so common in Japanese gardens were found
+by Professor Omori to have fallen in many different directions. Taking
+only those which had circular bases, twenty-nine were overthrown in
+directions between north and east, sixteen between east and south,
+eighty-one between south and west, and fourteen between west and
+north.[11] Fig. 8 represents Professor Omori's results graphically,
+the line drawn from O to any point being proportional to the number of
+lamps which fell in directions between 7-1/2° on either side of the
+line.
+
+   [Illustration: FIG. 7.--Model to illustrate the motion of an
+   earth-particle during an earthquake. (_Sekiya._)]
+
+   [Illustration: FIG. 8.--Plan of directions of fall of overturned
+   stone-lamps at Tokio during the earthquake of 1894.]
+
+It will be seen from this figure that most of the stone lamps fell in
+directions between west and south-west, and it is remarkable that the
+mean direction of fall is S. 70° W.,[12] which is exactly the same as
+that of the great oscillation. Somewhat similar results were obtained
+by this able seismologist at different places affected by the great
+Japanese earthquake of 1891 (Figs. 43 and 44), and the study of the
+apparent directions observed during the Hereford earthquake of 1896
+leads to the same conclusion.
+
+It thus appears that an isolated observation may give a result very
+different from the true direction. Indeed, if we may judge from
+Professor Omori's measurements in 1894, the chance that a single
+direction may be within five degrees of the mean direction is about 1
+in 9. But, on the other hand, it is equally clear from these and other
+observations that the mean of a large number of measurements will give
+a result that agrees very closely with the true direction.
+
+One other point may be alluded to before leaving Professor Omori's
+interesting observations. It would seem, from the list that he gives,
+that he exercised no selection in his measurements, but continued
+measuring the direction of every fallen lamp indifferently until he
+had obtained sufficient records for his purpose. Now, if the number of
+fallen lamps at his disposal had been small, say 12 instead of 144,
+the mean observed direction would probably have differed from the
+direction given from the seismograph.[13] But, on the other hand, a
+preliminary survey without any actual measurements would have revealed
+at once the predominant direction of overthrow, and a fairly accurate
+result might have been obtained by neglecting discordant directions
+and taking the mean of those only which appeared to agree with the
+mentally determined average.
+
+This, indeed, appears to have been the course followed, more or less
+unconsciously, by Mallet in his Neapolitan work. "When the observer,"
+he says, "first enters upon one of those earthquake-shaken towns, he
+finds himself in the midst of utter confusion. The eye is bewildered
+by 'a city become an heap.' He wanders over masses of dislocated stone
+and mortar, with timbers half buried, prostrate, or standing stark up
+against the light, and is appalled by spectacles of desolation....
+Houses seem to have been precipitated to the ground in every direction
+of azimuth. There seems no governing law, nor any indication of a
+prevailing direction of overturning force. It is only by first gaining
+some commanding point, whence a general view over the whole field of
+ruin can be had, and observing its places of greatest and least
+destruction, and then by patient examination, compass in hand, of many
+details of overthrow, house by house and street by street, analysing
+each detail and comparing the results, as to the direction of force,
+that must have produced each particular fall, with those previously
+observed and compared, that we at length perceive, once for all, that
+this apparent confusion is but superficial."
+
+   [Illustration: FIG. 9.--Meizoseismal area of Neapolitan
+   earthquake. (_Mallet._)]
+
+_Mallet's Determination of the Epicentre._--Within the third
+isoseismal line Mallet made altogether 177 measurements of the
+direction of the wave-path at 78 places. These are plotted on his
+great map of the earthquake; but, owing to the small scale of Fig. 9,
+it is only possible to represent, by means of short lines, the mean or
+most trustworthy direction at each place.[14] Producing these
+directions backwards, he found that those at sixteen places passed
+within five hundred yards of a point which is practically coincident
+with the village of Caggiano; those at sixteen other places passed
+within one geographical mile (1.153 statute miles) of this point; the
+directions at sixteen more places within two and a half geographical
+miles; while those at twelve places passed through points not more
+than five geographical miles from Caggiano. As the direction of the
+shock at places near the epicentre must have been influenced by the
+mere size of the focus, this approximate coincidence is certainly
+remarkable, and there can be little doubt, I think, that the
+epicentre, or, at any rate, _an_ epicentre must have been situated not
+far from the position assigned to it by Mallet's laborious
+observations.
+
+_Existence of Two Epicentres._--It is difficult, however, to realise
+that the impulse at the focus corresponding to Mallet's epicentre was
+the origin of all the destruction of life and property that occurred.
+The position of the epicentre close to the north-west boundary of the
+meizoseismal area, the extraordinary extension of that area towards
+the south-east, and especially the great loss of life at Montemurro
+and the adjoining towns, can hardly be accounted for in this manner.
+Mallet himself recognised that these facts required explanation, and
+he suggested that the situation and character of the towns were in
+part responsible for their ruin, and the physical structure of the
+country for the course of the isoseismal lines. But the comparative
+escape of places much nearer Caggiano, and the wide extent of the
+meizoseismal area, embracing many towns and villages of varied
+character and site and many different surface-features, point
+unmistakably to a different explanation.
+
+   [Illustration: FIG. 10.--Distribution of death-rate within
+   meizoseismal area of Neapolitan earthquake.]
+
+One clue to the solution of the problem is afforded by the seismic
+death-rate of the damaged towns. From a table given by Mallet (vol.
+ii. pp. 162-163), we know the population before the earthquake of the
+different communes in the province of Basilicata, and the loss of life
+in each due to the shock; and from these figures we can find the
+percentage of deaths at nearly every place of importance. As will be
+seen from Fig. 10, it varies from seventy-one at Montemurro and fifty
+at Saponara down to less than one at all the places marked to which
+figures are not attached. There is thus a group of places, with its
+centre near Montemurro, where the loss of life far exceeded that in
+the surrounding country; and also a slightly less-marked group, with
+its centre near Polla, in the north-west of the meizoseismal area;
+while in the intermediate region the death-rate was invariably small.
+Too much stress should not be laid upon the exact figures, for there
+were no doubt local conditions that affected the death-roll. But it
+seems clear that one focus was situated not far from Montemurro; while
+the north-westerly group of places, combined with Mallet's
+observations on the direction, point to a second focus near Polla,
+about twenty-four miles to the north-west. It will be seen in a later
+section that the observations on the nature of the shock also imply
+the existence of a double focus.
+
+
+DEPTH OF THE SEISMIC FOCUS.
+
+_Mallet's Method of Determining the Depth of the Focus._--In
+ascertaining the position of the epicentre, Mallet's work was
+remarkable only for the novelty of the method employed by him; but, in
+his attempt to calculate the depth of the seismic focus, he was
+breaking new ground. That the depth must be comparatively small had
+already been recognised, and was indeed obvious from the limited area
+disturbed by nearly every earthquake. No one, however, had tried to
+estimate the depth in miles; and it is impossible not to sympathise
+with Mallet while he accumulated his observations with feverish
+activity and subjected them to the first rough examination even if
+one cannot share his confidence that he had succeeded in measuring the
+depth "in miles and yards with the certainty that belongs to an
+ordinary geodetic operation."
+
+The method employed by him for the purpose is no less simple
+theoretically than that used for locating the epicentre. If the
+position of the latter (E) is known, one accurate measurement of the
+angle of emergence EPF, at any other point P would be sufficient to
+fix the depth of some point within the focus F (Fig. 11). Here, again,
+Mallet relied chiefly on fissures in walls that were fractured but not
+overthrown. In detail, these fissures are nearly always jagged or
+serrated, for they tend to follow the lines of joints rather than
+break through the solid stone, though they sometimes traverse bricks
+and mortar alike. But the general course of the fissures, he urged,
+would be at right angles to the wave-path, and their inclination to
+the vertical should be equal to the angle of emergence.
+
+   [Illustration: FIG. 11.--Diagram to illustrate Mallet's method of
+   determining depth of seismic focus.]
+
+In obtaining measurements of this angle, the buildings to be chosen
+are those of large size, with few windows or other apertures, and with
+walls made of brick or small short-bedded stones. The cathedral-church
+at Potenza perhaps satisfies these conditions more closely than any
+other structure examined by Mallet. The plan of the fissures in the
+walls and roof has been given in Fig. 5, and Fig. 12 represents the
+fissures In the vertical section along the axial line and looking
+north, as drawn by the cathedral architect. From these fissures Mallet
+calculated the mean angle of emergence at Potenza to be 23° 7'. The
+distance of Potenza from Caggiano being seventeen miles, and the
+height of the former being 2,580 feet, the depth of the focus
+resulting from this observation alone would be 6-3/4 miles below the
+level of the sea.
+
+   [Illustration: FIG. 12.--Vertical section of Cathedral Church at
+   Potenza. (_Mallet._)]
+
+_Objection to Mallet's Method._--The weakest point in Mallet's method
+is probably his assumption that the wave-paths are straight lines
+extending outward from the focus. Even if the depth of the focus is
+not more than a few miles, the waves must traverse rocks of varying
+density and elasticity, and, at every bounding surface, they must
+undergo refraction. If the rocks are so constituted that the velocity
+of the earth-waves in them increases with the depth, then the
+wave-paths must be bent continually outwards from the vertical, so
+that the angle of emergence at the surface may be considerably less
+than it would have been with a constant velocity throughout. In this
+case, the actual depth will be greater, perhaps much greater, than the
+calculated depth. For instance, if the angle of emergence at Potenza
+were diminished only 5° by refraction, the calculated depth of the
+focus would be too small by 1-3/4 miles.
+
+_Mallet's Estimate of the Depth of the Focus._--Mallet measured the
+angle of emergence at twenty-six places, the mean angle (_i.e._ the
+mean of the greatest and least observed angles) varying from 72° at
+Vietri di Potenza and 70° at Pertosa, which are about two miles from
+the calculated epicentre, to 11-1/2° at Salerno, distant about 40
+miles. Fig. 13 reproduces part of the diagram on which he plotted the
+mean angle of emergence at different places. The horizontal line
+represents the level of the sea, and the vertical line one passing
+through the epicentre and focus, called by Mallet the "seismic
+vertical." The lines on the left-hand side represent the commencing
+wave-paths (assumed straight) to the observing stations situated to
+the westward of the meridian through the epicentre, those on the
+right-hand side corresponding to places to the eastward of the same
+meridian. Small horizontal marks are added to indicate the depth in
+miles below the level of the sea.
+
+   [Illustration: FIG. 13.--Diagram of wave-paths at seismic
+   vertical of Neopolitan earthquake. (_Mallet._)]
+
+It will be seen, from this diagram, that all the wave-paths start from
+the seismic vertical at depths between three and nine miles; but the
+points of departure are clustered thickly within a portion, the length
+of which is about 3-1/2 miles and the mean depth about 6-1/2 miles. So
+great was Mallet's confidence in these calculations that he assigns
+the diverging origin of the wave-paths to different points of the
+focus, and thus concludes that, while the mean depth of the focus was
+about 6-1/2 miles, its dimensions in a vertical direction did not
+exceed 3-1/2 miles.
+
+How far Mallet's results should be accepted as correct, it is
+difficult to say in our ignorance of the constitution of the earth's
+interior. There can be no doubt that the focus was of considerable
+size, and that, in consequence, the wave-paths would diverge from
+different points of it. But that each wave-path should actually
+intersect the focus, and so enable its magnitude to be determined,
+would surely involve an approach to some law connecting the direction
+of a wave-path with the depth of its own origin, and no such law seems
+to be ascertainable. Nor can the limitation of these apparent origins
+between certain depths be held to argue that the focus, or any part of
+it, was equally confined, for the wave-paths would to a great extent
+be similarly refracted. I fear that the only conclusions that we can
+with safety draw from Mallet's admirable work are that his figures
+indicate the order of magnitude both of the vertical dimensions and of
+the mean depth of the focus.
+
+
+NATURE OF THE SHOCK.
+
+It is not easy to form any precise image of the earthquake as it
+appeared to the terrified witnesses within the meizoseismal area. To
+minds unbalanced by the suddenness of the shock and by the crash of
+falling houses, actuated too by the intense need of safety, the mere
+succession of events must have presented but little interest. The
+interval of two months that elapsed between the occurrence of the
+earthquake and its investigation was also unfavourable to the
+collection of accurate accounts from a wonder-loving people. Only one
+feature, therefore, stands out clearly in the few records given by
+Mallet--namely, the division of the shock into two distinct parts.
+
+In the central district, this division is perhaps less apparent than
+elsewhere. At Polla, for instance, which lies close to the north-west
+epicentre, the first warning was given by a rushing sound; almost
+instantly, and while it was yet heard, came a strong subsultory or
+up-and-down movement, succeeded after a few seconds, but without any
+interval, by an undulatory motion. At Potenza, which is not far from
+the same epicentre but a few miles outside the meizoseismal area, the
+separation was more pronounced. According to one observer, the first
+movement was from west to east; and, within a second or two
+afterwards, there was a less violent shock in a transverse direction,
+followed immediately by a shaking in all directions, called by the
+Italians vorticose. Naples lies sixty-nine miles from the north-west
+epicentre, and here more accurate observations could be made. Dr.
+Lardner, well known fifty years ago as a writer of scientific works,
+describes the first movement felt there as "a short, jarring,
+horizontal oscillation, that made all doors and windows rattle, and
+the floors and furniture creak. This ceased, and after an interval
+that seemed but a few seconds was renewed with greater violence, and,
+he thought, with a distinctly undulatory movement, 'like that in the
+cabin of a small vessel in a very short chopping sea.'"
+
+In five other earthquakes studied in this volume, the separation of
+the shock into two parts was a well-marked phenomenon. In the
+Neapolitan earthquake, the separation was so distinct that Mallet took
+some pains to account for its origin. He regarded it in every case as
+due to the reflection or refraction of the earth-waves by underlying
+rocks, though he does not explain why the reflected or refracted wave
+should be more intense than that transmitted directly. I shall refer
+to the subject in greater detail when describing the Andalusian,
+Charleston, Riviera, and Hereford earthquakes. For the present, it may
+be sufficient to urge that the double shock cannot have been due to
+the separation of the original waves by underground reflection or
+refraction, for then the second part should have been generally the
+weaker; nor to the succession of longitudinal and transverse waves,
+for, in that case, every earthquake-shock should be duplicated. The
+only remaining supposition is that there was a second impulse
+occurring either in the same or in a different focus.
+
+Which alternative should be adopted, the evidence on the nature of the
+shock is too scanty to determine. The defect is, however, supplemented
+by Mallet's observations on the direction of motion; for, at many
+places within and near the meizoseismal area, he met with the clearest
+signs of a double direction. Sometimes this was apparent to the senses
+of the observer; in other cases, damaged buildings presented two sets
+of fissures. At La Sala and near Padula, the first movement was roughly
+east and west, the second north and south. At Moliterno, there was
+evidence of a subordinate shock at right angles to the chief one; in
+the neighbourhood of Tramutola, its direction was from about E. 30° S.
+In these and other cases, Mallet saw the effects of earthquake-echoes;
+but the underground reflection of earth-waves would give rise to the
+second part of the shock, not the first as at La Sala and Padula.
+Moreover, the secondary directions, though they are seldom recorded
+accurately, point nearly to an epicentre not far from Montemurro. The
+observations on the nature and direction of the double shock thus
+confirm the conclusion, derived from the distribution of the seismic
+death-rate, that there were two detached foci, one near Polla and the
+other near Montemurro.
+
+This seems to be the best explanation of the facts recorded by Mallet.
+There is, however, a possible difficulty that should not be
+overlooked--namely, the apparently slight influence of the Montemurro
+focus on the mean direction of the shock (Fig. 9). At a few places, of
+course, the mean direction passes through both epicentres; at some
+others, as we have seen, one of the two observed directions points
+towards the Montemurro epicentre. It is not impossible, also, that
+Mallet, after the first few days' work, may occasionally have quite
+unconsciously selected and measured those fissures from the maze
+presented to him which agreed most closely with his early impressions
+obtained from the neighbourhood of Polla. But, for places nearer Polla
+than Montemurro (and these form the majority of those visited by
+Mallet), the probable explanation of the difficulty is that the
+Montemurro focus was not so deep as the Polla focus. This, as will
+appear more fully in the next chapter, would account for the
+comparatively great intensity in the immediate neighbourhood of
+Montemurro and for its rapid decline outwards; and it receives some
+support from an isolated reference by Mallet to two angles of
+emergence at Padula, one of 25° from the north, and the other of 8° or
+10° in the perpendicular walls.
+
+
+ELEMENTS OF THE WAVE-MOTION.
+
+The elements of the wave-motion, as mentioned in the introductory
+chapter, are four in number, namely, the period, amplitude, maximum
+velocity, and maximum acceleration. If any two of these are known for
+each vibration--and the first two are now given by every accurately
+constructed seismograph--the others can be determined if the
+vibrations follow the law of simple harmonic motion.[15]
+
+_Amplitude._--To ascertain the amplitude, Mallet had to rely chiefly
+on the fissures made in very inelastic walls. If the parts into which
+such a wall are fractured are free to move, and yet, being inelastic,
+obliged to remain in the farthest position to which they are carried
+by the wave, the distance traversed by the centre of gravity of one of
+the displaced parts should give a "rude approximate measure" of the
+horizontal amplitude of the earth-wave. At Certosa, near Padula, he
+thus found the amplitude to be about 4 inches, at Sarconi about 4-3/4
+inches, and at Tramutola about 4-1/2 inches. From somewhat similar
+evidence, the amplitude at Polla appears to have been about 2-1/2 or 3
+inches; and, from the oscillation of a suspended clock or watch on a
+rough wall, about 3-1/2 inches at La Sala and 1-3/4 inches at
+Barielle. With the exception of Barielle, these places lie nearly on a
+straight line passing through Mallet's epicentre, and he gives the
+following table, showing an increase in amplitude with the distance
+from the epicentre:--
+
+                     Polla.  La Sala.  Certosa.  Tramutola.  Sarconi.
+
+Distance in miles     4.0      13.4      19.0       23.8       30.8
+Amplitude in inches   2-1/2    3-1/2      4         4-1/2      4-3/4
+
+The existence of the Montemurro focus must, however, complicate any
+relation that may connect these two quantities.
+
+_Maximum Velocity._--The means at Mallet's disposal for determining
+the maximum velocity were more numerous than those available for the
+amplitude. From the dimensions of a fallen column of regular form we
+should be able, he remarks, to find an inferior limit to the value of
+the maximum velocity; while a superior limit at the same place may be
+obtained from some other regular solid which escaped being overthrown.
+If a loose body is projected by the shock at a place where the angle
+of emergence is known, the horizontal and vertical distances traversed
+by the centre of gravity will give the velocity of projection. Or, if
+two such bodies are projected at one place, the same measures for each
+will as a rule give both the angle of emergence and the velocity of
+projection. A third method depends on the fissuring of walls,
+supposing that we know the force per unit surface which, when suddenly
+applied, is just sufficient to produce fracture. Sometimes more than
+one method must be applied to the same object. The two gate-pillars
+near Saponara (illustrated in Fig. 6) for example required a
+horizontal velocity of 5.48 feet per second to fracture them, and an
+additional velocity of 5.14 feet per second to overthrow them.
+
+The well-known seismologist, Professor Milne, urges very forcibly that
+measurements obtained from the projection or fall of columns are
+unreliable, for the earlier tremors might cause the columns to rock,
+and their overthrow need not therefore measure accurately the maximum
+velocity of the critical vibration.[16] There can be no doubt that
+Mallet was alive to this difficulty, though he may not have
+appreciated it at its full value. Thus, at the Certosa de St. Lorenzo,
+a monastery near Padula, a vase projected from the summit of a slender
+gate-pier implied a velocity of 21-3/4 feet per second; and the excess
+of about 8-1/4 feet per second above the velocity determined by other
+means is attributed by him to the oscillation of the pier itself. How
+far this source of error enters into other observations it is
+impossible to say; but it is worth noticing how closely the velocities
+obtained by different methods agree with one another. Thus, from
+projection only, we have velocities of 11.5 feet per second at the
+Certosa, 11.8 at Moliterno and Monticchio, 14.8 at Tramutola, and 9.8
+feet per second at Sarconi; from overthrow alone, 11.0 feet per second
+at Viscolione, near Saponara, and 11.6 at Barielle; from overthrow and
+projection, 13.2 feet per second at Polla and 12.9 at Padula; from
+fracture and overthrow, 12.3 feet per second at Potenza and 15.6 at
+Saponara. The comparatively high values at Tramutola and Saponara,
+Mallet imagined might be due to the oscillation of the hills on which
+these towns are built. He therefore omits them in calculating the mean
+maximum velocity, which he finds to be twelve feet per second, a
+velocity less than that with which a man reaches the ground when he
+jumps off a table.
+
+With the same omissions, Mallet gives the following table, showing a
+general decrease in the maximum velocity as the distance from his
+epicentre increases:--
+
+              Polla. Padula. Certosa. Moliterno. Viscolione. Sarconi.
+
+Distance in
+miles           4.0   19.0    19.0      29.4        30.0       30.8
+
+Max. vel. in
+ft. per sec.   13.2   12.9    11.5      11.8        11.0        9.8
+
+On the north side of the epicentre we have:--
+
+                           Potenza.   Monticchio.   Barielle.
+
+Distance in miles            17.3        27.1         28.2
+Max. vel. in ft. per sec.    12.3        11.8         11.6
+
+It is not impossible that the high calculated velocities at Tramutola
+and Saponara were partly or entirely due to the impulse from the
+Montemurro focus.
+
+If we take 4 inches for the amplitude of the largest variation, and 12
+feet per second for the maximum velocity, and assume the motion to
+have been of a simple harmonic character, the period of a complete
+vibration would be less than one-fifth of a second.[17] Now, we know
+from seismographic records that this is roughly the period of the
+small tremors that form the commencement of an earthquake-shock, while
+the period of the largest vibrations may amount to as much as one or
+two seconds. We may therefore conclude either that the assumption of
+simple harmonic motion is incorrect, or that the maximum velocity is
+too great, or more probably perhaps that the amplitude is too
+small.[18]
+
+
+SOUND-PHENOMENA.
+
+Mallet was one of the first seismologists to realise the significance
+of the earthquake-sound; and he attended closely to the subject,
+though finding the sound even more elusive of precise observation
+than the shock.
+
+The chief result obtained by him was the comparative smallness of the
+area over which the sound was heard. He estimates it at little more
+than 3,300 square miles, or about one-twelfth of that over which the
+shock was felt. It extends north and south from Melfi to Lagonegro,
+and east and west from Monte Peloso to Duchessa and Senerchia. The
+sound was thus confined to the region in which the shock attained its
+most destructive character.
+
+Towards the north and south ends of the sound-area all observers
+described the sound as a low, grating, heavy, sighing rush, lasting
+from twenty to sixty seconds, some adding that it was also of a
+rumbling nature. Near the centre and the east and west boundaries, the
+sound was distinctly more rumbling; it was shorter in duration, and
+began and ended more abruptly.
+
+The earthquake, Mallet remarks, "began everywhere with tremors; the
+sounds generally arrived at the same time; the apparent direction of
+movement of the tremulous oscillations appeared rapidly to change, and
+still more rapidly to increase in amplitude; then the great _shove_ of
+the destructive shock arrived, in some places rather before, in some a
+little after, the moment of loudest sound, and it died away suddenly
+(_i.e._, with extreme rapidity) into tremors again, but differing in
+direction from that of the great shock itself."[19]
+
+The earthquake-sound will be described more fully in the chapter
+dealing with the Hereford earthquake of 1896, in which it will be
+found that the phenomena recorded by Mallet are equally characteristic
+of the slighter shocks felt in this country.
+
+
+VELOCITY OF THE EARTH-WAVES.
+
+In 1857 little was known about the velocity of earthquake-waves.
+Experiments had been made by Mallet himself in 1849 in the
+neighbourhood of Dublin. These gave 825 feet per second for the
+velocity in dense wet sand, 1,306 feet per second in discontinuous
+granite, and 1,665 feet per second in more solid granite.[20] The only
+earthquake for which the velocity had been calculated was the Rhenish
+earthquake of 1846, the value ascertained by Schmidt being 1,376
+French feet, or 1,466 English feet, per second.
+
+The accurate public measurement of time, which, as Mallet remarks, is
+one of the surest indications of advancing civilisation, was, however,
+unknown in the kingdom of Naples; and his attempt was therefore
+fettered by the rarity of precise estimates of the time of occurrence.
+Throughout the whole disturbed area only six good records could be
+obtained, and three of these (at Vietri di Potenza, Atella, and
+Naples) were derived from stopped clocks, witnesses of rather doubtful
+value. At Montefermo and Barielle the time was at once read from a
+watch, and at Melfi from an accurate pocket chronometer. The times
+given vary from 9h. 59m. 16s. P.M. (Naples mean time) at Vietri di
+Potenza to 10h. 7m. 44s. at Naples. Allowing for the supposed change
+of direction by refraction at the Monte St. Angelo range on the way
+to Naples, Mallet finds the mean surface velocity to be 787 feet per
+second. Omitting the Naples record, and taking account of the
+calculated depth of the focus, the mean velocity becomes 804 feet per
+second.
+
+
+MINOR SHOCKS.
+
+A great earthquake rarely, if ever, occurs without some preparation in
+the form of a marked increase of seismic activity. Perrey records
+several shocks during the two years 1856-57 that were felt at places
+as far apart as Naples, Melfi, and Cosenza. On December 7th, 1857, a
+slight shock, with a report from beneath like the explosion of a mine,
+was felt at Potenza. Then came the great earthquake on December 16th,
+at about 10 P.M.
+
+This was followed by numerous after-shocks--how numerous it is
+impossible to say, for the records are of the scantiest description.
+For some hours the ground within the meizoseismal area is said to have
+trembled almost incessantly. At Potenza many slight shocks, both
+vertical and horizontal, were felt during the night, and for a month
+or more they were so frequent as to render enumeration difficult.
+Mallet's last record is dated March 23rd, 1858, when four slight
+shocks were felt at La Sala and Potenza, but occasional tremors were
+reported to him until May 1859.
+
+The most important of all these after-shocks was one felt about an
+hour after the principal earthquake. Everywhere far less powerful, it
+was yet strong enough to shake down many buildings at Polla that had
+been shattered by the great shock. Towards the south at Moliterno,
+and towards the north at Oliveto and Barielle, it evidently attracted
+very little attention. So far as can be judged from the evidence given
+by Mallet, the disturbed area seems to have been approximately of the
+same form and dimensions as the meizoseismal area, and elongated in
+the same direction, but concentric with the north-west focus.
+
+On the other hand, if we may rely on too brief evidence, several
+after-shocks recorded only at Montemurro, Saponara, Viggiano, or
+Lagonegro, were probably connected with the south-east or Montemurro
+focus.
+
+
+ORIGIN OF THE EARTHQUAKE.
+
+Mallet's theories have suffered perhaps more than any other part of
+his work from the recent growth of our knowledge. From a historical
+point of view, some reference to his explanation of the origin of the
+Neapolitan earthquake seems desirable, and his own conscientious work
+demands it. On the other hand, his conclusions are, for the present at
+any rate, superseded, and it will therefore be sufficient to describe
+them briefly.
+
+Most of the wave-paths, as we have seen, pass within three miles of a
+point almost coincident with the village of Caggiano. Of the
+remainder, six traverse a spot about two miles farther to the
+south-west, and three cross another about two miles farther to the
+north-east. Neglecting other points of intersection, but taking
+account of the observed emergences at Vietri di Potenza, Auletta,
+Polla, etc., Mallet infers that the horizontal section of the focus
+was a curve (indicated by the dotted line in Fig. 9) not less than ten
+miles in length, and passing from near Balvano on the north, close to
+Vietri di Potenza, Caggiano, and Pertosa, to a point about two and a
+half miles west of Polla. Again, he remarks, the observed emergences
+at places near the epicentre indicate that the vertical section of the
+seismic focus was either more or less curved, or more probably a
+surface inclined towards the south-east. He concludes, therefore, that
+the seismic focus was a curved fissure, 10 miles long and 3-1/2 miles
+in height, and with its centre at a depth of 6-1/2 miles below the
+level of the sea.
+
+The production of this great fissure, accompanied, perhaps by the
+injection into it of steam at high pressure, was regarded by Mallet as
+the cause of the principal earthquake. He imagines that the rent would
+start at or near the central point of the focus and then extend
+rapidly outwards in all directions. In the initial stage, vibrations
+of very small amplitude would alone be transmitted, and these would
+give rise to the early sounds and tremors. As the rending proceeded,
+the vibrations would increase in strength up to a certain point when
+they produced the shock itself. After this, they would decrease; and,
+in the final stage, would give place to the small vibrations
+corresponding to the sounds and tremors that marked the close of the
+earthquake.
+
+The rush of steam at high pressure into the focus Mallet does not seem
+to have considered essential, though he evidently regarded it as
+possible, indeed probable; and he suggests that it may have been in
+part the cause of the earthquake which occurred an hour later. Though
+feeling sceptical as to the existence of any general law of increase
+of underground temperature, he assumes it, for the sake of
+illustration, to be 1° F. for every 60 feet of descent. This would
+give a temperature of 339° F. at the upper limit of the focus, 643° F.
+at its central point, and 884° F. at its lower margin. If the focus
+were filled with steam at each of these temperatures, the
+corresponding pressures on its walls would be 8, 149, and 684
+atmospheres, respectively. As the steam may be supposed to be admitted
+suddenly and to be unlimited in supply, Mallet infers that it might
+exist at the tension due to the highest of these temperatures, in
+which case it would be capable of lifting a column of limestone 8,550
+feet in height (or about one-half the depth of the upper margin of the
+focus), and would exert a pressure on the walls of the focus of 4.58
+tons per square inch, or of more than 640,528 millions of tons upon
+its whole surface.
+
+So many pages have already been given to this interesting earthquake
+that I must sketch still more briefly my own view as to its origin.
+There were, I believe, two distinct foci with their centres about
+twenty-four miles apart along a north-west and south-east line, and it
+was to this arrangement that the elongation of the meizoseismal area
+was chiefly, though not entirely, due. The evidence is insufficient to
+determine whether the earthquake was caused by fault-slipping; it is
+in no way opposed to this view, but if the Neapolitan earthquake stood
+alone, we should hardly be justified in drawing any further inference.
+Relying, however, on knowledge obtained from the study of more recent
+shocks, it seems to me probable that the two foci formed parts of one
+fault with a general north-west and south-east direction. The slip
+causing the first part of the double shock apparently took place
+within the south-east focus, and was followed after a few seconds by
+one within the north-west focus, greater in amount as well as more
+deeply seated. In consequence of these displacements there were local
+increases of stress, causing numerous small slips within or near both
+principal foci; and, if we may judge from some slight shocks felt at
+La Sala, accompanied also by other minor slips in the intermediate
+region of the fault.
+
+
+REFERENCE.
+
+  MALLET, R.--_The Great Neapolitan Earthquake of 1857: The First
+        Principles of Observational Seismology_, etc. 2 vols 1862.
+
+FOOTNOTES:
+
+[3] _Irish Acad. Trans._, vol. xxi., 1848, pp. 51-105 (read Feb. 9,
+1846).
+
+[4] _Brit. Assoc. Reports_, 1850, pp. 1-87; 1851, pp. 272-330; 1852,
+pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136.
+
+[5] _A Manual of Scientific Enquiry_, edited by Sir J.F.W. Herschel,
+1849, pp. 196-223.
+
+[6] _Irish Acad. Trans._, vol. xxii., 1855, pp. 397-410.
+
+[7] The linear dimensions of the isoseismal lines are obtained by
+measurements from Mallet's maps. The areas are given by him in
+geographical square miles.
+
+[8] Mallet, by some accident, omitted the losses at Polla and
+neighbouring towns from this estimate. Mercalli (_Geologia d'Italia_,
+pte. 3, p. 324) gives the number of killed as more than 12,300.
+
+[9] Mallet does not make use of the term _epicentre_; he speaks of the
+line FE as the _seismic vertical_. The modern and accepted terms are
+used above
+
+[10] _Japan Seismol. Soc. Trans._, vol. xi., 1887, pp. 175-177.
+
+[11] _Ital. Seismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.
+
+[12] Professor Omori gives the mean direction as S. 71° W., but this
+was obtained from observation on lamps with square, as well as with
+circular bases.
+
+[13] Twelve measurements chosen at random from Professor Omori's list
+gave a mean direction of S. 78° W.
+
+[14] When the accuracy of all the observations seemed equally
+probable, he adopted the mean of the two extremes as the true
+direction.
+
+[15] If _a_ be the amplitude of a simple harmonic vibration, _T_ its
+complete period, _v_ its maximum velocity, and _f_ its maximum
+acceleration, we have v=2*pi*a/T and f=4*pi^2*a/T^2
+
+[16] _Earthquakes and other Earth Movements_, pp. 81-82.
+
+[17] Obtained from the formula: T=2*pi*a/v=2*pi*x*(1/3)/12
+
+[18] If we take the maximum velocity to be 12 feet per second, and the
+period to be one second, the amplitude would be about 11-1/2 inches.
+
+[19] Vol. ii., p. 299. The punctuation of the original is not followed
+in the above extract.
+
+[20] _British Association Report_, 1851, pp. 272-320.
+
+
+
+
+CHAPTER III.
+
+THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY 28TH, 1883.
+
+
+Separated from Italy by a distance of not more than six miles, Ischia
+and the intermediate island of Procida strictly form part of the
+Phlegræan Fields, the well-known volcanic district to the north of
+Naples. Ischia, the larger of the two islands, is six miles long from
+east to west, and five miles from north to south, and contains an area
+of twenty-six square miles. In 1881, the total population was 22,170,
+that of Casamicciola, the largest town, being 3,963.
+
+
+VOLCANIC HISTORY OF ISCHIA.
+
+The central feature of Ischia is the great crater of Epomeo (_a_, Fig.
+14). On the south side, and partly also on the east, the crater-wall
+has been broken down and removed; the portion remaining is about 1-1/2
+mile in diameter from east to west, and reaches a height of 2,600 feet
+above the sea-level. All the upper part of the mountain is composed of
+a pumiceous tufa, rich in sanidine and of a characteristic greenish
+colour. At two points, to the west near Forio and to the north between
+Lacco and Casamicciola, this tufa is seen reaching down to the sea;
+but, in all other parts, it is covered by streams of trachitic lava,
+by more recent tufas, or by a deposit of marly appearance, which is
+regarded by Fuchs as resulting from the decomposition of the Epomean
+tufa.
+
+There are two distinct periods in the geological history of Ischia.
+The first, a submarine period, probably began with the dawn of the
+quaternary epoch, for all the marine fossils of the island belong to
+existing species. About this time, Epomeo seems to have originated in
+eruptions occurring in a sea at least 1,700 feet in depth--eruptions
+that preceded the formation of Monte Somma and were either
+contemporaneous or alternating with those that gave rise to the oldest
+trachitic tufas of the Phlegræan Fields. The destruction of the south
+wall may have occurred much later through some great eruptive
+paroxysm, but more probably, as Professor Mercalli suggests, through
+early marine erosion and subsequent subaerial denudation. To the
+submarine period must also be assigned the formation of the trachitic
+masses which compose Monti Trippiti, Vetta, and Garofoli (_b_, _c_,
+_d_, Fig. 14), on the east side of Epomeo; and, in part only, those of
+Monte Campagnano and Monte Vezza (_f_, _g_).
+
+At or near the close of the elevation, many violent eruptions occurred
+on the south-west of Epomeo, during which was formed the south-west
+corner of the island, including Monte Imperatore and Capo Sant' Angelo
+(_h_, _i_).
+
+In the second or terrestrial period, when the island had practically
+attained its present altitude, the eruptive activity was almost
+confined to the eastern and northern flanks of Epomeo. At the
+beginning Monte Lo Toppo (_j_) was formed by a lateral eruption. In
+the north-west corner of the island, Monte Marecocco and Monte Zale
+(_k_ and _l_) owe their origin to a gigantic flow of sanidinic
+trachite, issuing probably from the depression which now exists
+between them. Lastly, towards the north-east, are the recent lateral
+craters of Rotaro, Montagnone, Bagno, and Cremate (_m_, _n_, _p_,
+_s_), the first two being the most regular and best preserved in the
+island.
+
+   [Illustration: FIG. 14.--Geological sketch-map of Ischia.
+   (_Mercalli._)[21]]
+
+The earliest eruption of the historic, or rather human, period appears
+to have taken place from Montagnone, and probably also at about the
+same time from the secondary crater of Porto d'Ischia (_u_), about the
+beginning of the eleventh century B.C. The eruptions of Marecocco and
+Zale are referred to about B.C. 470; and those of Rotaro and Tabor
+(_q_) to between the years 400 and 352 B.C. Another eruption is said
+to have occurred in B.C. 89, but the site of it is unknown; and three
+others are recorded on doubtful authority about the years A.D. 79-81,
+138-161, and 284-305. The last outburst of all took place after the
+series of earthquakes in 1302 from a new crater, that of Cremate
+(_s_), which opened on the north-east flank of Epomeo, and from which
+a stream of lava, called the Arso (_t_), flowed down rapidly and,
+after a course of two miles, reached the sea.
+
+After the first eruptions to which it owed its origin, the central
+crater of Epomeo apparently remained inactive. All the later eruptions
+occurred either on the external flanks of the mountain or on radial
+fractures of the cone.[22] Trippiti, Lo Toppo, Montagnone and the Lago
+del Bagno (_b_, _j_, _n_, _p_) lie in one line, Vetta and Cremate
+(_c_, _s_) on another, and Garofoli and Vatoliere (_d_, _e_) on a
+third, all passing through a point near the town of Fontana, which
+occupies the centre of the old crater of Epomeo.
+
+Professor Mercalli points out that the lateral eruptions of Epomeo
+differ in one respect from those of Etna and Vesuvius. In these
+volcanoes the lava ascends to a considerable height in the central
+chimney, and by its own weight rends open the flanks of the cone. In
+Epomeo, it appears to traverse lateral passages at some depth, perhaps
+far below the level of the sea, and to rend the mountain by means of
+the elastic force of the aqueous vapour, etc., which it contains. It
+will be seen how important is the bearing of this difference on the
+occurrence of the Ischian earthquakes.
+
+The eruptions that have taken place during the last three thousand
+years agree in several particulars. They either occurred suddenly, or,
+at any rate, were not preceded by a stage of moderate Strombolian
+activity; they were always accompanied by violent earthquakes; and all
+succeeded intervals of long repose. As the eruption of 1302 happened
+after at least a thousand years of rest, the lapse of six more
+centuries does not justify us in concluding that Epomeo is at last
+extinct.
+
+We seem, on the contrary, to be drawing near another epoch of
+activity. During the four and a half centuries that followed the
+eruption of 1302, we have no record of Ischian earthquakes.[23] Then,
+suddenly, on the night of July 28-29, 1762, Casamicciola was visited
+by sixty-two shocks, some of which were very strong and damaged
+buildings. On March 18th, 1796, another severe shock took place, but
+destructive only in the neighbourhood of Casamicciola, where seven
+persons were killed. On February 2nd, 1828, the area of damage, though
+concentric with the former, enlarged its boundaries; 30 persons were
+killed and 50 wounded. On March 6th, 1841, and during the night of
+August 15-16, 1867, further shocks injured houses at Casamicciola, but
+without causing any loss of life. Slight tremors occurred at various
+dates in 1874, 1875, 1879, and 1880, leading up to the disastrous
+earthquakes here described, those of March 4th, 1881, when 127 persons
+were killed, and July 28th, 1883, which resulted in the death of 2,313
+persons and the wounding of many others.
+
+
+EARTHQUAKE OF MARCH 4TH, 1881.
+
+The Ischian earthquakes have been fortunate in their investigators. In
+the spring of 1881, Dr. H.J. Johnston-Lavis, the chronicler for many
+years of Vesuvian phenomena, was residing in Naples. Impressed by a
+recent perusal of Mallet's report on the Neapolitan earthquake, and
+wishing to test the value of the methods explained in the last
+chapter, he crossed over to Ischia on March 5th; and to his unwearied
+inquiries extending over more than three weeks and lasting from
+thirteen to sixteen hours a day, we are indebted for most of what we
+know about the earthquake of 1881.
+
+On March 4th, at 1.5 P.M., the great shock occurred abruptly, without
+any warning tremors. Its effects were aggravated by the faulty
+construction of the houses. The walls are of great thickness, loosely
+put together, and connected by mortar of the poorest quality. The
+chimneys and roofs also are massive, and the rafters are so slightly
+inserted in the walls that they were drawn out with the rocking of the
+houses. In such cases, the destruction was often so complete that no
+fissures were left available for measurement.
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+The isoseismal lines as drawn by Dr. Johnston-Lavis are represented by
+the curves in Fig. 15. The isoseismal marked 1 bounds the area of
+complete destruction; it is about 1 mile long from east to west, 2/3
+of a mile broad, and contains an area of not more than half a square
+mile. The next isoseismal (2) marks the area of partial, but still
+serious, destruction; this is nearly 2 miles long from east to west,
+1-1/4 miles broad, and 2 square miles in area. Within the isoseismal
+3, buildings were more or less slightly damaged. The course of this
+curve is somewhat doubtful, but, as drawn, it is about 3 miles long,
+2 miles wide, and 5 square miles in area.
+
+   [Illustration: FIG. 15.--Isoseismal lines of the Ischian
+   earthquake of 1881. (_Johnston-Lavis._)]
+
+Outside the last curve, the shock diminished rapidly in intensity. At
+Monte Tabor and Bagno, it was very slight; in the town of Ischia, only
+about half the people were conscious of any movement; and at Capella,
+a small village to the south, it was not felt at all. Again, the shock
+was perceptible, though only faintly, in the neighbourhood of
+Campagnano, at Serrara to the south of Epomeo, and at Panza near the
+south-west corner of the island. On the other hand, at Fontana, which
+occupies approximately the centre of the crater of Epomeo, there were
+evidences of a distinctly stronger shock. No house actually fell, and
+side walls were but little injured; but the roofs, which are of great
+weight, suffered considerable injury.
+
+In the adjacent island of Procida, the shock was felt distinctly by
+many people, and by some, though slightly, at Monte di Procida,
+Misenum, and Bacoli, on the coast of Italy. No record whatever was
+given by the seismographs in the university of Naples and the
+observatory on Vesuvius. We have of course no means of estimating the
+exact size of the disturbed area, but in this respect, disastrous as
+the earthquake was in the neighbourhood of Casamicciola, it was
+clearly inferior to all but the very weakest earthquakes felt in the
+British Islands.
+
+
+POSITION OF THE EPICENTRE.
+
+In determining the position of the epicentre, Mallet's method was
+closely followed. Fissures in buildings were used for the most part,
+in two out of every three cases; and occasional measurements were
+made from objects overthrown, projected, or shifted, and also from the
+personal experiences of observers. The attempt to apply the method
+was, however, fraught with difficulties. The heterogeneous structure
+of the island was no doubt responsible for many divergent azimuths;
+the irregularity of the buildings both in form and material and their
+variety of site furnished other sources of error; even the smallness
+of the area was a disadvantage in lessening the number of trustworthy
+records.
+
+Measurements were made at 55 places altogether, but in most cases they
+were the results of isolated observations, not the means of several at
+each place. On this account, I have not reproduced in Fig. 15 the
+azimuths shown in Dr. Johnston-Lavis's map of the earthquake. A large
+number of them clearly converge towards an area lying to the west of
+Casamicciola; and, from their arrangement, Dr. Johnston-Lavis
+concludes, though the evidence does not seem to me quite strong enough
+for the purpose, that they emanated from a fracture running from a
+little west of north to a little east of south.
+
+This conclusion is, however, justified by other evidence. In the
+centre of the injured district, Dr. Johnston-Lavis has traced a
+meizoseismal band, in which the shock must have been nearly or quite
+vertical. "The damage inflicted on buildings included within this band
+was," he says, "very characteristic of the nature of the shock; the
+walls having received but slight injury, whilst almost every floor and
+ceiling had been totally destroyed. In fact," he adds, "many houses
+would have required no other repairs than the replacing of the
+divisions between the different storeys." The shaded central area in
+Fig. 15 represents this band, passing in a nearly north and south
+direction from a point midway between Campo and the upper part of
+Lacco on the north, through the west part of Casamenella and Campo, to
+a point near Frasso on the south; the length of the band being thus
+about two-thirds of a mile.
+
+If the central line of this band is produced towards the south, as
+indicated by the dotted line, it grazes the west side of Fontana,
+where, as we have seen, there was a second meizoseismal area, much
+smaller than the other and surrounded by a district in which houses
+were almost uninjured. That the shock in this town was vertical or
+nearly so, is shown by the nature of the damage (p. 52) and also by
+the testimony of the inhabitants. I will give Dr. Johnston-Lavis's
+explanation of this detached meizoseismal area when discussing the
+origin of the Ischian earthquakes; but the evidence seems to me to
+favour either the existence of two distinct foci or, more probably
+perhaps, the extension of the fissure to the south with an increased
+impulse beneath the centre of Epomeo.
+
+
+DEPTH OF THE SEISMIC FOCUS.
+
+At nine places, Dr. Johnston-Lavis was able to make measurements of
+the angle of emergence, in every case from fissures in buildings, and
+therefore liable to sources of error already referred to. On the other
+hand, owing to the small depth of the focus, there would probably be
+less general refraction of the wave-paths than in the Neapolitan
+earthquake. The depths indicated by these observations vary between
+about 615 and 2,885 feet, a difference that is no greater than might
+be expected, as the size of the focus was no doubt comparable with
+that of the district in which observations were made. The mean depth
+Dr. Johnston-Lavis finds to be about 1,700 feet, or a little less than
+one-third of a mile.
+
+
+NATURE OF THE SHOCK.
+
+The limited depth of the focus is also evident from the nature of the
+shock. It was only within the actual meizoseismal band that the shock
+was subsultory or vertical throughout; at a short distance from the
+epicentre, the movement was both subsultory and undulatory; while near
+the third isoseismal, and in most of the region outside, the movement
+was entirely undulatory or lateral. An observer at Perrone (which lies
+1-2/3 miles east of the epicentre) gives the following account of the
+shock:--"I was standing on my balcony (this faces Casamicciola)
+admiring the scene ... when I felt the house rock, feeling at the same
+time as if something was rolling along beneath the ground. This
+movement was accompanied by a sound like this, Boob, boob -- boob --
+-- boob -- -- -- boob -- -- -- -- boob. Both noise and movement seemed to
+come from Casamicciola.... In a few seconds, in the distance over the
+town arose a terrific cloud of white dust, so that I imagined the town
+on fire.... I felt hardly any, if any, subsultory movement, but as I
+leant upon the balcony rails, I was alternately pressed against them
+and then drawn away."
+
+At Fontana, however, the undulatory shock was replaced by a vertical
+one. This was the universal experience, though one or two persons felt
+a slight lateral movement immediately after. At Valle (near Barano)
+and Piejo, both places about a mile from Fontana, the vertical
+component was also perceptible.
+
+
+AFTER-SHOCKS.
+
+The after-shocks were few and of slight intensity. Dr. Johnston-Lavis
+gives the following dates: March 7th, 12.5 A.M. and midday; March
+11-12, 15-16, 17-18, 27 (?), April 5th and 6th, and July 18th, 8.30
+P.M. The only shock of the series marked as strong occurred at
+midnight on March 15-16 at Casamicciola. The last of all, that of July
+18th, consisted of a rumble and slight shock, and was most perceptible
+at Fango.
+
+
+EARTHQUAKE OF JULY 28TH, 1883.
+
+Undeterred by the experience of 1881 or by the warnings of
+seismologists, Casamicciola was rebuilt, only to suffer more complete
+disaster. On July 28th, 1883, at 9.25 P.M., occurred the most
+destructive earthquake of which we have any record in Ischia. The
+shock lasted about fifteen seconds, and before it was over clouds of
+dust were rising above the ruins of Casamicciola, Lacco, and Forio;
+1,200 houses were destroyed, 2,313 persons were killed, nearly 1,800
+in Casamicciola alone, and more than 800 seriously wounded. "No better
+idea," says Dr. Johnston-Lavis, "of the absolute destruction of
+buildings could be conceived than what was actually realised at
+Casamicciola and Campo. Looking, on the following Monday, over the
+field of destruction, I could discover (with few exceptions) the
+wall-stumps only remaining."
+
+Dr. Johnston-Lavis again spent about three weeks in the island,
+examining the effects of the new shock with equal zeal and wider
+experience. His monograph is now our chief work of reference on
+Ischian earthquakes. Inquiries were also made by several Italian
+seismologists, among others by Professor M.S. de Rossi, the organiser
+of earthquake-studies in the peninsula; by Professor L. Palmieri, the
+founder of the Vesuvian observatory; and especially by Professor G.
+Mercalli, whose valuable memoir supplements the report of Dr.
+Johnston-Lavis in some important particulars.
+
+
+PREPARATORY SIGNS.
+
+The interval between July 18th, 1881, when the last shock of that year
+was felt, and July 28th, 1883, was one of almost complete quiescence.
+Early in March 1882, a few slight shocks were noticed at Casamicciola.
+On July 24th, 1883, a watch hanging from a nail in a wall was seen to
+swing at 6 A.M. and 9 A.M., and, on the same morning, at about 8.30, a
+slight shock, accompanied by a rumbling sound, was felt at
+Casamicciola. Again, on the 28th, about a quarter of an hour before
+the great shock, one observer at Casamicciola states that an
+underground noise was heard, and that some persons in consequence left
+their houses.
+
+Many assertions have been made with regard to variations witnessed a
+day or two before the shock in the hot springs, such as an increase of
+flow or temperature and changes in their volume and purity. Fumaroles
+are alleged to have burst out with violence, and even flames to have
+been seen. The statements, though widely quoted, can hardly be said
+to rest on satisfactory evidence. On the other hand, Dr.
+Johnston-Lavis arrived in the island within twenty-four hours after
+the shock, and, before another day had elapsed, he had examined most
+of the places where the phenomena were said to have occurred, but
+could find no remarkable change nor any signs of such having taken
+place. It is also known, as he remarks, that the temperature of the
+Ischian springs and fumaroles sometimes varies considerably without
+any earthquake following, that of the water of Gurgitello occasionally
+changing by as much as 30° or 40°. We may therefore, I think, conclude
+that, except for one or two shocks and underground noises too slight
+to cause general alarm, there were no decisive heralds of the great
+earthquake.
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+The curves in Fig. 16 represent the isoseismal lines as drawn by Dr.
+Johnston-Lavis. As in the earthquake of 1881, they bound respectively
+the areas of complete destruction, partial destruction and slight
+damage to buildings, the course of the outer line being to a great
+extent conjectural owing to the small extent of land traversed by it.
+The first isoseismal is about 2-1/2 miles long, 1-1/2 miles broad, and
+3 square miles in area; the second about 4 miles long, 3-1/2 miles
+broad, and 11 square miles in area; and the third about 6-1/2 miles
+long, 6 miles broad, and 30 square miles in area. The curve drawn by
+Professor Mercalli (Fig. 14) coincides nearly with the second of these
+lines.
+
+At Fontana, the damage exceeded that in the surrounding country,
+though the difference was of course less marked than on the previous
+occasion.
+
+   [Illustration: FIG. 16.--Isoseismal lines of the Ischian
+   earthquake of 1883.]
+
+Outside Ischia, the shock was felt distinctly in all the island of
+Procida and in Vivara; on the mainland, by some as far as Pozzuoli
+and by several persons in Naples, which is twenty miles from
+Casamicciola. The seismograph at the university of this city
+registered two small shocks, the first at 9.10 P.M., and the second
+and stronger at 9.25 P.M.; and De Rossi states that at about 9.30 P.M.
+the seismographs at Ceccano, Velletri, and Rome recorded a shock
+consisting of very slow undulations. There are again no materials for
+estimating the size of the disturbed area, but there can be no doubt
+that it was much less than that of a moderately strong British
+earthquake.
+
+
+POSITION OF THE EPICENTRE.
+
+Owing to the limited size of the disturbed area, time-observations,
+even had they been available, would not have sufficed to determine the
+position of the epicentre, and both Dr. Johnston-Lavis and Professor
+Mercalli therefore had recourse to Mallet's method, the former relying
+chiefly, as before, on fissures in damaged buildings, and the latter
+on the overthrow or displacement of columns and other objects.
+
+Dr. Johnston-Lavis measured the azimuth of the wave-paths at
+sixty-five places, and at about one-third of these was able to make
+two or more observations. The azimuths converge towards the same
+region as in 1881, but the area covered by their intersections is
+larger. The meizoseismal band of maximum vertical destruction
+indicated by shading in Fig. 16 is also of the same form and slightly
+greater extent, reaching from the upper part of Lacco to a little
+south of Frasso, and being therefore nearly a mile in length. The
+centre of maximum impulse was in the same position as in 1881, or
+possibly a little more to the south.
+
+Professor Mercalli's observations were made at forty-eight places, and
+in only six cases were they the same as those used by his predecessor.
+He also notices that most of the azimuths converge towards
+Casamenella, and intersect within an elongated area. This area runs in
+the same direction as Dr. Johnston-Lavis's meizoseismal band, but is
+less elongated, and situated a short distance farther to the south,
+though on the whole the agreement between the two areas is remarkably
+close.
+
+There was again apparently a second epicentre at Fontana. In this
+town, according to Dr. Johnston-Lavis, there were two distinct types
+of damage. As in 1881, there was evidence of a vertical blow, the only
+one that absolutely ruined houses; but, in addition, there was another
+independent set of fissures, quite as widely distributed as the
+others, though evidently caused by a less violent movement. These
+indicated a wave-path with a low angle of emergence coming from
+between north and north-north-west, or almost exactly in the line of
+meizoseismal band. To the south of Fontana, however, there is a group
+of places, including Panza, Serrara, Barano, etc., where the azimuths
+diverged rather widely from the epicentre at Casamenella. These
+azimuths are twelve in number, and it is worthy of notice that they
+all intersected the crater of Epomeo, while half of them passed within
+a few hundred yards of Fontana.
+
+
+DEPTH OF THE SEISMIC FOCUS.
+
+Measurements of the angle of emergence were made by Dr. Johnston-Lavis
+at twenty-four places, and in every case from fissured walls. The
+greater part of the diagram on which his results are depicted is
+reproduced in Fig. 17. The horizontal line, as in Fig. 13, represents
+the level of the sea, the longer vertical line one passing through the
+epicentre, and the shorter another through Fontana. The short lines
+on the left of the former show the incipient wave-paths to places
+lying east of the epicentre; those on the right, with one exception,
+represent the wave-paths to places west of the same meridian. Small
+horizontal marks are inserted on the vertical lines to show the depth
+in tenths of a mile below the level of the sea.
+
+   [Illustration: FIG. 17.--Diagram of wave-paths at seismic
+   vertical of Ischian earthquake of 1883. (_Johnston-Lavis._)]
+
+The six angles of emergence that would give the greatest depth below
+the epicentre were all measured at places in the south of the island
+close to the line joining Panza and Barano, and it will be noticed
+that five of these apparent depths are much greater than those
+obtained from the other wave-paths. Excluding these observations, the
+remaining eighteen give depths ranging from about 450 to about 3,350
+feet, and a mean depth of 1,730 feet,[24] or nearly one-third of a
+mile, that is, almost exactly the same as the mean depth found from
+the earthquake of 1881.
+
+The six exceptional angles of emergence come from the district of
+divergent azimuths to the south of Epomeo. Three of the corresponding
+azimuths pass within one-quarter of a mile from the centre of Fontana,
+and none of the other three more than three-quarters of a mile from
+the same point. Though disbelieving in a subsidiary focus below this
+town, Dr. Johnston-Lavis has calculated its mean depth, supposing it
+to exist, and found it to be about 1,560 feet below the sea level, a
+result which is remarkably close to the calculated mean depth of the
+focus near Casamenella.
+
+
+NATURE OF THE SHOCK.
+
+In the meizoseismal band, preliminary tremor and rumbling sound were
+alike absent. So sudden, indeed, was the onset of the earthquake, that
+the survivors generally found themselves beneath the ruins of their
+houses before they were conscious of any shock. The destruction,
+practically instantaneous, was wrought by four or five vertical blows,
+so powerful that, according to some observers, Casamicciola seemed to
+jump into the air. Then followed undulations, not noticed by all, that
+appeared to come from every direction. The shock lasted altogether
+fifteen seconds or more,[25] and was accompanied by a rumbling noise,
+in the midst of which were detonations as of thunder or of great blows
+given upon an empty barrel.
+
+In the immediate neighbourhood of the meizoseismal area, at Perrone,
+Pennella, and Lower Lacco, the subsultory movement was still the more
+prominent; but, farther away, as at Panza, Testacchio, Barano, Ischia,
+and Bagno, the subsultory motion was followed by distinctly horizontal
+undulations, while outside the island of Ischia only slow undulatory
+movements were perceptible.
+
+
+LANDSLIPS.
+
+The dotted areas in Fig. 16 indicate the sites of the only landslips
+of importance that were precipitated by the earthquake of 1883. Two of
+these occurred on the north slope of Epomeo, and the third on the
+west flank of Monte Rotaro. The materials of the Epomean landslips had
+evidently been separated for some time by shallow fissures from the
+adjoining rock, for the surfaces of the fissures were discoloured by
+fumarolic action. Immediately after the earthquake a cloud of dust was
+seen to rise from the spots; the masses, already detached laterally,
+were merely set in motion by the shock; and they continued to slide
+down during the following days either through the action of the
+after-shocks or of the heavy rains that followed.
+
+All over the island, however, fissures and minor landslips occurred.
+At two places on the north coast the steep cliffs of incoherent tufa
+were so much damaged that, according to Dr. Johnston-Lavis, "large
+quantities of their materials were thrown into the sea. The water then
+sorted out the pieces of pumice, which in many cases were of very
+large size, and were seen floating about in the neighbourhood for some
+days," giving rise to the supposition that a submarine eruption had
+taken place to the north of the island.
+
+
+AFTER-SHOCKS.
+
+The after-shocks in 1883 were much more numerous than in 1881. Between
+9.25 P.M. on July 28th and noon on August 3rd, twenty-one slight
+shocks were recorded at Casamicciola. At 2.15 P.M. on August 3rd, a
+violent shock occurred that caused further damage at Forio, and even
+at places so far from the epicentre as Fiaiano, Barano, and Fontana,
+and increased the displacements of the landslips on Epomeo. This
+shock was also registered at the observatory on Vesuvius.
+
+After this the shocks became less frequent and slighter, twelve being
+felt at Casamicciola during the remainder of the year, and six in the
+first half of 1884. Several shocks and rumbling noises were also
+observed in other parts of the island. Among them may be mentioned
+noises heard at Fontana on August 12th and 15th, and a slight shock at
+the same place on August 17th; also on September 4th, at 10.30 and
+10.40 A.M., slight shocks at Barano, Serrara, and Forio. On March
+27th, 1884, at 2.7 P.M., another strong shock occurred; strongest at
+Serrara, where the shock was subsultory and accompanied by noise; and
+less strong, though still subsultory, at Ciglio, Panza, Forio,
+Fiaiano, and Casamicciola, and very slight at Ischia. The series seems
+to have ended during the following summer, with a slight shock at
+Casamicciola on July 21st, and a stronger one on July 23rd, felt from
+Casamicciola on the north to Serrara on the south.
+
+Most of the after-shocks must have originated in the neighbourhood of
+Casamicciola, but it is worthy of notice that more than one centre was
+in action. Several were recorded at Ischia only. Others, as mentioned
+above, affected chiefly the south part of the island, and especially
+the small towns of Serrara and Fontana.
+
+
+CHARACTERISTICS OF ISCHIAN EARTHQUAKES.
+
+After the eruption of 1302, there succeeded a period of comparative
+repose in Ischia. The revival of activity dates from 1762, and, since
+that year, there have been four great earthquakes, namely, those of
+1796, 1828, 1881, and 1883. In every respect but that of increasing
+intensity, these earthquakes were apparently identical; each, as
+Professor Mercalli says, was merely a replica on a different scale of
+those that preceded it. The principal features in which they resemble
+one another, and differ from the average tectonic earthquake, are the
+coincidence of the epicentres, the small depth of the foci, and the
+sudden onset of the principal shock.
+
+1. _Coincidence of Epicentres._--In Fig. 14, which is copied from
+Professor Mercalli's map, are shown the areas in which buildings were
+seriously damaged by these four earthquakes. The curves for 1796,
+1828, and 1881 are approximately concentric. In 1796, the shock was
+disastrous only to the west of Casamicciola; in 1828, according to
+Covelli, "the ground most injured was not precisely the region of
+Casamicciola, but that which lies between the district called Fango
+and that known as Casamenella, situated to the west of Casamicciola,
+and a short distance from it."[26] The epicentres may have varied
+slightly in size, but, in position, it is clear that all four were
+nearly or quite coincident. The meizoseismal bands in 1881 and 1883
+were also similar in form and elongated in the same direction.
+
+In the last two earthquakes there was, as we have seen, very distinct
+evidence of a secondary meizoseismal area surrounding Fontana, and it
+is remarkable that this was also noticeable in the earthquake of 1828.
+"Besides the centre of vibration in the district of Fango," says
+Covelli, "another less powerful centre showed itself in the locality
+of Fontana; this made itself felt more heavily than in surrounding
+localities; as if another centre of movement had taken place from that
+part, independent of the former."
+
+2. _Small Depth of the Foci._--Mallet's method, as noted above, cannot
+be trusted to yield accurate estimates of the focal depth, or to
+indicate more than its order of magnitude. But it is remarkable that
+the depths calculated by Dr. Johnston-Lavis for the last two
+earthquakes are both only a little less than a third of a mile, and it
+is probable that the actual depth did not differ very greatly from
+this amount. The nature of the shock, vertical or nearly so close to
+the epicentre and horizontal at a short distance from it, is merely
+personal testimony of the same character as fissures in masonry, and
+of course points to the same result.
+
+   [Illustration: FIG. 18.--Diagram showing connection between depth
+   of focus and rate of decline in intensity.]
+
+But the most conclusive evidence on which we have to rely is the
+extraordinary intensity of the shock at the centre of a very small
+distributed area. In Great Britain, an earthquake felt over a district
+of equal size would hardly at the centre exceed the trembling produced
+in a station platform by a passing train. The curves in Fig. 18 show
+how the rate of decline in intensity depends on the depth of the
+focus. They are drawn on the supposition that the intensity at any
+point on the surface varies inversely as the square of its distance
+from the focus; the curves _a_, _b_, _c_ corresponding to foci
+situated at depths of one-third of a mile, one mile, and two miles
+respectively, and the figures below the horizontal line denoting the
+distance in miles from the epicentre. Thus, the rapid decline of
+intensity from the epicentre outwards shows that, in each of the four
+great Ischian earthquakes, the depths of the focus must have been very
+small.
+
+3. _Suddenness of the Shocks._--In 1796, we have no record of
+preparatory shocks, but the evidence is scanty; in 1828 and 1881, none
+are mentioned; in 1883, one or two tremors and underground noises,
+possibly of seismic origin, gave warning to a few. Fore-shocks, for
+all practical purposes, were conspicuous by their absence.
+
+Still more remarkable is the sudden advent of the great shocks. There
+were no preliminary tremors or rumbling sound, no animals showed signs
+of uneasiness and no birds fluttered screaming from trees or ground.
+The shock of 1828, says Covelli, "was announced by three powerful
+blows coming almost vertically, from below upwards;" and the same
+words apply equally well to the earthquakes of 1881 and 1883. The
+destruction of houses in every case was practically instantaneous, and
+coincident with the first vibration.
+
+In all respects, tectonic earthquakes differ widely from the Ischian
+shocks. The epicentres of successive earthquakes are rarely
+coincident, but show a distinct tendency to migration along certain
+lines; the decline in intensity outwards from the epicentre is nearly
+always very gradual, and therefore indicative of a comparatively
+deep-seated focus; they are almost invariably preceded either by a
+series of slight shocks and rumbling sounds, or, in an unstable
+district, by a marked increase in their frequency. Distinctions, so
+great as these are, evidently remove the Ischian shocks from the
+category of tectonic earthquakes.
+
+
+ORIGIN OF THE ISCHIAN EARTHQUAKES.
+
+On the other hand, the Ischian earthquakes possess several features
+which connect them closely with true volcanic earthquakes.
+
+1. They originate beneath the northern slope of Epomeo--a volcano that
+we have no reason to consider absolutely extinct, but rather as one
+subject to eruptions at long intervals of time--in a region as yet
+unoccupied by parasitic craters, but having the same relation to the
+central cone of Epomeo as those in which the recent craters of Monte
+Rotaro, Montagnone and Cremate are situated.
+
+2. In both the earthquakes of 1881 and 1883, the epicentre is an
+elongated band, the axis of which, if produced, would pass through the
+centre of the old crater of Epomeo. Along the line of this band, occur
+the fumaroles of Monte Cito and Ignazio Verde and the thermal springs
+of the Rita and Capitello. These facts, as Professor Mercalli
+suggests, lead us to believe that the foci of the earthquakes coincide
+with a radial fracture of the volcano, the course of which, as traced
+by him, is represented by the continuous line in Fig. 14.[27]
+
+3. Except in their relations with actual eruptions, the Ischian
+earthquakes resemble closely the true volcanic earthquakes which from
+time to time shake the flanks of Etna. These are marked by great
+intensity of the shock at the centre of a comparatively small
+disturbed area, epicentres often elongated radially to the cone,
+frequent repetition with similar characters in the same districts; and
+as a rule they precede by a short interval, but sometimes accompany or
+follow, volcanic eruptions.[28]
+
+Two other phenomena may be referred to as probably indicating some
+connection between Ischian earthquakes and the structure and history
+of Epomeo.
+
+We have seen that, in the three earthquakes of 1828, 1881, and 1883,
+there is distinct evidence of a second meizoseismal area at Fontana,
+within which the shock was mainly subsultory. Dr. Johnston-Lavis,
+though recognising the possibility of the existence of two epicentres,
+prefers another explanation.[29] But the wide extension of the
+southern boundary of the area of destruction in 1883, and the
+limitation of several of the after-shocks to the south of the island,
+seem to me to favour the existence of a second focus beneath the
+crater of Epomeo, though, it may be, not entirely detached from the
+chief focus beneath Casamenella.
+
+Again, as Professor Mercalli remarks, all historic eruptions on the
+flanks of Epomeo were accompanied by very violent earthquakes; while,
+previously to 1302, only one disastrous earthquake, so far as known,
+occurred in the island without being attended by an eruption. It
+should be noticed also that the principal shocks during the recent
+revival of activity (_i.e._, since 1762) show a continual increase in
+intensity, whether this be measured by the damage to buildings, the
+loss of life, or the extent of the area of destruction (Fig. 14).
+
+It therefore seems legitimate to conclude that, in the recent Ischian
+earthquakes, we have merely so many unsuccessful attempts to force a
+new volcanic eruption. The passages once existing through Epomeo and
+its parasitic craters having become blocked, the highly heated magma
+beneath is compelled to find a new outlet. Its tension slowly
+increasing, the crust above is at last rent, or an incipient rent is
+enlarged, the fluid rock is injected almost instantaneously with great
+force into the open fissure, and its sudden arrest by the containing
+walls is the ultimate cause of an earthquake. With the expansion of
+the magma, its tension is at once correspondingly reduced, and some
+time must elapse before it can again reach the critical point at which
+a further rupture, resulting in a second shock, takes place.[30]
+
+Thus, with each great Ischian earthquake, we are, I believe, advancing
+a step nearer the time, which may be close at hand or may be very
+remote, when the fracture will at last reach the surface, and above
+the site of Casamenella a new parasitic cone will rise, from which, as
+from Cremate in 1302, a stream of lava may flow down towards the sea.
+
+
+REFERENCES.
+
+  1. BALDACCI, L.--"Alcune osservazioni sul terremoto avvenuto all'
+        Isola d'Ischia il 28 luglio 1883." _Ital. Com. Geol. Boll._,
+        vol. xiv., 1883, pp. 157-166.
+
+  2. DAUBRÉE, A.--"Rapport sur le tremblement de terre ressenti à
+        Ischia le 28 juillet, 1883; causes probables des tremblements
+        de terre." Paris, _Acad. Sci._, _Compt. Rend._, vol. xcvii.,
+        1883, pp. 768-778.
+
+  3. DU BOIS, F.--"The Earthquakes of Ischia." _Japan Seism. Soc.
+        Trans._, vol. vii., pt i., 1883-84, pp. 16-42.
+
+  4. ---- "Farther Notes on the Earthquakes of Ischia." _Ibid._, vol.
+        viii., 1885, pp. 95-99.
+
+  5. JOHNSTON-LAVIS, H.J.--_Monograph of the Earthquakes of Ischia_
+        (1885).
+
+  6. MERCALLI, G.--_Vulcani e fenomeni vulcanici in Italia_ (vol. iii.
+        of _Geologia d'Italia_, by G. Negri, A. Stoppani, and G.
+        Mercalli), 1883, pp. 46-50, 331-332.
+
+  7. ---- _L'Isola d'Ischia ed il terremoto del 28 luglio 1883_
+        (Milano, 1884).
+
+  8. PALMIERI, L., E A. OGLIALORO.--"Sul terremoto dell' Isola
+        d'Ischia della sera del 28 luglio 1883." Napoli, _R. Accad.
+        Atti_, vol. i., 1884, pp. 1-28.
+
+  9. ROSSI, M.S. DE.--"Il terremoto di Casamicciola del 4 marzo 1881."
+        _Bull. del Vulc. Ital._, anno viii., 1881, pp. 5-12. (In the
+        same volume are brief notices by different writers on pp. 22,
+        38-42, 52-53, 67-68, 70-74.)
+
+  10. ---- "Raccolta di fatti, relazioni, bibliografie sul terremoto
+        di Casamicciola del 28 luglio 1883, con brevi osservazioni."
+        _Bull. del Vulc. Ital._, anno xi., 1884, pp. 65-172.
+
+  11. ---- "Intorno all' odierna fase dei terremoti in Italia e
+        segnatamente sul terremoto in Casamicciola del 4 Marzo 1881."
+        _Ital. Soc. Geogr. Boll._, 1881.
+
+  12. SERPIERI, A.--"Sul terremoto d'Ischia il 28 luglio 1883."
+        _Scritti di Sismologia_, Pte. ii., pp. 207-216.
+
+  13. ---- "Sul terremoto dell' Isola d'Ischia il 28 luglio 1883."
+        _Ibid._, pp. 217-232.
+
+FOOTNOTES:
+
+[21] The shaded areas indicate the principal trachytic masses, the
+broken lines represent the boundaries of the craters that are still
+recognisable, and the dotted lines the boundaries of the areas within
+which buildings were damaged by the earthquakes of 1796, 1828, 1881,
+and 1883 (according to Mercalli). The continuous curved line shows the
+position of the radial fracture with which the earthquakes were
+probably connected. The trachytic masses and craters are denoted by
+the following tables:--
+
+    _a._ Epomeo.
+    _b._ Trippiti.
+    _c._ Vetta.
+    _d._ Garofoli.
+    _e._ Vatoliere.
+    _f._ Campagnano.
+    _g._ Vezza.
+    _h._ Imperatore.
+    _i._ C. St. Angelo.
+    _j._ Lo Toppo.
+    _k._ Marecocco.
+    _l._ Zale.
+    _m._ Rotaro.
+    _n._ Montagnone.
+    _p._ Bagno.
+    _q._ Tabor.
+    _r._ P. Castiglione.
+    _s._ Cremate.
+    _t._ Arso.
+    _u._ Porto d'Ischia.
+
+[22] It is possible that Monte Campagnano may form an exception to
+this statement.
+
+[23] Shocks were felt in the island in 1559 and 1659, but one at least
+was of external origin.
+
+[24] Prof. Mercalli, from the five estimates of the angle of emergence
+which he considered most reliable, found the mean depth to be about
+3,280 feet.
+
+[25] Professor de Rossi estimated the mean duration as not much
+exceeding ten seconds. Dr. Johnston-Lavis, on the other hand,
+considers the general estimate of fifteen seconds as far too low. In
+one case, at Casamicciola, he ranks it as high as thirty-one seconds.
+
+[26] Quoted from the useful translation of Covelli's memoir given by
+Dr. Johnston-Lavis.
+
+[27] Baldacci supposes that the thermal springs and fumaroles of
+Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie along
+a tangential fracture starting from Forio and passing by Casamicciola
+to near Punta di Castiglione. Mercalli, however, argues forcibly
+against this inference.
+
+[28] Professor Mercalli adds, as a fourth point of contact between
+Ischian earthquakes and volcanic phenomena, the changes in the
+fumaroles and hot springs which preceded or accompanied or followed
+the earthquakes of 1828, 1881, and 1883.
+
+[29] "Fontana," he says, "occupies the centre of the great crater of
+Epomeo..., and therefore lies immediately over the ancient chimney,
+which in all probability is filled by an old plug of consolidated
+trachyte, which must descend to the igneous reservoir. Any mass of
+igneous matter, that might determine the further rupture of a
+collateral fissure, would result in the conduction of any changes of
+pressure or vibrations, along the column of highly elastic trachyte;
+whilst the same earth-waves would be annulled or absorbed by the
+inelastic tufas surrounding it, so that the blow would be struck
+perpendicularly to the surface, and in a small area with well defined
+limits. The undulatory sensations, after the principal local shock,
+were those that arrived from the great centre of impulse beneath
+Casamenella."
+
+[30] The above paragraph is a summary of the reasoning stated with
+admirable clearness by Dr. Johnston-Lavis. It should be mentioned that
+the late Professor Palmieri, relying on the extremely limited
+disturbed area, dissented from this view; but his difficulty is met by
+supposing the focus to be small as well as shallow, a supposition that
+is supported by the shortness of the meizoseismal band, as well as by
+the elongation of the isoseismal lines in the direction perpendicular
+to this band.
+
+
+
+
+CHAPTER IV.
+
+THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884.
+
+
+In most countries the principal seismic districts are of limited
+extent. Thus, in central Japan, the east coast is frequently visited
+by earthquakes, while the west coast is relatively undisturbed. Of the
+earthquakes felt in the kingdom of Greece during the years 1893-98, 63
+per cent. were observed in Zante, and were for the most part confined
+to that island. In the interior of the Iberian peninsula--in Leon and
+in New and Old Castile--destructive earthquakes are practically
+unknown; while the littoral regions of central and southern Portugal,
+Andalusia, and Catalonia are noted for their disastrous shocks.
+
+During the eighteenth century seismic activity was chiefly
+concentrated in Portugal, and culminated in the great Lisbon
+earthquake of 1755. In the following century the seat of disturbance
+was transferred from the west to the south of the peninsula; Portugal
+remained throughout in comparative repose, while Almeria experienced
+destructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and
+1864, leading up to the Andalusian earthquakes of 1884-85, described
+in the present chapter.
+
+The preparation for the principal earthquake of December 25th, 1884,
+was unusually indistinct. For a day or two before, shocks were felt
+here and there in Andalusia, but so weak were they that they passed
+almost unperceived. During the night of December 24-25, one slight
+shock was noticed at Colmeñar (Fig. 19) and another at Zafarraya. On
+the 25th, a faint movement of the ground was noticed at Malaga, and a
+few weak tremors at Periana; and shortly after came the great shock at
+about 8.50 P.M. mean time of Malaga, or about 9.8 P.M. Greenwich mean
+time.
+
+This earthquake was investigated by no fewer than three official
+committees. The first in the field was nominated by the Spanish
+Government on January 7th, 1885, and consisted of four members, the
+President being Señor M.F. de Castro, the director of the Geological
+Survey of Spain. The report of this commission was presented to the
+Minister of Agriculture, etc., on March 12th. Early in February a
+French Commission, appointed by the Academy of Sciences, proceeded to
+the scene of the disaster. With Professor F. Fouqué as chief, and MM.
+Lévy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Bréon as
+members, this committee resolved itself after a time into one for
+studying the geology of the central area; and, of their voluminous
+report of more than 700 quarto pages (published in 1889), only 55 are
+immediately concerned with the earthquake. At the beginning of April,
+Professors Taramelli and Mercalli, sent by the Italian Government,
+arrived in Andalusia; and their memoir, read a few months later before
+the Reale Accademia dei Lincei, forms by far the most valuable
+contribution to our knowledge of the earthquake.
+
+
+DAMAGE CAUSED BY THE EARTHQUAKE.
+
+The meizoseismal area (see Figs. 19 and 20) lies in a mountainous
+district, almost equidistant from the cities of Malaga and Granada. In
+this area, which contains nearly 900 square miles, the shock was
+disastrous to all but well-built houses. Whole villages were
+overthrown. In the surrounding zone many buildings escaped serious
+damage, and only a few were completely destroyed. It is estimated by
+the Spanish Commission that, in the province of Granada, 3,342 houses
+were totally, and 2,138 partially, ruined; in the province of Malaga,
+1,057 houses were totally, and 4,178 partially, ruined; while in the
+two provinces together 6,463 houses were damaged; making a total of
+17,178 buildings more or less seriously injured.
+
+As usual in the South of Europe, bad construction and narrow streets
+were largely responsible for the loss of property, houses that were
+regularly built and made of good materials being only slightly
+injured. But, in this case, the great slope of the ground, the bad
+quality of the foundations, and the nature of the underlying rocks
+were contributing factors. Many buildings also had been damaged by
+previous shocks, and their ruin was only completed by the earthquake
+of 1884.
+
+The total loss of life is variously estimated. According to the
+Spanish Commission, 690 persons were killed and 1,426 wounded in the
+province of Granada, while 55 were killed and 59 wounded in that of
+Malaga, making a total of 745 persons killed and 1,485 wounded. The
+Italian seismologists, having additional materials at their disposal,
+raise the total figures to 750 persons killed and 1,554 severely
+wounded. Careful inquiries were also made on this subject by the
+conductors of the newspaper _El Defensor de Granada_. In Granada
+alone, they reckon that 828 persons were killed and 1,164 wounded.
+
+From the table given in the Italian report, it appears that 330
+persons were killed at Alhama, 118 at Arenas del Rey, 102 at
+Albuñuelas, 77 at Ventas de Zafarraya, and 40 at Periana; the
+percentage of mortality being 9 at Arenas del Rey, about the same at
+Ventas de Zafarraya, and 3 or 4 at Alhama, Albuñuelas and Periana.
+Comparing these latter figures with the death rates of 71 per cent. at
+Montemurro, caused by the Neapolitan earthquake, and of about 45 per
+cent. at Casamicciola, by the Ischian earthquake of 1883, it will be
+seen that the loss of life during the Andalusian earthquake was
+comparatively small--an exemption which is attributed by the Italian
+commissioners to the absence of inhabited places from the immediate
+neighbourhood of the epicentre, and to the fact that the destructive
+vibrations occurred towards the end of the shock, thus allowing
+opportunity for escape.
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+Fig. 19 shows the principal isoseismal lines as drawn by the Italian
+commissioners. The meizoseismal area, which included all places at
+which the shock was disastrous, is bounded by an ellipse (marked 1 on
+the map) 40 miles long from east to west, 28 miles wide, and about 886
+square miles in area. The next isoseismal (2) includes the places in
+which some buildings were ruined, but not as a rule completely, and in
+which there was no loss of life. Its bounding line is also elliptical,
+the longer axis being about 71 miles long and running nearly east and
+west. Towards the south this zone is interrupted by the sea. It will
+be noticed that these isoseismals are not concentric, the second
+extending much farther to the west and south-west than in the
+opposite direction. A third isoseismal (not shown in the map) encloses
+the district in which the shock was "very strong," or just capable of
+producing cracks in the walls of houses. It is similar in form to the
+second isoseismal, reaching as far as Estepone to the south-west,
+Osuna, Cordova, and Seville to the west, Jaen to the north, while
+towards the east it stops short of Almeria.
+
+   [Illustration: FIG. 19.--Isoseismal lines of Andalusian
+   earthquake. (_Taramelli and Mercalli._)]
+
+The French Commission have also published a map of the earthquake,
+and, though the work of an experienced seismologist like Professor
+Mercalli is probably more trustworthy, it is interesting to compare
+his isoseismal lines with those obtained by his French colleagues,
+which are reproduced in Fig. 20. The curves in this figure are drawn
+so as to include the places that were, respectively, ruined, seriously
+damaged, and slightly damaged, by the shock. They should therefore
+correspond with the lines in Fig. 19. It will be seen that they differ
+considerably in form, but at the same time they present certain points
+of agreement, such as the east and west elongation of the meizoseismal
+area, and the great extension of the two outer isoseismals towards the
+west and south-west The greatest difference is to be found in the
+eastern portion of the third isoseismal, which, according to the
+Italians, extends beyond the limits included in Fig. 20, and,
+according to the French, is bayed back by the great masses of the
+Sierra Nevada.
+
+Outside Andalusia the earthquake was sensibly felt to the north as far
+as Madrid and Segovia, to the west at Huelva, Cárceres and Lisbon, and
+to the east at Valencia and Murcia. Towards the south, the greater
+part of the disturbed area was cut off by the Mediterranean, and there
+are no records forthcoming from the opposite coast of Africa. The
+total area disturbed by the earthquake is roughly estimated by the
+French Commission at about 154,000 square miles, and by the Italian
+Commission at about 174,000 square miles; but, as the shock was
+strong enough to stop clocks and ring bells at Madrid, it is evident
+that even the greater of these values is too small.
+
+   [Illustration: FIG. 20.--Isoseismal lines of Andalusian
+   earthquake. (_Fouqué, etc._)]
+
+
+THE UNFELT EARTHQUAKE.
+
+Far beyond the limits of the disturbed area, however, the long slow
+waves sped over the surface, disturbing magnetographs and other
+delicate instruments. More than a century before, the great Lisbon
+earthquake of 1755 had caused oscillations in Scottish lakes, and on
+other occasions the effects of remote earthquakes had been witnessed
+at isolated places. But, in 1884, the concurrent registration of the
+Andalusian earth-waves at distant observatories attracted general
+attention, and in part suggested the world-wide network of
+seismological stations, the foundation of which was laid before
+another decade had passed.
+
+In Italy, probable records of the earthquake were obtained at two
+observatories, but, owing to the approximate times given, their
+connection with it is not established. At Velletri, near Rome,
+Professor Galli's seismodynamograph registered a very slight movement
+at 10 P.M., and at Rome itself Professor de Rossi found a tromometer
+making unusual oscillations at 10.15 P.M.[31]
+
+The most interesting records, however, are those furnished by the
+magnetographs at Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
+Wilhelmshaven. At Lisbon, the records are extremely clear. The curves
+of the declination, horizontal force and vertical force magnets, as
+seen in Fig. 21, are abruptly broken at 8.33 P.M. (Lisbon time, or 9h.
+9m. 45s., G.M.T.). The disturbances, which are greatest on the
+declination curve and least on the vertical force curve, lasted in all
+three for about 12 minutes, and are quite distinct from the ordinary
+magnetic perturbations. At Parc Saint-Maur, the magnetographs seem to
+be ill-adapted to act as seismographs, for only a slight mark was
+discovered on a re-examination of the curves, beginning at 9.24 P.M.
+(Paris time, or 9h. 14m. 39s., G.M.T.) At Greenwich, Mr. W. Ellis
+writes, there was "a small simultaneous disturbance of the declination
+and horizontal force magnets, occurring at 9h. 15m.... Both magnets
+were at this time set into slight vibration, the extent of vibration
+in the case of declination being about 2' of arc, and in horizontal
+force equivalent to .001 of the whole horizontal force nearly." Of the
+three instruments at Wilhelmshaven, only one showed any movement at
+the time of the earthquake. The declination magnet was undisturbed,
+the horizontal force curve was accidentally interrupted, but the
+vertical force curve indicated a very perceptible shock. Beginning at
+9.52 P.M. (Wilhelmshaven mean time, or 9h. 29m. 29s., G.M.T.), the
+curve was broken for four minutes, for the rapid swinging of the
+needle could not be registered until the motion became fainter.
+Further disturbances also occurred at 9.59, 10, 10.2, and 10.5
+P.M.[32]
+
+   [Illustration: FIG. 21.--Magnetograph records of Andalusian
+   earthquake at Lisbon. (_Fouqué, etc._)]
+
+
+POSITION OF THE EPICENTRE.
+
+The innermost isoseismal being too large, and the time-records too
+inaccurate, to give the position of the epicentre, both Commissions
+resorted to observations of the direction, Professor Fouqué and his
+colleagues depending chiefly on the oscillation of hanging lamps, and
+Professors Taramelli and Mercalli on the fall or displacement of
+statues and other objects, and all avoiding as far as possible the
+evidence of fissures in buildings.
+
+The Italian observers point out that, among the divergent directions
+visible at any place, there is generally one more distinctly marked
+than the others, and this, they consider, corresponds to the movement
+coming almost directly from the centre of disturbance. Plotting these
+directions (36 in number), they find that they converge as a rule
+within the triangle formed by joining Ventas de Zafarraya, Alhama, and
+Jatar, while a large number of them traverse the elliptical area,
+whose boundary is represented by the dotted line in Fig. 19. This area
+is about 9 miles long and 2-1/2 miles wide, its longer axis runs
+nearly east and west, and its centre coincides with the western focus
+of the ellipse which forms the boundary of the meizoseismal area. It
+lies, moreover, close to Ventas de Zafarraya and Arenas del Rey, the
+two places where the seismic death-rate was highest, while its major
+axis almost coincides with the line joining them.
+
+The evidence of hanging lamps collected by the French Commission was
+more consistent than that of the fallen objects. At every place, the
+plane in which the lamps oscillated was nearly constant, the
+deviations being generally attributable to irregularities in the mode
+of suspension. The azimuths again intersect within an elliptical area,
+which, according to the Commission, differs little from the central
+region of the earthquake (Fig. 20). It Is clear, however, from the map
+accompanying the French report, that the majority converge towards a
+narrow band extending east and west from near Arenas del Rey to near
+Ventas de Zafarraya, and therefore agreeing closely with the
+epicentral area as determined by Professors Taramelli and
+Mercalli.[33]
+
+
+DEPTH OF THE SEISMIC FOCUS.
+
+If the depth of the seismic focus amounts to several miles, one of the
+most serious objections to Mallet's method lies in the varying
+refractive power of the different strata traversed by the earth-waves
+(p. 28). At present we have no way of meeting this objection, and all
+calculations of the depth of the focus are therefore more or less
+doubtful. A difficulty in practice has also been urged, depending on
+the widely differing inclinations of the fractures at any place; but
+the Italian observers found that the errors from this source were
+greatly reduced by avoiding all fissures in poorly-built houses, or
+which start from windows or other apertures, and selecting only those
+which occur in homogeneous walls directed towards the epicentre. The
+best angles of emergence thus measured by them are thirteen in number,
+all made at places lying within 5 and 23 miles from the centre of the
+epicentral area, and, with two exceptions, inside the meizoseismal
+zone (Fig. 19). The depths corresponding to the different wave-paths
+vary from 5.3 to 23.0 miles, the mean depth of the focus given by all
+thirteen observations being 7.6 miles.
+
+The only estimate made by the French Commission--and it is one that
+they rightly regarded with considerable doubt--was based on a method
+devised by Falb. As the sound generally precedes the shock, Falb
+assumes that it travels with a greater velocity. If the velocities of
+both series of waves are known, and if they start at the same instant
+and from the same region, the interval that elapses between the
+arrivals of the sound and shock should give the distance traversed by
+them and consequently the depth of the focus. It is unnecessary to
+mention more than two of the serious objections to this method. The
+duration of the preliminary sound should increase rapidly with the
+distance from the focus, and of this there is not the slightest
+evidence. Moreover, the sound-vibrations that are first heard do not
+necessarily come from the same part of the focus as those which cause
+the shock, but, as will be seen in Chapter VIII., probably from its
+nearer lateral margin. The French Commission, finding the average
+duration of the fore-sound near the epicentre to be 5 seconds,
+estimate the depth of the focus at about 7 miles--a result which
+agrees remarkably with that obtained from the angles of emergence, but
+which is not, on that account, entitled to credit.
+
+
+NATURE OF THE SHOCK.
+
+In the nature of the shock, there was a singular uniformity throughout
+the whole disturbed area, the chief variation noticed being evidently
+dependent on the observer's distance from the epicentre.
+
+For instance, in the meizoseismal area (Fig. 19), at Ventas de
+Zafarraya, a loud sound like thunder was first heard, and before it
+ceased there came a violent subsultory movement preceded by a very
+brief oscillation, then a pause of one or two seconds, and lastly a
+more intense and longer series of undulations, the whole movement
+lasting 12 seconds. At Cacin, three phases were distinguished, the
+first a slight undulatory movement coincident with the sound, followed
+immediately by the subsultory motion, a pause, and stronger
+undulations, the total duration being 15 seconds. The variations
+noticeable in this zone seem to have been apparent only, sensitive
+observers perceiving a tremulous motion before the vertical
+vibrations, and in the pause between them and the concluding
+undulations. In both phases, the intensity increased to a maximum and
+then gradually decreased. The movement at Ventas de Zafarraya and
+Cacin is represented by Professors Taramelli and Mercalli by the
+curves _a_ and _b_ in Fig. 22.
+
+In the second zone (Fig. 19), the same two phases were universally
+observed, but the subsultory movement was less pronounced or the
+movement was partly subsultory and partly undulatory, and occasionally
+both phases are described as undulatory. The motion near Malaga is
+represented by the curve _c_ in Fig. 22.
+
+   [Illustration: FIG. 22.--Nature of shock of Andalusian
+   earthquake. (_Taramelli and Mercalli._)]
+
+Outside the ruinous zone, the first phase rapidly lost what remained
+of its subsultory form, and the pause between the two parts was
+noticeably longer than near the epicentre. Thus, at Seville and
+Cordova, two shocks were felt, separated by an interval of some
+seconds; the second according to some observers at Seville,
+terminating with vertical tremors. At Madrid, also, the two parts were
+perceived, the interval between them being 3 or 4 seconds in length;
+but, as a rule, outside Andalusia, only a single undulatory shock was
+felt, without any preliminary sound.
+
+That the changes observed in the shock were merely an effect of less
+or greater distance, will be obvious from Fig. 23, in which the
+intensity at any moment is that represented by the distance of the
+corresponding point on the curve from the different base-lines, the
+base-line _a_ corresponding to a place near the epicentre, and _b_,
+_c_, _d_, etc., to places at gradually increasing distances. Thus, at
+a place corresponding to the base-line _b_, the intensity of the
+tremors during the intervening pause (represented by the short line
+PN) was so slight that they frequently escaped notice, while the
+preliminary tremors observed by some near the epicentre were
+altogether imperceptible. At the places corresponding to the
+base-lines _c_, _d_, _e_, _f_, the duration of the whole shock and of
+each part gradually diminished, while the interval between the two
+parts increased owing to the gradual extinction of the final
+vibrations of the first part and of the initial vibrations of the
+second. At the farthest of these places (_f_) the first part was so
+weak that it sometimes passed unobserved. Lastly, at a place
+corresponding to the base-line _g_, the first part was imperceptible
+to all observers, and the shock consisted of a single series of
+horizontal undulations.
+
+   [Illustration: FIG. 23.--Diagram to illustrate variation in
+   nature of shock of Andalusian earthquake.]
+
+_Origin of the Double Shock._--If the double shock were observed at
+only a few places, we should naturally look for some local explanation
+of the peculiarity. The second shock, for instance, might be a
+subterranean echo, the earth-waves being reflected at the bounding
+surface of two different kinds of rock. In the case of the Andalusian
+earthquake, such an explanation is precluded by the almost universal
+observation of the double shock, the greater intensity of the second
+part, and the longer period of its vibrations.
+
+The Italian observers, who paid considerable attention to the double
+shock, give a more general explanation. They regard the two parts of
+the shock as corresponding in the main to longitudinal and transversal
+waves starting simultaneously from the same focus (see p. 13). The
+former vibrations would be vertical at the epicentre and would
+gradually become horizontal in spreading outwards; the latter would be
+horizontal at the epicentre and at a distance from it (_e.g._ at
+Seville) nearly vertical. Also, as the longitudinal waves travel more
+rapidly than others, the interval between the two parts of the shock
+would increase with the distance from the origin. Owing again, to the
+large size of the focus, the first part of the shock would at no place
+be instantaneous, and its later vibrations might coalesce with the
+earlier transverse vibrations, so that, within and near the
+meizoseismal area, the second part of the shock might be stronger than
+the first. A similar result might be produced in the same district if
+the transverse vibrations coincided with reflected longitudinal
+vibrations, and Professors Taramelli and Mercalli think that such
+reflection would occur from the old crystalline rocks of the Sierra de
+Almijara and possibly also from the calcareous and crystalline rocks
+to the south-west of Cartama.
+
+Satisfactory as it seems to be in some respects, this explanation is
+open to serious objections, of which I will mention only two. The
+first is that, though the pause between the two parts of the shock
+does increase with the distance, it does not increase rapidly enough;
+at Seville, it should be two or three minutes, instead of "some
+seconds" in length. A more fatal objection, however, is that, if the
+explanation were correct, every earthquake-shock should consist of two
+parts, and this is only the case with a small minority.
+
+On the other hand, if the velocities of the waves composing each part
+were the same, the slight increase in the length of the interval is
+readily accounted for, as we have seen, by the gradual extinction of
+its weak terminal vibrations. But in any case, the long interval that
+elapsed between the beginnings of the two parts at a place so near the
+epicentre as Ventas de Zafarraya, shows that each part was due to a
+distinct impulse; and, judging from the directions of the respective
+movements, it would seem that the focus of the first impulse was
+situated at a greater depth than the focus of the second. Whether the
+epicentres corresponding to the two foci were coincident or more or
+less separate is not clear from the nature of the shock; but it is
+probable that they were nearly or quite detached, and that a second
+epicentre was situated near the eastern focus of the ellipse bounding
+the meizoseismal area.
+
+
+SOUND-PHENOMENA.
+
+In the Neapolitan earthquake, the sound was only heard in a district
+of about 3,300 square miles immediately surrounding the epicentres,
+while the whole area disturbed by the shock was not less than 39,000
+square miles. A similar limitation was noticed in the Andalusian
+earthquake. According to the Spanish Commission, the sound was heard
+at only one place (Cordova) outside the provinces of Granada and
+Malaga; and its audibility was a rule confined to the area within
+which buildings were damaged by the shock. It was compared at
+different places to the noise of a passing train or a carriage heavily
+laden running on a paved road, of distant thunder, a great storm, or
+the discharge of heavy guns.
+
+At every place where the sound was heard, it distinctly preceded the
+shock, frequently allowing time for escape from houses that were
+afterwards ruined. Its duration within the meizoseismal area was on an
+average about five or six seconds, rarely perhaps did it exceed ten
+seconds. At some places in the same area, it overlapped the beginning
+of the shock, but generally it was separated from the latter by a very
+short interval, estimated at a second. From this precedence of the
+sound, the Italian Commission conclude that the sound-waves travelled
+more rapidly than those which formed the shock, an inference that
+depends on the assumption that both waves started simultaneously from
+within precisely the same focal limits. A different explanation, not
+based on these assumptions, will be considered more fully in Chapter
+VIII, dealing with the recent earthquakes of Hereford and Inverness.
+
+
+VELOCITY OF THE EARTH-WAVES.
+
+If, in a highly-civilised country, the time-records of an earthquake
+vary within wide limits, it is not surprising that those given for the
+Andalusian earthquake should be wholly untrustworthy. Even the clocks
+in public buildings and railway stations differed by as much as 25
+minutes in their indications. An interesting observation is, however,
+described in the French report and is worth repeating, though it does
+not lead to any accurate result. At the time of the principal shock,
+two telegraph-clerks were in communication, one at Malaga and the
+other at Velez-Malaga. The latter, surprised by the shock, suddenly
+stopped his message; and, about six seconds later, the arrival of the
+earth-waves at Malaga explained the interruption to his colleague. As,
+according to the French report, Velez-Malaga is 9 kms. (or about 5-1/2
+miles) nearer than Malaga to the mean epicentral point, it follows
+that the velocity of the earth-waves must have been about 1.5 kms., or
+nearly a mile, per second.[34]
+
+The only observations of any real value in determining the velocity
+are those given by the stopped clock at the observatory of San
+Fernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur,
+Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon,
+Greenwich, and Wilhelmshaven at 9.18, 9.19, 9.25, and 9.29 P.M.
+respectively (Paris mean time) and the mean epicentral point as
+coinciding with Alhama, the French Commission estimates roughly the
+mean surface-velocity between Cadiz and Lisbon at 3.6 kms. per second,
+between Cadiz and Greenwich at 4.5 kms. per second, between Cadiz and
+Wilhelmshaven at 3.1 kms. per second, and between Greenwich and
+Wilhelmshaven at 1.6 kms. per second. Dr. Agamennone, however,
+notices that the distances from Alhama are not correctly measured, and
+substitutes for the above figures 4.83, 3.43, 2.82, and 1.75 kms. per
+second respectively.
+
+These results apparently show a decrease in the velocity with the
+outward spread of the earth-waves, but, as Dr. Agamennone again points
+out, a comparatively small error in the time at Cadiz would neutralise
+the apparent decrease. It is not to be supposed that the astronomical
+clock at this observatory was wrong by more than a second or two, but
+the behaviour of clocks during an earthquake is so irregular--some
+stopping at once, others staggering on for some seconds before
+arrest--that the Cadiz time may differ from the true time by several
+seconds.
+
+Besides this possible error, there is also considerable uncertainty in
+the records from the magnetic observatories, owing to the slow rate at
+which the photographic paper travels. At Parc Saint-Maur this rate is
+only 10 mm. per hour, and at the other observatories about 15 mm. per
+hour. Allowing, therefore, for an error of half-a-minute in the
+time-record at Cadiz, of one minute in those of Lisbon, Greenwich, and
+Wilhelmshaven, and of two minutes in that at Parc Saint-Maur, and
+taking the mean epicentral point as determined by the Italian
+observers, Dr. Agamennone, applying the method of least squares, finds
+the probable value of the velocity of propagation to be 3.15 kms. (or
+nearly 2 miles) per second, with a possible error of .19 kms. per
+second. This result agrees closely with the value found for the long
+slow undulations of more recent earthquakes.
+
+
+MISCELLANEOUS PHENOMENA.
+
+_Connection between Geological Structure and the Intensity of the
+Shock._--While a great part of the injury to buildings must be
+attributed to their faulty construction, the connection between the
+nature of the underlying rock and the amount of damage was very
+clearly marked. Other conditions being the same, houses built on
+alluvial ground suffered most of all; and the destruction was also
+great in those standing on soft sedimentary rocks such as clays and
+friable limestones. On the other hand, when compact limestones or
+ancient schists formed the foundation-rock, the amount of damage was
+conspicuously less than in other cases.
+
+The members of both the French and the Italian Commissions agree in
+ascribing the peculiar form and relative positions of the isoseismal
+lines to geological conditions. To the east of the epicentre, the
+schists and crystalline limestones form a deep, uniform, and compact
+mass; while, to the west, the old crystalline rocks are covered by
+jurassic, cretaceous, and eocene formations, constituting a less
+homogeneous and less elastic mass, in which the intensity of the shock
+would fade off much more rapidly, with the result that the epicentre
+occupies the western focus of the elliptical boundary of the
+meizoseismal area (Fig. 19).[35]
+
+That mountain-ranges have an important influence on the form of
+isoseismal lines is evident from both maps (Figs. 19 and 20), but
+especially from that published by the French Commission (Fig. 20).
+The resistance offered by the Sierra Nevada to the propagation of the
+earth-waves is shown in the former map by the approximation of the
+first and second isoseismals at the east end, and in the latter by the
+great bay in the third isoseismal line. Whichever interpretation of
+the evidence is the more accurate, the action of the mountainous mass
+is clearly to lessen rapidly the intensity of the shock--an effect
+which is probably due to the abrupt changes in the direction and
+nature of the strata encountered normally by the earth-waves. On the
+opposite side of the epicentre, the waves meet the Sierra de Ronda
+obliquely. In traversing this range, the shock lost a great part of
+its strength, while it continued to be felt severely along its eastern
+foot, thus giving rise to the south-westerly extension of the third
+isoseismal in Fig. 20, and, though to a less extent, that of the
+second in Fig. 19.
+
+_Fissures, Landslips, etc._--The earthquake resulted in many
+superficial changes, such as fissures, landslips, and derangement of
+the underground water-system--all changes of the same order as the
+destruction of buildings--but, so far as known, in no fault-scarps or
+other external evidence of deep-seated movements.
+
+Some of the fissures were of great length. One of the most remarkable
+occurred at Guevejar, a village built on the south-west slope of the
+Sierra de Cogollos. It was in the form of a horse-shoe, and was about
+two miles long, from ten to fifty feet wide, and of great depth. In
+its neighbourhood, innumerable small cracks appeared, some
+perpendicular and others parallel to the great fissure. The ground
+within, a bed of clay resting on limestone, also slid down towards the
+river. Houses near the centre of the fissured tract were shifted as
+much as thirty yards within the first month, and others near its
+extremity about ten feet; while the accumulation of the material at
+the south end of the fissure resulted in the formation of a small
+lake, of about 250 to 350 square yards in area and about 30 feet deep.
+All streams within the fissured zone disappeared, and the spring,
+which provided the drinking-water of the village, ceased to flow.
+
+The underground water-system was generally affected throughout the
+central area. In some places, mineral springs disappeared; in others,
+new springs broke out or old ones flowed more abundantly. At Alhama,
+the increased flow was accompanied by a permanent rise in temperature
+from 47° to 50° C., and by a marked change in character.
+
+
+AFTER-SHOCKS.
+
+Frequent after-shocks are a characteristic of the earthquakes of
+Southern Spain. After the Cordova earthquake of 1170, they continued
+for at least three years. The Murcian earthquake of 1828 was followed
+by 300 minor shocks during the next twenty-four hours, and for more
+than a year slight tremors were often felt. For some time after the
+great earthquake of 1884, the movements of the ground were extremely
+numerous in the immediate neighbourhood of the epicentre, farther away
+they were rarer and of less intensity, and outside the area of damaged
+buildings they were nearly absent.
+
+Thus, during the night of December 25-26, 110 after-shocks were
+counted at Jatar, from 14 to 17 at Alcaucin, Ventas de Huelma, Motril,
+Cacin, Durcal, Malaga, etc.; about 11 at La Mala and Albuñuelas; 9 at
+Velez-Malaga and Lenteje; and from 5 to 7 at Frigiliana, Riogordo, and
+Cartama. The strongest of these shocks occurred at 2.20 A.M., and,
+though none was violent, several helped to complete the ruin of many
+houses that had been damaged by the principal shock.
+
+From this time, after-shocks occurred almost daily until the end of
+May, after which they became much less frequent. According to the list
+given in the Italian report, which closes at the end of January 1886,
+237 shocks were felt, 23 up to the end of December, 30 in January
+1885, 25 in February, 27 in March, 46 in April, and 43 in May. In June
+1885, only three are recorded, and the average number during each of
+the following seven months lies between five and six. This list,
+however, does not include the very weak shocks,[36] for nearly all
+those contained in it were felt as far as Malaga or its neighbourhood.
+
+The shocks varied considerably in intensity as well as in frequency,
+five of them being much more violent than the rest. One that occurred
+on December 30th was felt strongly in all the damaged area, two others
+on January 3rd and 5th caused fresh injury to buildings, a fourth, on
+February 27th, disturbed an area bounded roughly by the second
+isoseismal of the principal earthquake (Fig. 19), while the fifth and
+strongest, that of April 11th, was felt over a large part of the zone
+beyond.
+
+At places within and near the meizoseismal area, earth-sounds were
+sometimes heard without any sensible shock; occasionally, also,
+tremors were felt with no attendant sound; but, as a rule, the shocks
+were accompanied by sound, and in every such case, as in the principal
+earthquake, the sound preceded the shock, or at most was partly
+contemporaneous with it.
+
+Several of the after-shocks resembled the principal earthquake in
+their division into two parts separated by an interval of rest or
+weaker movement from half a second to a second in length, though the
+whole duration of the shock itself in no case exceeded five or six
+seconds. Occasionally, the likeness was still closer, in the
+succession of sound, subsultory motion and concluding horizontal
+undulations.
+
+
+GEOLOGY OF THE MEIZOSEISMAL AREA AND ORIGIN OF THE EARTHQUAKES.
+
+The meizoseismal area and surrounding zones lie in the midst of the
+mountainous region that separates the basin of the Guadalquiver from
+that of the Mediterranean, the essential structure of which, according
+to the geologists of the French Commission, is outlined in Fig. 24. In
+this sketch-map, the lightly-shaded bands correspond to an upper
+series of crystalline schists, and the cross-shaded bands to the lower
+series of mica-schists and dolomites that form the anticlinal folds of
+the Sierra de Ronda, the Sierra de Mijas, and the Sierra Tejeda.
+
+In addition to the faulting and intense folding in the direction of
+their strikes, these rocks are also intersected by three nearly
+parallel transverse faults of post-Triassic age, which, aided by
+subsequent denudation, have cut up the whole range into a number of
+distinct sierras. They are represented by the broken lines in Fig. 24.
+
+   [Illustration: FIG. 24.--Structure of meizoseismal area of
+   Andalusian earthquake. (_Fouqué, etc._)]
+
+One of these faults, that which passes near Motril, traverses the
+meizoseismal area, whose boundary, as laid down by the French
+Commission, is indicated by the dotted line on the sketch-map.[37] In
+the neighbourhood of Zafarraya, the fault intersects the broken
+anticlinal fold of the Sierra Tejeda, and the epicentre is thus
+situated in one of the most disturbed tracts of the whole region. The
+evidence, both seismic and geological, is insufficient to support any
+precise view as to the origin of the earthquake, but there can be
+little doubt that it was closely connected with movements along one or
+more of the system of faults that intersect not far from Zafarraya.
+
+
+REFERENCES.
+
+  1. AGAMENNONE, G.--"Alcune considerazioni sui different metodi fino
+        ad oggi adoperati nel calcolare la velocità di propagazione
+        del terremoto andaluso del 25 dicembre 1884." Roma, _R.
+        Accad. Lincei, Rend._, vol. iii., 1894, pp. 303-310.
+
+  2. ---- "Velocità superficiale di propagazione delle onde sismiche
+        in occasione della grande scossa di terremoto dell' Andalusia
+        del 25 dicembre 1884." _Ibid._, vol. iii., 1894, pp. 317-325.
+
+  3. CASTRO, M.F. de.--_Terremotos de Andalucía: Informe de la
+        comision nombrada para su estudio dando cuenta del estado de
+        los trabajos en 7 de marzo de 1885._ (Madrid, 1885; 107 pp.)
+
+  4. FOUQUÉ, F., etc.--"Mission d'Andalousie: Études relatives au
+        tremblement de terre du 25 décembre 1884, et à la constitution
+        géologique du sol ébranlé par les secousses." Paris, _Acad.
+        Sci. Mém._, vol. xxx., pp. 1-772.
+
+  5. MACPHERSON, J.--"Tremblements de terre en Espagne." Paris, _Acad.
+        Sci., Compt. Rend._, vol. c., 1885, pp. 397-399.
+
+  6. NOGUÉS, A.F.--"Phénomènes géologiques produits par les
+        tremblements de terre de l'Andalousie, du 25 décembre 1884 au
+        16 janvier 1885." _Ibid._, pp. 253-256.
+
+  7. ROSSI, M.S. de.--"Gli odierni terremoti di Spagna ed il loro eco
+        in Italia." _Bull. Vulc. Ital._, anno xii., 1885, pp. 17-31.
+
+  8. TARAMELLI, T., and G. MERCALLI.--"I terremoti Andalusi cominciati
+        il 25 dicembre 1884." Roma, _R. Accad. Lincei, Mem._, vol:
+        iii., 1885, pp. 116-222.
+
+  9. Paris, _Acad. Sci., Compt. Rend._, vol. c., 1885, pp. 24-27,
+        136-138, 196-197, 256-257, 598-601, 1113-1120, 1436 (the last
+        three by F. Fouqué).
+
+FOOTNOTES:
+
+[31] These times correspond to about 9.10 and 9.25 P.M., Greenwich
+mean time. The earthquake stopped a clock at the Royal Observatory of
+San Fernando (Cadiz), at 8h. 43m. 54.5s. mean local time,
+corresponding to 9h. 8m. 44s., G.M.T.
+
+[32] The earthquake is also said to have been registered at the
+observatory of Moncalieri, near Turin, but I have not been able to
+ascertain the time of occurrence. A movement felt at about 10.20 P.M.
+at Ramsbury, in Wiltshire, was attributed to the earthquake, though
+the time is about an hour too late. On December 26th, an astronomical
+clock was stopped at Brussels and its pillar displaced; and, on the
+evening of the same day, the large telescope at the observatory was
+also found to have been shifted. These effects, it is suggested, were
+caused by the Andalusian earthquake, but the connection between them
+seems to me very doubtful.
+
+[33] The French observers have also applied a method depending on the
+time of occurrence of the shock. Joining places where the recorded
+times were the same, they notice that the perpendicular bisectors of
+these lines intersect within an area which agrees practically with
+that determined by the azimuths. The inaccuracy of the time-records
+must, however, lessen the significance of this result.
+
+[34] Dr. Agamennone points out that, according to the Italian report,
+the difference in distance is 22 kms. (or 13-3/4 miles), leading to a
+velocity of about 3.6 kms., or 2.3 miles per second.
+
+[35] It should be remembered that it is not improbable that there were
+two detached epicentres, coinciding roughly with the two foci of this
+curve.
+
+[36] Only eight are recorded during the night of December 25-26. On
+several occasions during April and May 1885, groups of slight shocks
+were felt; but as their individual times are not given, they are
+regarded as equivalent to one shock each in the above totals.
+
+[37] The boundary, as drawn in this figure, differs slightly from that
+given in Fig. 20.
+
+
+
+
+CHAPTER V.
+
+THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886.
+
+
+The Charleston earthquake stands alone among the great earthquakes
+described in this volume, and indeed among nearly all great
+earthquakes, in visiting a region where seismic disturbances were
+almost unknown. Calabria and Ischia, the Riviera and Andalusia, Assam
+and the provinces of Mino and Owari in Japan, are all regions where
+earthquake-shocks are more or less frequent and occasionally of
+destructive violence. But, from the foundation of Charleston in 1680
+until 1886, that is, for more than two centuries, it is probably not
+too much to say that few counties in Great Britain were so free from
+earthquakes as the State of South Carolina.[38]
+
+The practical isolation of the earthquake of 1886 left its trace on
+the character of the investigation. Not only were the observers
+untrained, but the investigators themselves were unprepared. For
+instance, the scale of intensity used in drawing the isoseismal lines
+was not adopted until after the first letters of inquiry were issued.
+On the other hand, nothing could exceed the energy and ability with
+which the epicentral tracts were examined by Mr. Earle Sloan and the
+collection of time-records made by Mr. Everett Hayden. To them, and to
+Major C.E. Dutton, whose valuable monograph supersedes all other
+accounts, we are indebted for the two chief additions to our knowledge
+resulting from the study of the Charleston earthquake. These are the
+determination of the double epicentre, and the measurement of the
+velocity with which the earth-waves travelled.
+
+
+DAMAGE CAUSED BY THE EARTHQUAKE.
+
+The land-area disturbed by the earthquake and the isoseismal lines are
+shown in Fig. 25, the small black oval area (which Includes
+Charleston) being that within which the greatest damage to buildings
+occurred. The chief part of the epicentre, however, lies from 12 to 15
+miles to the west and north-west of Charleston, in a forest-clad
+district, containing only two villages and various scattered houses.
+
+The city of Charleston, whose population between 1880 and 1891
+increased from fifty to fifty-five thousand, is built on a peninsula
+between the Cooper River on the east and the Ashley River on the
+south-west. Originally, this was an irregular tract of comparatively
+high and dry land, intersected by numerous creeks, which, as the city
+grew, were filled up to the general level of the higher ground. It is
+on this "made land" as a rule that the more serious damage to
+buildings occurred.
+
+At 9.51 P.M. (standard time of the 75th meridian), the great
+earthquake occurred, and, one minute later, there was left hardly a
+building in the city that was not injured more or less seriously. "The
+destruction," as Major Dutton remarks, "was not of that sweeping and
+unmitigated order which has befallen other cities, and in which every
+structure built of material other than wood has been levelled
+completely to the earth in a chaos of broken rubble, beams, tiles, and
+planking, or left in a condition practically no better." The number of
+houses entirely demolished was not great, but several hundred lost a
+large part of their walls, and many were condemned as unsafe and
+afterwards pulled down. A board of inspectors, appointed to
+investigate the condition of the houses, reported that not one hundred
+out of fourteen thousand chimneys examined by them escaped damage, and
+that 95 per cent. of those injured were broken off at the roof. The
+total cost of the necessary repairs, it was estimated, would amount to
+about one million pounds.
+
+According to the official records, 27 persons were killed in
+Charleston during the earthquake, but, by cold, exposure, etc., this
+number was brought up to not less than 83. The number of persons
+wounded was never ascertained.
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+In drawing the isoseismal lines (represented by the continuous curves
+in Fig. 25), Major Dutton made use of the well-known Rossi-Forel scale
+of seismic intensity, a translation of which is given below.[39] The
+curves range from the highest degree, 10, corresponding to disastrous
+effects on buildings, down to the lowest but one, 2, which would be
+applied to a shock felt only by a small number of persons at rest. It
+is evident, I think, that these lines cannot be regarded as drawn with
+great accuracy. The number of records (nearly 4000, from about 1,600
+places), great as it is, is hardly sufficient for the purpose; and
+many were collected from newspapers. The circulars of inquiry also
+contained no distinct questions corresponding to the different degrees
+of the scale employed, and therefore it is not always certain that the
+intensity recorded was the maximum observed. But, if the curves might
+have varied in detail with a larger and more accurate series of
+observations, they must represent in their main features the
+distribution of seismic intensity throughout the disturbed area. One
+point of importance is the partial earthquake-shadow in the region of
+the Appalachian Mountains shown by the southward incurving of the
+isoseismals 4, 5, and 6, and especially by the first two of these
+lines. Another is the close grouping of the isoseismals in the State
+of Mississippi, illustrating a rapid fading of intensity as the
+earth-waves crossed the unconsolidated materials of the Mississippi
+delta.
+
+   [Illustration: FIG. 25.--Isoseismal lines of Charleston
+   earthquake. (_Dutton, etc._)]
+
+Owing to the short distance between the epicentre and the sea-coast,
+it is impossible to make more than a rough estimate of the extent of
+the disturbed area. Even when the boundary lies on land, it traverses
+some districts which are thinly populated and others where the
+inhabitants are unobservant, and unlikely to notice the slow
+oscillations which were alone perceptible at great distances. The
+shock was, however, felt at Boston (800 miles from the epicentre), La
+Crosse on the upper Mississippi (950 miles to the north-west), at
+several places in Cuba (between 700 and 710 miles), and in Bermuda
+(950 miles). To the south, the limits are unknown, there being no
+report from Yucatan, the nearest point of which is distant about 930
+miles. If we assume the disturbed area to have a mean radius of 950
+miles, then it must have covered no less than 2,800,000 square miles.
+And, that this estimate is not excessive, will be evident from the
+fact that the land-area disturbed (including parts of the great lakes
+and inlets in the sea-coast) amounted to about 920,000 square miles.
+
+
+PREPARATION FOR THE EARTHQUAKE.
+
+The preparation for the earthquake seems to have begun about three
+months before. During June, and even earlier, slight but undoubted
+tremors are said to have been felt in Charleston, but no record of
+them was kept until about 8 A.M. on August 27th, when a decided
+earthquake occurred at Summerville, a village twenty-two miles to the
+north-west. The shock and sound were simultaneous, the shock a single
+jolt or heavy jar, the sound loud and sudden; they were such as might
+have been caused by the firing of a heavy cannon or the explosion of a
+boiler or blast of gunpowder. At 4.45 A.M. on August 28th, the shock
+and sound were repeated, only more strongly, the former being
+distinctly felt as far as Charleston. During that day and the next,
+there were several other shocks at Summerville, and then rest and
+quiet succeeded until the evening of August 31st.
+
+
+NATURE OF THE SHOCK.
+
+At 9.51 P.M. (to take one of the best descriptions), the attention of
+an observer in Charleston was "vaguely attracted by a sound that
+seemed to come from the office below, and was supposed for a moment to
+be caused by the rapid rolling of a heavy body, as an iron safe or a
+heavily-laden truck, over the floor. Accompanying the sound there was
+a perceptible tremor of the building, not more marked, however, than
+would be caused by the passage of a car or dray along the street. For
+perhaps two or three seconds the occurrence excited no surprise or
+comment. Then by swift degrees, or all at once--it is difficult to say
+which--the sound deepened in volume, the tremor became more decided,
+the ear caught the rattle of window-sashes, gas-fixtures, and other
+movable objects; the men in the office ... glanced hurriedly at each
+other and sprang to their feet.... And then all was bewilderment and
+confusion.
+
+"The long roll deepened and spread into an awful roar, that seemed to
+pervade at once the troubled earth and the still air above and around.
+The tremor was now a rude, rapid quiver, that agitated the whole
+lofty, strong-walled building as though it were being shaken--shaken
+by the hand of an immeasurable power, with intent to tear its joints
+asunder and scatter its stones and bricks abroad....
+
+"There was no intermission in the vibration.... From the first to the
+last it was a continuous jar, adding force with every moment, and, as
+it approached and reached the climax of its manifestation, it seemed
+for a few terrible seconds that no work of human hands could possibly
+survive the shocks. The floors were heaving under-foot, the
+surrounding walls and partitions visibly swayed to and fro, the crash
+of falling masses of stone and brick and mortar was heard overhead and
+without....
+
+"For a second or two it seemed that the worst had passed, and that the
+violent motion was subsiding. It increased again and became as severe
+as before. None expected to escape. A sudden rush was simultaneously
+made to endeavor to attain the open-air and fly to a place of safety;
+but, before the door was reached all stopped short, as by a common
+impulse, feeling that hope was vain--that it was only a question of
+death within the building or without, of being buried beneath the
+sinking roof or crushed by the falling walls. The uproar slowly died
+away in seeming distance. The earth was still, and oh! the blessed
+relief of that stillness."
+
+If somewhat sensational in form, this report gives an extremely vivid
+and generally accurate account of the great shock. Other observers in
+Charleston concur in dividing the movement into five phases. The
+preliminary tremors and murmuring sound lasted about twelve seconds,
+and, although they increased in strength, they were succeeded somewhat
+suddenly by the violent oscillations of the second phase, followed by
+a third phase of much less intensity and a fourth of stronger
+oscillations, these three phases lasting altogether about fifty
+seconds. The fifth phase, in which the tremors died out rather
+rapidly, continued about eight seconds; so that the total duration of
+the earthquake was not less than seventy seconds. The variation of the
+intensity with the time is represented roughly by the curve in Fig.
+26.
+
+   [Illustration: FIG. 26.--Curve of intensity at Charleston.
+   (_Dutton._)]
+
+At Charleston, there were thus two decided maxima of intensity, nearly
+equal in strength, though the first seems to have been slightly more
+powerful than the second. As in the Andalusian earthquake, the
+intervening tremors were imperceptible at a distance from the
+epicentre, and the earthquake appeared in the form of two distinct
+shocks, separated by an interval the average duration of which was
+estimated at slightly less than half a minute. At most places, the
+first shock is described as the stronger, but the difference in
+intensity of the two parts could not have been great, for both were
+noticed at several places more than 600 miles from the epicentre.
+
+_Visible Earth-Waves._--Many persons in the meizoseismal area assert
+that they saw waves moving along the surface of the ground. At
+Charleston, according to an observer who was facing a street-lamp at
+the time, "the progress of the waves as they passed the house, going
+towards the south-east, was plainly observed, although they travelled
+with incomparable swiftness. The shadow of each moving ridge cast from
+the gas-light was distinctly seen. The waves were not in long rollers,
+but had rather the appearance of 'ground-swells' in deep water," the
+height of which from crest to trough he estimated at not less than two
+feet. In the words of another observer, "The vibrations increased
+rapidly and the ground began to undulate like the sea. The street was
+well lighted, having three gas-lamps within a distance of 200 feet,
+and I could see the earth waves as they passed as distinctly as I have
+a thousand times seen the waves roll along Sullivan's Island beach.
+The first wave came from the south-west, and as I attempted to make my
+way ... I was borne irresistibly across from the south side to the
+north side of the street. The waves seemed then to come from both the
+south-west and north-west, and crossed the street diagonally,
+intersecting each other, and lifting me up and letting me down as if I
+were standing on a chop sea. I could see perfectly, and made careful
+observations, and I estimate that the waves were at least two feet in
+height."
+
+
+THE DOUBLE EPICENTRE.
+
+For seismological purposes, it is unfortunate that the epicentral
+district should be one containing so few buildings and other objects
+that could preserve the effects of the shock. It is for the most part
+a barren, forest-clad region, in places swampy, with occasional
+scattered houses. But it is crossed by three lines of railway
+diverging from Charleston, and the damage which they suffered
+supplements to some extent the defects arising from the scarcity of
+buildings. These railway lines are the South Carolina, the
+North-Eastern, and the Charleston and Savannah, denoted by the letters
+A, B, and C, respectively, in Figs. 28 and 29.[40] It will be
+convenient to follow Major Dutton, and trace the variation of
+intensity exhibited along each line.
+
+For six miles along the South Carolina Railway (A) the damage to the
+line, though indicative of a strong shock, was of little consequence.
+In the first half of this distance no repairs were required, but at
+3-2/3 miles the rails were bent and the joints between them opened; at
+5 miles, the fish-plates were torn from their fastenings and the
+joints between the rails opened seven inches; and at nearly 6 miles
+the joints were again opened, and the road-bed depressed six inches.
+After this point, deflections of the line and elevations and
+depressions of the road-bed were no longer rare. Near the 9-mile
+point, the intensity of the shock seemed to increase most rapidly;
+lateral displacements of the line became more frequent as well as
+greater in amount. The distortions of the lines were probably greatest
+between 10 and 11 miles; here they were often displaced laterally,
+sometimes depressed or elevated, and occasionally twisted into
+S-shaped curves, while many hundred yards of the track were shoved
+bodily towards the south-east. "The buckling always took place when
+this lateral shoving encountered a rigid obstacle, usually a long
+rigid trestle. At the north-western end of the trestle the
+accumulation of rails resulted in a sharp kink. Corresponding
+extensions of the track by the opening of the joints and shearing of
+the fish-plate bolts occurred some distance to the north-westward." At
+11-1/2 miles, the lines were again stretched and the joints opened by
+about seven inches; but, from this point for more than four miles, the
+sharp kinks revealing a sliding of the track were entirely absent,
+though there were still long slight flexures in the lines and changes
+of level in the road-bed. The railway in this section traverses a
+district which is partly a swamp and partly a rice-field; and thus it
+may be, as Major Dutton suggests, that the ground was less fitted to
+preserve the effects of the shock.[41] At about 18 miles, the line
+reaches higher and firmer ground; and, from here to Summerville
+(21-2/3 miles), there were many sinuous flexures. For six miles
+farther, violent distortions of the rails ceased to occur, the rate of
+decrease in intensity being most marked near the 23-mile point. The
+last flexure occurred at Jedburgh (27-1/2 miles) at the south end of a
+long, heavy trestle (Fig. 27).
+
+   [Illustration: FIG. 27.--Flexure of rails at Jedburgh.
+   (_Dutton._)]
+
+There is thus a certain symmetry in the damage to this line with
+respect to a point about 15 or 16 miles from the Charleston terminus.
+The changes of intensity are most rapid at distances of about 9 and 23
+miles from the terminus. Also, on the south-east side of the 16-mile
+point, the longitudinal displacements of the line are always to the
+south-east; on the other side, always to the north-west. Major Dutton
+therefore infers that the epicentre must be on a line drawn nearly
+through the 16-mile point at right angles to the railway.
+
+Somewhat similar changes were noted along the North-Eastern Railway
+(B), the Charleston terminus of which is about three-quarters of a
+mile to the south-east of that of the South Carolina Railway. Slight
+flexures in the line occurred at distances of 1-1/2 and 4 miles from
+the terminus, and at about 6 miles the road-bed was depressed, in one
+part by as much as 22 inches. At about 6-1/3 miles, the joints between
+the rails were opened 14 inches, and there were slight sinuous
+flexures in the line near the 7-mile and 8-mile points. The
+indications of great intensity then rapidly increased, the rate of
+change being greatest near the 9-mile point. Here, there was a long
+lateral flexure with a shift of 4 inches eastward. Half-a-mile
+farther, the fish-plates were broken and the rails parted 8-1/2
+inches. A little beyond the 10-mile point, an embankment 15 feet high
+was pushed 4-1/2 feet eastward along a chord of 150 feet. At the
+12-mile point and beyond, fish-plates were broken, lines were bent and
+the joints opened; the road-bed was cut by a series of cracks, one of
+which was 21 inches wide, while the beginning of a long trestle was
+shifted 8-1/3 feet to the west. From 12-1/2 to 14-1/2 miles, several
+buildings were damaged or destroyed by a movement which was clearly
+more vertical than horizontal. Near the 16-mile point, the ground was
+fissured and thrown into ridges, the rails being similarly bent in a
+vertical plane. Soon after this, the line reaches a broad, sandy
+tract, and, though the thickness of the sand is probably not much more
+than 40 feet in any place, the disturbances diminish almost at once,
+and, for a distance of more than two miles, there was little damage
+done to the line. At Mount Holly Station (18 miles), the intensity was
+so slight that the houses suffered no injury more serious than the
+loss of chimneys. Half-a-mile farther, the ground becomes less sandy,
+and the effects of the shock more distinct. The lines were bent in
+places for about a quarter of a mile, after which they again pass into
+the sandy area with a decrease of damage, the last flexure being near
+the 21-mile point. The rate of change of intensity in this part of the
+line appears to have been greatest at a distance of about 19-1/2 miles
+from the terminus, but the exact distance is obviously somewhat
+uncertain.
+
+There is again a rough symmetry in the damage to the line, the central
+point being about 14 miles from the Charleston terminus. A line drawn
+through this point at right angles to the North-Eastern Railway (or
+rather to that part of it between the 9-mile and 19-1/2-mile points)
+should pass through the epicentre. It meets the corresponding line for
+the South Carolina Railway in a point which is indicated in Figs. 27
+and 28 by a small circle (W). Houses and other buildings are rare in
+the surrounding district; but, as the intensity of the shock
+diminished outwards in all directions, this point must mark
+approximately the position of the epicentre. As it is close to the
+Woodstock Station on the South Carolina Railway, it is called by Major
+Dutton the Woodstock epicentre.
+
+The Charleston and Savannah Railway (C) uses the same lines as the
+North-Eastern for the first seven miles from Charleston, and then
+turns off in a south-westerly direction. For 4-1/2 miles from the
+junction the signs of disturbance were few and unimportant. The
+railway then crosses the Ashley River, the banks of which slid towards
+one another and jammed the drawbridge; but for four miles farther
+there was no serious damage done to the lines. At about 16-1/2 miles
+the effects of the shock became rapidly more apparent. For nearly
+1-1/2 mile the entire railroad was deflected into an irregular curve,
+the displacement being greatest at the bridge, where it crosses the
+Stono River. Here, it was as much as 37 inches to the south. After
+Rantowles Station (18 miles), there were many displacements, both
+lateral and vertical. At 18-1/2 miles, a long southward deflection
+began, the amount of which reached 25 inches at the 19-mile point, 50
+inches half-a-mile farther on, and was still greater at 20-2/3 miles.
+For two miles more, sinuous flexures were continuous, but, at the
+22-2/3-mile point, they rapidly disappeared, the railroad passing on
+to higher and firmer ground. Between 25 and 27 miles, there were
+occasional slight flexures in the line or depressions of the railroad;
+but, after the 27-1/4-mile point, they seldom occur, and, when they
+do, are of little consequence.
+
+Some of the effects described in the last paragraph may, as Major
+Dutton suggests, be due to the varying nature of the surface-rocks. It
+is important to notice, however, that disturbances of the lines were
+exceedingly rare in the section that lies nearest to the Woodstock
+epicentre, and that they increase in violence for some distance from
+that region, the maximum intensity being reached a mile or two to the
+west of Rantowles Station. This points clearly to the existence of a
+second focus. Unfortunately, there are very few houses or other
+objects in the neighbourhood, and the position of the corresponding
+epicentre cannot be determined accurately. Major Dutton places it in
+the position indicated by a small circle (R), and calls it the
+Rantowles epicentre from its vicinity to the station of that name.
+
+If the meizoseismal area had been a thickly populated one, the
+evidence of ruined and damaged houses would have provided materials
+for the construction of isoseismal lines surrounding the two
+epicentres. It is difficult, as it is, to gauge the equality of the
+effects on objects so different as railway-lines and buildings; and
+the isoseismals shown in Figs. 28 and 29 can therefore lay no claim to
+accuracy.
+
+Fig. 28 shows the epicentral isoseismals as they are drawn by Mr.
+Earle Sloan. They do not correspond to the degrees of any definite
+scale of seismic intensity; but they may be taken as representing the
+impressions of a very careful observer, who traversed the district
+immediately after the occurrence of the earthquake, and who, when
+drawing these lines, was biassed by no preconceived theory.
+
+Major Dutton, relying chiefly on Mr. Sloan's written notes, interprets
+the evidence differently, and obtains the series of curves shown in
+Fig. 29. In this case, also, the isoseismals correspond to no
+expressed standard of intensity. They are intended merely to represent
+the forms of the curves, and, by their less or greater distance apart,
+the more or less rapid rate at which the intensity varied.
+
+The chief difference between the two maps concerns the form of the
+Woodstock isoseismals. Major Dutton draws them approximately
+circular, leaving the map blank towards the north, where hardly any
+evidence was forthcoming. Mr. Sloan attributes the scantiness of
+effects here to a diminution of intensity, and makes the lines curve
+in towards the epicentre. They certainly must do so in crossing the
+North-Eastern Railway; and the somewhat southerly trend of Mr. Sloan's
+curves to the east of this railway seems to me to furnish the better
+representation of the distinctly greater intensity in that region.
+
+   [Illustration: FIG. 28.--Epicentral isoseismal lines of
+   Charleston earthquake according to Mr. Sloan. (_Dutton._)]
+
+   [Illustration: FIG. 29.--Epicentral isoseismal lines of
+   Charleston earthquake according to Major Dutton. (_Dutton._)]
+
+More important, however, than this divergence of opinion is the
+agreement in one respect between the two sets of curves. Both show a
+marked expansion around the points known as the Woodstock and
+Rantowles epicentres, especially about the former, and a contraction
+in the intermediate region. The evidence of these isoseismals
+therefore confirms that of the damaged railway lines, and establishes
+Major Dutton's inference that there were two distinct foci, the
+epicentres of which were about thirteen miles apart.
+
+
+ORIGIN OF THE DOUBLE SHOCK.
+
+In the last chapter, it was shown that the double shock of the
+Andalusian earthquake could be due only to two distinct impulses
+taking place either within the same focus or, more probably, in two
+detached foci. Similar reasoning applies to the Charleston earthquake.
+The double maximum or double shock was observed in no less than
+fourteen States. Moreover, the interval between the two maxima at
+Charleston appears from Fig. 26 to have been about 34 seconds in
+length. Thus, the duplication of the shock cannot have been a merely
+local phenomenon, nor can it have resulted from the separation into
+two parts of the earth-waves proceeding from a single disturbance.
+Each maximum must therefore be connected with a distinct impulse.
+
+Combining this inference with Major Dutton's discovery of the double
+focus, no doubt can remain as to the origin of the repeated shock. It
+is clear, also, that the impulse at the Woodstock focus was the
+stronger of the two; for the isoseismals spread out more widely round
+the corresponding epicentre, and there was no rapid decline of
+intensity from that point, such as might be associated with a weaker
+disturbance within a shallow focus.
+
+   [Illustration: FIG. 30.--Planes of oscillation of stopped
+   pendulum clocks at Charleston.]
+
+Again, since the earlier part of the shock is almost uniformly
+described as the stronger, it follows that the Woodstock focus was the
+first in action. A curious fact recorded by Major Dutton supports this
+inference. In Charleston, four clocks were stopped by the shock, the
+errors of which at the time were certainly less than eight or nine
+seconds. The planes in which their pendulums oscillated are shown by
+the lines lettered A, B, C, and D in Fig. 30, the broken lines W and R
+representing respectively the directions from Charleston of the
+Woodstock and Rantowles epicentres. Clock A stopped at 9h. 51m. 0s., B
+at 9h. 51m. 15s., C at 9h. 51m. 16s., and D (which had been reset to
+the second earlier in the day) at 9h. 51m. 48s. Now, if the plane of
+oscillation coincided nearly with the direction of the shock, the only
+effect would be a temporary change in the period of oscillation; but
+if it was at right angles to the direction of the shock, the clock
+might be stopped by the point of the pendulum catching behind the
+graduated arc in front of which it oscillated. The planes of the first
+three clocks, it will be seen, were approximately at right angles to
+the direction of the Woodstock epicentre, and B and C were indeed
+stopped in the manner just described; while the plane of shock D was
+nearly perpendicular to the direction of the Rantowles epicentre. As
+the pendulums of B and C might make a few staggering oscillations
+before their final arrest, Major Dutton assigns 9h. 51m. 12s. as the
+epoch of the first maximum at Charleston; and, as the interval between
+the two maxima was about 34 seconds, this would give about 9h. 51m.
+46s. for the epoch of the second maximum--a time which agrees very
+closely with that given by clock D. Thus, clocks A, B, and C must have
+been stopped by the Woodstock vibrations, and clock D about
+half-a-minute later by those coming from the Rantowles focus.
+
+
+DEPTH OF THE SEISMIC FOCI.
+
+Two methods of estimating the depth of the seismic focus have been
+described in the preceding pages--namely, Mallet's, depending on the
+angle of emergence, and Falb's, based on the interval between the
+initial epochs of the sound and shock. To these, Major Dutton adds a
+third method, in which he relies on the rate at which the intensity of
+the shock varies with the distance from the epicentre.
+
+_Dutton's Method of determining the Depth of the Focus._--If the
+seismic focus is either a point or a sphere, and the initial impulse
+equal in all directions, and if the intensity of the shock diminishes
+inversely as the square of the distance from the focus, then the
+continuous curve in Fig. 31 will represent the variation of intensity
+along a line passing through the epicentre E. The form of the curve on
+these assumptions does not depend in any way on the initial intensity
+of the impulse; it is governed solely by the depth of the focus. The
+deeper the focus, the flatter becomes the curve, as we have seen in
+discussing the Ischian earthquakes (p. 68). In all directions from the
+epicentre, the intensity at first diminishes slowly; but the rate of
+change of intensity with the distance soon becomes more rapid, until
+it is a maximum at the points C, C; after which it again diminishes
+and dies out very slowly when the distance becomes great. It will be
+evident from Fig. 18 that the deeper the focus the greater also is the
+distance EC of the points where the intensity of the shock changes
+most rapidly. It may be easily shown, indeed, that this distance
+always bears to the depth of the focus the constant ratio of 1 to
+sqrt(3), or about 1 to 1.73.[42]
+
+Now, if a series of isoseismals could be drawn corresponding to
+intensities which differ by constant amounts, we should have a series
+of circles like those surrounding the Woodstock epicentre in Fig. 29,
+the distance between successive lines at first decreasing gradually
+until it is a minimum at the dotted circle and afterwards gradually
+increasing. This dotted circle is obviously that which passes through
+all points where the intensity of the shock changes most rapidly.
+Major Dutton calls it the _index-circle_ and, when its radius is
+known, the depth of the focus is at once obtained by multiplying the
+radius by 1.73.
+
+In 1858, Mallet proposed a method which bears some resemblance to the
+above,[43] but depending only on the intensity of the longitudinal
+waves. Major Dutton claims for his method that the effects of the
+longitudinal and transverse waves are not separated, that it takes
+account of the "total energy irrespective of direction or kind of
+vibration."
+
+   [Illustration: FIG. 31.--Diagram to illustrate Dutton's method of
+   determining depth of seismic focus.]
+
+_Objections to Dutton's Method._--I have described this method
+somewhat fully, though it seems to me open to more serious objections
+than Mallet's first method which it is intended to replace.
+
+We have, in the first place, no reason for supposing that the focus is
+either a point or a sphere, or that the initial impulse is uniform in
+all directions. If the earthquake were caused by fault-slipping, both
+assumptions would be untrue, and it is for those who employ the method
+to prove their validity.
+
+But of greater consequence is the fact that, if the method were
+correct, all earthquakes originating at the same depth must have
+index-circles of equal radii. If the depth of the focus were, say, ten
+miles, then the index-circle must have a radius of about six miles,
+whether the initial disturbance be of extreme violence or so weak that
+it is not felt at the surface at all, much less so far as six miles
+from the epicentre. The law of the inverse square is of course only
+true for a perfectly elastic and continuous medium, and the real curve
+of intensity is not that of the continuous line in Fig. 31, but
+something of the form represented by the dotted line. In this case,
+the rate of change of intensity is greatest at some point C', nearer
+than C to the epicentre, and the application of Major Dutton's rule
+would give a point F', nearer the surface than F, for the focus. Thus,
+assuming that the method can be applied in practice--and the test
+involved is one so delicate that it would be difficult to apply except
+with refined measurements--then all that we can assert is that the
+calculated depth is certainly less than the true depth.
+
+_Dutton's Estimate of the Depth of the Seismic Foci._--In applying the
+method, the chief difficulty is to obtain a series of isoseismal lines
+corresponding to equidistant degrees of intensity. As already pointed
+out, those given in Fig. 29 are merely diagrammatic; but the
+index-circle of the Woodstock focus, represented by the dotted line,
+is made to pass through the places where the rate of change of
+intensity was found to be greatest. The radius of this circle being
+very nearly seven miles, it follows that the resulting depth of the
+Woodstock focal point would be about twelve miles. Major Dutton
+regards this estimate as probably correct within two miles.
+
+In the neighbourhood of the Rantowles epicentre, the isoseismals in
+both Figs. 28 and 29 are elongated in form. The _index-circuit_, as it
+would be called in such a case, cannot be drawn completely, but its
+radius parallel to the shorter axis of the curves is about 4-1/2
+miles, and the resulting depth of the Rantowles focal point would be
+nearly eight miles.
+
+
+VELOCITY OF THE EARTH-WAVES.
+
+The recognition of the double epicentre is, from a geological point of
+view, the most important fact established by the investigation of the
+Charleston earthquake. But of equal interest, from a physical point of
+view, is the estimate of the velocity of the earth-waves, which is
+probably more accurate than that determined for any previous shock.
+Owing to the existence of the standard time system in the United
+States, the exact time is transmitted once a day to every town and
+village within reach of a telegraph line; and the effect of small
+errors in the observations is considerably lessened by the great
+distance traversed by the earth-waves, sixty good reports coming from
+places more than 500 miles from the epicentre, and ten from places
+more than 800 miles distant.
+
+The total number of time-records collected is 316, but of these 130
+had to be rejected, either because they were obviously too early or
+too late, or because they were only given to the nearest
+five-minutes' interval. There remain 186 observations which are
+divided by Major Dutton into four classes according to their probable
+value.
+
+In an earthquake of such great duration (about 70 seconds at
+Charleston), it is necessary in the first place to select some special
+phase of the movement as that to which the records mainly refer, and
+then to determine as accurately as possible the time of occurrence of
+this phase at the origin.
+
+There can be little doubt as to which phase should be chosen. The
+shock began with a series of tremors, which passed somewhat abruptly
+into the oscillations that formed the first and stronger maximum.
+These were clearly felt all over the disturbed area, and, as the
+beginning of the first maximum at places near the epicentre and the
+beginning of the shock at distant stations were probably due to the
+same vibrations, this particular phase may be fairly selected as that
+to which the time-measurements refer.
+
+The time of this phase at the origin can only be ascertained from the
+time at which it reached Charleston, and our knowledge of this depends
+chiefly on the evidence of stopped clocks. How unreliable this may be
+is well known. Clocks may indeed be stopped at almost any phase of the
+movement; and, whenever stopped clocks can be compared with really
+good personal observations, they almost invariably show a later time.
+At Charleston three good clocks were stopped by the vibrations from
+the Woodstock focus, two of them being in close agreement (p. 121);
+and, allowing for a few oscillations before their final arrest, Major
+Dutton places the time of arrival of the selected phase at Charleston
+at 9h. 51m. 12s. P.M. The evidence of these clocks is also supported
+by that of other observations, which show that 9.51 was certainly the
+nearest minute to the time of arrival, and favour a somewhat later
+instant much more strongly than one a little earlier.
+
+Now, the distance of Charleston from the Woodstock epicentre is
+sixteen miles, and from the corresponding focus (with the calculated
+value of its depth) twenty miles. A first estimate of the velocity
+gives a value of a little more than three miles a second, and the time
+at the Woodstock focus may therefore be taken as 9h. 51m. 6s. with a
+probable error of a few seconds.[44]
+
+Proceeding to the observations at a distance, we find them, even after
+all rejections, to be very different in value. They were therefore
+divided into groups consisting of observations which are as nearly as
+possible homogeneous.
+
+The first group contains five records from places between 452 and 645
+miles from the Woodstock epicentre. They give the time to within 15
+seconds, obtained from an accurately kept clock, or from a clock or
+watch that was compared with such within a few hours of the
+earthquake. The resulting velocity is 3.236 plus or minus .105 miles
+(or 5205 plus or minus 168 meters) per second.[45]
+
+In the second group there are eleven observations (between distances
+of 438 and 770 miles) which satisfy the same conditions as those in
+the first group, except that the time is only given to the nearest
+minute or half-minute. The velocity obtained from them is 3.226 plus
+or minus .147 miles (or 5192 plus or minus 236 metres) per second.
+
+The third group included all but the above records and those obtained
+from stopped clocks. They are 125 in number (between distances of 80
+and 924 miles), but it is uncertain whether they correspond to the
+selected phase of the movement, and the errors of the clocks and
+watches used were unknown. They give a mean velocity of 3.013 plus or
+minus .027 miles (or 4848 plus or minus 43 metres) per second.
+
+In the fourth group, we have the evidence of 45 stopped clocks (at
+places between 20 and 855 miles), which apparently give a velocity of
+2.638 plus or minus .105 miles (or 4245 plus or minus .168 metres) per
+second. At six places, however, the times indicated by stopped clocks
+can be compared with good personal observations; and these show that
+the time of traverse from the origin obtained from the former is on an
+average 1.28 times the time of traverse obtained from the latter. If a
+similar correction be made for all the stopped clocks, the corrected
+velocity of the earth-waves would be from 3.17 to 3.23 miles (or 5100
+to 5200 metres) per second.
+
+In obtaining the mean value of the velocity from all the observations,
+those of the fourth group are omitted, and the weights of the first
+three groups are taken inversely as the squares of the probable
+errors--that is, as 2: 1: 4. The resulting mean velocity is 3.221 plus
+or minus .050 miles (or 5184 plus or minus 80 metres) per second; and,
+though it does not follow that all other estimates are erroneous (for
+the velocity may vary with the strength of the earthquake and with
+other conditions), it is probable that this result is more nearly
+accurate than any other previously obtained.
+
+
+MISCELLANEOUS PHENOMENA.
+
+_Fissures and Sand-Craters._--In point of size, there was nothing
+remarkable about the fissures in the ground produced by the Charleston
+earthquake. The largest were only a few hundred yards long, and,
+except near the river-banks, they rarely exceeded an inch in width.
+They seem, however, to have been unusually abundant; for, within an
+area of nearly 600 square miles surrounding the two epicentres, they
+were almost universal, and over a much wider area they still occurred
+in great numbers, though with somewhat less continuity.
+
+From many of these fissures water was ejected, carrying with it large
+quantities of sand and silt, and so abundantly that every stream-bed,
+even though generally dry in summer, was flooded. By the passage of
+the water, some part of the fissures was often enlarged into a round
+hole of considerable size, ending in a craterlet at the surface.
+Certain belts within the fissured area contained large numbers of
+these craterlets, of all sizes up to twenty feet or more in diameter.
+One near Ten-Mile Hill was twenty-one feet across. In this district,
+they were apparently larger and more numerous than elsewhere; many
+acres of ground being covered with sand, which, close to the orifices,
+was two feet or more in depth.
+
+Here and there, the water was ejected with considerable violence, as
+was manifest from the heights to which it spurted. The testimony of
+witnesses on this point is of course doubtful, for the earthquake
+occurred after nightfall, but in a few places the branches and leaves
+of trees overhanging the orifices were smirched with sand and mud up
+to a height of fifteen or twenty feet.
+
+_Effects on Human Beings._--It is interesting to notice the behaviour
+of different races under the influence of a violent earthquake, and
+perhaps no greater contrast could be observed than between the
+calmness exhibited by the Japanese in the presence of disaster and the
+wild fear merging into helpless panic that characterised the
+residents, and especially the negroes, of Charleston. "As we dashed
+down the stairway," says a writer already quoted (p. 108), "and out
+into the street, from every quarter arose the shrieks, the cries of
+pain and fear, the prayers and wailings of terrified women and
+children, commingled with the hoarse shouts of excited men.... On
+every side were hurrying forms of men and women, bareheaded, partially
+dressed, some almost nude, and all nearly crazed with fear and
+excitement.... A few steps away, under the gas-lamp, a woman lies
+prone and motionless on the pavement, with upturned face and
+outstretched limbs, and the crowd which has now gathered in the street
+passes her by, none pausing to see whether she is alive or dead ... no
+one knows which way to turn, or where to offer aid; many voices are
+speaking at once, but few heed what is said."
+
+Between the selfish rush for safety here described and the calm
+interest of the most distant observers, Major Dutton records nearly
+every possible variety of mental effects, the actions resulting from
+which may be roughly classified as follows:
+
+A. No persons leave their rooms.
+
+B. Some persons leave their houses.
+
+C. Most persons run into the streets, which are full of excited
+people.
+
+D. People rush wildly for open spaces and remain all night
+out-of-doors.
+
+In the map of the isoseismal lines (Fig. 25), the dotted curves bound
+the areas in which the effects corresponding to the three highest
+degrees of the above scale were observed. The curve for the first
+degree (A) coincides of course with the isoseismal line of intensity
+2.
+
+It will be seen that there is a certain rough correspondence between
+these curves and the isoseismal lines. The curve D and the isoseismal
+8 are close together; in other words, people thought it wiser to camp
+out-of-doors for the night if the shock was strong enough to damage
+buildings slightly. The curve C and the isoseismal 6 are similarly
+connected; that is, if the movement made pictures swing, etc., people
+rushed into the streets. On the whole, the curve B and the isoseismal
+3 roughly coincide, or, if the shock was not strong enough to make
+doors and windows rattle, some persons left their houses and public
+meetings were dispersed.
+
+_Feeling of Nausea._--A feeling of nausea was experienced by many
+persons at the time of the earthquake, somewhat rarely it appears in
+the neighbourhood of the epicentre and even outside the isoseismal 7,
+but more frequently beyond these limits, and perceptible as far as the
+broken line in Fig. 25. The most distant places at which it was
+noticed are Blue Mountain Creek (New York) and Dubuque (Iowa), which
+are respectively 823 and 886 miles from Charleston.
+
+
+AFTER-SHOCKS.
+
+As Summerville lies six miles to the north-west of the Woodstock
+epicentre and Charleston 16 miles to the south-east, it is probable
+that many of the after-shocks were unfelt and a still greater number
+unrecorded. In Charleston, seven shocks, all much slighter than the
+principal shock, were felt during the night of August 31--September 1,
+and thirty before the end of September. Of these, the shock of
+September 3rd, at 11 P.M., was the strongest, but those which occurred
+on September 1st, 2nd, 21st, and 27th were also described as severe,
+and the remainder as moderate or slight. For weeks after the great
+shock, curious sensations were distinctly perceptible during the still
+hours of the night "as though the crust of the earth were resting on a
+gelatinous mass in constant motion." The last shock felt in Charleston
+seems to have been one recorded on March 18th, 1887.
+
+At Summerville, many shocks occurred that were scarcely perceptible in
+Charleston, and those noticed in both places were usually stronger,
+and the motion more nearly vertical, at Summerville. "The peculiar
+characteristic of all of them was the deep, solemn, powerful boom,
+like the report of a heavy cannon, usually accompanied by a quick,
+short jar. Sometimes it was prolonged into a heavy roar or rumble, as
+if many reports were delivered in a volley. The number of them was
+never recorded." On September 3rd, Mr. W.J. McGee, of the United
+States Geological Survey, arrived at Summerville. During the evening
+of that day, detonations were heard at intervals, averaging perhaps
+half-an-hour, accompanied occasionally by very slight spasmodic
+tremors of an instant's duration. They were much like peals of thunder
+at a distance of half-a-mile or more, though rather more muffled. "It
+was my impression," Mr. McGee remarks, "that the sound was sometimes
+about as grave as the ear can perceive, resembling somewhat the
+tremulous roar sometimes accompanying combustion in locomotives."
+These sounds continued, but with diminishing frequency, throughout the
+remainder of the year and as late as July 1st, 1887.
+
+
+ORIGIN OF THE EARTHQUAKE.
+
+Major Dutton's valuable monograph is a record of the
+earthquake-phenomena. He offers no theory as to the cause of the
+shock, and is therefore in no way responsible for the account given in
+the remaining part of this chapter.
+
+That there were two seismic foci he has shown, I think, conclusively;
+and my object is now to trace out briefly the probable nature of the
+movements that produced the double shock.
+
+Referring to Figs. 28 and 29, it will be seen that, according to both
+Mr. Sloan and Major Dutton, the isoseismals surrounding the Rantowles
+epicentre are distorted along a line which runs from a few degrees
+east of north to a few degrees west of south. Their oval form is in
+all probability connected with a focus elongated in about the same
+direction. Near the Woodstock epicentre, the isoseismals are drawn
+differently in the two maps, and in neither case do they offer any
+sure guide as to the form of the seismic focus. If, however, we follow
+Mr. Sloan's interpretation of the evidence, and suppose the earthquake
+to have been fault-formed, then it is probable that in this region the
+fault bends round slightly towards the east.
+
+The only other evidence on this point is that afforded by the regions
+of defective intensity, real or apparent, along the three
+railway-lines diverging from Charleston. One of these occurred near
+Mount Holly Station on the North-Eastern Railway (B, Figs. 28 and 29),
+another for four miles starting from the 11-1/2-mile point on the
+South Carolina Railway (A), and a third along the Charleston and
+Savannah Railway (C) over a distance of four miles from the Ashley
+River. In the first two cases, Major Dutton suggests that the less
+amount of damage was due to the nature of the soil traversed by the
+railway; but it is on the softer ground that the effects of an
+earthquake-shock are generally the more disastrous. On the whole, it
+seems to me probable that the three tracts referred to are really
+regions of less intensity, and it is worthy of notice that they lie
+along a nearly straight line.
+
+To show the bearing of these remarks, let CD (Fig. 32) represent the
+section of a fault and EF that of the surface of the earth, and
+suppose the rock-mass A to slip slightly and suddenly downwards. Then
+the particles of A at the surface of the fault will, by impulsive
+friction, be drawn sharply upwards, and those of B correspondingly
+downwards; so that the earth-waves in the two rock-masses will start
+in opposite phases of vibration. Along the line of fault, every
+particle of rock, being urged upwards and downwards almost equally,
+will remain practically at rest. Thus, regions of defective intensity
+may arise from partial interference by the spreading of either
+earth-wave in the adjoining rock-mass.
+
+   [Illustration: FIG. 32.--Diagram to explain origin of regions of
+   defective intensity.]
+
+If this be the correct explanation, the path of the originating fault
+may be taken as that indicated by the broken line in Fig. 28, a line
+which is nearly parallel to the chief branches of the isoseismal
+curves.[46] As both epicentres lie on the west side of this line, the
+fault must hade or slope in this direction. The distortion of the
+Woodstock isoseismals towards the north-west confirms the latter
+inference, for the intensity of the shock is greater on the side
+towards which the fault hades.
+
+From the comparative absence of earthquakes in South Carolina, we may
+infer that the fault is one subject to displacements at wide intervals
+of time. The gradually increasing stress along its surface was
+relieved at one or two points in or near the Woodstock focus on August
+27th and 28th, and perhaps during the preceding months. But the first
+great slip took place suddenly in that focus, and spread gradually
+southwards--for there was no interruption in the movement--until about
+half-a-minute later it reached the Rantowles focus, where the second
+great slip occurred. Eight or ten minutes afterwards there was another
+slip--in what part of the fault is uncertain--and this was followed at
+irregular intervals by many small movements gradually diminishing in
+frequency and in focal area. Within a year from the first disturbance,
+the fault-system attained once more its usual condition of rest.
+
+
+REFERENCES.
+
+  1. DUTTON, C.E.--"The Charleston Earthquake of August 31st, 1886."
+        _Amer. Geol. Survey, Ninth Annual Report_, pp. 209-528.
+
+  2. _Nature_, vol. xxxv., 1887, pp. 31-33; vol. lxiii., 1901, pp.
+        165-166.
+
+FOOTNOTES:
+
+[38] The authorities for this statement are Mallet's Catalogue of
+Recorded Earthquakes (_Brit. Assoc. Rep._, 1852, pp. 1-176; 1853, pp.
+117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs'
+_Statistik der Erdbeben von 1865-1885_. According to Mallet, there was
+an earthquake in S. Carolina in November 1776, and the New Madrid
+earthquake of December 16th, 1811, was felt at Charleston. Fuchs
+records two earthquakes at Charleston on May 12th, 1870, and December
+12th, 1876; and two in S. Carolina on December 12th and 13th, 1879.
+
+[39] 1. Recorded by a single seismograph, or by some seismographs of
+the same pattern, but not by several seismographs of different kinds,
+the shock felt by an experienced observer.
+
+2. Recorded by seismographs of different kinds; felt by a small number
+of persons at rest.
+
+3. Felt by several persons at rest; strong enough for the duration or
+direction to be appreciable.
+
+4. Felt by several persons in motion; disturbance of movable objects,
+doors, windows; creaking of floors.
+
+5. Felt generally by every one; disturbance of furniture and beds;
+ringing of some bells.
+
+6. General awaking of those asleep; general ringing of bells;
+oscillation of chandeliers, stopping of clocks; visible disturbance of
+trees and shrubs; some startled persons leave their dwellings.
+
+7. Overthrow of movable objects, fall of plaster, ringing of church
+bells, general panic, without damage to buildings.
+
+8. Fall of chimneys, cracks in the walls of buildings.
+
+9. Partial or total destruction of some buildings.
+
+10. Great disasters, ruins, disturbance of strata, fissures in the
+earth's crust, rock-falls from mountains.
+
+[40] In order to simplify these figures, the rivers, most of the
+inlets, and other details are omitted. Small figures are added along
+the railway lines to denote the distance in miles from the stations in
+Charleston.
+
+[41] If this were so, the decrease in intensity would be only
+apparent; but it may have been real, and a possible explanation on
+this supposition is given later on (p. 135).
+
+[42] If _c_ be the depth of the focus, _a_ the intensity at unit
+distance from the focus, and _y_ the intensity on the surface at
+distance _x_ from the epicentre, then
+
+y=a/(c^2+x^2)
+
+The inclination of the curve at any point is given by
+
+dy/dx=-2*a*x/(c^2+x^2)^2,
+
+and this is a maximum when
+
+d^2y/dx^2 or (3*x^2-c^2)/(c^2+x^2)^3
+
+is zero, which is satisfied when c=x*sqrt(3)
+
+[43] _British Association Report_, 1858, pp. 101-103.
+
+[44] The above time would have to be increased by one second if the
+depth of the focus were very small, and diminished by one second if it
+were as great as 23 miles; the difference in either case being less
+than the probable error.
+
+[45] The method employed is as follows: Let t_0 be the computed time
+(9h. 51m. 6s.) at the focus, _x_ seconds the error in this estimate,
+_t_ the reported time at a given place, _D_ its distance from the
+focus in miles, and _y_ the number of seconds required to travel one
+mile; then, assuming that _y_ does not vary with the distance, we have
+x+Dy=t+t_0. An equation of this form is obtained from each
+observation, and the method of least squares is then applied to
+determine the most probable values of _x_ and _y_.
+
+[46] This seems to me the more probable course. It is possible,
+however, that the fault-line may pass from Mount Holly Station to the
+east of the Woodstock epicentre as shown in Fig. 28, and then to the
+west of the Rantowles epicentre, the fault changing its direction of
+hade in the intermediate district.
+
+
+
+
+CHAPTER VI.
+
+THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887.
+
+
+Few earthquakes have aroused a more widespread interest than those
+which struck the thronged cities of the Riviera on February 23rd,
+1887. The first and greatest of the shocks occurred at about 6.20
+A.M., the second nine minutes later, and the third, intermediate in
+strength, at about 8.51 A.M.[47] All three shocks were of destructive
+violence, the damage wrought by them extending along the coast and for
+a short distance inland from Nice to beyond Savona. Most of the injury
+to property and nearly all the loss of life were, however,
+concentrated on the eastern side of the frontier; and it therefore
+fell to the lot of the Italian Government to provide for the
+scientific investigation of the earthquakes, as well as to meet the
+wants of those deprived of home and support. Professors Taramelli and
+Mercalli, who two years before had studied the earthquakes in
+Andalusia, were again nominated, the former to examine the geology of
+the central regions, and the latter to report on the seismic
+phenomena. Their joint memoir forms one of the most complete accounts
+that we possess of any earthquake, and is the chief authority for the
+description given in this chapter. Another valuable monograph is that
+prepared by Professor A. Issel, of Genoa, who received an independent
+appointment from the same Ministry. A third official commission was
+also sent to estimate the amount of damage caused by the earthquakes
+in the Italian towns and villages. In France, the destruction of
+property was much less serious, and attention was confined chiefly to
+the records of the shock provided by magnetographs and other
+instruments in distant observatories. In Switzerland, the effects
+remarked were merely those due to the evanescent vibrations of a
+remote earthquake; but many interesting records were collected by the
+permanent seismological commission established in that country.
+
+
+DAMAGE CAUSED BY THE EARTHQUAKES.
+
+Owing to variations in the nature, foundation, and site of buildings,
+there is always great diversity in the destructive effects of an
+earthquake. In one and the same town, most of the houses may be razed
+to the ground, while in their midst may be found some that are
+shattered but still standing, and others perhaps that are practically
+unharmed. The stronger after-shocks often complete the ruin of the
+partially damaged houses; though in such cases the real loss is as a
+rule comparatively small.
+
+The close succession of the two strong after-shocks of February 23rd
+made it impossible as a rule to separate their effects from those due
+to the first shock; but it has been roughly estimated that about
+one-quarter of the total damage was caused by the two after-shocks
+together. To them also must be referred in part the comparatively
+small number of wounded, many persons buried beneath the ruins having
+no doubt perished from subsequent falls before they could be
+extricated.
+
+Taking all three shocks together, the total loss to property,
+according to Professor Mercalli, must be valued at about 22 million
+francs in Italy alone. For the province of the Alpes Maritimes in
+France, full details are wanting, but the loss there cannot fall far
+short of three million francs. The total amount of damage must
+therefore be placed at about a million pounds. From the figures given
+by the official commissions, it appears that the earthquakes were most
+disastrous at Diano Marina and Diano Castello; while other places,
+such as Oneglia, Bussana, Baiardo, Pompeiana, and Vallecrosia,
+suffered only a little less severely. At Mentone about 155 houses, and
+at Nice about 61 houses, were rendered uninhabitable, and many others
+were badly injured.
+
+In Italy, 633 persons were killed, 432 seriously wounded, and 104
+slightly wounded; in France, 7 persons were killed and 30 seriously
+wounded, the number of persons slightly wounded being unknown. The
+majority of the deaths occurred in two or three places. Thus, at Diano
+Marina, 190 persons were killed and 102 wounded; at Baiardo, 220 were
+killed and 60 wounded; at Bussana, there were 53 killed and 27
+wounded. The death-rates were, however, comparatively small, amounting
+for the above places to not more than 8-1/2, 14, and 6-1/2 per cent.,
+respectively; figures which only slightly exceed those obtained for
+places in the meizoseismal area of the Andalusian earthquake.
+
+Though the damage can hardly be regarded as excessive, it was
+nevertheless largely due to the peculiar architecture prevalent in the
+Riviera. Arches in the walls are common even in the upper storeys,
+and, in Oneglia and Diano Marina, if not also in other places, the
+floors are nearly always brick arches abutting against the walls and
+without other lateral support. Professor Mercalli believes that, in
+private houses, more than 90 per cent. of the dead bodies were crushed
+beneath these fallen arches. The height of the buildings is also great
+in proportion to the foundation and to the thickness of the walls; and
+the main walls are interrupted by numerous apertures, from the corners
+of which nearly all the fissures sprang. In some of the coast towns,
+the houses are built of rounded stones gathered from the beach, or of
+rubble with stones of all shapes and sizes, bound by cement of the
+poorest quality. Lastly, as much of the damage due to previous
+earthquakes had been badly repaired, it is evident that the
+destructiveness of the Riviera earthquakes must to a great extent be
+referred to preventable causes.
+
+The occurrence of the principal shock shortly after six on the morning
+of Ash Wednesday must also have increased the death-rate; for many
+persons, after a night of amusement, had lain down for a short time
+and were sleeping heavily; while others had already risen and were
+collected in the churches; the circumstances in either case rendering
+escape more difficult.
+
+Taking account, however, of this accidental increase in the number of
+victims, Professor Mercalli considers that the earthquake of 1887 was
+the most disastrous of all those which have visited either the
+Riviera or northern Italy in the last three centuries; though, during
+the nineteenth century, there were three Italian earthquakes of far
+greater destructive power, but all confined to the southern part of
+the peninsula--namely, the Neapolitan earthquakes of 1805 and 1857,
+and the Ischian earthquake of 1883.
+
+
+PREPARATION FOR THE EARTHQUAKES.
+
+It is difficult, as usual, to specify the exact moment when the first
+earthquake of the 1887 series took place; but it is evident that the
+preparation for the great shock was very brief. At Oneglia, it is
+alleged that faint shocks and sounds were observed many times, chiefly
+at night, during the month preceding February 23rd; though they were
+not at the time supposed to be of seismic origin. A slight shock is
+also reported from Diano at about midnight on February 21-22.
+
+The first undoubted shock occurred on February 22nd, at about 8.30
+P.M., or ten hours before the principal earthquake. Though very
+slight, it was felt throughout the Riviera and in part of Piedmont.
+Another shock, also weak, took place at about 11 P.M.; and a third,
+sensible only in the eastern part of the Ligurian Apennines, on
+February 23rd, at about 1 A.M.; at which time the tide-gauge at Genoa
+recorded some abnormal oscillations. An hour later, a more important,
+though by no means a strong, shock occurred; this was perceptible all
+over the Riviera, in Piedmont, and in Corsica; in other words, it
+disturbed a region agreeing closely with the central area of the
+disastrous shock. At about 5 A.M., a fifth shock, somewhat weaker
+than the preceding, was felt over the same area, concurrently, or
+nearly so, with another abnormal oscillation of the tide-gauge at
+Genoa; while a sixth shock was noticed at several places a few minutes
+before the great shock.
+
+During the night of February 22-23, nervous persons in many towns and
+villages were agitated without apparent reason. Birds and animals,
+more sensitive than human beings to faint tremors, were more
+distinctly affected, especially for some minutes before the
+earthquake. Horses refused food, were restless or tried to escape from
+their stables, dogs howled, birds flew about and uttered cries of
+alarm. As these symptoms were noticed at more than one hundred and
+thirty places within the Italian part of the central area, there can
+be little doubt that they were caused by microseismic movements for
+the most part insensible to man.
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+The only complete map of the isoseismal lines is that drawn by
+Professor Mercalli.[48] In this map, reproduced in Fig. 33, the
+continuous curves represent the principal isoseismal lines; the dotted
+curves define the disturbed areas of two of the stronger after-shocks.
+
+The meizoseismal area, bounded by the curve marked 1 in Fig. 33, is
+also shown on a larger scale in Fig. 34. At the places denoted by
+small circles in the latter figure, the principal shock was
+"disastrous," some of the houses in each being either totally or
+partially ruined. At those marked by a small cross, the shock was
+"almost ruinous"; in other words, numerous houses were damaged, but in
+no case was the injury of a serious character. The meizoseismal area
+is thus a narrow band, skirting the Riviera coast from Mentone to
+Albissola, a distance of 106 miles, and extending inland for not more
+than from nine to twelve miles. The greatest intensity, corresponding
+to the ruin of many houses with considerable loss of life, was
+reached at only a few places between Bussano and Diano Marina, all
+lying within a littoral band about twenty miles in length and three to
+three and a half miles in width. If, however, the epicentre had lain
+on land, the area would have been much greater, Professor Mercalli
+estimates about four times greater, than its actual amount.
+
+   [Illustration: FIG. 33.--Isoseismal lines of the Riviera
+   earthquake. (_Mercalli._)]
+
+The curve marked 2 (Fig. 33) bounds the "almost ruinous" zone; its
+expansion towards the north and contraction towards the west,
+north-west, and east, being its most noteworthy features. The next
+zone, that of slight damage, is contained between the isoseismals 2
+and 3, the latter curve probably grazing the north end of Corsica.
+Beyond this lies the "strong" zone, in which the shock was generally
+felt without causing any damage to buildings. Its boundary (marked 4)
+passes near Marseilles, Como, and Parma, and includes nearly the whole
+of Corsica; towards the north-west, in the valley of Aosta, it curves
+in towards the isoseismal 3.
+
+In the outermost zone of all the shock was "slight," and towards the
+margin was only just perceptible. The boundary, which of course
+defines that of the disturbed area, reaches as far north as Basle and
+Dijon, to Perpignan on the west, Trento, Venice, and Pordenone on the
+east, and to the south as far as Tivoli (near Rome) and the northern
+end of Sardinia. In eastern Switzerland, it shows a marked curve
+inwards; possibly, as Professor Mercalli suggests, from the vibrations
+having to cross the northern Apennines in a direction nearly at right
+angles to their axis. Except for this bay, however, the curve differs
+little from a circle, the centre of which lies in the sea, a little to
+the south of Oneglia, close to the position assigned by other evidence
+to the epicentre. The radius of this circle being about 264 miles, it
+follows that the disturbed area must have contained about 219,000
+square miles--by no means a large amount for so strong an earthquake.
+
+
+POSITION OF THE EPICENTRES.
+
+It is evident, from the form of the meizoseismal area shown in Fig.
+33, that a mere fringe of it lies upon land, and that the epicentre
+must be situated some distance out at sea. Other facts may be
+mentioned which point to the same conclusion. There were, for
+instance, no purely vertical movements observed, even in the districts
+where the damage done by the shock was greatest. Nor were any large
+landslips to be seen in those areas; there were no lasting changes in
+the underground water-system; and in general, as Professor Mercalli
+remarks, all the superficial distortions of the ground which are so
+characteristic of the epicentral area of a great earthquake were
+conspicuous by their absence. There is evidence, again, of some
+disturbance of the sea-bed in the death and flight of fishes from
+great depths and in the seismic sea-waves recorded by the tide-gauges
+at Genoa and Nice. These phenomena will be described in a later
+section, but reference should be made here to an interesting
+observation at Oneglia on the occurrence of some of the stronger
+after-shocks. Persons on the coast, it is said, saw the sea curling
+and moving, and immediately afterwards the shock was felt.
+
+In determining the position of the epicentre, Professor Mercalli had
+recourse as usual to observations on the direction of the shock,
+especially those derived from the oscillation of lamps or other
+suspended objects, the projection or fall of bodies free to move,
+fractures, etc., in damaged houses, and the stopping of pendulum
+clocks. Such observations were made at 120 places--72 in the western
+Riviera and the Alpes Maritimes, and 48 at Piedmont, Lombardy, and
+Tuscany.
+
+At many of these places the movement was extremely complicated. In
+nearly all parts of the area most strongly shaken, for instance, the
+direction of the shock changed more than once; and it was therefore
+necessary to select whenever possible the principal direction of the
+shock at each place. In some towns, such as Oneglia, Mentone, Antibes,
+Cuneo, etc., the shock had two dominant directions, and these appeared
+to be sensibly at right angles to one another; an inclination which,
+as Professor Mercalli suggests, may be due in part to the
+approximation of the real directions to those of the principal walls
+of the houses in which the observations were made.
+
+Most of the lines of direction, when plotted on the map, converge
+towards an area lying between the meridians of Oneglia and San Remo,
+and between nine and fifteen miles from the coast. For places near the
+epicentre, the most trustworthy, in Mercalli's opinion, are those made
+at Oneglia, Mentone, Taggia, Bordighera, Castel Vittorio, Nice, and
+Genoa; and the points in which these lines Intersect one another are
+Indicated by small crosses on the map of the meizoseismal area (Fig.
+34). All of them lie at sea at distances between six and fifteen miles
+to the south of Oneglia. The most probable position of the principal
+epicentre is that marked by the small circle A, which is situated
+about fifteen miles south of Oneglia.
+
+   [Illustration: FIG. 34.--Meizoseismal area of the Riviera
+   earthquake. (_Taramelli and Mercalli._)]
+
+There are, however, several lines of direction which can have no
+connection with this epicentre. Besides the east and west lines at
+Nice, Mentone, and Antibes, there are others at the same places which
+run north and south or nearly so. Professor Mercalli believes that
+they were due to vibrations coming from a second focus lying to the
+south of Nice, and there are also several lines of direction at more
+distant places which converge towards the neighbourhood of the
+corresponding epicentre.
+
+This conclusion receives unexpected support from some of the best
+time-records. At the railway-stations of Loano and Pietra Ligure, the
+times of occurrence were given as 6h. 20m. 5s. and 6h. 20m.
+respectively--estimates which are probably accurate to within a few
+seconds; for, at the moment of the shock, the officer who brought the
+exact time along the railway-line from Genoa was at Loana, and had
+just passed through Pietra Ligure. On the other hand, the estimates
+for Mentone and Nice--namely, 6h. 18m. 35s. and 6h. 19m. 43s., if not
+equally exact, cannot err by many seconds, certainly not by so much as
+one minute. Since the distances of Loana and Pietra Ligure from the
+principal epicentre are 31 and 32 miles, and those of Mentone and Nice
+28 and 37 miles, it is therefore clear that the vibrations which
+arrived first at Nice and Mentone must have come from a local focus,
+where the impulse preceded that at the principal focus by several
+seconds.
+
+
+DEPTH OF THE PRINCIPAL FOCUS.
+
+Inaccurate as are all the methods of determining the depth of focus,
+it seems probable, as Professor Issel argues, that the principal
+Riviera focus was situated at a considerable distance from the
+surface. In no part of the meizoseismal area was the shock a really
+violent one; yet its intensity must have faded very slowly outwards,
+for it was strong enough to stop clocks at places in Switzerland and
+elsewhere not less than 250 miles from the origin.
+
+Professor Mercalli regards Mallet's method with greater favour than
+most seismologists. He points to the gradual increase in the angle of
+emergence from the outer zones disturbed by the Riviera earthquake
+towards the meizoseismal area, where several good observations were
+made from fissures in walls parallel to the dominant direction of the
+shock. The angles of emergence which he considers as most trustworthy
+are those of 35° at Taggia, 40° at Oneglia, and about 30° at
+Bordighera. The corresponding depths for the focus are 10.4, 10.4, and
+11.6 miles, giving an average of about 10-3/4 miles.
+
+There are no similar observations forthcoming for the depth of the
+secondary focus near Nice and Mentone; but Professor Mercalli observes
+that it must have been shallower than the other, for the vertical
+component of the vibrations from this focus was much less sensible
+than that of the motion coming from the principal focus.
+
+
+NATURE OF THE SHOCK.
+
+_The Double Shock._--In the valuable collection of records made by
+Professors Taramelli and Mercalli there appears at first sight to be
+the utmost diversity in the evidence with regard to the nature of the
+shock. Thus, in the province of P. Maurizio alone, the shock was
+described as subsultory first and then undulatory or vorticose at 25
+places, undulatory and then subsultory at 22, undulatory and then
+subsultory and again undulatory or vorticose at 13, and subsultory
+first, then undulatory, and finally subsultory and vorticose at two
+places. It is clear that the shock was of considerable duration, not
+less than half-a-minute as a rule, and that there were several phases
+in the movement; and it would seem that one or more of these phases
+may have passed unnoticed owing to the alarm occasioned by the shock,
+and to the fact that most of the observers were asleep when the
+earthquake began. Defects of memory must also have an influence not to
+be neglected, for, even with the simple shocks felt in the British
+Isles, persons in the same or neighbouring places differ greatly in
+their testimony.
+
+But, if we confine ourselves to the accounts of careful persons alone,
+the discrepancies to a large extent disappear. Indeed, all over the
+ruinous area (Fig. 33) the shock maintained a nearly uniform
+character. At Oneglia, for instance, there were two well-marked
+phases, the first of which began with a brief subsultory movement,
+followed by more horizontal undulations of longer period; a pause,
+lasting but for an instant, was succeeded by vibrations which, though
+not vertical, were highly inclined to the horizon; they continued
+throughout the second phase, but, towards the end, new undulations
+were superposed, and these, coming from different directions, resulted
+in an apparently vorticose movement. Professor Mercalli represents the
+motion diagrammatically by the curve _a_ in Fig. 35. At Diano Marina,
+as will be seen from the curve _b_, the shock again consisted of two
+phases, each beginning with a few subsultory vibrations and ending
+with horizontal undulations of much longer period. In the first phase,
+the undulations were marked by a dominant direction, but, towards the
+close of the second phase, there was no determinate direction, and the
+impression was again that of a vorticose shock. At Savona, the
+movement, which is represented by the curve _c_, must have lasted from
+twenty-five to thirty seconds. It also consisted of two phases, with
+subsultory vibrations and undulations in the same order; and it was
+noticed that the second part of the shock was much stronger than the
+first. According to some observers, the concluding movements were
+vorticose.
+
+   [Illustration: FIG. 35.--Nature of shock of Riviera earthquake.
+   (_Taramelli and Mercalli._)]
+
+In the zone surrounding the ruinous area, the vertical component of
+the motion was observed to diminish with the intensity; but, in other
+respects as well as in duration, the shock retained the same general
+form. At Genoa, Turin, Acqui, Alessandria, Antibes, and other places,
+two distinct phases were perceived, occasionally separated by a brief
+pause, the first being invariably the weaker. At some places, the
+observers speak of two shocks at about 6.20 A.M., separated by an
+interval of a few seconds; and this division was noticeable as far as
+Salò on the shore of Lake Garda and Vicenza in Venetia. Only in
+Switzerland and other districts near the boundary of the disturbed
+area did the weaker part of the shock become insensible, the other
+consisting of horizontal oscillations, remarkable for their slowness
+and regularity, and lasting for as much as twenty or thirty seconds.
+
+We may thus conclude, with Professor Mercalli, that the earthquake
+resulted from the almost immediate succession of two distinct shocks,
+in each of which the nearly vertical vibrations were more marked at
+the beginning, while the slower undulations predominated towards the
+close, those of the second phase generally becoming vorticose through
+the superposition of movements coming from different directions. The
+second part of the shock in all of the more carefully written accounts
+is described as the stronger, especially as regards the subsultory
+vibrations in the meizoseismal area; except in the immediate
+neighbourhood of Nice, where the second phase was generally regarded
+as the weaker, or at any rate as not stronger than the first.
+
+_Origin of the Double Shock._--These observations show, not only that
+the principal earthquake consisted of two distinct shocks, but also
+that the shocks originated in different foci. For, if the vibrations
+of both had started from one focus, the second shock would have been
+everywhere the stronger; instead of which there was a small area near
+Nice where the intensity of the first was the greater. This points
+clearly to the existence of another focus situated not far from Nice;
+and it is evident that the greater intensity of the first part in that
+district was due solely to the proximity of this focus, for, still
+farther to the west, at Antibes, the second part was again the
+stronger.
+
+There is thus a striking agreement in the inferences drawn from
+observations on the direction, time of occurrence, and nature of the
+shock. In the face of such concurring testimony, little doubt can
+remain as to the existence of two foci, one to the south of Oneglia
+and the other to the south of Nice, the initial impulse at the latter
+being decidedly the weaker, and preceding that at the eastern focus by
+an interval of some seconds, long enough at any rate for the resulting
+vibrations to reach the Oneglia focus and to spread beyond it before
+the vibrations from that focus started on their outward journey.
+
+_Seismographic Records._--In 1887, the Riviera and the districts
+adjoining it were unprovided with accurately constructed seismographs.
+The observatories at Alessandria, Milan, Monza, Parma, Florence, and
+other places in Italy contained seismoscopes and other pendulums, and
+these all registered the fact that an earthquake had occurred, and in
+many cases traced a series of elliptical or elongated curves. A record
+of the shock was also given by a Cecchi seismograph at Perpignan in
+France, but the distance from the epicentre was too great to allow
+details to be shown. The most valuable record was that obtained from a
+Cecchi seismograph at the observatory of Moncalieri, near Turin, about
+ninety miles north of the principal epicentre.
+
+In this seismograph, the pendulums are provided with pointers, the
+tips of which touch vertical sheets of paper attached to the sides of
+an upright rectangular box. When an earthquake occurs, this box is
+made to descend slowly with a uniform velocity, while the moving
+pointers trace curves upon the smoked paper. The north-and-south
+component of the horizontal motion is inscribed on the sheet of paper
+facing west, and the east-and-west component on the paper facing
+south.
+
+   [Illustration: FIG. 36.--Seismographic record of the Riviera
+   earthquake at Moncalieri. (_Denza._)]
+
+During the principal Riviera earthquake, the former pendulum furnished
+an indistinct record, while the other traced the diagram reproduced in
+Fig. 36. The movement, as here represented, began at about 6h. 21m.
+50s. A.M. (mean time of Rome) with a series of small tremors, which
+lasted for about twelve seconds. Then followed some large
+oscillations, always in a nearly east-and-west direction, which at 6h.
+22m. 21s. gave place to a second series of tremors similar to those at
+the beginning of the shock, but of greater amplitude. These continued
+for at least twelve seconds, at the end of which time the motion of
+the smoked paper ceased. The total duration of the movement at
+Moncalieri cannot therefore have been less than forty-three seconds.
+
+Interesting as this record is, it is doubtful how far it represents
+accurately the movement of the ground. The Moncalieri instrument was
+erected before the modern type of seismograph was designed, in which
+some part remains steady, or very nearly steady, during the
+complicated movements of the ground that take place in an earthquake.
+It will be noticed that the curve in Fig. 36 shows no sign of the
+division of the shock into two distinct parts, and this may perhaps be
+due to the swinging of the pendulum itself; in which case, the curve
+described by the pointer would be the resultant of the oscillations of
+the ground and the proper motion of the pendulum.
+
+
+SOUND-PHENOMENA.
+
+The sounds that preceded and accompanied the Riviera earthquake have
+attracted but little study, although they seem to have been widely
+observed. No attempt was made to define the limits of the area over
+which they were audible; but Professor Mercalli states that in the two
+outer zones (Fig. 33) the sound generally passed unobserved. It was,
+however, heard near Piacenza in Lombardy and Reggio in Emilia, places
+which are about 115 and 140 miles from the principal epicentre.
+
+In the area in which the shock was most violent, the sound resembled
+that of trains and vehicles in motion; while, outside this area it
+generally appeared to be like the hissing of a violent wind. In only a
+few places was it compared to detonations, the crashes of artillery or
+distant thunder. Some observers describe the sound as appearing at
+first as if a strong wind were rising, and then as the roaring of a
+heavy railway-train passing.
+
+Nearly all the observers, who were awake at the beginning of the
+earthquake, agree in asserting that the sound distinctly preceded any
+movement of the ground. From this, as in the case of the Andalusian
+earthquake, Professor Mercalli infers that the sound-vibrations
+travelled with the greater velocity; but, as will be shown in Chapter
+VIII., the general precedence of the sound admits of another and more
+probable explanation.
+
+
+THE UNFELT EARTHQUAKE.
+
+If the Andalusian earthquake first drew general attention to the
+distant spread of unfelt earth-waves, the Riviera earthquake showed
+that this was no isolated phenomenon. We know now that the propagation
+of such waves is only limited by the surface of the earth, but in 1887
+some doubt was felt at first as to the nature of the disturbance,
+whether it was magnetic or mechanical in its origin.
+
+In 1884, the only observatories at which magnetographs were disturbed
+were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
+Wilhelmshaven. In 1887, the magnetographs registered the Riviera
+earthquake at these and several other observatories, the distribution
+of which is shown in Fig. 37. In this sketch-map, the position of the
+principal epicentre is represented by the small cross, while the
+nearly circular line shows the boundary of the disturbed area.
+
+   [Illustration: FIG. 37.--Distribution of observatories at which
+   magnetographs were disturbed by the Riviera earthquake.]
+
+Three of the observatories, those of Nice, Lyons, and Perpignan, lie
+inside this area. At Nice (which is thirty-seven miles from the
+principal epicentre), M. Perrotin states that the magnetograph curves
+show nothing of any interest, except a notable magnetic perturbation
+on the vertical force curve, the time of which, however, is not
+stated.[49] At Lyons (211 miles), the declination, horizontal force
+and vertical force, magnets were all disturbed at 6h. 25m. 47s. A.M.,
+and Perpignan (264 miles), all three magnets, but especially those for
+the declination and horizontal force, were set abruptly oscillating
+at 6h. 25m. 20s.
+
+Elsewhere in France, the disturbances were noticed at the
+observatories of Parc Saint-Maur and Montsouris, near Paris (about 447
+miles), and at Nantes (538 miles). At Parc Saint-Maur, all three
+curves show a very clear trace of the earthquake at 6h. 25m. 35s., the
+oscillations lasting several minutes, and at Montsouris they also
+began at the same time. At Nantes, the perturbations were so slight
+that they escaped notice on a first examination.
+
+In Austria, disturbances were observed at Pola (295 miles) and Vienna
+(506 miles), beginning at 6h. 28m. 35s. and 6h. 30m. 35s.,
+respectively. They reached Brussels (522 miles) at 6h. 29m. 27s., and
+Utrecht (600 miles) at 6h. 28m. 38s.[50] At Wilhelmshaven (690 miles),
+only the vertical force magnet was affected, the oscillations
+beginning at 6h. 30m. 35s., and lasting for fourteen minutes. At 6h.
+27m. 55s., the declination and horizontal force magnets of Greenwich
+observatory (642 miles) were set vibrating, but no similar
+disturbances were revealed by the vertical force curve or by the two
+earth-current registers. At Kew (652 miles), the horizontal force
+magnetograph was moved by the earthquake at about 6h. 29m. 55s. The
+curves at Stonyhurst and Falmouth show no sign of any disturbance, nor
+do those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles),
+however, the three curves indicate disturbances at 6h. 32m. 35s., but
+so feeble are they that they would have escaped discovery if the
+occurrence of the earthquake had been unknown.
+
+The effects registered on the magnetograms are quite different from
+those which correspond to ordinary magnetic perturbations; but they
+are not unlike those produced by the action of the momentary currents
+which are used for making the hour-marks, except that the
+earthquake-oscillations lasted several minutes (see Fig. 21). In each
+case, then, the magnetic bars must have received a succession of
+several or many impulses.
+
+Now, the effect of these impulses on each magnet must depend on the
+relations which exist between the period of oscillation of the magnet,
+the rate of damping of such oscillations, and the interval between the
+successive impulses. Also, the apparent commencement of the phenomena
+may be delayed if two impulses of contrary sense should follow one
+another before the bar is perceptibly displaced. It is therefore to be
+expected, as M. Mascart points out, that the disturbances of the three
+instruments need not be of the same order of magnitude, that with
+different forms of apparatus the effects may be very variable, and
+that the deflection of one instrument may precede that of another at
+one and the same place.
+
+In all the magnetographs, the record is made on photographic paper,
+which travels so slowly that the time of a movement can only be
+ascertained to the nearest minute. As the disturbances on the French
+curves were apparently almost simultaneous, and as no two of the
+others differed in time of occurrence by more than five minutes, there
+is thus some colour for M. Mascart's contention that the magnetic
+apparatus registered, not the movements of the ground, but the passage
+of electric currents produced in the ground at a certain epoch of the
+earthquake.[51]
+
+On the other hand, it is important to notice that, in the central part
+of the disturbed area, at Nice, two, if not all three, of the
+magnetographs were unaffected at the time of the earthquake.
+
+At first sight, this fact seems equally opposed to a mechanical
+explanation of the disturbance. But, when the vibrations are very
+rapid, as they are in the neighbourhood of the epicentre, the magnetic
+bars, owing to their mode of suspension, have not sufficient time to
+be sensibly deflected in the brief interval between successive phases
+of the impulse. The magnetograms of the Montsouris observatory show,
+for instance, hardly any perceptible trace of disturbance during the
+passage of railway trains along two adjacent lines. The farther,
+however, the earth-waves travel from the origin, the longer becomes
+the period of their vibrations. In Switzerland, they were remarkable
+for their slowness, even to the unaided senses. Thus, at places more
+or less remote from the Riviera, the magnets would receive impulses at
+intervals approximating to their own periods of vibration, and they
+would then oscillate freely for some time.
+
+Again, notwithstanding some variations, it will be remarked that on
+the whole the retardation of the initial epoch of the disturbances
+increases with the distance from the epicentre. It thus seems clear, I
+think, that the cause of the disturbances must be sought in the shock
+itself; although their initial epochs at different places are too
+roughly defined for ascertaining the velocity with which the
+earth-waves travelled.
+
+
+EFFECTS OF THE EARTHQUAKE AT SEA.
+
+The Riviera earthquake, owing to its submarine origin, was marked by
+certain phenomena that were absent from the other earthquakes
+described in this volume.
+
+_Nature of the Earthquake at Sea._--At the time of the earthquake,
+several vessels were close to the epicentral area. One, about three
+miles off Diano Marina, was shaken twice at about 6.20 A.M., and so
+violently that it seemed as if the masts would be broken off. Another,
+about ten miles south of P. Maurizio, also experienced two shocks, a
+few minutes apart, as if each time it had struck the bottom. These
+observations are chiefly interesting in showing that the double shock
+was felt at sea as well as on land. As transverse vibrations are not
+propagated through water, it follows that the second part of the shock
+cannot, as some maintain, have been composed of transverse vibrations.
+
+_Destruction of Fishes._--During the days immediately following the
+earthquake, a large number of deep-sea fishes were found dead or
+half-dead either in shallow water or stranded on the beach, especially
+in the neighbourhood of Nice. Among them were numerous specimens,
+mostly dead and floating, of _Alepocephalus rostratus_, a typical
+deep-sea form, several of _Pomatomus telescopium_, _Scopelus
+elongatus_, and _S. humboldti_, and many of _Dentex macrophthalmus_
+and _Spinax niger_. The death and flight of these fishes must have
+been due to a sudden shock, almost like that caused by the explosion
+of dynamite, and communicated simultaneously to the whole surface of
+their bodies.
+
+_Seismic Sea-Waves._--Immediately after the earthquake, the sea
+retired a short distance, variously estimated at from ten to thirty
+metres, laying bare some rocks that were usually immersed. At P.
+Maurizio, the surface was lowered by a little more than a metre; and
+after a few minutes it rose to nearly a metre above its original
+level, returning to it after a series of continually-decreasing
+oscillations. At San Remo, a fall of about the same amount took place,
+the sea returning after five minutes, and a ship anchored in the
+harbour broke from her moorings. Again, at Antibes, the sea was
+suddenly lowered by about a metre, so that ships afloat in the harbour
+were aground for some instants, and then returned with some
+impetuosity to its original level.
+
+   [Illustration: FIG. 38.--Record of tide-gauge at Nice.
+   (_Issel._)]
+
+The evidence of eye-witnesses is confirmed by that of the tide-gauges
+at Nice and Genoa, the curves of which are reproduced in Figs. 38 and
+39. At Nice, the first arrest of the curve in its usual course
+occurred at 6.30 A.M.;[52] the sea-level sank somewhat abruptly, and
+after a few marked oscillations gradually returned to its normal
+position at 7.50 A.M. At Genoa, the shock caused the writing-pen of
+the tide-gauge to dent the paper on which the record is made, and soon
+afterwards the curve shows a series of irregular oscillations, about
+eight taking place every hour, and gradually decreasing until they
+ceased to be perceptible about two hours after the principal
+earthquake.
+
+   [Illustration: FIG. 39.--Record of tide-gauge at Genoa.
+   (_Issel._)]
+
+
+MISCELLANEOUS PHENOMENA.
+
+_Connection between Geological Structure and the Intensity of the
+Shock._--As with the Andalusian earthquake, faulty construction and
+defective materials were responsible for much of the damage caused by
+the Riviera earthquake. But, if we may judge from the sharp local
+variations in its amount, the nature of the surface-rocks must have
+exerted a still more potent influence. At Cervo, for example, the
+injury to property amounted to less than £3 per head of the
+population; at Diano Marina, only two or three miles to the west, it
+rose to £22 per head. The death-rate at Cervo was about one-tenth, and
+at Diano Marina about 8-1/2 per cent. Again, at Mentone, the damage
+must have been considerable, for about 155 houses were rendered
+uninhabitable; while Monte Carlo, only a few miles farther west,
+escaped almost unharmed. Now, Mentone and Diano Marina are for the
+most part built on clay or alluvial deposits, and Monte Carlo on a
+foundation of limestone.
+
+Even within the limits of a single town, variations no less striking
+were perceptible. In Mentone, the greatest damage occurred to houses
+of two storeys built on alluvial soil in the low-lying parts near the
+sea and in the valleys. The effect of the foundation in this part was
+well shown in the case of two equally well-built houses not more than
+300 yards apart. One in the valley, with doubtful foundations, was
+very much shattered; the other, built on rock, was uninjured. The
+large hotels, especially those on high ground, suffered least, few of
+them having their main walls seriously damaged. These buildings rise
+to heights of from four to six storeys, and of necessity have a firm
+and solid foundation.
+
+Professors Taramelli and Mercalli have made a careful study of the
+subject of this section. The general conclusions at which they arrive
+are that the intensity of the shock was greatest at places built on
+pliocene conglomerates, beds of clay superposed on compact old rocks,
+patches of alluvium, miocene formations of some thickness formed of
+repeated alternations of strata of incoherent marls and limestones or
+compact sandstones, beds of chalk, or somewhat rotten dolomite.
+
+The shock was also more destructive on the summits of isolated hills
+and ridges and on the steep slopes of mountains. The influence of the
+form of the ground was, however, subordinate to that exerted by the
+nature of the subsoil. Thus, at Mentone, as we have seen, and also at
+Nice and Genoa, houses built on rock in elevated positions suffered
+much less than those situated on the plains below that are composed of
+sand and recent alluvium.
+
+_Observations of the Earthquake in Railway-Tunnels._--Observations
+made in mines at various times and places have proved that an
+earthquake is felt less strongly in deep workings, if felt at all,
+than on the surface of the ground. In the railway-tunnels of the
+Riviera, as Professor Issel has shown, the same result was established
+during the earthquake of 1887.
+
+On the line which runs northward from Genoa to Piedmont, a tunnel more
+than five miles in length pierces the hilly ground between Ponterosso
+and Ronco, the greatest thickness of rock above being about a thousand
+feet. At the time of the earthquake, the tunnel was not everywhere
+opened out to its full width, and men were at work in different
+sections. Outside, the shock was strong enough to damage buildings.
+Inside, at about 200 yards from the south end, only a feeble shock was
+felt; at 1,350 and 1,625 yards, some bricks were seen to fall from the
+facing, but the shock was not otherwise perceived, and only a few
+yards farther nothing unusual was noticed by the men at work.
+
+Again, in an unfinished tunnel, about three-quarters of a mile long,
+between the harbour of Genoa and the eastern railway-station, the
+vibrations were very slightly felt. Even in the tunnels traversed by
+the coast railway from Genoa to Nice--that is, in those situated
+within the meizoseismal area--the shock was either very weak or not
+felt at all, and not one of the tunnels suffered the slightest injury.
+
+To men at work inside a long tunnel, the conditions for observing
+earthquakes are somewhat imperfect, but these facts, nevertheless,
+bring out very clearly the inferior intensity of the shock at some
+depth below the surface.
+
+
+AFTER-SHOCKS.
+
+While the unfelt earth-waves of the great earthquake were still
+wending their way over the zone that surrounds the disturbed area, the
+central regions were again shaken, at 6.29 A.M., by a shock strong
+enough to produce fresh ruins in the stricken towns along the coast.
+Nearly two and a half hours of quiet followed, broken only by a few
+subterranean rumblings in the central part of the meizoseismal area.
+Then, at 8.51 A.M., occurred another shock, short and sharp, and
+inferior in strength only to the principal earthquake. Both of these
+after-shocks were felt in Western Switzerland; indeed, they were
+perceptible nearly as far as the great shock; the second, however, a
+little farther than the first, for it alone was noticed at such places
+as Vicenza, Forlì, and Florence. The shock at 6.29 was usually
+described as long and its vibrations as undulatory only; that at 8.51
+as rather subsultory than undulatory and of very brief duration. The
+latter, however, was followed after an interval of a few seconds by
+another shock so weak that it generally passed unobserved. Both shocks
+were preceded by a rumbling sound.
+
+During the next two days, tremors and earth-sounds were frequent in
+the Riviera; once an hour, on an average, the greater part of the
+meizoseismal area was shaken by vibrations more or less slight. But,
+between one shock and another, at Diano Marina and Alassio, and even
+as far as Nice, it only required attention from a careful observer to
+perceive an almost continual throbbing of the ground.
+
+Only one of these shocks, that of February 24th, at 2.10 A.M., was
+strong enough to cause slight damage to buildings. It disturbed an
+area, not exceeded by any of the later shocks, the boundary of which,
+shown by the dotted line A in Fig. 33, extends to the north and east
+as far as Piacenza and Spezia, while to the west it includes Cannes.
+The centre of the curve so drawn lies on land, but, as the shock was
+not felt in Corsica, there is no evidence as to the southerly
+extension of the disturbed area; and it is probable, as Professor
+Mercalli suggests, that the shock originated in the eastern or Oneglia
+focus of the great earthquake.
+
+After February 25th, slight shocks were felt during the next
+fortnight, at the rate of three or four a day, until March 11th, when
+the last after-shock resulting in slight damage occurred at about 3.12
+P.M. The boundary of its disturbed area, represented in Fig. 33 by the
+dotted line B, passes a little to the east of Savona, and then through
+Alessandria, Moncalieri, and Marseilles. The shock, however, was not
+observed in Corsica, so that the exact position of the epicentre is
+unknown; but Professor Mercalli believes it to coincide with the
+western or Nice epicentre of the principal earthquake. At the moment
+of the shock, the sea was observed from Alassio to curl and to rise
+slightly, while the tide-gauge at Nice, which had traced a continuous
+curve earlier in the day, showed a characteristic notch about 3.7 P.M.
+
+Of the remaining after-shocks, only two attained any notable degree of
+strength. One, on May 20th at about 8.15 A.M., disturbed an area
+nearly concentric with that of the great earthquake, and with a
+boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on
+July 17th at 11.30 P.M., occurred a shock felt over an area nearly as
+large as that disturbed on February 24th at 2.10 A.M., and situated in
+the same part of the country.
+
+Altogether, during the year following the Riviera earthquake,
+Professor Mercalli records 190 after-shocks, most of them slight or
+only just felt. With the exception of the first two (on February
+23rd), none was observed outside the isoseismal 4 of the principal
+earthquake (Fig. 33); and, of the rest, only the four whose dates are
+given above disturbed an area of more than one-eighth of that of the
+great shock. Some of them, like the shock of March 11th, were stronger
+in the western part of the meizoseismal area; but the majority
+affected most the eastern portion and seem to be closely associated
+with the Oneglia focus.
+
+From February 26th to April 20th, Professor Rumi made observations on
+the after-shocks by means of the Foucault pendulum erected at Genoa
+for demonstrating the rotation of the earth. In nearly every case, the
+oscillations took place along a north-east and south-west line, or in
+the same direction as the first great shock--a resemblance which
+supports the inference that many of the after-shocks originated
+within the Oneglia focus.
+
+
+ORIGIN OF THE EARTHQUAKES.
+
+_Recent Movements in the Riviera._--The earliest movements that
+resulted in the great range of the Maritime Alps and the Ligurian
+Apennines date from pre-Carboniferous times, when the central
+crystalline massifs in part emerged. At the end of the Liassic epoch,
+the secondary formations of the district were uplifted, and it was at
+this time that the range assumed its characteristic curved form. Later
+still, at the close of the Eocene period, an elevation of more than
+9000 feet took place, for upper Eocene beds are found at this height
+in the Maritime Alps.
+
+Since that time, other important movements have occurred. Pliocene
+deposits have been found in the Riviera at an altitude of 1,800 feet.
+Recent soundings in the Gulf of Genoa have also shown that all the
+valleys of the Riviera between Nice and Genoa are continued far below
+the level of the sea to depths of not less than 3000 feet. Thus, at
+the end of the Pliocene or beginning of the Quaternary period, there
+was an elevation of nearly 5000 feet, accompanied or followed by the
+erosion of the valleys which, later on, during the Quaternary period,
+were submerged about 3000 feet. Even in still more recent times,
+probably in the Palæolithic age, minor movements continued. Traces of
+recent elevation, varying in amount from a few feet to sixty feet or
+more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi,
+and in Genoa; while evidences of submergence are to be found near
+Monaco, at Beaulieu and at Diano Marina. It is important to notice
+that the great movements dating from the end of the Eocene period are
+almost confined to the Maritime Alps and the western portion of the
+Riviera. In the parts of Piedmont lying to the north of Cuneo and in
+the eastern Riviera, they produced hardly any sensible effect.
+
+_Seismic History of the Riviera._--The movements just referred to are
+those which, in course of time, have become sensible to the eye. They
+represent the sum of a long-continued series of displacements that may
+once have been on a large scale, but are now comparatively small. The
+earthquakes that occur in the Riviera show, however, that the final
+stage has not yet been reached. Their epicentres indicate the regions
+in which slips are still taking place, and the magnitude of these
+slips is roughly measured by the intensity of the resulting shocks.
+
+The map in Fig. 40 is one of a series drawn by Professor Mercalli to
+represent the distribution of seismic activity in Piedmont and the
+Riviera. It corresponds to the period from 1801 to 1895. The whole
+area is divided into a number of seismic districts, each of which is
+distinguished by a particular degree of activity. In estimating this
+quantity, Professor Mercalli takes intensity as well as frequency into
+account. Thus, the lowest degree, represented by the lightest tint of
+shading, corresponds to one or two strong earthquakes with a few
+moderate or slight shocks; the eighth and highest to four or five
+ruinous or disastrous earthquakes followed by trains of after-shocks.
+The map shows very clearly that, during the last century, the seismic
+activity was greatest in the Maritime Alps and the western
+Riviera--that is, in the very districts in which the recent
+mountain-making movements have been most conspicuous.[53]
+
+   [Illustration: FIG. 40.--Distribution of seismic activity in the
+   Riviera. (_Mercalli._)]
+
+In all these districts, Professor Mercalli distinguishes several
+well-marked seismic centres, to each of which he traces the origin of
+two or more earthquakes. In the districts with which we are at present
+concerned, those of the Alpes Maritimes and the western Riviera, the
+most important centres are situated near Oneglia (in the sea), near
+Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in
+the sea to the south of Nice. To the first of these centres belongs
+the disastrous earthquake of February 23rd, 1887, as well as its
+after-shocks on February 24th, May 20th, July 17th, and September 30th
+of the same year, also the ruinous earthquakes of 1612 and 1854, and
+several others of a lesser degree of intensity. All of these were
+longitudinal earthquakes, the axes of their meizoseismal areas being
+parallel to the neighbouring mountain-ranges. A few miles to the west
+of Oneglia lies the Taggia centre, with which were connected the
+disastrous earthquake of 1831, the violent earthquake of 1874, and
+other strong or very strong shocks. These were for the most part
+transversal earthquakes, their axes being perpendicular to those of
+the Oneglia centre.
+
+Some of the strongest earthquakes in this region originated in a
+centre lying to the north of Nice in the valleys of the Vesubia and
+Tinea. Among them may be mentioned the ruinous earthquakes of 1494,
+1556, 1564, and 1644, and probably also the disastrous earthquake of
+1227. A fourth centre, and one of considerable interest, is that which
+lies at sea, a short distance to the south of Nice, and nearly along
+the continuation of the valleys above-mentioned. This is the secondary
+centre of the earthquake of 1887, and probably also of that of
+December 29th, 1554. It is occasionally in action apart from the
+Oneglia centre, as on November 27th, 1771, June 19th, 1806, and
+December 21st, 1861; but such shocks, though rather strong, never
+reach a high degree of intensity.
+
+_Origin of the Earthquakes of 1887._--The most important feature in
+the principal earthquake of 1887 is its origination in two distinct
+foci, which are sometimes in action almost simultaneously, but more
+often separately. The earthquakes belonging to the two foci differ
+greatly in intensity and number, and the stronger part of the shock in
+1887 originated in the focus associated with the more disastrous and
+more frequent earthquakes.
+
+The existence of two foci would of course give rise to a meizoseismal
+area elongated in the direction of the line joining them. It is clear,
+however, that the Oneglia focus was also extended in the same
+direction; for, in the after-shock of February 24th, the isoseismals
+drawn by Professor Mercalli are parallel to this line; and this was
+also the case in the shock of March 11th. As both foci were under the
+sea, it is difficult to locate them with precision; but it seems very
+probable that they occupy portions of a submarine fault that runs
+parallel or nearly so to the Apennine axis between the meridians of
+Oneglia and Nice.
+
+A brief period of preparation is a characteristic of the Riviera
+earthquakes. In 1887, two at least of the preliminary shocks on
+February 23rd (those of about 2 and 5 A.M.) originated in the Oneglia
+focus. At 6.20 A.M. the first and weaker movement took place in the
+western focus; and, a few seconds after the resulting vibrations
+reached the eastern focus, the second and greater slip took place
+there. The occurrence of seismic sea-waves is probably evidence of the
+formation of a small, though sensible, fault-scarp in the same region.
+To relieve the additional stresses thus brought into action along the
+fault-surface, numerous small slips took place in different parts,
+some as far to the west as the Nice focus, but the greater number
+probably within or close to the focus in the neighbourhood of Oneglia.
+
+
+REFERENCES.
+
+  1. BERTELLI, T.--"Osservazioni fatte in occasione di una escursione
+        sulle Riviera Ligure di ponente dopo i terremoti ivi seguiti
+        nell' anno 1887." _Boll. Mens. dell' Oss. di Moncalieri_,
+        vol. viii., 1888, Nos. 6, 7, 8.
+
+  2. CHARLON, E.--"Note sur le tremblement de terre du 23 février
+        1887." _Bull. del Vulc. Ital._, anno xiv., 1887, pp. 18-23.
+
+  3. DENZA, F.--_Alcune notizie sul terremoto del 23 febbraio 1887_
+        (Turin).
+
+  4. ISSEL, A.--"Il terremoto del 1887 in Liguria." _Boll. del R. Com.
+        Geol. d'Italia_, anno 1887, supplemento, pp. 1-207.
+
+  5. MERCALLI, G.--_I terremoti della Liguria e del Piemonte_.
+        (Naples, 1897, 146 pp.)
+
+  6. ODDONE, E.--"I dati sismici della Liguria in rapporto alla
+        frequenza ed alla periodicità." _Boll. della Soc. Sismol.
+        Ital._, vol. ii., 1896, pp. 140-151.
+
+  7. OFFRET, A.--"Sur le tremblement de terre du 23 février 1887.
+        Discussion des heures observés dans la zone épicentrale."
+        Paris, _Acad. Sci., Compt. Rend._, vol. civ., 1887, pp.
+        1150-1153.
+
+  8. ----. "Tremblements de terre du 23 février 1887. Heures de
+        l'arrivée des secousses en dehors de l'épicentre." _Ibid._,
+        pp. 1238-1242.
+
+  9. ROSSI, M.S. DE.--"Relazione sui terremoti del febbraio 1887."
+        _Bull. del Vulc. Ital._, anno xiv., 1887, pp. 5-17.
+
+  10. ----. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."
+        _Ibid._, pp. 60-62, 107-112, 115-128.
+
+  11. TARAMELLI, T., and G. MERCALLI.--"Il terremoto ligure del 23
+        febbraio 1887." _Annali dell' Uff. Centr. di Meteor. e di
+        Geodin._, vol. viii., parte iv., 1888. (Roma, 298 pp.)
+
+  12. UZIELLI, G.--_Le commozioni telluriche e il terremoto del 23
+        febbraio 1887_ (Turin).
+
+  13. _Nature_, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi.,
+        1887, pp. 4, 151-152.
+
+  14. Paris, _Acad. Sci. Compt. Rend._, vol. civ., 1887, pp. 556-557,
+        606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766,
+        822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245,
+        1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol.
+        cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274,
+        660.
+
+FOOTNOTES:
+
+[47] The above times and all others in this chapter are given in Rome
+mean time, which is 50m. earlier than Greenwich mean time.
+
+[48] Professor Uzielli has also published a map of the isoseismal
+lines for the Italian part of the disturbed area.
+
+[49] It seems doubtful whether this movement was connected with the
+earthquake. M. Offret does not include Nice in his list of
+observatories at which magnetographs were disturbed.
+
+[50] This is the time given by M. Offret. According to M. Mascart, it
+should be 6h. 25m. 40s.
+
+[51] In order to test the truth of this explanation, M. Moureaux
+suspended a bar of copper at the Parc Saint-Maur observatory by two
+threads in the same way as the horizontal force-magnet. The direction
+of this bar was also registered photographically, and it remained
+unmoved during the Verny earthquake of July 12th, 1889, and the
+Dardanelles earthquake of October 25th, 1889, while one or more of the
+magnets were disturbed. The experiment, however, was ineffective; for,
+in order that the magnet may rest in a horizontal position, its centre
+of gravity must be at unequal distances from the two points of
+support.
+
+[52] The hour-marks in Fig. 38 refer to Paris mean time, and those in
+Fig. 39 to Genoa mean time.
+
+[53] In the seventeenth century, the maximum seismic activity was
+manifested in the neighbourhood of Nice, and in the eighteenth century
+in Piedmont.
+
+
+
+
+CHAPTER VII.
+
+THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891.
+
+
+Although several years have elapsed since the occurrence of the
+greatest of Japanese earthquakes, the final report that will embody
+the labours of all its investigators is yet to be written. Several
+important contributions to it, however, have already been made.
+Professor Koto, in an admirable memoir, has traced the course of the
+great fault-scarp and discussed the origin of the earthquake;
+Professor Omori, with equal care and thoroughness, has investigated
+the unrivalled series of after-shocks; Mr. Conder studied the damaged
+buildings from an architect's point of view; Professor Tanakadate and
+Dr. Nagaoka devoted themselves to a re-determination of the magnetic
+elements of the central district,[54] while, by the compilation of his
+great catalogue of Japanese earthquakes during the years 1885-92,
+Professor Milne has provided the materials for a further analysis of
+the minor shocks that preceded and followed the principal earthquake.
+
+The part of Japan over which the earthquake was sensibly felt is
+shown in Fig. 41. The small black area in the centre is that in which
+the shock was most severe and the principal damage to life and
+property occurred. The other bands, more or less darkly shaded
+according to the greater or less intensity of the shock, will be
+referred to afterwards. Fig. 45 represents the meizoseismal area on a
+larger scale; and, as the greater part of it lies within the two
+provinces of Mino and Owari, the earthquake is generally known among
+the Japanese themselves as the Mino-Owari earthquake of 1891.
+
+   [Illustration: FIG. 41.--Sketch-Map of Disturbed Area and
+   Isoseismal Lines. (_Masato._)]
+
+
+THE MEIZOSEISMAL AREA.
+
+More than half of the meizoseismal area occupies a low flat plain of
+not less than 400 square miles in extent. On all sides but the south,
+the plain, which is a continuation of the depression forming the Sea
+of Isé, is surrounded by mountain ranges, those to the west, north,
+and north-east being built up mainly of Palæozoic rocks, and those on
+the east side of granite. A network of rivers and canals converts what
+might otherwise have been unproductive ground into one of the most
+fertile districts in Japan. A great garden, as it has been aptly
+termed, the whole plain is covered with rice-fields, and supports a
+population of about 787 to the square mile--a density which is
+exceeded in only six counties of England. As a rule, the soil is a
+loose, incoherent, fine sand, with but little clayey matter; and it
+is, no doubt, to its sandy nature that the disastrous effects of the
+earthquake were largely due. In the northern half of the district, the
+meizoseismal area is much narrower, and here it crosses a great
+mountain-range running from south-west to north-east and separating
+the river-systems of the Japan sea from those of the Pacific. To the
+north, the meizoseismal area terminates in another plain, in the
+centre of which lies the city of Fukui, where the destructiveness of
+the earthquake was only inferior to that experienced in the provinces
+of Mino and Owari. There is also a detached portion of the area lying
+to the east of Lake Biwa, but it is uncertain whether the exceptional
+intensity there was due to the nature of the ground or to the
+occurrence of a secondary or sympathetic earthquake in its immediate
+neighbourhood.
+
+   [Illustration: FIG. 42.--General Plan of Geological Structure of
+   Meizoseismal Area. (_Koto._)]
+
+The general plan of the geological structure of the central district
+is represented in Fig. 42. The thick line, partly continuous and
+partly broken, shows the course of the great fault, to the growth of
+which the earthquake chiefly owed its origin; while the thin
+continuous lines represent the changing direction of strike of the
+Palæozoic rocks which surround the Mino-Owari plain, and the
+arrowheads the direction of the dip. It will be seen that the
+direction of the strike forms an S-shaped curve, and it is clear that
+the present torsion-structure of the district could not have been
+produced without the formation of many fractures at right angles and
+parallel to the lines of strike. Professor Koto points out that the
+regular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, and
+Itatori, indicated by broken lines in Fig. 42, have probably been
+excavated along a series of transverse fractures running from
+north-west to south-east; while fractures which are parallel to the
+line of strike may be responsible for the zigzag course of the
+valleys.
+
+
+DAMAGE CAUSED BY THE EARTHQUAKE.
+
+The great earthquake occurred at 6.37 A.M., practically without
+warning, and in a few seconds thousands of houses were levelled with
+the ground. Within the whole meizoseismal area there was hardly a
+building left undamaged. The road from Nagoya to Gifu, more than
+twenty miles in length, and formerly bordered by an almost continuous
+succession of villages, was converted into a narrow lane between two
+long drawn-out banks of _débris_. "In some streets," says Professor
+Milne, "it appeared as if the houses had been pushed down from the
+end, and they had fallen like a row of cards." Or, again, a mass of
+heaped-up rubbish might be passed, "where sticks and earth and tiles
+were so thoroughly mixed that traces of streets or indications of
+building had been entirely lost." At Gifu, Ogaki, Kasamatsu, and other
+towns, fires broke out after the earthquake. In Kasamatsu the
+destruction was absolutely complete; nothing was left but a heap of
+plaster, mud, tiles, and charred timbers. At Ogaki, not more than
+thirty out of 8000 houses remained standing, and these were all much
+damaged. Within the whole district, according to the official returns,
+197,530 buildings were entirely destroyed, 78,296 half destroyed, and
+5,934 shattered and burnt; while 7,279 persons were killed, and 17,393
+were wounded.
+
+Next to buildings, the embankments which border the rivers and canals
+suffered the most serious damage, no less than 317 miles of such works
+having to be repaired. Railway-lines were twisted or bent in many
+places, the total length demolished being more than ten miles. In
+cuttings, twenty feet or more in depth, both rails and sleepers were
+unmoved; it was on the plains that the effects of the earthquake were
+most marked. The ground appeared as if piled up into bolster-like
+ridges between the sleepers, and in many places the sleepers had moved
+end-ways. When the line crossed a small depression in the general
+level of the plain, the whole of the track was bowed, as if the ground
+were permanently compressed at such places. "Effects of compression,"
+says Professor Milne, "were most marked on some of the embankments,
+which gradually raise the line to the level of the bridges. On some of
+these, the track was bent in and out until it resembled a serpent
+wriggling up a slope.... Close to the bridges the embankments had
+generally disappeared, and the rails and sleepers were hanging in the
+air in huge catenaries."
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+The land area disturbed by the earthquake and the different isoseismal
+lines are shown in Fig. 41. The "most severely shaken" district, that
+in which the destruction of buildings and engineering works was
+nearly complete, contains an area of 4,286 square miles, or about
+two-thirds that of Yorkshire. This is indicated on the map by the
+black portion. Outside this lies the "very severely shaken" district,
+17,325 square miles in area, extending from Kobe on the west to
+Shizuoka on the east, in which ordinary buildings were destroyed,
+walls fractured, embankments and roads damaged, and bridges broken
+down. The third or "severely shaken" district contains 20,183 square
+miles; and in this some walls were cracked, pendulum clocks stopped,
+and furniture, crockery, etc., overthrown. Tokio and Yokohama lie just
+within this area. In the fourth region the shock was "weak," the
+motion being distinctly felt, but not causing people to run
+out-of-doors; and in the fifth it was "slight," or just sufficient to
+be felt. These two regions together include an area of 51,976 square
+miles.
+
+Thus, the land area disturbed amounts altogether to 93,770 square
+miles--_i.e._, to a little more than the area of Great Britain.
+According to Professor Omori, the mean radius of propagation was about
+323 miles, and the total disturbed area must therefore have been about
+330,000 square miles, or nearly four times the area of Great Britain.
+Considering the extraordinary intensity of the shock in the central
+district, this can hardly be regarded as an over-estimate.
+
+The isoseismal lines shown in Fig. 41 are not to be regarded as drawn
+with great accuracy; for there is no marked separation between the
+tests corresponding to the different degrees of the scale of
+intensity. The seismographs at Gifu and Nagoya were thrown down
+within the first few seconds, and failed to record the principal
+motion. But a great number of well-formed stone lanterns and
+tombstones were overturned, and, from the dimensions of these,
+Professor Omori calculated the maximum horizontal acceleration
+necessary for overturning them at fifty-nine places within the
+meizoseismal area.[55] At five of these it exceeded 4000 millimetres
+per second per second, an acceleration equal to about five-twelfths of
+that due to gravity. Making use of these observations, Professor Omori
+has drawn two isoseismal lines within the central district, which are
+shown in Fig. 44. At every point of the curve marked 2, the maximum
+acceleration was 2000 millimetres per second per second, and of that
+marked 1, 800 millimetres per second per second. The dotted line
+within the curve marked 2 represents the boundary of the meizoseismal
+area, which, it will be observed, differs slightly from that given by
+Professor Koto (see Fig. 45). The difference, however, is apparently
+due to the standard of intensity adopted, Professor Koto's boundary
+agreeing rather closely with the curve marked 2 in Fig. 44.
+
+
+NATURE OF THE SHOCK.
+
+Little has yet been made known with regard to the nature of the shock,
+and the published records of the accompanying sound are so rare that
+it seems as a rule to have passed unheard. The seismographs at Gifu
+and Nagoya registered the first half-dozen vibrations, and were then
+buried beneath the fallen buildings. In the following table, the data
+from these two stations are therefore incomplete:--
+
+
+PRINCIPAL MEASUREMENTS OBTAINED FROM SEISMOGRAPHIC RECORDS.
+
+-------------------+------------+----------+----------+---------------
+                   |            |          |          | Tokio
+                   | Gifu.      | Nagoya.  | Osaka.   | (Imp. Univ.).
+-------------------+------------+----------+----------+---------------
+Maximum horizontal |            |          |          |
+motion             | > 18 mm.   | > 26 mm. | 30 mm.   | > 35 mm.
+                   |            |          |          |
+Period of ditto    | 2.0 secs.  | 1.3 sec. | 1.0 sec. | 2.0 secs.
+                   |            |          |          |
+Maximum vertical   |            |          |          |
+motion             | > 11.3 mm. | 6.2 mm.  | 8 mm.    | 9.5 mm.
+                   |            |          |          |
+Period of ditto    | 0.9 sec.   | 1.5 sec. | 1.0 sec. | 2.4 secs.
+-------------------+------------+----------+----------+---------------
+
+If the period of the principal vibrations were known, the observations
+of Professor Omori on the overturning of bodies would enable us to
+determine the range of motion at different places. For instance, the
+maximum acceleration at Nagoya was found by these observations to be
+2,600 millimetres per second per second, and if we take the period of
+the greatest horizontal motion to be the same as that of the initial
+vibrations--namely, 1.3 second, the total range (or double amplitude)
+would be 223 millimetres, or 8.8 inches. With the same period, and the
+maximum acceleration observed (at Iwakura and Konaki) of more than
+4,300 millimetres per second per second, the total range would be
+greater than 14.5 inches.[56]
+
+In the meizoseismal area, many persons saw waves crossing the surface
+of the ground. At Akasaka, according to one witness, the waves came
+down the streets in lines, their height being perhaps one foot, and
+their length between ten and thirty feet. To the north of the same
+area, we are told that "the shoreline rose and fell, and with this
+rising and falling the waters receded and advanced." Even at Tokio,
+which is about 175 miles from the epicentre, the tilting of the ground
+was very noticeable. After watching his seismographs for about two
+minutes, Professor Milne next observed the water in an adjoining tank,
+80 feet long and 28 feet wide, with nearly vertical sides. "At the
+time it was holding about 17 feet of water, which was running across
+its breadth, rising first on one side and then on the other to a
+height of about two feet." Still clearer is the evidence of the
+seismographs in the same city. Instead of a number of irregular waves,
+all the records show a series of clean-cut curves. The heavy masses in
+the horizontal pendulums were tilted instead of remaining as steady
+points. They were not simply swinging, for the period of the
+undulations differed from that of the seismograph when set swinging,
+and also varied in successive undulations. It was ascertained
+afterwards, by measurement with a level, that to produce these
+deflections, the seismograph must have been tilted through an angle of
+about one-third of a degree.
+
+_Direction of the Shock._--Shortly after the earthquake, Professor
+Omori travelled over the meizoseismal area and made a large number of
+observations on the directions in which bodies were overturned, taking
+care to include only those in which the direction of falling would not
+be influenced by the form of the base, such as the cylindrical stone
+lanterns so frequently found in Japanese gardens. At some places
+these bodies fell in various directions, at others with considerable
+uniformity in one direction. For instance, at Nagoya, out of 200 stone
+lanterns with cylindrical stems, 119 fell between west and south, and
+36 between east and north; the numbers falling within successive
+angles of 15° being represented in Fig. 43. The mean direction of fall
+is W. 30° S., coinciding with that in which the majority of the
+lanterns were overturned. Similar observations were made at forty-two
+other places within and near the meizoseismal area, and the resulting
+mean direction for each such place in the Mino-Owari district is shown
+by short lines in Fig. 44, the arrow indicating the direction towards
+which the majority of bodies at a given place were overturned. It will
+be seen from this map that the direction of the earthquake motion was
+generally at right angles, or nearly so, to that of the neighbouring
+part of the meizoseismal zone, and that on both sides of it, the
+majority of overturned bodies at each place fell towards this zone.
+
+   [Illustration: FIG. 43.--Plan of Directions of Fall of Overturned
+   Bodies at Nagoya.]
+
+   [Illustration: FIG. 44.--Map of Mean Directions of Shock and
+   Isoseismal Lines in Central District. (_Omori._)]
+
+
+VELOCITY OF THE EARTH-WAVES.
+
+The times of the great earthquake and of sixteen minor shocks on
+October 28th and 29th and November 6th were determined at the Central
+Meteorological Observatory at Tokio, and at either two or three of the
+observatories of Gifu, Nagoya, and Osaka, each of which is provided
+with a seismograph and chronometer. The after-shocks referred to
+originated near a point about 6 miles west of Gifu, and the difference
+between the distances of Tokio and Osaka from this point is 89-1/2
+miles, of Tokio and Nagoya 147 miles, and of Tokio and Gifu 165 miles.
+The mean time-intervals between these three pairs of places were 67,
+111, and 128 seconds respectively; and these give for the mean
+velocity for each interval 2.1 kilometres (or 1.3 mile) per second.
+Thus there appears in these cases to be no sensible variation in the
+velocity with the distance from the origin.
+
+As might be expected, an earthquake of such severity was recorded by
+magnetometers at several distant observatories. Disturbances on the
+registers of Zikawei (China), Mauritius, Utrecht, and Greenwich have
+been attributed to the Japanese earthquake, but the times at which
+they commenced are too indefinite to allow of any determination of the
+surface-velocity of the earth-waves to great distances from the
+origin.
+
+
+THE GREAT FAULT-SCARP.
+
+As in all disastrous earthquakes, the surface of the ground was
+scarred and rent by the shock. From the hillsides great landslips
+descended, filling the valleys with _débris_; and slopes which were
+formerly green with forest, after the earthquake looked as if they had
+been painted yellowish-white. Innumerable fissures cut up the plains,
+the general appearance of the ground, according to Professor Milne,
+being "as if gigantic ploughs, each cutting a trench from 3 to 12 feet
+deep, had been dragged up and down the river-banks." But by far the
+most remarkable feature of the earthquake was a great rent or fault,
+which, unlike the fissures just referred to, pursued its course
+regardless of valley, plain, or mountain. Although at first sight
+quite insignificant in many places, and some time hardly visible to
+the untrained eye, Professor Koto has succeeded in tracing this fault
+along the surface for a distance of forty miles, and he gives good
+reasons for believing that its total length must be not less than
+seventy miles.
+
+   [Illustration: FIG. 45.--Map of Meizoseismal Area. (_Koto._)]
+
+   [Illustration: FIG. 46.--Ploughshare Appearance of the Fault near
+   Fujitani. (_Koto._)]
+
+   [Illustration: FIG. 47.--The Fault-scarp at Midori. (_Koto._)]
+
+The general character of the fault-scarp changes with the surface
+features. On flat ground, where the throw is small, it cuts up the
+soft earth into enormous clods, or makes a rounded ridge from one to
+two feet high, so that it resembles, more than anything else, the
+pathway of a gigantic mole (Fig. 46). When the throw is
+considerable--and in one place it reaches from 18 to 20 feet--the
+fault-scarp forms a terrace, which from a distance has the appearance
+of a railway embankment (Fig. 47). Or, again, where the rent traverses
+a mountain ridge or a spur of hills, "it caused extensive landslips,
+one side of it descending considerably in level, carrying the forest
+with it, but with the trees complicatedly interlocked or prostrate on
+the ground."
+
+   [Illustration: FIG. 48.--Displacement of Field Divisions by the
+   Fault near Nishi-Katabira. (_Koto._)]
+
+At its southern end, the fault was seen for the first time crossing a
+field near the village of Katabira. The field was broken into clods of
+earth, and swollen up to a height of 5-1/2 yards, while a great
+landslip had descended into it from an adjoining hill. A little
+farther to the north-west, the ground was sharply cut by the fault,
+the north-east side having slightly subsided and at the same time been
+shifted horizontally through a distance of 3-1/4 to 4 feet to the
+north-west Adjoining fields were formerly separated by straight mounds
+or ridges running north and south and east and west, and these mounds
+were cut through by the fault and displaced, as shown in Fig. 48. From
+this point the fault runs in a general north-westerly direction, the
+north-east side being always slightly lowered with respect to the
+other and shifted to the north-west. Near Seki it takes a more
+westerly direction, and continues so to a short distance east of
+Takatomi, where the north side is lowered by five feet, and moved
+about 1-1/4 feet to the west. At the north end of Takatomi, a village
+in which every house was levelled with the ground, the fault is
+double, and the continuous lowering towards the north has converted a
+once level field into sloping ground. At this point, the small river
+Toba, flowing south, is partially blocked by the fault-scarp, and an
+area of about three-quarters of a square mile, on which two villages
+stand, was converted into a deep swamp (Fig. 49), so that, as the
+earthquake occurred at the time of the rice-harvest, the farmers were
+obliged to cut the grain from boats. After passing Takatomi, the fault
+again turns to the west-north-west, but, the throw being small, it
+resembles here the track of an enormous mole. At Uméhara it crosses a
+garden between two persimmon trees, appearing on the hard face of the
+ground as a mere line; but the trees, which were before in an
+east-and-west line, now stand in one running north and south, without
+being in the least affected by the movement (Fig. 50). From here to
+Kimbara, where the fault enters the Neo valley, the north side is
+always depressed and shifted westwards by about 6-1/2 feet.
+
+   [Illustration: FIG. 49.--Map of Swamp formed by stoppage of River
+   Toba by Fault-scarp. (_Koto._)]
+
+   [Illustration: FIG. 50.--Shifting of Trees by fault at Uméhara.
+   (_Koto._)]
+
+It was in the Neo valley that the supreme efforts of the earthquake
+were manifested. Landslips were so numerous that the greater part of
+the mountain slopes had descended into the valley, the whole
+appearance of which had changed. "Unfamiliar obstacles," remarks
+Professor Koto, "made themselves apparent, and small hills covered
+with forest had come into sight which had not been seen before." But
+the ground was not only lowered and shifted by the fault; it was
+permanently compressed, plots originally 48 feet in length afterwards
+measuring only 30 feet. In fact, "it appears," in the words of
+Professor Milne, "as if the whole Neo valley had become narrower."
+
+A few miles after entering the Neo valley, the throw of the fault
+reaches its maximum at Midori. But instead of the relative depression
+of the east side, which prevails throughout the rest of the line, that
+side is here about 20 feet higher than the other. It is, however,
+shifted as usual towards the north, by about 13 feet; and this
+displacement is rendered especially evident by the abrupt break in the
+line of a new road to Gifu (Fig. 47). That the east side has really
+risen is clear, for, a little higher up, the river has changed from a
+shallow rapid stream 30 yards wide into a small lake of more than
+twice the width, and so deep that a boatman's pole could not reach the
+bottom. At Itasho, about a mile north of Midori, both sides are nearly
+on the same level, the fault appearing like a mole's track; and seven
+miles farther, at Nagoshima, the east side is relatively depressed by
+more than a yard, and at the same time shifted about 6-1/2 feet to the
+north.
+
+   [Illustration: FIG. 51.--Daily frequency of after-shocks at Gifu
+   and Nagoya.]
+
+At Nogo, the main Neo valley turns off at right angles to the east,
+and the fault continues its course up a side valley, the east side,
+with respect to the other, being continually depressed and shifted
+towards the north. It was traced by Professor Koto through Fujitani
+(Fig. 46), where there were many unmistakable evidences of the
+violence of the shock, as far as the eastern shoulder of Haku-san; and
+here, after following the fault for 40 miles, the lateness of the
+season compelled him to return. There can be no doubt, however, that
+it runs as far as Minomata; and it is probable, from the linear
+extension of the meizoseismal area, that it does not entirely die out
+before reaching the city of Fukui, 70 miles from its starting-point at
+Katabira.
+
+
+MINOR SHOCKS.
+
+For some hours after the earthquake, shocks were so frequent in the
+meizoseismal area that the ground in places hardly ever ceased from
+trembling. Without instrumental aid, detailed record was of course
+impossible; but fortunately the buried seismographs at Gifu and Nagoya
+were uninjured, and in about seven hours both were once more in
+working order. To the energy by which this result was accomplished, we
+owe our most valuable registers of the after-shocks of a great
+earthquake.
+
+   [Illustration: FIG. 52.--Monthly frequency of after-shocks at
+   Gifu. (_Omori._)]
+
+Until the end of 1893--that is, in little more than two years--the
+total number of shocks recorded at Gifu was 3,365, and at Nagoya
+1,298. None of these approached the principal earthquake in severity.
+Nevertheless, of the Gifu series, 10 were described as violent and 97
+strong; while of the remainder, 1,808 were weak, 1,041 feeble, and 409
+were sounds alone without any accompanying shock. The slight intensity
+of most of the shocks is also evident from the inequality in the
+numbers recorded at Gifu and Nagoya, from which it appears that nearly
+two-thirds were imperceptible more than about 25 miles from the chief
+origin of the shocks. Only 70 of the after-shocks during the first two
+years were registered at Osaka, and not more than 30 at Tokio.
+
+_Distribution of After-shocks in Time._--The decline in frequency of
+the after-shocks was at first extremely rapid, the numbers recorded at
+Gifu during the six days after the earthquake being 303, 147, 116, 99,
+92, and 81, and at Nagoya 185, 93, 79, 56, 30, and 31; in fact, half
+of the shocks up to the end of 1893 occurred by November 23rd at Gifu,
+and by November 6th at Nagoya. The daily numbers at these two places
+are represented in Fig. 51, in which the crosses correspond to the
+numbers at Gifu, and the dots to those at Nagoya; and the curves drawn
+through or near the marks represent the average daily number of shocks
+from October 29th to November 20th. It will be seen that these curves
+are hyperbolic in form, the change from very rapid to very gradual
+decline in frequency taking place from five to ten days after the
+great earthquake. Fig. 52 illustrates the distribution in time of the
+after-shocks at Gifu to the end of 1893, the ordinates in these cases
+representing the number of shocks during successive months.[57]
+
+A similar rapid and then gradual decline in frequency characterises
+the strong and weak shocks recorded at Gifu. Of the ten violent
+shocks, only one occurred after the beginning of January 1892; and of
+the 97 strong shocks, only three after April 1892. But at the
+commencement of the series, feeble shocks (_i.e._, shocks that could
+just be felt) and earth-sounds without any accompanying movement were
+comparatively rare, and did not become really prominent until two
+months had elapsed. Of the 308 after-shocks recorded in 1893, none
+could be described as strong, only 10 were weak, while 263 were feeble
+shocks and 35 merely earth-sounds.
+
+The last two diagrams show at a glance that the decline in frequency
+of after-shocks is very far from being uniform. Some of the
+fluctuations are due to the occurrence of exceptionally strong shocks,
+each of which is followed by its own minor train of after-shocks.[58]
+Others seem to be periodic, and possibly owe their origin to external
+causes unconnected with the earthquake.[59]
+
+_Method of representing the Distribution of After-shocks in
+Space._--The maps in Figs. 54-57 show the distribution of the
+after-shocks in space during four successive intervals of two months
+each. They are founded on Professor Milne's great catalogue of
+Japanese earthquakes, which give, among other data, the time of
+occurrence and the position of the epicentre for every shock until the
+end of 1892. For the latter purpose, the whole country is divided by
+north-south and east-west lines into numbered rectangles, each
+one-sixth of a degree in length and breadth; and the position of an
+epicentre is denoted by the number of the rectangle in which it
+occurs. The area included within the maps is bounded by the parallels
+34° 40' and 36° 20' lat. N., and by the meridians 2° 10' and 3° 50'
+long. W. of Tokio, so that ten rectangles adjoin each side of the map.
+The number of epicentres lying within each rectangle having been
+counted, curves are then drawn through the centres of all rectangles
+containing the same number of epicentres, or through points which
+divide the line joining the centres of two rectangles in the proper
+proportion. Taking, for example, the curve marked 5, if the numbers in
+two consecutive rectangles are 3 and 7, the curve bisects the line
+joining their centres; if the numbers are 1 and 6, the line joining
+their centres is divided into five equal parts, and the curve passes
+through the first point of division reckoned from the centre of the
+rectangle in which six epicentres are found. Thus the meaning of the
+curve marked, say, 5 may be stated as follows:--If any point in the
+curve be imagined as the centre of a rectangle whose sides are
+directed north-south and east-west, and are respectively one-sixth of
+a degree of latitude and longitude in length; then the number of
+epicentres within this rectangle is at the rate of 5 for the time
+considered.
+
+_Preparation for the Great Earthquake._--At first sight, there appears
+to have been but little direct preparation for the great earthquake.
+Except for a rather strong shock on October 25th, at 9.14 P.M., it
+occurred without the warning of any preliminary tremors. But a closer
+examination of the evidence shows, as we should indeed expect, that
+there was a distinct increase in activity for many months beforehand.
+The region had become "seismically sensitive." Of the hundred
+rectangles included in the maps in Figs. 53-57, there are thirteen
+lying along the meizoseismal area of the earthquake of 1891, in which
+nearly all the after-shocks originated. During the five years 1885-89,
+53 out of 125 earthquakes (or 42 per cent.) had their epicentres lying
+within the thirteen rectangles; or, in other words, the average
+frequency in one of the rectangles of the meizoseismal area was five
+times as great as in one of those outside it. In 1890 and 1891 (until
+October 27th), the percentage in the thirteen rectangles rose to 61,
+and the average frequency in one of them to ten times that of one of
+the exterior rectangles.
+
+The curves in Fig. 53 illustrate the distribution of epicentres during
+the latter interval. It will be seen that they follow roughly the
+course of the meizoseismal area southwards to the Sea of Isé, and that
+to the south-east they continue for several miles the short branch of
+the meizoseismal area which surrounds the southern end of the
+fault-scarp.
+
+   [Illustration: FIG. 53.--Distribution of preliminary Shocks in
+   Space. (_Davison._)]
+
+Thus, the preparation for the great earthquake is shown, first, by the
+increased frequency of earthquakes originating within its meizoseismal
+area; and, secondly, by the uniformity in the distribution of
+epicentres throughout the same region, the marked concentration of
+effort which characterises the after-shocks being hardly perceptible
+during the years 1890-91.
+
+   [Illustration: FIG. 54.--Distribution of After-shocks in Space
+   (November-December 1891). (_Davison_).]
+
+_Distribution of After-shocks in Space._--We have seen that the
+after-shocks were subject to a fluctuating decline in frequency, rapid
+at first, and more gradual afterwards. It is evident, from Figs.
+54-57, that a similar law governs the area within which the
+after-shocks originated. During the first two months, epicentres occur
+over nearly the whole of the meizoseismal area, but afterwards they
+are confined to a smaller district, which slowly, though not
+continually, decreases in size.
+
+   [Illustration: FIG. 55.--Distribution of After-shocks in Space
+   (January-February, 1892). (_Davison._)]
+
+   [Illustration: FIG. 56.--Distribution of After-shocks in Space
+   (March-April). (_Davison._)]
+
+The most important feature in the distribution of the epicentres is
+the central region of extraordinary activity; but there are also
+districts of minor and more short-lived activity near the three
+extremities of the meizoseismal band. The seat of chief seismic action
+shifts slightly from one part to another of the epicentral region,
+especially about the end of 1891, as will be seen by comparing the
+innermost curves of Figs. 54 and 55. Thus, with the decline in
+frequency of the after-shocks and the decrease in their sphere of
+action, there took place concurrently a gradual but oscillating
+withdrawal of that action to a more or less central region of the
+fault.
+
+   [Illustration: FIG. 57.--Distribution of After-shocks in Space
+   (May-June, 1892). (_Davison._)]
+
+_Sound Phenomena of After-shocks._--While comparatively few observers
+seem to have noticed any noise with the principal earthquake, many of
+the after-shocks were accompanied by sounds. Professor Omori describes
+them as belonging to two types. They were either rushing feeble noises
+like that of wind, or loud rumbling noises like those of thunder, the
+discharge of a gun, or the fall of a heavy body. In the Neo valley,
+sounds of the second type were most frequent and distinct, but they
+either occurred without any shock at all, or the attendant tremor was
+very feeble; while, on the other hand, severe sharp shocks were
+generally unaccompanied by distinctly audible sounds.
+
+It is remarkable, also, that sounds were less frequently heard with
+the early than with the later after-shocks. In November 1891, the
+percentage of audible shocks was 17, and from December to the
+following April always lay between 10 and 12. In May the percentage
+suddenly rose to 39, and until the end of 1892 was always greater than
+32, while in November 1892, it rose as high as 49. This, of course,
+agrees with Professor Omori's observation that sounds attended feeble
+shocks more often than strong ones.
+
+The distribution of the audible after-shocks in space is shown in Fig.
+58. These curves are drawn in the same way as those in Figs. 53-57,
+but they represent the percentages, not the actual numbers, of shocks
+accompanied by sound. It will be noticed that all three groups of
+curves lie along the meizoseismal area, or the continuation of the
+south-east branch; while the axis of the principal group of curves
+lies to the west of the central regions in which most after-shocks
+originated.
+
+   [Illustration: FIG. 58.--Distribution of Audible After-shocks in
+   Space (November 1891-December 1892). (_Davison_.)]
+
+The explanation of these peculiarities is no doubt connected with the
+comparative inability of the Japanese people to perceive the deep
+sounds which in Europe are always heard with earthquake shocks. The
+sounds are rarely heard by them more than a few miles from the
+epicentre.[60] We may therefore conclude that slight after-shocks
+originated nearer the surface than strong ones, that the mean depth
+of the foci decreased with the lapse of time, and that the axes of the
+systems of curves in Fig. 58 mark out approximately the lines of the
+growing faults. The separation of the two westerly groups of curves
+appears to show that the main branch of the meizoseismal area is
+connected with a fault roughly parallel to that traced by Professor
+Koto, but of which no scarp (if it existed) could be readily
+distinguished among the superficial fissures produced by the great
+shock.
+
+
+EFFECT OF THE EARTHQUAKE ON THE SEISMIC ACTIVITY OF THE ADJOINING
+DISTRICTS.
+
+So great and sudden a displacement as occurred along the fault-scarp
+could hardly take place without affecting the stability of adjoining
+regions of the earth's crust, and we should naturally expect to find a
+distinct change in their seismic activity shortly after October 28th.
+In Fig. 59 two such regions are shown, bounded by the straight dotted
+lines. The district in which the principal earthquake and its
+after-shocks originated is enclosed within the undulating dotted
+lines. The continuous lines inside all three districts are the curves
+corresponding to 10 and 5 epicentres for the years 1885-92. Not far
+from the axes of the outer groups of curves there are probably
+transverse faults, approximately parallel to the great fault-scarp and
+the main branch of the meizoseismal band, and distant from them about
+45 and 55 miles respectively.
+
+   [Illustration: FIG. 59.--Map of Adjoining Regions in which
+   Seismic Activity was affected by the Great Earthquake.
+   (_Davison._)]
+
+In the district represented in the north-east corner of Fig. 59, 29
+earthquakes originated between January 1st, 1885, and October 27th,
+1891, and 30 between October 28th, 1891, and December 31st, 1892, 7 of
+the latter number occurring in November 1891. In the south-west
+district, the corresponding figures before and after the earthquake
+are 20 and 36, 8 of the latter occurring in November 1891. Thus, in
+the north-east district, for every shock in the interval before the
+earthquake there were six in an equal time afterwards, and at the rate
+of 10 during November 1891; and in the south-west district, for every
+shock before the earthquake there were 10 afterwards, and at the rate
+of 16 during November 1891.
+
+Now, it is unlikely that the gradual increase of stress should be so
+nearly proportioned everywhere to the prevailing conditions of
+resistance as to give rise to a marked and practically simultaneous
+change in seismic activity over a large area; whereas the paroxysmal
+occurrence of a strong earthquake might alter the surrounding
+conditions with comparative rapidity, and so induce a state of seismic
+excitement in the neighbourhood. It therefore seems very probable
+that the increased activity in the two districts here described was a
+direct consequence of the occurrence of the great earthquake.
+
+
+ORIGIN OF THE EARTHQUAKE.
+
+The preponderance of preliminary earthquakes within the meizoseismal
+area and the outlining of the fault-system by the frequency curves of
+1890-91 (Fig. 53) point to the previous existence of the originating
+fault or faults, and to the earthquake being due, not to the formation
+of a new fracture, as has been suggested, but to the growth of an old
+fault.
+
+The last severe earthquake in the Mino-Owari plain occurred in 1859,
+so that for more than thirty years there had been but little relief to
+the gradually increasing stresses. Now, the distribution of stress
+must have been far from uniform throughout the fault-system, and also
+the resistance to displacement far from proportional to the stresses
+at different places. At certain points, therefore, the effective
+stress would be greater than elsewhere, and it would be at these
+points that fault-slips would first occur. Such slips tend to remove
+the inequalities in effective stress. Thus, the function of the slight
+shocks of 1890 and 1891 was, briefly, to equalise the effective stress
+over the whole fault-system, and so to clear the way for one or more
+great slips throughout its entire length.
+
+As to which side of the fault moved during the great displacement, or
+whether both sides moved at once, we have no direct evidence but as
+regards the neighbourhood of Midori, and there the conditions were
+exceptional. Professor Koto thinks that it was probably the rock on
+the north-east side that was generally depressed and always shifted to
+the north-west But the disturbance in reality seems to have been more
+complicated. That this was the case, that displacement occurred along
+more than one fault, is probable from the branching of the
+meizoseismal area, the isolation of the audibility curves of the
+after-shocks (Fig. 58), and the sudden increase in seismic activity
+both to the north-east and south-west of the epicentre. The detached
+portion of the meizoseismal area near Lake Biwa may also point to a
+separate focus. The whole region, indeed, was evidently subjected to
+intense stresses, and the depression on the north-east side of the
+fault-scarp can hardly fail to have been accompanied by other
+movements, especially along a fault running near the western margin of
+the main branch of the meizoseismal area.
+
+The later stages of the movements are somewhat clearer. From a study
+of the after-shocks, we learn that the disturbed masses began at once
+to settle back towards the position of equilibrium. At first the slips
+were numerous and took place over the whole fault-system, but chiefly
+at a considerable depth, where no doubt the initial displacement was
+greatest. After a few months, stability was nearly restored along the
+extremities of the faults; slips were confined almost entirely to the
+central regions, while a much larger proportion of them took place
+within the superficial portions of the faults.
+
+The official records bring down the history to the end of 1893. Since
+that time more than one strong shock has been felt in the Mino-Owari
+plain; but the stage of recovery from the disturbances of 1891 is
+probably near its end, and we seem rather to be entering on a period
+in which the forces are once more silently gathering that sooner or
+later will result in another great catastrophe.
+
+
+REFERENCES.
+
+  1. CONDER, J.--"An Architect's Notes on the Great Earthquake of
+        October 1891." _Japan Seismol. Journ._, vol. ii., 1893, pp.
+        1-91.
+
+  2. DAVISON, C.--"On the Distribution in Space of the Accessory
+        Shocks of the Great Japanese Earthquake of 1891." _Quart.
+        Journ. Geol. Soc._, vol. liii., 1897, pp. 1-15.
+
+  3. ---- "On the Effect of the Great Japanese Earthquake of 1891 on
+        the Seismic Activity of the Adjoining Districts." _Geol.
+        Mag._, vol. iv., 1897, pp. 23-27.
+
+  4. ---- "On the Diurnal Periodicity of Earthquakes." _Phil. Mag._,
+        vol. xiii., 1896, pp. 463-476, especially pp. 466-468.
+
+  5. ---- "On Earthquake-Sounds." _Phil. Mag._, vol. xlix., 1900, pp.
+        31-70--especially pp. 49-53, 60-61.
+
+  6. KOTO, B.--"The Cause of the Great Earthquake in Central Japan,
+        1891." _Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893,
+        pp. 295-353.
+
+  7. MASATO, H.--"Report on Earthquake Observations in Japan." _Cent.
+        Meteor. Obs. of Japan_ (Tokio, 1892), pp. 16-18, 41, and map
+        30.
+
+  8. MILNE, J.--"A Note on the Great Earthquake of October 28th,
+        1891." _Japan Seismol. Journ._, vol. i., 1893, pp. 127-151;
+        _Brit. Assoc. Rep._, 1892, pp. 114-128.
+
+  9. ---- "A Catalogue of 8,331 Earthquakes recorded in Japan between
+        1885 and 1892." _Japan Seismol. Journ._, vol. iv., 1895, pp.
+        1-367--especially pp. 134-234, 303-353.
+
+  10. ---- "On Certain Disturbances in the Records of Magnetometers
+        and the Occurrence of Earthquakes." _Brit. Assoc. Rep._, 1898,
+        pp. 226-251--especially pp. 227, 232, 234, 241, and 245.
+
+  11. MILNE, J., and W.K. BURTON.--"The Great Earthquake in Japan."
+        _Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893, pp.
+        295-352.
+
+  12. OMORI, F.--"On the After-shocks of Earthquakes." _Journ. Coll.
+        Sci. Imp. Univ. Japan_, vol. vii., 1894, pp. 111-200; abstract
+        in _Japan Seismol. Journ._, vol. iii., 1894, pp. 71-80.
+
+  13. ---- "A Note on the Great Mino-Owari Earthquake of October 28th,
+        1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_,
+        No. 4, Tokio, 1900, pp. 13-24.
+
+  14. ---- "Sulla velocità di propagazione e sulla lunghezza delle
+        onde sismiche." _Ital. Soc. Sismol. Boll._, vol. i., 1895, pp.
+        52-60--especially pp. 52-57.
+
+  15. ---- "Sull' intensità e sull' ampiezza del movimento nel gran
+        terremoto giapponese del 28 ottobre 1891." _Ital. Soc. Sismol.
+        Boll._, vol. ii., 1896, pp. 189-200.
+
+  16. ---- "Note on the After-shocks of the Mino-Owari Earthquake of
+        Oct. 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign
+        Languages_, No. 7, Tokio, 1902, pp. 27-32.
+
+  17. ---- "Note on the relation between Earthquake Frequency and the
+        Atmospheric Pressure." _Tokyo Phys.-Math. Soc. Reports_, vol.
+        ii., 1904, No. 8.
+
+  18. TANAKADATE, A., and H. NAGAOKA. "The Disturbance of Isomagnetics
+        attending the Mino-Owari Earthquake of 1891." _Journ. Coll.
+        Sci. Imp. Univ. Japan_, vol. v., 1893, pp. 149-192.
+
+FOOTNOTES:
+
+[54] I have not referred to the results of this survey, for, though
+changes in all the magnetic elements (especially in horizontal
+intensity) have taken place between 1887 and 1891-92, these changes
+cannot be ascribed with confidence to the earthquake in the absence of
+a thorough knowledge of the secular variation.
+
+[55] From the formula a=x*g/y, where _a_ is the maximum horizontal
+acceleration, _g_ the acceleration due to gravity, _y_ the height of
+the centre of gravity, and _x_ its horizontal distance from the edge
+about which the body was overturned.
+
+[56] These estimates are made, on the supposition of simple harmonic
+motion, from the formula 2*a=alpha*t^2/(2*pi^2), where 2_a_ is the
+total range or double amplitude, _a_ the maximum acceleration, and _t_
+the period of the vibration.
+
+[57] Professor Omori finds that the mean daily number of earthquakes
+_y_ during the month _x_ (reckoned from November 1891) may be
+approximately represented by the equation--
+
+y = 16.9 / (x + 0.397);
+
+or, taking the semi-daily earthquake numbers during the five days
+between October 29th and November 2nd, 1891, by the equation--
+
+y = 440.7 / (x + 2.314),
+
+where _y_ denotes the number of earthquakes observed during the twelve
+hours denoted by _x_, the time being measured from the first half of
+October 29th. It is interesting to notice that, taking account of the
+mean annual frequency of earthquakes in ordinary years, the number of
+shocks observed at Gifu during the two years 1898-99 should, according
+to the latter formula, be 163; the actual number recorded was 160.
+
+[58] The last violent shock before the end of 1893 occurred on
+September 7th, 1892, and its effects on the frequency of after-shocks
+is shown by the daily numbers recorded at Gifu during the first
+fortnight in September. These are--2, 2, 2, 3, 5, 5, 28 (on September
+7th), 8, 8, 5, 4, 3, 2, 4, 3.
+
+[59] The periodicity of after-shocks is discussed in the papers
+numbered 4, 12, 16, and 17 at the end of this chapter. In these, the
+existence of diurnal and other periods is clearly established.
+Professor Omori also shows that the mean daily barometric pressure is
+subject to fluctuations with maxima occurring on an average every
+5-1/2 days, and that earthquakes are least frequent on the days of the
+barometric maxima and minima, and more frequent in the days
+immediately preceding and following them.
+
+[60] Of the Japanese earthquakes of 1885-92 originating beneath the
+land, twenty-six per cent. were accompanied by a recorded sound; but
+less than one per cent. of those originating beneath the sea and not
+more than ten miles from the coast.
+
+
+
+
+CHAPTER VIII.
+
+THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND THE INVERNESS
+EARTHQUAKE OF SEPTEMBER 18TH, 1901.
+
+
+Among the earthquakes described in this volume, the Hereford and
+Inverness earthquakes hold but a minor place. The damage to buildings,
+though unusual for this country, was slight when compared with that
+caused by the preceding shocks; there was no loss of life, not a
+single person was injured by falling masonry. The interest of the
+earthquakes lies entirely in the detailed study rendered possible by
+numerous observations of the shock and sound,[61] and in the bearing
+of this evidence on the general theory of the origin of earthquakes.
+
+
+THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896.
+
+The principal earthquake of this series occurred at 5.32 A.M. on
+December 17th, and was preceded by at least nine minor shocks (the
+first of which was felt at about 11 or 11.30 P.M. on December 16th),
+and followed by two others on the same day, and by a third and last on
+July 19th, 1897. The accounts of these preliminary movements will be
+found on a later page, as their bearing will be more fully apparent
+after the discussion of the principal shock.
+
+   [Illustration: FIG. 60.--Isoseismal and Isacoustic lines of
+   Hereford earthquake. (_Davison._)]
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+On the map in Fig. 60, the continuous curves represent isoseismal
+lines corresponding to the degrees 8, 7, 6, 5, and 4 of the
+Rossi-Forel scale. The isoseismal 8, which is the most accurately
+drawn of the series, is an elongated oval, 40 miles long, 23 miles
+broad, and containing an area of 724 square miles. The longer axis is
+directed W. 44° N. and E. 44° S. Within this curve, there are 73
+places where buildings are known to have been damaged, 55 places being
+in Herefordshire, 17 in Gloucestershire, and one in Worcestershire.
+
+The most important damage occurred in the city of Hereford, which, in
+1901, contained 4,565 inhabited houses. Here, no fewer than 218
+chimneys had to be repaired or rebuilt. The Cathedral was slightly
+injured. The finial of a pinnacle of the Lady Chapel was thrown down,
+a fragment of a stone fell from one of the arches in the south
+transept, and the three pinnacles of the western front were fractured.
+Several churches suffered to a similar extent, while, at the Midland
+Railway Station, all the seven chimney-stacks were shattered. At
+Dinedor, Fownhope, Dormington, Withington, and a few other villages,
+the damage was also relatively greater than elsewhere, these places
+all lying within a small oval about 8-1/2 miles long, which surrounds,
+not the centre, but rather the north-west focus, of the isoseismal 8.
+
+The isoseismal 7, which includes places where the shock was strong
+enough to overthrow ornaments, vases, etc., is also very nearly an
+ellipse, whose axes are 80 and 56 miles in length, and whose area is
+3,580 square miles. Its longer axis, running from W. 42° N. to E. 42°
+S., is practically parallel to that of the inner curve. Next in
+succession comes the isoseismal 6, surrounding those places where the
+shock was strong enough to make chandeliers, pictures, etc., swing;
+but, as most of the observers seem to have slept in darkened rooms,
+the number of determining points for this curve is less than usual,
+and its course is therefore laid down with a somewhat inferior degree
+of accuracy. The error, however, is probably small, and we may
+therefore regard the isoseismal 6 as another ellipse, 141 miles long,
+116 miles broad, and containing an area of 13,000 square miles. Its
+longer axis is again nearly parallel to those of the preceding
+isoseismals.
+
+The next two isoseismals are nearly circular in form. It will be
+noticed that large portions of them, and especially of the isoseismal
+4, traverse the sea. In these parts, the paths of the curves are to
+some extent conjectural. In drawing them, the chief guides are their
+trend before leaving the land and the known intensity along the
+neighbouring coastlines. The isoseismal 5 bounds the area within which
+the shock was perceptible as a sensible displacement and not merely a
+quiver. Its dimensions are 233 miles from north-west to south-east,
+and 229 miles from south-west to north-east, and its area 41,160
+square miles. The isoseismal 4, which includes places where the shock
+was strong enough to make doors, windows, etc., rattle, is 356 miles
+from north-west to south-east, and 357 miles from south-west to
+north-east, and 98,000 square miles in area; its centre coincides
+nearly with that of the small oval area in the neighbourhood of
+Hereford, where the damage to buildings was relatively greater than
+elsewhere.
+
+Outside the isoseismal 4, the earthquake was observed at several
+places. The shock was certainly felt at Middlesbrough, 12-1/2 miles
+from the curve, and probably at Killeshandra (in Ireland), 65 miles
+distant. Thus, if we consider the boundary of the disturbed area to
+coincide with the isoseismal 4, its area would be 98,000 square
+miles, or 1-2/3 that of England and Wales; if it were a circle
+concentric with the isoseismal 4, and passing through Middlesbrough,
+its area would be 115,000 square miles, or nearly twice that of
+England and Wales; while, if it passed through Killeshandra, its area
+would be 185,000 square miles, or more than three times the area of
+England and Wales.[62]
+
+_Position of the Originating Fault._--The form, directions, and
+relative positions of the isoseismal lines furnish important evidence
+with regard to the originating fault. We conclude in the first place
+that its mean direction is parallel to the longer axes of the three
+innermost isoseismal lines--that is, north-west and south-east, or,
+more accurately, W. 43° N. and E. 43° S.[63] In this case, the
+elongated forms of the isoseismal lines cannot be attributed to
+variations in the nature of the surface rocks. The district embraced
+contains about 13,000 square miles, and it is improbable that the axes
+of the three isoseismals should retain their parallelism over so large
+an area, if these variations had any considerable effect. Moreover, in
+the same district, an earthquake occurred in 1863, whose meizoseismal
+area was elongated from north-east to south-west, or almost exactly
+perpendicular to the direction in 1896.
+
+Secondly, it will be noticed (Fig. 60) that the isoseismal lines are
+not equidistant from one another. On the north-east side, they are
+separated by distances of 20, 34, 55, and 51 miles; and on the
+south-west side by distances of 13-1/4, 25, 60, and 77 miles. It
+follows from this that the fault-surface must hade or slope towards
+the north-east; for, near the epicentre, the intensity is greatest and
+dies out more slowly on the side towards which the fault hades.
+
+If we could ascertain any one place through which the fault passed,
+its position would thus be completely determined. Unfortunately, there
+is no decisive evidence on this point. There are, however, several
+places to the south-west of Hereford where the intensity of the shock
+was distinctly less than in the surrounding district, and it is
+possible that this was due to their neighbourhood to the fault-line
+(see p. 135). If so, the originating fault must have extended from a
+point about a mile and a half west of Hereford for a distance of about
+16 miles to the south-east; and a fault in this position would
+certainly satisfy all the details of the seismic evidence.
+
+
+NATURE OF THE SHOCK.
+
+Throughout the disturbed area, considerable variations were observed
+in the nature of the shock. These changes were due to the mere size of
+the focus, to its elongated form and, as will be seen, to its
+discontinuity, and also to the distance of the place of observation
+from the epicentre.
+
+At places near the epicentre, rapid changes in the direction of the
+shock were observed owing to the large angle subtended by the focus;
+while, at considerable distances, this angle being small, the changes
+of direction were imperceptible. A further variation with the distance
+was an increase in the period of the vibrations. Close to the
+epicentre, the general impression was that of crossing the wake of a
+steamer in a very short rowing-boat, or of riding in a carriage
+without springs. At distances of a hundred miles or more, the movement
+is described as being of a pleasant, gentle, undulating character,
+like that felt during the rocking of a ship at anchor or in a carriage
+with well-appointed springs.
+
+The most remarkable feature of the shock, however, was its division
+into two distinct parts or series of vibrations, separated by an
+interval, lasting two or three seconds, of absolute rest and quiet.
+And this was no mere local phenomenon. With the exception of a narrow
+band that will be referred to presently, records of the double shock
+come from nearly all parts of the disturbed area, even from districts
+so remote as the Isle of Man and the east of Ireland. The two parts
+differed in intensity, in duration, and in the period of their
+constituent vibrations. For instance, at Oaklands (near Chard), a
+shivering motion was first felt, and then, after about three or four
+seconds, a distinct rocking from side to side. At Exeter, there was a
+sudden tremor lasting about two seconds, followed, after two or three
+seconds, by another and more severe shaking lasting four or five
+seconds. Again, at West Cross (near Swansea), an undulatory movement
+for about four seconds was followed soon after by a tremulous shock.
+At Liverpool, the durations of the first part, interval, and second
+part were respectively estimated at about six, two, and four seconds.
+
+As a first result of the observations, then, it appears that in the
+south-east half of the disturbed area, the second part of the shock
+was the stronger, of greater duration and consisted of longer-period
+vibrations (as at _a_, Fig. 61); while, in the north-west half, the
+same features characterised the first part of the shock (_b_, Fig.
+61). A closer examination of the records shows, however, that the
+boundary between the two portions of the disturbed area was not a
+straight line, but slightly curved, the concavity facing the
+south-east. The broken line on the map (Fig. 60), which is hyperbolic
+in form, represents roughly the position of this curved boundary.[64]
+
+   [Illustration: FIG. 61.--Nature of shock of Hereford earthquake.]
+
+Along this hyperbolic boundary-line, or rather within a narrow band of
+which it is the central line, the shock lost its double character, and
+was manifested as a single series of vibrations gradually increasing
+in intensity and then dying away. Close to the edges of this band,
+careful observers were able to distinguish two maxima of intensity
+connected by a continuous series of tremors (_c_, Fig. 61). Thus,
+within the band, the two series of vibrations, which elsewhere were
+isolated, must have been superposed on one another; while, near the
+edges of the band, the concluding tremors of the first series
+overlapped the initial tremors of the second.
+
+_Origin of the Double Series of Vibrations._--The Hereford earthquake
+thus belongs to the same class as the Neapolitan, Andalusian,
+Charleston, and Riviera earthquakes. As in these cases, the hypothesis
+of a single focus is inadmissible. The division of the disturbed area
+into two regions of opposite relative intensity, duration, etc., is
+sufficient proof that a single series of vibrations was not duplicated
+by reflection or refraction, or by separation into longitudinal and
+transverse waves. It is equally conclusive against a repetition of the
+impulse within the same focus. We must therefore infer that the focus
+consisted of two nearly or quite detached portions arranged along a
+north-west and south-east line, and that the impulse at the north-west
+focus was the stronger of the two. The only question that remains to
+be decided is whether the impulses at the two foci were simultaneous
+or not.
+
+Now, if the impulses occurred at the same instant, the waves from the
+two foci would travel with the same velocity, and would therefore
+coalesce along a straight band which would bisect at right angles the
+line joining the two epicentres. But we have already seen that this
+band is curved, and it thus follows that the two impulses were not
+simultaneous. Again, since the concavity of the hyperbolic band faces
+the south-east, the waves from the north-west focus must have
+travelled farther than those from the south-east focus before the two
+met along the hyperbolic band; in other words, the impulse at the
+north-west focus must have occurred two or three seconds before the
+impulse at the other.
+
+_Position and Dimensions of the Two Foci._--There can be little doubt
+that the impulse at the north-west focus was responsible for the
+greater damage to buildings at Hereford, Dinedor, Fownhope, etc. The
+centre of its epicentral area must therefore lie about three miles
+south-east of Hereford. It is probable, also, that the corresponding
+centre of the other focus is similarly placed with respect to the
+south-east portion of the isoseismal 8--that is, about two or three
+miles north-east of Ross. These two points are eight or nine miles
+apart. Now, since, as we shall see, the mean surface-velocity of the
+earth-waves was about 3000 feet per second, and the mean duration of
+the quiet interval between the two series was 3-1/2 seconds, the
+nearest ends of the two foci must have been separated by a distance of
+not less than two miles. Moreover, since the series of vibrations from
+the north-west or Hereford focus lasted a few seconds longer than that
+from the south-east or Ross focus, the former must have been about two
+miles longer than the latter, and we may therefore estimate their
+lengths at about eight and six miles respectively. Including the
+undisturbed intermediate portion, this would give a total length of
+focus of about 16 miles, a result we have already inferred from the
+dimensions of the isoseismal 8.
+
+
+DIRECTION OF THE SHOCK.
+
+Although no question was asked with regard to the direction of the
+shock, no fewer than 469 observers made notes on this point. As a
+general rule, their determinations are extremely rough, few referring
+to more than the eight principal points of the compass. Moreover, in
+any one place, the directions assigned to the shock are very varied.
+For instance, in the city and suburbs of Birmingham, eight observers
+give the direction along a north and south line, eight east and west,
+eleven north-west and south-east, and five north-east and south-west,
+while there are five other intermediate estimates. But, when these
+directions are plotted on a map of the district, it is seen at once
+that they are either nearly parallel or perpendicular to the roads in
+which the observers were living; that is, the apparent direction of
+the shock was at right angles to one of the principal walls of the
+house. This, of course, is a result to be anticipated, for, whatever
+be the direction of the earthquake-motion, a house tends to oscillate
+in a plane perpendicular to one or other of its walls.
+
+It is extraordinary to how great a distance the direction of the shock
+is perceptible. Records come from Brighton (137 miles from the
+epicentre), Maldon in Essex (144 miles), Harrogate (147 miles),
+Douglas in the Isle of Man (167 miles), Dublin (176 miles), and
+Baltinglass in Co. Wicklow (180 miles).
+
+Nevertheless, whatever the distance may be, the sense of direction
+must be most perceptible in those houses whose principal walls are at
+right angles to the true direction of the earthquake-motion, and we
+should therefore expect to find the observations of direction most
+frequently made in such houses, or in others which approximate to this
+situation. Thus, the average of all the observations within a fairly
+small area should give a result not very far from the true direction
+of the shock; and, the smaller the area and the farther from the
+epicentre, the more reliable should be the result. Now, in Birmingham
+the mean direction of the shock is E. 39° N., which differs only by 2°
+from the line joining the city to the epicentre; in London it is E.
+21° S., the difference being again 2°. In other cases, the
+observations from different counties are grouped together, and the
+mean direction is taken to correspond to the centre of the county.
+Yet, even then, there is often a close agreement between the mean
+direction of the shock and the direction of the county-centre from the
+epicentre; the difference being not more than two or three degrees in
+the counties of Buckingham, Devon, Stafford, Warwick, and York. In
+other cases, where the deviation exceeds this amount, either the
+number of observations is small or the county is near the epicentre
+and so subtends a large angle.
+
+Two results of some importance follow from this analysis: (1) that
+while, with a few isolated observations, the "method of directions" is
+almost sure to fail, with a large number of observations closely
+grouped, the position of the epicentre may be determined with a fair
+approach to accuracy; and (2) that, at any rate outside a radius of
+forty miles, the earth-waves travelled in approximately straight lines
+outwards from the epicentre.
+
+
+COSEISMAL LINES AND VELOCITY OF EARTH-WAVES.
+
+Coseismal lines were defined by Mallet as long ago as 1849, but, owing
+to the difficulty of ascertaining the correct time, they have so far
+been of little service in the investigation of earthquakes. In the
+case of the Hereford earthquake, the distances traversed by the
+earth-waves are small; but, on the other hand, the time-records are
+numerous and frequently trustworthy to the nearest minute. Rejecting
+all estimates earlier than 5.32 A.M., and later than 5.36, as well as
+a number at 5.35, there remain fairly good observations from 381
+places, and exceptionally accurate ones from 33 places. The latter
+were obtained from signalmen and other careful observers who were in
+possession of Greenwich time, or who compared their watches shortly
+afterwards with well-regulated watches.
+
+With evidence so abundant, a new method of drawing coseismal lines
+becomes possible. According to this method, each place of observation
+is indicated on the map by a mark corresponding to the particular
+minute recorded. If the records were quite correct, there would be a
+central area occupied by the marks corresponding to 5.32 A.M.,
+surrounded by a series of zones in which the times were respectively
+5.33, 5.34, and 5.35. The curves separating these zones would be
+coseismal lines corresponding to the times 5.32-1/2, 5.33-1/2, and
+5.34-1/2.
+
+Owing, however, to the inevitable inaccuracy of all the time-records,
+these different zones intrude on one another, and the coseismal lines
+have therefore to be drawn about half-way through the overlapping
+regions, special weight being attributed to the apparently more
+accurate observations.
+
+   [Illustration: FIG. 62.--Coseismal lines of the Hereford
+   earthquake. (_Davison._)]
+
+The coseismal lines obtained in this manner are represented by the
+continuous curves in Fig. 62. The isoseismals, which are added for the
+sake of comparison, are indicated by the dotted lines. It will be seen
+that the coseismal lines are elongated in the same direction as the
+isoseismals, but to a less extent, and this no doubt is due to the
+fact that the epoch selected by the majority of observers was one not
+far from, and slightly preceding, that of the maximum intensity of
+the shock.
+
+Now, the average distance between the two inner coseismals is 32-3/4
+miles, between the two outer ones (so far as drawn) 35-1/6 miles, and
+between the first and third 67-1/6 miles. The mean surface-velocity
+between the two inner coseismals is therefore 2,882 feet per second,
+and between the two outer ones 3,095 feet per second. There is thus an
+apparent increase in the velocity with the distance, but the accuracy
+of the coseismal lines is unequal to establishing this as a fact. The
+mean surface-velocity of 2,955 feet per second between the first and
+third coseismals is probably, however, the most accurate estimate of
+the surface-velocity yet made in a slight earthquake.
+
+
+SOUND-PHENOMENA.
+
+_Nature of the Sound._--The sound which accompanied the shock was of
+the same character as that heard during all great earthquakes. It is
+often described in such terms as a deep booming noise, a dull heavy
+rumble, a grating roaring noise, or a deep groan or moan; more rarely
+as a rustling or a loud hissing rushing sound. As a rule, it began
+faintly, increased gradually in strength, and then as gradually died
+away; and this no doubt is the reason why it sometimes appeared as if
+an underground train or waggon were approaching quickly, rushing
+beneath the observer, and then receding in the opposite direction.
+Occasionally, the sound was very loud, being compared to the noise of
+many traction-engines heavily laden passing close at hand, or to a
+heavy crash or peal of thunder. But its chief characteristic was its
+extraordinary depth, as if it were almost too low to be heard.
+According to one observer, it was a low rumbling sound, much lower
+than the lowest thunder; and another compared it to the pedal notes of
+a great organ, only of a deeper pitch than can be taken in by the
+human ear, a noise more _felt_ than heard. It will be seen presently
+how the sound, from its very depth, was inaudible to many persons.
+
+A few observers described the sound in terms like those quoted above,
+but by far the larger number compared it to some more or less
+well-known type, and in many cases the resemblance was so close that
+the observer at first attributed it to the object of comparison. The
+descriptions, which present great varieties in detail, may be
+classified as follows: (1) One or several traction-engines passing,
+either alone or heavily laden, sometimes driven furiously past; a
+steam-roller passing over frozen ground or at a quicker pace than
+usual; heavy waggons driven over stone paving, on a hard or frosty
+road, in a covered way or narrow street, or over hollow ground or a
+bridge; express or heavy goods trains rushing through a tunnel or deep
+cutting, crossing a wooden bridge or iron viaduct, or a heavy train
+running on snow; the grating of a vessel over rocks, or the rolling of
+a lawn by an extremely heavy roller; (2) a loud clap or heavy peal of
+thunder, sometimes dull, muffled or subdued, but most often distant
+thunder; (3) a moaning, roaring, or rough, strong wind; the rising of
+the wind, a heavy wind pressing against the house; the howling of wind
+in a chimney, a chimney or oil-factory on fire; (4) the tipping of a
+load of coal, stones, or bricks, a wall or roof falling, or the crash
+of a chimney through the roof; (5) the fall of a heavy weight or tree,
+the banging of a door, only more muffled, and the blow of a wave on
+the sea-shore; (6) the explosion of a boiler or cartridge of dynamite,
+a distant colliery explosion, distant heavy rock-blasting and the boom
+of a distant cannon; (7) sounds of a miscellaneous character, such as
+the trampling of many men or animals, an immense covey of partridges
+on the wing, the roar of a waterfall, the passage of a party of
+skaters, and the rending and settling together of huge masses of rock.
+
+The total number of comparisons made was 1,264. Of these, 45.4 per
+cent. refer to passing waggons, etc., 15.0 per cent. to thunder, 15.5
+to wind, 3.9 to loads of stones falling, 2.7 to the fall of a heavy
+body, 7.2 to explosions, and 10.3 per cent. to miscellaneous sounds.
+
+Generally, the sound adhered throughout to one of the types mentioned
+above, and, if it varied at all, varied only in intensity. At some
+places, however, the character of the sound was observed to change.
+For instance, one person described it as like the rumbling of a train
+going over a bridge, with a terrific crash, such as is heard in a
+thunderstorm at the instant when the shock was strongest, the rumbling
+dying away afterwards for some seconds.
+
+_Inaudibility of the Sound to some Observers._--The total number of
+observers who give a detailed account of the earthquake is 2,681, and,
+of these, 59 per cent. state that they heard the sound, 23 per cent.
+give no information, while 18 per cent. distinctly say that they
+heard no sound; that is, roughly, out of every five observers, three
+heard the sound, one made no reference to it, and one failed to hear
+the sound.
+
+In a few cases, no doubt, this failure was due to the distance of the
+observer, but this is far from being a complete explanation; for, in
+Herefordshire, six out of 179, and in Gloucestershire 17 out of 227,
+observers heard no sound. Nor is the peculiarity a local one, for at
+Clifton two out of five observers who were awake did not hear the
+sound, at Birmingham four out of 23, and in London, eight out of 18.
+Even in the same house, it would happen that one observer would hear a
+sound as of a heavily-laden traction-engine passing, while to another
+it was quite inaudible.
+
+Again, a large number of observers who heard the sound expressly state
+that they were unconscious of any while the shock lasted. The noise at
+first resembled the approach of a steam-roller or traction-engine up
+the street, it became gradually louder, and then ceased more or less
+suddenly as the shock began; while, to others in the same places, the
+sound continued to grow in loudness until the strongest vibrations
+were felt.
+
+Even when observers in the same place agreed in hearing the sound, it
+presented itself to them under different aspects. Thus, at Hereford, a
+crash or bomb-like explosion was noticed by some, but not by all,
+observers; at Ledbury, the sound according to one began like a rushing
+wind and culminated in a loud explosive report, another heard a noise
+like distant thunder, which ended when the shock began, while a third
+heard no sound at all. At places more distant from the epicentre, the
+same diversity, both in character and intensity, is manifested. Thus,
+at Birmingham, the accounts refer on the one hand to the distant
+approach of a train and the rising of the wind, on the other to the
+reports of large cannons and to a noise as if tons of _débris_ had
+been hurled against the wall of the house; at Bangor, to muffled
+thunder, wind through trees, and a loud rumbling sound.
+
+The first explanation of these apparent anomalies which presents
+itself is inattention on the part of the observers; but it is one that
+will not bear examination, though it may apply in some cases. The
+sound is too loud, at any rate near the epicentre, to escape notice,
+and it is generally heard before the shock begins to be felt.
+Moreover, as described in the last chapter, three out of every four
+earthquakes in Japan are unaccompanied by recorded sound, and the
+Japanese as a race cannot be accused of such constant inattention. The
+defect, it can hardly be doubted, is inherent to the observer, and not
+dependent on the conditions in which he is placed.
+
+That the higher limit of audibility varies with different persons has
+long been known; and there can be no reason for doubting that there is
+a similar variability in the lower limit. Thus, to some observers, the
+sound remains inaudible throughout, however intently they may be
+listening. Again, it is found that, the deeper the sound, the greater
+must be the strength of the vibrations required to render them
+audible. As the vibrations which reach an observer increase in period,
+it may therefore happen that, sooner or later, the strength of some
+does not attain or exceed that limiting value, and, at that moment,
+the sound will cease to be heard. Moreover, for vibrations of a given
+period, this limiting value varies for different persons. Thus, to one
+observer, the sound may become inaudible, while another may continue
+to hear it. Lastly, the vibrations which affect an observer at any
+moment are of various strength and period. One may hear all perhaps,
+while a second may be able to hear some and not others. Thus, to one
+observer, the sound may be like a rising wind, to another like a heavy
+traction-engine passing; one may hear the crashes which accompanied
+the strongest part of the shock, while a second may be deaf to the
+same vibrations; to one the sound may become continually louder and
+cease abruptly, to another it may increase to a maximum and then die
+away.
+
+_Sound-Area._--While the sound was a very prominent feature of the
+earthquake in and near the epicentral area, records at a great
+distance are naturally difficult to obtain, and, on this account, the
+number of stations for determining the boundary of the sound-area is
+too small to allow of it being accurately drawn. As a rule, however,
+it must lie between the isoseismals 5 and 4, but it is less nearly
+circular than either of these lines. Its length, from north-west to
+south-east, is 320 miles, its breadth 284 miles, and the area
+contained by it about 70,000 square miles, or roughly two-thirds that
+of the disturbed area.
+
+_Isacoustic Lines._--The dotted lines in Fig. 60 represent isacoustic
+lines--that is, lines which pass through all places where the
+percentage of observers who recorded their perception of the sound is
+the same. For instance, if we take any point in the line marked 80 and
+describe a small circle with that point as centre, then 80 per cent.
+of the observers within that circle would hear the earthquake-sound.
+The isacoustic lines thus show how the audibility of the sound varies
+throughout the sound area. To draw the curves with a close approach to
+accuracy, the unit of area should be small and of constant dimensions;
+but, in the present case, owing to the comparative paucity of the
+observations, a smaller unit than the county would give unreliable
+results.[65] At the centre of each county, the sound audibility may be
+regarded as proportional to the percentage of the total number of
+observers within the county who distinctly heard the sound. To draw
+the curve marked 50, the centre of every county in which the average
+percentage is less than 50 is joined to the centres of those adjoining
+counties in which it is above 50, and these lines are then divided in
+the proper ratio so as to give a point where the percentage would be
+exactly 50. A number of points at which the percentage is 50 is thus
+obtained, and the curve drawn through them is the required isacoustic
+line. The percentage of audibility varies from 87 in Herefordshire to
+23 in Essex and the east of Ireland, but the only isacoustic lines
+which can be completely drawn are those that correspond to the
+percentages between 80 and 50 inclusive.
+
+The peculiar form of the isacoustic lines will be evident at a glance.
+They bear little relation to the isoseismal lines. Their greatest
+extensions are not along the axes of those lines, but in two
+directions which are a little east of north-east and south of
+south-west. They lie indeed along a hyberbolic line which, towards
+the south-west, agrees closely with the curvilinear axis of the
+hyperbolic band represented by the broken line in Fig. 60. Towards the
+north-east, the coincidence is not so close, but this is chiefly owing
+to the magnitude of the northern counties, which causes a deflection
+of the isacoustic lines towards the north.
+
+It will be remembered that the hyperbolic band is the area within
+which the vibrations from the two foci were superposed. Now, the sound
+accompanied each part of the shock, and ceased entirely during the
+interval between them. Also, the stronger series of vibrations was
+accompanied by the louder sound; but, while the difference in strength
+was considerable between the two parts of the shock, it was very
+slight between the two sounds. There is therefore no marked distortion
+of the isoseismal lines when crossing the hyperbolic band, while the
+isacoustic lines are completely diverted from their normal course.
+
+Thus, the study of the isacoustic lines strongly confirms the
+conclusions at which we have arrived above (p. 223)--namely, that
+there were two distinct foci arranged in a north-west and south-east
+line, and that the impulse at the former focus occurred a few seconds
+earlier than that at the latter.[66]
+
+_Variations in the Nature of the Sound throughout the Sound-area._--In
+one respect, the sound exhibited a marked uniformity all over the
+sound-area--namely, in its great depth; the word "heavy" being used in
+one out of every four accounts of the sound, whether close to the
+epicentre or near the boundary of the sound-area.
+
+The type of comparison employed varies in different parts of the
+sound-area. As we recede from the origin, the sound becomes on the
+average less like thunder or explosions and more like wind. The
+references to passing waggons, etc., are so numerous that it is
+possible to draw curves, in the same way as isacoustic lines, which
+represent equal percentages of comparison to this type out of the
+total number of comparisons. The curves are somewhat incomplete, but
+it is noteworthy that those corresponding to the higher percentages
+cling to the extremities of the hyperbolic band, probably because the
+uninterrupted duration of the sound is greater there than elsewhere.
+
+The effect of distance from the epicentre, however, is most noticeable
+in connection with changes in the character of the sound. It is only
+on the immediate neighbourhood of the origin that the explosive
+reports or crashes were heard in the midst of the rumbling sound. At a
+moderate distance, the sound before and after the shock became
+smoother, while the sound which accompanied the shock retained to a
+certain extent its rougher and more rumbling or grating character.
+Close to the boundary of the sound-area, the irregularities were still
+further smoothed away, and the only sound heard was like the low roll
+of distant thunder.
+
+The explanation of these changes depends on the fact that, as we
+recede from the epicentre, the vibrations of every period tend to
+become inaudible. The limiting vibrations of the whole series will be
+the first to be lost, especially those of the longest period. Thus,
+near the epicentre, sound-vibrations of many different periods will be
+heard, and the sound will be more complex than it is elsewhere. The
+greater the distance, the narrower are the limits with regard to
+period between which the audible vibrations lie, until, near the
+boundary of the sound-area, the sound becomes an almost monotonous
+deep growl of nearly uniform intensity.
+
+_Time-relations of the Sound and Shock._--The principal epochs to be
+compared are the beginning, the epoch of maximum intensity, and the
+end. The beginning of the sound preceded that of the shock in 82 per
+cent. of the observations on this epoch, coincided with it in 12, and
+followed it in 6 per cent.; the epoch of maximum intensity preceded
+that of the shock in 21 per cent. of the records, coincided with it in
+73, and followed it in 6 per cent.; while the end of the sound
+preceded that of the shock in 22-1/2 per cent., coincided with it in
+27-1/2, and followed it in 50 per cent. Thus, as a general rule, the
+beginning of the sound preceded that of the shock, the sound was
+loudest when the shock was strongest, and the end of the sound
+followed that of the shock. In other words, the duration of the sound
+was in most cases greater than that of the shock.
+
+
+MINOR EARTHQUAKES.
+
+Of the twelve undoubted minor earthquakes, nine occurred before, and
+three after, the principal shock, the times of the first eleven lying
+between limits about seven hours apart. With three exceptions, the
+records are insufficient to determine the positions of the epicentre
+with any approach to exactness.
+
+The first occurred at about 11 or 11.30 P.M. on December 16th. The
+boundary of the disturbed area, which coincides nearly with that of
+the fifth shock (E, Fig. 63), is 97 miles long from north-west to
+south-east, 83 miles wide, and contains about 6,300 square miles. The
+focus was apparently situated between the two foci of the principal
+earthquake and partly coincided with them.
+
+   [Illustration: FIG. 63.--Map of minor shocks of Hereford
+   earthquake. (_Davison._)]
+
+Then came three slight shocks (at about 1 A.M. on December 17th, 1.30
+or 1.45 A.M., and 2 A.M.), about which little is known except that
+they probably originated somewhere near the Ross focus.
+
+The fifth shock (E, Fig. 63) occurred at about 3 A.M., and disturbed
+an area 104 miles in length, 79 miles in width, and about 6,400 square
+miles in area. Its boundary occupies approximately the position that
+would be taken by an isoseismal of intensity between 7 and 6 of the
+principal earthquake. We may therefore infer that this shock and the
+principal earthquake were caused by slips along the same fault and in
+about the same region of the fault. Also, as there is no evidence of
+discontinuity in the vibrations of the minor shock, it is probable
+that the focus was continuous, and occupied the space between the two
+foci of the principal earthquake, as well as part or the whole of both
+these foci.
+
+The next four shocks occurred at about 3.30, 4, 5, and 5.20 A.M., and
+were more closely associated with the Ross than with the Hereford
+focus, and then followed the principal earthquake at 5.32 A.M.
+
+A few minutes later, at 5.40 or 5.45 A.M., a very slight shock was
+felt, the focus of which was possibly situated in the central region
+between the two foci. The next, at about 6.15 A.M. (K, Fig. 63),
+disturbed an area 41 miles long, 27 miles broad, and containing about
+870 square miles. Its focus must have coincided approximately with the
+Ross focus of the principal earthquake, and this was also the case
+probably with the last shock of all, which occurred on July 19th,
+1897, at 3.49 A.M.
+
+
+ORIGIN OF THE EARTHQUAKES.
+
+The greater part of the epicentral district is covered by a sheet of
+Old Red Sandstone (Fig. 64), but, just to the north-east of the
+position laid down for the originating fault (indicated by the
+straight broken line), is the well-known Woolhope anticlinal, by which
+Silurian beds are brought to the surface. The anticlinal axis runs
+approximately north-west and south-east, and is thus roughly parallel
+to the earthquake-fault. Moreover, the thinning-out and occasional
+disappearance of some of the Silurian beds on the south-west side of
+the anticlinal (as compared with those on the north-east side) is
+suggestive of a north-west and south-east fault or rapid flexure at or
+near the south-west junction of the Old Red Sandstone and the
+Silurian strata. If it be a fault, it must hade to the north-east, and
+would therefore satisfy two of the conditions determined by the
+seismic evidence. It would lie, however, about two miles too far to
+the north-east, being in fact to the north-east of the villages which
+suffered most from the earthquake.
+
+   [Illustration: FIG. 64.--Geology of meizoseismal area of Hereford
+   earthquake. (_Davison._)]
+
+But only a few miles to the south-east of the Woolhope anticlinal, and
+almost in the same line with it, there is a second anticlinal, that of
+May Hill. This is a triangular area, and is known to be bounded on all
+three sides by faults. The fault on the north-east side has an average
+north-west and south-east direction, and, if it were continued through
+the Old Red Sandstone towards the north-west, but bending at first a
+few degrees more to the west, it would pass through a point about
+1-1/2 miles west of Hereford. It is worthy of notice that both this
+fault and another nearly parallel to it, about half-a-mile farther
+north-east, stop, according to the Geological Survey map, at the
+points where they enter the Old Red Sandstone. The latter is an area
+which has never been investigated with thoroughness by modern
+stratigraphical methods, and in which it is difficult to trace faults.
+It therefore appears not improbable that the earthquakes were due to
+slips along a continuation of this fault.
+
+Whether this be the case or not, however, it is clear that the
+earthquake-fault must pass between the anticlinal areas of Woolhope
+and May Hill, the former being on the north-east, and the latter on
+the south-west, side of the fault. At the Hereford focus, the fault
+must hade to the north-east; and, at the Ross focus, it is probable,
+from the distribution of places where damage occurred to buildings,
+that it hades to the south-west If this be the case, the fault must
+change in hade between the two foci.
+
+How long a time had elapsed since the last sign of growth in the
+earthquake-fault took place, it is impossible to say; but it must be
+many years in length. During this interval, the stresses tending to
+produce movement along the fault-service had been gradually
+increasing, until they were sufficient to overcome the resistance
+opposed to them. It is worthy of notice that the earliest perceptible
+movements were slight. Their function seems to have been to prepare
+the way for the great slips by equalising the difference between
+stress and resistance over a large area of the fault-surface. We
+cannot trace with accuracy the transference of the seat of movement
+from one part of the fault-surface to another. The first slip seems to
+have taken place chiefly in the region between the two foci of the
+principal earthquake; possibly it overlapped both of them partly. The
+next three slips were apparently in the neighbourhood of the Ross
+focus, and were followed by a fifth in the same area as the first.
+Then came a series of small movements that we cannot locate further
+than by saying that they were more closely connected with the Ross
+focus than the other.
+
+In consequence of the preliminary slips within and near the Ross
+focus, the effective stress in that portion of the fault was
+diminished; and this may be the reason why the first great slip took
+place at the Hereford focus. The immediate result of such a movement
+would naturally be an increase of stress in and beyond the terminal
+regions, and the next slip might have been expected in an area partly
+overlapping the Hereford focus, and either to the north-west or
+south-east of it. Instead of this, for a distance of two miles in the
+latter direction, there was not the least perceptible movement during
+the principal earthquake, and the second great slip occurred in the
+region beyond occupied by the Ross focus. This second slip, moreover,
+occurred within two or three seconds after the other; that is, before
+the earth-waves had time to travel from the Hereford to the Ross
+focus. In other words, the slip at the Ross focus was not a
+consequence of the slip at the Hereford focus; but both were due to a
+single generative effort.
+
+Now, a section drawn parallel to the earthquake-fault and on the
+north-east side of it, would show an anticline near the Hereford focus
+and a corresponding syncline near the Ross focus, with an undisplaced
+portion in the intermediate region; while a parallel section on the
+other side of the fault would show a syncline near the Hereford focus,
+an anticline near the Ross focus, and again an undisplaced portion in
+the intermediate region. If further movements tending to accentuate
+such a structure were to occur (that is, if the anticlinals were to be
+made more anticlinal and the synclines more synclinal), there would
+therefore be two slips, one in each focus; while, along the
+fault-surface between, there would be practically no displacement. At
+any rate, the earlier stresses in that region may have been fully
+relieved by two slight preliminary slips (those causing the first and
+fifth minor earthquakes), and those resulting from the great
+displacements by the first after-slip which followed in about ten
+minutes.
+
+Half-an-hour later, another slip took place at the Ross focus, and by
+this the equilibrium of the rock-masses was almost completely
+restored; for we have no certain evidence of any further movements
+until seven months have elapsed (July 19th, 1897), when there was a
+final slip in the same region of the fault.
+
+
+THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901.
+
+Between the north-east end of Loch Ness and the Moray Firth at
+Inverness, there lies a tract of land not more than seven miles in
+length, which is notable as one of those most frequently shaken by
+earthquakes in the British Islands. In the intensity of its shocks it
+is inferior to the south-east of Essex and the centre of
+Herefordshire, and, in mere number, to the celebrated village of
+Comrie in Perthshire. But, in the interest of its seismic phenomena,
+in the light which they cast on the development of the earth's crust,
+the neighbourhood of Inverness has no equal in Great Britain, and not
+many superiors in any part of the world.
+
+For this importance from a seismological point of view, the district
+is indebted to the great fault which traverses Scotland along the line
+of the Caledonian Canal, and to the fact that this fault, although it
+dates from Old Red Sandstone times, has not yet finished growing. As
+results of its formation, we have the almost straight cliff along the
+south-east coast of Rossshire, and the long chain of lakes, beginning
+with Loch Dochfour and Loch Ness, and ending with Loch Oich, Loch
+Lochy, and Loch Linnhe. As evidences of its persistent though
+intermittent growth, we have the slight tremors and earth-sounds
+occasionally observed at and near Fort William, and the much stronger
+shocks felt in the neighbourhood of Inverness.
+
+During the nineteenth century there were three strong earthquake
+shocks in this district. The first and most severe occurred on August
+13th, 1816, and was felt over the greater part of Scotland; the second
+on February 2nd, 1888; and the third and weakest on November 15th,
+1890. This last shock was followed by several slighter ones, the
+series ending with a rather smart shock on December 14th. Between this
+date and the summer of 1901 no earthquakes seem to have been felt at
+or anywhere near Inverness.
+
+
+PREPARATORY SHOCKS.
+
+The date of the first shock of 1901 is not quite certain. One is said
+to have been felt at Aldourie (see Fig. 66) some time in June, and a
+second at Dochgarroch in July. These may have been succeeded by others
+too slight to attract much notice, but the first to be generally
+observed occurred on September 16th at 6.4 P.M. A weak tremor,
+accompanied by a faint sound, was perceived over a nearly circular
+area about 12 miles in diameter, and with its centre about 1-1/2 miles
+south of Dochgarroch. On the next day, at 11 P.M., a quivering lasting
+two seconds was felt at Inverness, and a weak tremor, accompanied by
+sound, at Dochgarroch at 1.15 A.M. on September 18th. Nine minutes
+later, at 1.24 A.M., occurred the principal earthquake, the shock of
+which would be called a strong one, even in Italy and Japan.
+
+
+EFFECTS OF THE SHOCK.
+
+In Inverness, the damage to buildings, though seldom serious, was by
+no means inconsiderable. One brick building used as a smithy was
+destroyed, several chimneys or parts of them fell, and many
+chimney-cans were displaced or overthrown. At Dochgarroch and other
+places within the meizoseismal area, walls were cracked, chimneys
+thrown down, and lintels loosened.
+
+But, for this country, an unusual effect of the earthquake was a long
+crack made in the north bank of the Caledonian Canal near Dochgarroch
+Lochs. It occurred in the middle of the towing-path, and could be
+traced at intervals for a distance of 200 yards to the east of the
+Lochs, and 400 yards to the west, being often a mere thread, and in no
+place more than half-an-inch wide. Soon after its formation, however,
+the fissure was obliterated by heavy showers of rain.
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+The map (Fig. 65) shows the area over which the earthquake was
+perceptible. The isoseismal lines are drawn partly continuous and
+partly dotted--continuous where some confidence can be placed in their
+accuracy, and dotted where their course must be regarded as doubtful,
+owing to the rarity or absence of observations.
+
+The innermost isoseismal (shown on a larger scale in Fig. 66)
+corresponds to the intensity 8 of the Rossi-Forel scale, and includes
+the places where the shock was strong enough to cause slight
+structural damage to buildings. It is elliptical in form, 12 miles
+long, 7 miles broad, and 67 square mile in area, with its centre at a
+point about 1-1/2 mile east-north-east of Dochgarroch, and its longer
+axis running N. 33° E. and S. 33° W.
+
+   [Illustration: FIG. 65.--Isoseismal lines of the Inverness
+   earthquake. (_Davison._)]
+
+The remaining isoseismals are less accurately drawn, owing to the
+scarcity of observations made in the west of Scotland. Except towards
+the west, however, the course laid down for the isoseismal 7 may be
+trusted. Its length is 53-1/2 miles, width 35 miles, and area 1,500
+square miles. Its longer axis is almost exactly parallel to that of
+the preceding isoseismal, but the distance between the two curves is 9
+miles on the north-west, and 14 miles on the south-east, side. The
+isoseismal 6 is 105 miles long, 87 miles wide, and contains 7,300
+square miles; and the isoseismal 5, 157 miles long, 143 miles wide,
+and about 17,000 square miles in area.
+
+The isoseismal 4 may be regarded as the boundary of the disturbed area
+of the earthquake, for, so far as known, the shock was not noticed at
+any point outside it. Towards the north, it was felt at Wick,
+Castletown, and other intermediate places; towards the west at
+Tobermory in the island of Mull; and, towards the south, at Skelmorlie
+(in Ayrshire), Paisley, Belsyde (near Linlithgow), Gullane (near North
+Berwick), and Dunbar. Along the east coast of Scotland, between Wick
+and Dunbar, there are few places of any size where the shock was not
+felt. The disturbed area of the earthquake is thus 215 miles long from
+north-east to south-west, 198 miles wide, and contains about 33,000
+square miles.
+
+_Position of the Originating Fault._--The only isoseismals which are
+drawn accurately enough to determine the earthquake-fault are the two
+inner ones, those marked 8 and 7; but these are sufficient for the
+purpose. It is clear, from the direction of their longer axes, that
+the average direction of the fault must be N. 33° E. and S. 33° W.
+Again, the isoseismals are farther apart towards the south-east than
+towards the north-west, implying that the fault hades to the
+south-east. Lastly, as the intensity of the shock is greater on the
+side towards which the fault hades, it follows that the fault-line
+must lie a short distance (about a mile or so) on the north-west side
+of the centre of the isoseismal 8.
+
+Now, the great fault alluded to above occupies almost exactly the
+position indicated by the seismic evidence. Its mean direction from
+Tarbat Ness to Loch Linnhe is N. 35° E. and S. 35° W., it hades to the
+south-east, and the fault-line passes through a point about
+three-quarters of a mile to the north-west of the centre of the
+isoseismal 8 (Fig. 66). There can be little doubt, therefore, that the
+earthquake was caused by a slip of this fault; and the evidence of the
+after-shocks, as will be seen, offers additional support to this
+conclusion.
+
+The region in which the slip took place may be determined roughly from
+the position and form of the innermost isoseismal. Its centre must
+have been close to the point marked A in Fig. 66, which corresponds to
+a point about 1-1/2 mile east-north-east of Dochgarroch. In a
+horizontal direction, its length must have been at least five or six
+miles; otherwise, the isoseismal 8 would have been less elongated. It
+must therefore have reached from about half-a-mile north-east of Loch
+Ness to about half-a-mile south-west of Inverness. Its width, measured
+along the dip of the fault-surface is unknown; but the small distance
+between the centre of the isoseismal and the fault-line shows that the
+principal movement took place at a depth which was probably under,
+rather than over, one mile.
+
+
+NATURE OF THE SHOCK.
+
+We come now to the evidence afforded by the nature of the shock, in
+which there was but little variation throughout the disturbed area. At
+Inverness, a gentle movement was first felt, followed by an
+extraordinary quivering, which increased in force for two or three
+seconds, and then decreased for two or three seconds; just as the
+quivering was about to cease, there was a distinct lurch or heave,
+after which the vibration was much more severe than before and lasted
+several seconds longer than the first part of the shock. Dalarossie
+lies about fourteen miles south-east of Inverness, and here the first
+indication was a loud sound, as of an express train, coming from the
+east, rushing close to, and then under, the house; this lasted for a
+few seconds, and towards the end of it the house vibrated. Then
+succeeded an interval of quietness for about a second, followed by a
+terrific burst or crash, not unlike the crash of a loud thunder peal,
+of about two seconds' duration, during which the house distinctly
+heaved up once and then sank back. After another brief interval of
+quietness, there was a low rumble, like the sound of a dying peal of
+thunder.
+
+It will be noticed, in this account, that the two parts of the shock
+were no longer consecutive. There was a short interval of rest between
+them, the intermediate vibrations observed at Inverness being too weak
+to be felt at Dalarossie. Still farther away, the extinction became
+more marked. At Aberdeen, for instance, the shock consisted of two
+parts, the first a tremble, followed, after an interval of a few
+seconds, by a swinging movement of longer duration than the tremble.
+
+In all parts of the disturbed area, the shock maintained the same
+character of division into two parts, the second of which was of
+greater duration and intensity than the first and consisted of
+vibrations of longer period. A phenomenon of such wide occurrence was
+clearly not due to local influences. It must have been caused by two
+separate initial impulses, the stronger succeeding the other after an
+interval of a few seconds and taking place in nearly the same region
+of the fault.[67]
+
+
+SOUND-PHENOMENA.
+
+Outside the isoseismal 5, there are but few records of the
+earthquake-sound; but it was heard faintly at Skelmorlie (in
+Ayrshire), Belsyde (near Linlithgow), and Gullane (near North
+Berwick). Towards the north, it was not observed beyond Wick and
+Wathen (in Caithness). The boundary of the sound-area cannot be laid
+down with any approach to accuracy, but it must have included a
+district containing about 27,000 square miles.
+
+Throughout the whole disturbed area, 84 per cent. of the observers
+heard the sound. The percentage varies in different counties, from 93
+in Inverness-shire to 77 in the counties of Perth and Aberdeen; but
+the records in the more distant regions are too few to allow of the
+construction of isacoustic lines.
+
+In its character, the sound resembled that usually heard with strong
+earthquakes, 39 per cent. of the observers having compared it to
+passing waggons, traction-engines, etc., 25 per cent. to thunder, 14
+to wind, 8 to loads of stones falling, 3 to the fall of heavy bodies,
+4 to explosions or the firing of heavy guns, and 7 per cent. to
+miscellaneous sounds. The intensity of the sound gradually diminished
+outwards from the epicentre, and most rapidly near the isoseismal 7,
+which abounds approximately the area in which the sound was very loud
+from that in which it was distinctly fainter, and also includes nearly
+all the places at which loud explosive crashes were heard with the
+strongest vibrations.
+
+In the time-relations of the sound and shock, the Inverness earthquake
+resembles the Hereford earthquake of 1896. The beginning of the sound
+preceded that of the shock in 72 per cent. of the records, coincided
+with it in 20, and followed it in 8 per cent.; the epoch of maximum
+intensity of the sound preceded that of the shock in 20 per cent. of
+the records, coincided with it in 73, and followed it in 7 per cent.;
+while the end of the sound preceded that of the shock in 15 per cent.
+of the records, coincided with it in 34, and followed it in 52 per
+cent.
+
+Somewhat similar proportions hold over the greater part of the
+disturbed area, the percentages being nearly the same in the counties
+of Inverness, Ross, Nairn, Elgin, Banff, and the most distant
+counties. But in Aberdeenshire an exception occurs, the three epochs
+of sound and shock in most cases coinciding with one another. The
+majority of the observations in this county come from the southern
+part, and the line joining this district to the epicentre is nearly
+perpendicular to the line of the earthquake-fault. This result has an
+important bearing on the origin of the sound-vibrations. For, if the
+general precedence of the sound with respect to the shock were due to
+its superior velocity, the percentage of records in which the
+beginning of the sound preceded that of the shock would vary only with
+the distance, and not with the direction from the origin. Indeed,
+with increasing distance from the origin, this percentage should
+continually approach 100; while that in which the end of the sound
+followed that of the shock should diminish to zero. There is, however,
+no trace of either tendency, the sound being heard after the shock at
+places close to the boundary of the sound-area. On the other hand, it
+the sound-vibrations were to start simultaneously, or nearly so, from
+all parts of the focus, but especially from its marginal regions,
+then, in the greater part of the disturbed area, the sound would be
+heard both before and after the shock; for the lateral margins of the
+focus would be the portions nearest to, and farther from, most
+observers; while, at places near the line through the epicentre at
+right angles to the earthquake-fault, the three principal epochs of
+the sound and shock should approximately coincide.
+
+The inference that the sound-vibrations heard before and after the
+shock come from the margins of the focus is also supported by the
+observations on the relative duration of the sound and shock. If we
+take only those records which are free from doubt, in 78 per cent. of
+the total number, the duration of the sound was greater than that of
+the shock; while, in Aberdeenshire, according to 93 per cent. of the
+observers, the durations of sound and shock were equal.
+
+We may imagine, then, that the slip within the seismic focus would be
+greatest in a central region, and that it would die outwards in all
+directions towards the edges. The friction arising from the slipping
+in the central region would produce chiefly the comparatively large
+oscillations that formed the perceptible shock; the evanescent creep
+within the marginal regions would produce the small and rapid
+vibrations that were sensible only as sound.
+
+
+ORIGIN OF THE EARTHQUAKE.
+
+While the seismic evidence enables us to determine the
+surface-position and the horizontal dimensions of the seismic focus,
+it unfortunately throws no light whatever on a point of some
+importance--namely, the direction of the movement which caused the
+earthquake. We cannot infer from it whether it was the rock on the
+south-east or north-west side of the fault that slipped or whether
+both sides slipped at once; nor, if that point had been settled, do we
+know if the movement of the displaced side was upward or downward. In
+the formation of the fault, however, it is clear that either the
+south-east side has been depressed or the north-west side elevated;
+and, as the bed of Loch Ness is below the level of the sea, that the
+former movement has predominated. If the displacements which gave rise
+to the earthquake were merely a continuation of the original series of
+movements--and this is, to say the least, a very probable view to
+take--then we may imagine that, for a distance of five or six miles,
+and at a depth of about a mile or less, there was a sudden sag
+downwards of the rock on the south-east side of the fault through a
+distance which perhaps in no part exceeded a fraction of an inch.
+
+Fig. 66 is an attempt to represent roughly the displacement which
+caused the principal earthquake. The diagram makes no pretence to
+accuracy, and the scale in the vertical direction is enormously
+greater, perhaps a hundred thousand times greater, than that in the
+horizontal direction. The straight line is supposed to represent a
+straight line drawn before the earthquake on the surface of the rock
+adjoining the fault on the south-east side and at a depth of about a
+mile, and the curve the form of the same line after the earthquake.
+
+   [Illustration: FIG. 66.--Diagram to illustrate supposed
+   fault-displacement causing Inverness earthquake.]
+
+The effect of this great slip would obviously be to relieve the stress
+in the central region A, and to increase it suddenly in the parts
+denoted by the letters B and C. It is, therefore, in these parts
+especially that we should expect future slips to occur. Each slip
+would of course give rise to an after-shock, and would in like manner
+result in an increase of stress in its own terminal regions, though
+chiefly on the side remote from the centre A.
+
+
+THE AFTER-SHOCKS AND THEIR ORIGIN.
+
+It is difficult to form any estimate of the total number of
+after-shocks. The list, compiled from the records of careful observers
+only, includes forty-six shocks and ten earth-sounds, the last of all
+occurring on November 21st. But the list is certainly incomplete. It
+contains, for instance, only one entry on September 18th between 3.56
+and 9 A.M.; whereas, during the same interval, no fewer than eighteen
+slight shocks were felt by one observer at Dochgarroch, while another
+near Aldourie estimates the number of shocks up to October 23rd at
+about seventy. The total number probably did not fall short of one
+hundred.
+
+The majority were certainly very slight, and, at another time, would
+hardly have attracted any notice. There were, however, three of much
+greater importance than the rest. These occurred on September 18th at
+3.56 and 9 A.M., and on September 30th at 3.39 A.M. The isoseismal
+lines of all three are elongated ovals, their longer axes are parallel
+to the fault, and their centres lie on the south-east side of the
+fault-line. The shocks were therefore evidently due to slips several
+miles in length along the fault. At present, we are concerned more
+with the position of their epicentres. These are indicated by the dots
+lettered B, C, D in Fig. 67; the dot marked A denoting the centre of
+the principal earthquake, and the continuous line the path of the
+fault.
+
+Thus, within two and a half hours, the great slip was followed by one
+with its centre at B, near the south-west margin of the principal
+focus. About five hours later, the scene of action was suddenly
+transferred to a region with its centre at C on the north-east margin.
+Both slips affected a portion of the fault-surface several miles in
+length, and must therefore have increased the area of displacement,
+slightly towards the north-east and considerably towards the
+south-west. Only small movements occurred during the next twelve days
+until 3.39 A.M. on September 30th, when another long slip took place,
+with its centre at D, still farther to the south-west, and therefore
+again extending the area and amount of displacement in this direction.
+
+   [Illustration: FIG. 67.--Map of epicentres of after-shocks of
+   Inverness earthquakes. (_Davison._)]
+
+Turning now to the weaker after-shocks and earth-sounds, we find them
+affecting chiefly three regions of the fault. One of these is close to
+Dochgarroch, another near Inverness, and the third between Aldourie
+and Drumnadrochit; the effects of the slips in the last two districts
+being, as before, to extend the area of displacement a short distance
+(perhaps half a mile) to the north-east and not less than six miles to
+the south-west underneath Loch Ness.
+
+The unequal division of the after-shocks between the two sides of the
+principal centre (A, Fig. 67) is very marked. The positions of the
+epicentres of forty-four shocks and earth-sounds can be determined
+with more or less accuracy, and, of these, only ten lie to the
+north-east of the principal centre, while thirty-four lie to the
+south-west, six or seven of the latter being beneath Loch Ness.
+
+One other point may be referred to before leaving these minor shocks.
+So far as regards the stronger shocks, there was a continual decrease
+in the depths of the seismic foci. This is shown by the progressive
+approach of their epicentres towards the fault-line; the distances in
+the three chief after-shocks being 1.7, 1.0, and 0.5 miles
+respectively; and in one of the latest shocks (that of October 13th at
+4.24 P.M., E, Fig. 67) the distance is no more than one-tenth of a
+mile. The focus of this shock must, indeed, have been quite close to
+the surface near Dochgarroch. This constant diminution in the depth of
+the foci shows that the great slip was followed by a sudden increase
+of stress upwards as well as laterally, and explains why that slip did
+not leave any perceptible trace, either as fault-scarp or fissure, at
+the surface.
+
+
+SYMPATHETIC EARTHQUAKES.
+
+It is remarkable that, of the 56 recorded after-shocks, at least six
+were felt or heard only at Dalarossie and other places in the valley
+of the Findhorn, a valley which lies about 13 or 14 miles to the
+south-east of the great fault. That they had no connection with that
+fault is certain, for two of them were so strong that, if they were so
+connected, they could not have escaped the notice of one or more of
+the watchful observers between Drumnadrochit and Inverness. The
+probable explanation of these after-shocks is that they were due to
+slips of a fault running along the Findhorn valley;[68] and that the
+great displacement near Inverness on September 18th led to a sudden
+increase of stress within the rocks for many miles around, which, at
+and near Dalarossie, was sufficient to precipitate the slips referred
+to.
+
+
+CONCLUSION.
+
+At first sight, two earthquakes could hardly be more unlike than the
+Japanese earthquake of 1891 and the Inverness earthquake of 1901. In
+the rice-fields of central Japan, as we have seen, the roads for many
+leagues were edged with ruins, the fault-slip was prolonged up to the
+surface and visible as a scarp forty, if not seventy, miles in length,
+plots of ground were compressed and their boundaries altered, the
+hillsides were scored by landslips, places can now be seen from one
+another that formerly were hidden by a mountain ridge, and the total
+number of after-shocks within little more than two years amounted to
+above three thousand. On the other hand, when we examine the
+distribution of the after-shocks in space, we find that, though no
+part of the fault was exempt from slips, they favoured three regions
+in particular--one, the most important, a central region, yet not
+coincident with that in which the principal shock was most intense;
+and the other two surrounding the extremities of the fault. With the
+lapse of time, the after-shocks on the whole became weaker and
+occurred less frequently, and the average depth of the foci gradually
+diminished. Moreover, in two districts distant forty-five and
+fifty-five miles from the fault, the frequency of the shocks during
+the month succeeding the earthquake was suddenly increased to ten and
+sixteen times the normal rate.
+
+It is interesting to notice so close a similarity in character,
+subsisting with so vast a difference in the scale of intensity. The
+identity of the powers at work in shaping the structure of both
+islands Is manifest. In Japan, we see the mountain-making forces
+acting with violence and producing effects that are only too apparent
+to the eye. In Scotland, whatever may have happened in former
+geological epochs, the changes in surface-structure are now taking
+place with almost infinite slowness, and hundreds or thousands of
+years must elapse before Loch Ness makes any visible progress in its
+march towards the sea.
+
+
+REFERENCES.
+
+  1. DAVISON, C.--_The Hereford Earthquake of December 17, 1896._
+        (Birmingham, 1899.)
+
+  2. ---- "The Inverness Earthquake of Sept. 18, 1901, and its
+        accessory shocks." _Quart. Journ. Geol. Soc._, vol. lviii.,
+        1902, pp. 377-397.
+
+FOOTNOTES:
+
+[61] The study of the Hereford earthquake is based on 2,902 records,
+coming from 1,943 places; that of the Inverness earthquake on 710
+records from 381 places.
+
+[62] The disturbed area of the Hereford earthquake of 1896 was
+probably greater than that of any other British earthquake of the
+nineteenth century; that of the Pembroke earthquake of 1892 being more
+than 56,000 square miles, of the Pembroke earthquake of 1893 about
+63,600 square miles, while that of the Essex earthquake of 1884 (a far
+stronger shock in the meizoseismal area) is estimated at about 50,000
+square miles.
+
+[63] The approximate circularity of the two outer isoseismals is due
+to the fact that the vibrations propagated to such great distances are
+those which start from the comparatively small central region of the
+focus.
+
+[64] The above statement summarises the evidence of the majority of
+the observers in each portion of the disturbed area. In this, as in
+other similar cases, discrepancies in the observations are
+unavoidable; but it is important to notice that they are least
+frequent in the observations evidently made with the greatest care.
+
+[65] Except in the case of Yorkshire, where the three Ridings are
+regarded as separate counties.
+
+[66] The Derby earthquake of March 24th, 1903, was also a twin
+earthquake. The centres of the two foci were situated near Ashbourne
+and Wirksworth, above eight or nine miles apart, along a line running
+N. 33° E. and S. 33° W. The two parts of the shock coalesced along a
+rectilineal band about five miles wide running centrally across the
+lower isoseismals in a direction at right angles to their longer axes.
+The isacoustic lines are also elongated in the direction of this band.
+In this case, the impulses at the two foci must have taken place at
+the same instant. (_Quart. Journ. Geo. Soc._, vol. lx., 1904, pp.
+215-232.)
+
+[67] If the foci of the two impulses had been detached, there would,
+with so small an interval between the two parts, have been a variation
+in the nature of the shock like that observed during the Hereford
+earthquake.
+
+[68] This part of Inverness-shire has not yet been mapped by the
+Geological Survey, but a fault is known to exist in the Findhorn
+valley near Drysachan Lodge, which lies about eleven miles down the
+valley from Dalarossie.
+
+
+
+
+CHAPTER IX.
+
+THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897.
+
+
+Very different from the shocks of Britain was the earthquake that
+overwhelmed so large a part of its great dependency on June 12th,
+1897--an earthquake which, if it is not without a rival, is certainly
+one of the most disastrous and most widely-felt of which we possess
+any record. That it was of the first magnitude was evident at once in
+Calcutta from the extensive injury to buildings, and its investigation
+was undertaken without delay by the members of the Geological Survey
+of India. The four officers who were at the headquarters in Calcutta
+were despatched to the area of greatest damage, letters and circulars
+were distributed as widely as possible, a large number of observers
+were induced to co-operate by keeping records of the after-shocks,
+and, later on, during the cold weather of 1897-98, Mr. R.D. Oldham,
+one of the superintendents of the Survey, made a tour through the
+epicentral district. To him, moreover, fell the much harder task of
+discussing the very numerous observations collected by himself and
+others; and the least that can be said of the valuable report prepared
+by him is that it is worthy of a great subject. Professor Omori also
+spent several months in studying the earthquake on behalf of the
+Japanese Government; but the account, which is written in his own
+language, unfortunately remains a sealed book to western
+seismologists.
+
+   [Illustration: FIG. 68.--Isoseismal Lines of Indian Earthquake.
+   (_Oldham._)]
+
+
+ISOSEISMAL LINES AND DISTURBED AREA.
+
+In Fig. 68, which shows the area disturbed by the earthquake, Mr.
+Oldham has drawn two series of curves. In the absence of detailed
+records of the intensity--records that could not have been obtained
+from some parts of the disturbed area, and would have been difficult
+to procure in sufficient number from others--he has represented by the
+dotted curves a group of isoseismals in the form which he believes
+they would have assumed had the earth-waves been propagated in a
+homogeneous medium. The first includes all places, such as Shillong
+and Goalpara, where the destruction of brick and stone buildings was
+practically universal; the second, those, like Darjiling, in which
+damage to buildings was universal and often serious; the third,
+places, like Calcutta, where the earthquake was strong enough to
+injure all or nearly all brick buildings. Inside the fourth
+isoseismal, the shock was strong enough to disturb furniture and loose
+objects, but not to cause more than slight damage; within the fifth,
+it was generally noticed; and, beyond this, and as far as the sixth
+isoseismal, the earthquake was perceived only by a small number of
+sensitive persons at rest. The approximation of the curves towards the
+east and south-east, Mr. Oldham believes to be partly real, and not
+due to imperfect information.
+
+The continuous curves represent more closely the actual variation of
+intensity. The innermost curve A indicates the probable boundary of
+the epicentral tract, which is about 200 miles in length and more than
+6000 square miles in area. This will be referred to afterwards in
+greater detail. The next curve B bounds the region within which
+serious damage to brick houses was common. Its irregular course is
+closely connected with the geological structure of the country, and is
+due to the fact, of which we have already met with several examples,
+that earthquakes are more destructive to houses built on alluvial
+ground than to those founded on rock. The area included within this
+curve is not less than 145,000 square miles; and, if we include the
+parts from which reports were not obtainable, it must amount to about
+160,000 square miles.
+
+The curve C represents the boundary of the disturbed area, so far as
+known, for about one-third of the area lies in regions from which no
+information was procurable, while another third is inhabited by
+ignorant and illiterate tribes. But, notwithstanding this, the shock
+is known to have been felt over an area of at least 1,200,000 square
+miles. If we include the detached region to the west, near Ahmedabad,
+the portion of the Bay of Bengal in which the shock would have been
+felt had the sea been replaced by land, and a large part of Thibet or
+Western China, from which no reports have come, but in which the shock
+was certainly sensible, this estimate, great as it is, must be raised
+to about 1,750,000 square miles.[69]
+
+Figures, such as those given above, convey but little idea of the
+vastness of the area concerned. Transferring them to countries with
+which we are more familiar, we may say that the disturbed area was
+only a little less than half the size of Europe; the region in which
+serious damage occurred to masonry was more than twice as large as the
+whole of Great Britain; while, if the centre of the epicentral tract
+had been in Birmingham, nearly every brick and stone building between
+York and Exeter would have been levelled with the ground.
+
+
+NATURE OF THE SHOCK.
+
+Few and slight were the forerunners of the greatest of modern
+earthquakes. Early in June, faint tremors were felt by sensitive
+persons at Shillong. Others at the same place heard a rumbling sound
+for ten or fifteen seconds before the shock began, and at Silchar
+birds were seen to rise suddenly from trees before the movement became
+sensible to man. Except for these almost imperceptible warnings, the
+earthquake broke abruptly over the whole district.
+
+"At 5.15," writes one observer at Shillong, "a deep rumbling sound,
+like near thunder commenced, apparently coming from the south or
+south-west.... The rumbling preceded the shock by about two seconds
+... and the shock reached its maximum violence almost at once, in the
+course of the first two or three seconds. The ground began to rock
+violently, and in a few seconds it was impossible to stand upright,
+and I had to sit down suddenly on the road. The shock was of
+considerable duration, and maintained roughly the same amount of
+violence from the beginning to the end. It produced a very distinct
+sensation of sea-sickness.... The feeling was as if the ground was
+being violently jerked backwards and forwards very rapidly, every
+third or fourth jerk being of greater scope than the intermediate
+ones. The surface of the ground vibrated visibly in every direction,
+as if it was made of soft jelly; and long cracks appeared at once
+along the road.... The road is bounded here and there by low banks of
+earth, about two feet high, and these were all shaken down quite flat.
+The school building, which was in sight, began to shake at the first
+shock, and large slabs of plaster fell from the walls at once. A few
+moments afterwards the whole building was lying flat, the walls
+collapsed, and the corrugated iron roof lying bent and broken on the
+ground. A pink cloud of plaster and dust was seen hanging over every
+house in Shillong at the end of the shock.... My impression at the end
+of the shock was that its duration was certainly under one minute, and
+that it had travelled from south to north.... The violence of the
+shock may be imagined when it is stated that the whole of the damage
+done was completed in the first ten or fifteen seconds of the shock."
+
+Other estimates of the duration are generally higher than that given
+above, ranging from three to five or even more minutes at Tura,
+Dhubri, Silchar, Calcutta, and other places. In some cases, it is
+possible that the immediately succeeding tremors were included as part
+of the great shock; but, in the central area, it is probable that the
+average duration of the shock did not differ much from three or four
+minutes.
+
+In this district, the movement was most complicated. Changes of
+direction were frequently noticed. At Silchar, for instance, the
+earthquake began with an undulatory movement from north to south, like
+the swinging of a suspension bridge; it closed with a motion like
+that of a boat tossed in a choppy sea, or by the crossing of great
+waves which, whatever their dominant direction may have been,
+certainly did not travel from north to south. The vertical component
+of the motion must have been considerable; for, at Shillong, loose
+stones lying on the roads were tossed in the air "like peas on a
+drum," But this was even less pronounced than the horizontal movement,
+the range of which was at least eight or nine inches, and during which
+people felt as if they were being shaken like a rat by a terrier. The
+period of these vibrations was estimated at about a second.
+
+As they left the central region, the period of the waves lengthened,
+so that, at a distance, the shock no longer consisted of short jerks,
+but became a gentle rocking motion, causing in some people a sensation
+of nausea. At Calcutta, the undulations were regular and resembled the
+rolling of a mighty ship, the period being between one and two
+seconds. At Balasor, the motion was a long rolling one, such as would
+be felt on the deck of a ship in a fairly heavy sea; and, farther to
+the south as far as the limit of the disturbed area, the same
+undulatory movements were observed, gradually decreasing in intensity,
+and usually compared to the easy motion of a ship in a gentle sea.
+
+_Visible Earth-Waves._--A few examples have already been given of the
+observation of visible waves on the surface of the ground. They were
+seen at Charleston during the earthquake of 1886 (p. 110), and at
+Akasaka and other places in the meizoseismal area during the Japanese
+earthquake of 1891 (p. 186). But they were more than usually
+prominent in the Indian earthquake; indeed, much of the difficulty
+experienced in standing during the shock seems to have been due to the
+passage of these surface-waves.
+
+At Shillong, according to an observer quoted above (p. 266), the
+surface of the ground vibrated visibly in every direction, as if it
+were made of soft jelly. Another describes it as presenting "the aspect
+of a storm-tossed sea, with this difference that the undulations were
+infinitely more rapid than any seen at sea." Near Maimansingh,
+earth-waves were watched approaching, exactly like rollers on the
+sea-coast, and, as they passed, the observers had a difficulty in
+standing. At Nalbari, the rice in the fields could be seen rising and
+falling at intervals during the transit of the waves. In the Assam
+valley, near Mangaldai, there were seen "waves coming from opposite
+directions and meeting in a great heap and then falling back; each time
+the waves seemed to fall back the ground opened slightly, and each time
+they met, water and sand were thrown up to a height of about 18 inches
+or so." Even as far as Midnapur, the ground was "distinctly billowy,"
+and at Allahabad a series of waves was observed to cross the ground
+from south-south-west to north-north-east.
+
+It is obviously difficult to judge in any case of the magnitude of
+such waves. In the epicentral area, Mr. Oldham believes that, on an
+average, they were probably about thirty feet long and one foot in
+height, though some may have been both shorter and higher. These
+movements must have been comparatively slow, for their progress could
+be easily followed by the eye; indeed, their rate, as one witness
+remarks, "though decidedly faster than a man could walk, was not so
+fast as he could run."
+
+
+ELEMENTS OF THE WAVE-MOTION.
+
+In his study of the Neapolitan earthquake, Mallet showed how the
+amplitude and maximum velocity of the vibrations could be determined
+roughly from the displacement, projection, or overthrow of various
+bodies by the earthquake. Somewhat similar methods were employed by
+Mr. Oldham in the absence of seismographs from the epicentral area.
+His results are of course only approximate, but they lead nevertheless
+to a conclusion of great value and interest.
+
+   [Illustration: FIG. 69.--Section of Tombs in the Cemetery at
+   Cherrapunji. (_Oldham._)]
+
+_Amplitude._--The best measure of the amplitude was obtained at the
+cemetery at Cherrapunji, situated near the southern margin of the
+epicentral area. Here were two oblong masonry tombs (Fig. 69),
+standing close together with their longer axes pointing north and
+south. Their inner sides were partially destroyed. "On the outer
+sides, they are almost intact, but the tombs have been driven bodily
+down into the ground, and on either side to east and west, there is a
+depression with a vertical side parallel to the outer surface of the
+tomb and a smooth flat bottom over which the base of the tomb has
+slid.... The edge of the western depression has the grass growing
+undisturbed up to the edge of it, and along the edge small fragments
+of lime and plaster show that this was originally in contact with the
+edge of the tomb, which has now moved away to a distance of 18 inches.
+On the east the edge of the depression is raised and the grass and
+earth forced upwards by the thrust of the tomb against it; the breadth
+of this depression is 10 inches."
+
+During the movement of the ground, the tombs, owing to their inertia,
+remained comparatively stationary, and the depressions were formed by
+the backward and forward movement of the ground against them. The
+movement on the east side was clearly arrested in some manner, and the
+range therefore cannot have been less than 10 inches. It may have been
+as much as 18 inches, and was probably, in Mr. Oldham's opinion, the
+mean of these two amounts--namely, 14 inches. This would give an
+amplitude of about 7 inches, a value which may be in excess of the
+average amount elsewhere in the district, as the cemetery is situated
+near the edge of a high sandstone scarp.
+
+At Tura, also within the epicentral area, a range of not less than 10
+inches was given by the sliding of a wooden house over the posts on
+which it rested. Six months after the shock, Mr. Oldham frequently
+noticed vacant spaces four or five inches across by the side of large
+boulders scattered over the Khasi hills, and he infers that
+"throughout the whole tract lying west of Shillong and Gauhati, as far
+as the hills extend, and probably over a large area of the plains
+besides, the amplitude of the wave-motion was nowhere less than 3
+inches, while in many places it was over 6 inches."
+
+_Maximum Velocity._--The most trustworthy measure of the maximum
+velocity are those obtained from the projection of bodies. Mr. Oldham
+selects the following as most deserving of notice:--At Goalpara, an
+obelisk surmounting a tomb was broken off and thrown to one side,
+giving a maximum velocity of not less than 11 feet per second. At
+Gauhati, the coping of a small gate-pillar was shot off and fell at a
+distance of 4 feet 4 inches from the centre of the pillar; in this
+case the maximum velocity must have exceeded 8 feet per second. The
+highest velocity, of more than 16 feet per second, was measured at
+Rambrai, where a small group of monoliths were shot out of the ground,
+one of them to a distance of 6-1/2 feet. Lastly, at Silchar, a bullet
+was projected from the corner of a wooden post, acting as a rough form
+of seismometer, from which a maximum velocity of at least 1-1/2 feet
+per second was deduced.
+
+_Maximum Acceleration._--Estimates of the maximum horizontal
+acceleration were made from 28 overthrown pillars by means of
+Professor West's formula (p. 184, footnote). The measures obtained at
+the same place show some variation, but Mr. Oldham considers as fair
+average values those of 14 feet per second per second at Goalpara, 12
+at Gauhati, Shillong, and Sylhet, 10 at Cherrapunji, 9 at Dhubri, and
+4 feet per second per second at Silchar.
+
+Of the vertical component of the acceleration, not even the roughest
+numerical estimate can be formed. We know, however, that at Shillong,
+Gauhati, and indeed throughout the epicentral area, stones were
+projected upwards, and this is evidence that the vertical component
+was greater than that of gravity--namely, 32 feet per second per
+second.
+
+Violent as the shock was at the places just mentioned, it must have
+been still greater in certain parts of the epicentral area. At Dilma,
+in the Garo hills, the shock seems to have been strong enough to
+disable men; and, in the neighbourhood of the faults that will be
+described in a later section, forest trees were snapped in two.
+Fortunately, as Mr. Oldham remarks, there were in these districts no
+towns or populous settlements to feel the full power of the earthquake
+to destroy.
+
+_Anomalies in the above Measurements._--If the movements of the ground
+followed the law of simple harmonic motion, any two of the four
+elements (period, amplitude, maximum velocity, and maximum
+acceleration) would suffice to determine the others (p. 4). Applying
+the usual formulæ to the quantities obtained at Gauhati--namely, 8
+feet per second for the maximum velocity and 12 feet per second per
+second for the maximum acceleration, it follows that the amplitude
+would be 5 feet and the period 4 seconds--values, which are evidently
+inadmissible. Or, taking the maximum vertical component at 32 feet per
+second per second, the corresponding values would be 2 feet and 1-1/2
+seconds, that of the amplitude being still too great. Again, at
+Rambrai, the maximum velocity was found to exceed 16 feet per second.
+The other elements are unknown, but, if the amplitude were one foot,
+Mr. Oldham shows that the maximum acceleration would be 256 feet per
+second per second; or, taking the amplitude at the impossible amount
+of two feet, that the maximum acceleration would be 128 feet per
+second per second.
+
+It follows, therefore, that only part of the high velocities at
+Rambrai and elsewhere can be due to the elastic waves provoked by the
+initial disturbances. The remaining portion must be attributed to a
+bodily displacement of the earth's crust within the epicentral area--a
+displacement of which the fault-scarps and other distortions of that
+region furnish ample evidence.
+
+
+SOUND-PHENOMENA.
+
+In the epicentral area, the sound that accompanied the earthquake was
+remarkable for its extraordinary loudness. At Shillong, the crash of
+houses falling within thirty yards was completely drowned by the roar
+of the earthquake.
+
+The sound was generally compared to distant thunder, the passage of a
+train or cart, etc.; but, whatever the type may be, it always implies
+a sound of deep pitch, close to the lower limit of audibility--a
+continuous rumbling or rattling noise, as a rule gradually becoming
+louder and then dying away. There was the usual conflict in the
+evidence of different observers due to the depth of the sound. In
+Calcutta, which lies well within the sound-area, some persons asserted
+that they heard a rumbling noise; others were positive that the only
+noise was that caused by falling buildings and furniture. Some, again,
+noticed that the shock was preceded by a loud roar; while others were
+certain that there was no sound of any kind until the earthquake had
+become severe.
+
+As in the case of the disturbed area, it is impossible to define the
+boundary of the region over which the sound was heard. Like the shock,
+also, it seems to have been observed farther to the west than towards
+the east. Leaving out of account records that are possibly doubtful,
+the sound was heard for a distance of 330 miles to the west and
+south-west, and 290 miles to the east of the epicentral area--that is,
+allowing for the dimensions of that area, it must have been
+perceptible over a region measuring not less than 800 miles from east
+to west.
+
+
+VELOCITY OF THE EARTH-WAVES.
+
+It is somewhat doubtful whether a more accurate estimate of the
+velocity is to be obtained from a violent earthquake or from one of
+moderate intensity. In the former case, the vast distances to which
+the shock is noticed lessen the effects of errors in the
+time-determinations, but this advantage is to a great extent
+compensated by the considerable duration of the shock and the
+consequent uncertainty whether all observers have timed the same phase
+of the movement. Also, in the Indian earthquake, there are further
+sources of error in the variety of standard times employed throughout
+the country and in the magnitude of the epicentral area.
+
+Of the numerous time-records collected by Mr. Oldham, the best are
+those which were obtained from a few self-recording instruments, from
+the more busy telegraph offices, from the larger railway stations, and
+in some cases from private individuals. All records were in the first
+place subjected to a rigid process of selection; a large number were
+rejected on various grounds, and those only were retained which bore
+internal evidence of accuracy, due either to the conditions of the
+reporter's occupation or to the care taken by him to ensure exactness.
+To guard against any unconscious bias in making the selection, this
+process was carried out before the distances were calculated, and even
+before the position of the epicentral area was known.
+
+The boundary of this area is shown by the continuous line A in Fig.
+68. Its greatest length being about 200 miles from east to west, it is
+necessary in the first place to fix upon an equivalent centre within
+it, which may be regarded for this special purpose as the point of
+departure of the earth-waves. The more natural course perhaps would be
+to assume this point to coincide with the centre of the area. But, as
+the rate at which the initial movement spread over that area would
+probably differ little from the velocity of the earth-wave, and as all
+the time-stations lie towards the west, Mr. Oldham regards a point
+near the western boundary of the area (in lat. 25° 45' N. and long.
+90° 15' E.) as a sufficiently exact approximation to the position of
+the equivalent centre.
+
+The nearest place at which good time-observations were made is
+Calcutta, distant 255.5 miles from the assumed centre. One is
+indicated on the recording tide-gauge by a sudden rise of the water,
+while the others were obtained from the central telegraph office, the
+terminal railway stations, and from two careful readings by interested
+observers. They vary from 4h. 27m. 0s. to 4h. 28m. 37s. P.M., all
+being liable to an error of half-a-minute. The arithmetic mean for the
+beginning of the shock is 4h. 27m. 49s., and this is probably as
+accurate an estimate as the conditions allow.[70]
+
+Bombay lies outside the disturbed area, 1208.3 miles from the
+equivalent centre; and, for the time of arrival in that city, we have
+to depend on the records of the barograph and the three magnetographs.
+The horizontal force magnet was set in motion two and a half minutes
+before the others, no doubt by the advance tremors. The times given by
+the barograph and the vertical force-instrument differ by only one
+minute, and the best result seems to be that obtained by taking their
+mean--namely, 4h. 35m. 43s., which is probably accurate to within a
+minute.
+
+Assuming, then, that the time-interval between Calcutta and Bombay
+does not err by more than half-a-minute, it follows that the
+intervening velocity must lie between 2.8 and 3.2 kilometres per
+second, its probable value being 3 kilometres, or 2 miles, per second.
+
+The remaining records, which are of less value than those obtained in
+these cities, fall into two groups, the first consisting of a number
+of stations along a line running north and south between Calcutta and
+Darjiling or within a hundred miles on either side of the same, and
+the second a long series of stations crossing Northern India in a
+nearly westerly direction. The observations made at the Burmese
+stations were unfortunately affected by an error arising from the
+retardation of the Madras time-signals through frequent repetition
+along the line.
+
+   [Illustration: FIG. 70.--Time-curve of Indian earthquake.
+   (_Oldham._)]
+
+Individually, these records are not exact enough to be used in
+determining the velocity, but they may be employed collectively for
+the construction of the time-curve in Fig. 70. In this diagram,
+distances in hundreds of miles from the equivalent centre are
+represented along the horizontal line, and the time of occurrence in
+minutes past 4 P.M. along the perpendicular line. The small circles
+represent the observations at Calcutta and Bombay, the dots those at
+places lying nearly west of the origin, and the crosses those at
+places situated to the south or north-west. The continuous curve
+passes in an average manner through the series of points, and probably
+does not differ much from the true curve of the time of arrival of the
+shock at different places. The curve, it will be noticed, is at first
+concave, and afterwards convex, upwards; indicating that the times
+required to traverse successive equal distances at first increased,
+and then decreased. Thus, if the curve is an accurate representation
+of the facts, it would follow that the surface-velocity was subject to
+a continual decrease outwards from the centre, until it was a minimum
+at a distance of about 280 miles, after which it increased.
+
+The deviation of the curve from a straight line is, however, so slight
+that we cannot feel much confidence in this conclusion. If we join the
+points corresponding to Calcutta and Bombay by a straight line (drawn
+dotted in Fig. 70), it does not in any part vary from the continuous
+line by a distance equivalent to more than half-a-minute. Indeed, if a
+very few discordant records are excluded, and if less weight is given
+to those times which are multiples of five minutes, the straight line
+represents the mean quite as fairly as the curved line does; and that
+this is the more probable interpretation will appear from the
+observations on the unfelt earthquake described in the next section.
+We may therefore conclude that the earth-waves travelled along the
+surface at an approximately uniform rate of 3 kilometres per second,
+or about 120 miles a minute--a result which Mr. Oldham considers may
+be accepted as accurate to within five per cent.
+
+If the two time-curves in Fig. 70 are continued to the right until
+they meet the time-scale, it will be seen that they intersect it near
+the point corresponding to 4.26 P.M., implying that this would be
+approximately the time at which the shock was felt within the
+epicentral area. This agrees closely with the observed times of about
+4.25 at Parbatipur and Kuch Bihar, 4.26 at Siliguri, and 4.27 at
+Shillong and Goalpara; and it is probable that the error is not more
+than a quarter of a minute in defect or half-a-minute in excess. Thus,
+the time of arrival of the first sensible waves at the surface would
+lie between 4h. 25m. 45s., and 4h. 26m. 30s. P.M., Madras time, or
+between 11h. 4m. 45s. and 11h. 5m. 30s. A.M., Greenwich mean time.
+
+
+THE UNFELT EARTHQUAKE.
+
+Of the crowd of vibrations that agitate the ground during an
+earthquake, part only combine to form the perceptible shock. Some are
+insensible owing to their small amplitude, others to the slowness of
+the motion. An interesting observation belonging to the latter class
+was made by an engineer near Midnapur, a place which lies just within
+the area of damage. At the time of the earthquake, he was taking
+levels on a railway bank, and was about to take a reading when he
+noticed the bubble of the level oscillating. In five or ten seconds
+the shaking began and appeared to last three or four minutes; but, for
+more than five minutes after it had apparently ceased, the level
+showed that the ground continued to rock.
+
+Again, in Burmah, at a place nineteen miles east of Tagaung and close
+to the border of the disturbed area, the water in a shallow tank,
+about 300 yards in length, was seen lapping up against the side in a
+manner that was at first attributed to elephants bathing. No shock was
+felt, but the shaking of the trees at the same time showed that the
+disturbance was due to the earthquake.
+
+Far beyond the limits of the disturbed area, however, the earthquake
+was recorded by many of the delicate instruments which have been
+employed during the last few years for the registration of distant
+shocks. Among the more important of these instruments are long
+vertical pendulums, horizontal pendulums of various forms, and
+magnetographs. In the vertical, and some of the horizontal,
+pendulums, especially in those used in the Italian observatories, the
+masses carried are heavy, and the movements of the ground are
+magnified by lightly-balanced levers ending in points which trace
+their records on bands of smoked paper driven by clockwork. In the
+other horizontal pendulums and in the magnetographs, the method of
+registration is photographic. The paper required for the mechanical
+records being inexpensive, a high velocity (half-an-inch or more per
+minute) can be given to it, and the resulting diagrams are open and
+detailed. The Italian instruments also respond more readily than the
+others to the earlier and slighter tremors: while the apparatus in
+which photographic methods are used are sometimes so violently
+disturbed by the later undulations that the spot of light fails to
+leave any trace on the photographic paper. It is therefore from the
+Italian observatories that the more interesting records come. One of
+these, given by a horizontal pendulum at Rocca di Papa near Rome, is
+reproduced in Fig. 71; while the curve of the bifilar pendulum at
+Edinburgh (Fig. 72) is a good example of those obtained by the
+photographic method of registration.[71]
+
+All over Italy, from Ischia and Catania in the south to Pavia in the
+north, the different instruments employed began, one after the other,
+to write their records of the movement as the unfelt earth-waves sped
+outwards from the centre. Italy passed, the tale was taken up by
+magnetographs at Potsdam and Wilhelmshaven, Pawlovsk (near St.
+Petersburg), Copenhagen, Utrecht, and Parc St. Maur (near Paris); by
+horizontal pendulums at Strassburg and Shide (in the Isle of Wight),
+and by a bifilar pendulum at Edinburgh. Shide is 4,891 miles from the
+centre of disturbance, but, as we shall see, the movement could be
+traced for a distance greater even than this.
+
+   [Illustration: FIG. 71.--Seismographic Record of Indian
+   Earthquake at Rocca di Papa. (_Cancani._)]
+
+In the more complete records, and especially in those given by the
+Italian apparatus, Mr. Oldham distinguishes three phases of motion.
+The first consists of rapid and nearly horizontal movements of the
+ground. In Italy, it begins at about 11.17 A.M.--that is, about 12-1/2
+minutes after the commencement of the shock at the epicentre (Fig. 71,
+_a_). Without any break in the movement, and after a further interval
+of about 8-1/2 minutes, the second phase begins; the vibrations are
+similar to the preceding, but they are larger and more open, and are
+accompanied by an unmistakable tilting of the surface of the ground
+(Fig. 71, _b_). Lastly, after the lapse of about twenty minutes more,
+the second phase gives place, without interruption, to the third (Fig.
+71, _c_),[72] consisting of well-marked slow undulations, which have
+been aptly compared by Professor Milne to the movements caused by an
+ocean-swell. As they travelled across Europe, the surface of the
+ground was thrown into a series of flat waves, 34 miles in length, and
+20 inches in maximum height, the complete period of each wave being 22
+seconds. This phase is by far the longest of the three; in the more
+sensitive instruments, two or three hours elapsed before their traces
+ceased to show any sign of movement.
+
+   [Illustration: FIG. 72.--Seismographic Record of Indian
+   Earthquake at Edinburgh. (_Heath._)]
+
+Knowing the distances of the different observatories from the
+epicentre, and the times taken by each phase to reach them, we can
+form some idea of the rates at which they travelled. If the early
+tremors moved in straight lines, their mean velocity for the first
+phase was 9.0, and for the second 5.3, kilometres per second; but, if
+they moved along curved paths through the body of the earth, their
+mean velocities must have exceeded these amounts. For the first
+undulations of the third phase, the velocity would be 2.9 kilometres
+per second if they travelled along straight lines, and 3.0 kilometres
+per second if they were confined to the surface of the earth.
+
+The existence of the second phase was noticed for the first time by
+Mr. Oldham in the records of the Indian earthquake, but he has since
+detected it in those of other shocks. He believes, in common with most
+seismologists, that the first phase corresponds to waves of elastic
+compression, or longitudinal waves, travelling through the body of the
+earth; and the second phase he attributes to waves of elastic
+distortion, or transversal waves, travelling in the same way, in which
+the particles move at right angles to the direction in which the wave
+travels, thus causing a slight tilting of the surface. It is probable
+that the waves of both phases move along curved, rather than straight,
+lines through the earth, that the curves are concave towards the
+surface, and that the velocity of the waves increases with the depth
+of their path below the surface.
+
+On the other hand, the surface-velocity of the first undulations of
+the third phase is practically constant for all distances from the
+epicentre, and, in the case of the Indian earthquake, it agrees almost
+exactly with that obtained for the velocity within the disturbed area,
+and as far as Bombay. It is therefore difficult to resist the
+conclusion that the third phase consists of undulations which travel
+along the surface of the earth. Diverging in two dimensions only, they
+fade away much more slowly than the vibrations of the other two
+phases.
+
+We may thus imagine these surface-undulations speeding outwards from
+the epicentre in ever-widening circles until they have passed over a
+quarter-circumference of the earth, when they should begin to converge
+towards the antipodes. Here they should cross each other, and again
+spread out as circular waves, once more in their course passing the
+same observatories where they were first recorded, but in the opposite
+order. It has been reserved for the most violent earthquake of modern
+times to verify this interesting conclusion. Faint, but decided, are
+the traces of the second crossing. At Edinburgh, they occur at 2.6
+P.M., at about the same time at Shide, at Leghorn 2.10, Catania
+2.12-3/4, while at Ischia there are several movements between 2 and 3
+P.M. At Rocca di Papa, near Rome, the time is slightly earlier, but
+the undulations, like those at the first crossing, have a complete
+period of about 20 seconds. The distances traversed by the waves are
+more than 20,000, instead of less than 5000 miles; but the mean
+velocity with which they travelled is almost exactly the same as at
+first--namely, 2.95 kilometres per second.
+
+
+EARTH-FISSURES, SAND-VENTS, ETC.
+
+_Earth-Fissures._--Among the superficial effects of the earthquake,
+none take a more important place than the fissures formed in alluvial
+plains. Not only were they remarkably abundant, more so than in any
+other known earthquake, but they occurred over an unusually wide area.
+Wherever the necessary conditions prevailed, they were found to be
+numerous over a district bounded approximately by the isoseismal 1
+(Fig. 68), and measuring about 400 miles from east to west, and about
+300 miles from north to south; and they were present, though in
+smaller numbers, over an area nearly 600 miles long in an
+east-north-east and west-south-west direction. They were naturally
+more frequent near river-channels and reservoirs, on account of the
+absence of lateral support, and as a rule were parallel to the edge of
+the bank, a few hundred yards in length, and in width varying from
+some inches to four or five feet.
+
+Fissures in such positions are formed with every violent earthquake,
+and even with some of those more moderate shocks that visit the
+British Islands (see p. 247). But an interesting point established by
+the Indian earthquake is that they also occurred at a distance from
+any water-channel or excavation, often running parallel to, and along
+either side of, a road or embankment. In other situations, they showed
+a distinct tendency to range themselves parallel to one another; and,
+in these cases, it is possible that their formation was connected with
+the passage of the visible surface-waves. In an account already quoted
+(p. 247), it is stated that these waves came from opposite directions
+and that, as they separated after meeting, the ground opened slightly.
+
+Among the Khasi and Garo hills (see Fig. 75), wherever the alluvium of
+the plains runs up to the foot of the hills, another form of fissure,
+represented in Fig. 73, was constantly noticed. Close to the
+junction, there was a sudden drop, as at _a_, of from one to five
+feet, the vertical face having the appearance of a fault, but
+distinguished from one by following the windings of the hills. Then
+came a depressed band _b_, from ten to twenty feet wide, and outside
+this a low rounded ridge _c_ raised above its former level, and
+merging beyond at _d_ into the undisturbed plain. When Mr. Oldham
+visited the district in March 1898, the natives had flooded the
+rice-fields, and the features described were clearly depicted by the
+gathering of the water in the depression and the isolation of the
+ridge.
+
+   [Illustration: FIG. 73.--Displacement of alluvium at foot of a
+   hill. (_Oldham._)]
+
+The explanation of these peculiarities is evidently that given by Mr.
+Oldham. During the passage of repeated waves of compression, the
+thrust of the hill and plain against one another caused the heaping up
+of the alluvium in the ridge _c_; while the return movements resulted
+in the tearing of the alluvium away from the hillside, leaving the
+scarp _a_ and the depression _b._
+
+_Displacements of Alluvium._--Many other remarkable evidences of
+compression were observed. Telegraph posts, originally set up in a
+straight line, were displaced, occasionally as much as ten or fifteen
+feet; sometimes without any apparent connection with neighbouring
+river-channels. In one part of the Assam-Bengal Railway, for nearly
+half a mile, the whole embankment, including borrow-pits and trees on
+either side, was shifted laterally without any sign of wrenching from
+the adjoining ground, the maximum distance amounting to 6-3/4 feet. As
+the displacement took place parallel to the only river-course in the
+neighbourhood, Mr. Oldham attributes it to the sliding of the
+surface-layers over some yielding bed beneath. Again, throughout large
+areas of Northern Bengal, Lower Assam, and Maimansingh, rice-fields,
+which had been carefully levelled so that they might be uniformly
+flooded, were thrown into gentle undulations, the crests of which were
+occasionally two or three feet above the hollows. The piers of bridges
+were also moved parallel to, as well as towards, the streams, showing
+that the displacements extended to the depth of the foundations.
+
+The buckling of railway lines was often violent and took place over a
+large area. In the Charleston earthquake, every such bend was
+accompanied by a corresponding extension elsewhere (p. 113); but, in
+the Baluchistan earthquake of 1892, the neighbouring fish-joints were
+jammed up tight.[73] In the one case, there was merely local
+compression; in the other, a permanent displacement of the earth's
+crust. The distortion of the Indian lines seems to belong to the
+former class. Repairs were of course generally made without delay; but
+all the information that could be obtained on this point showed that
+the compression causing the crumpling of the lines was accompanied by
+a compensating expansion, generally at a distance of about 300 yards.
+
+_Sand-Vents._--Shortly after the earthquake, large quantities of water
+and sand issued from fissures in the ground. At Dhubri, "innumerable
+jets of water, like fountains playing, spouted up to heights varying
+from 18 inches to quite 3-1/2 or 4 feet. Wherever this had occurred,
+the land was afterwards seen to occupy a sandy circle with a
+depression in its centre. These circles ranged from 2 to 6 and 8 feet
+in diameter, and were to be seen all over the country. In some places,
+several were quite close together; in others they were at a distance
+of several yards." Near Maimansingh, they seem to have been almost as
+numerous, fifty-two, of four feet and less in diameter, being counted
+within an area 100 yards long and about 20 feet wide.
+
+The sand and water were ejected from the vents with some force. A few
+observers estimated the height of the spouts at about 12 feet, but
+this probably refers to stray splashes. It is clear, however, that the
+sand and water were forced not only up to the surface, but even in a
+continuous stream to heights of from two to ten feet above it. In many
+districts, trunks of trees or lumps of coal and fossil resin were
+washed up with the water, and even, in one or two cases, pebbles of
+hard rock weighing as much as half-a-pound.
+
+The origin of the sand-vents is to be sought in the presence of a
+water-bearing bed situated not far below the surface. In the central
+area, where there was a marked vertical component in the motion, this
+bed during the earthquake was compressed between those above and below
+it, and the resulting pressure was in places sufficient to force the
+water and sand, through the fissures formed by the earthquake, up to
+and beyond the surface. The gradual settling of the upper layer, cut
+up by the fissures, into the underlying quicksand, prolonged the
+process for some time after the shock was over; and, when the pressure
+was at last relieved, some of the water was sucked back and so
+produced the crateriform hollows.
+
+_Rise of River-Beds, etc._--Over a large area, river-channels, tanks,
+wells, etc., were filled up, partly by the outpouring of the sand from
+vents, but chiefly, as shown by the forcing up of the central piers of
+bridges, by the elevation of the beds of the excavations. In the
+lowlands which lie between the Garo hills and the Brahmaputra, there
+were numerous channels from 15 to 20 feet in depth, the beds of which
+were pressed up until they became level with the banks, while a
+compensating subsidence took place close to the streams on either
+side. The general tendency of the earthquake was thus to obliterate
+the surface inequalities, so that, when the rivers rose later on, the
+district was extensively flooded.
+
+Besides these deferred floods, there occurred immediately after the
+earthquake a sudden rise in many rivers, amounting to from two to ten
+feet, followed by a gradual decline to the former state in two or
+three days. At Gauhati, for instance, the river-gauge showed that, at
+about three-quarters of an hour after the earthquake, the water stood
+7 feet 7 inches higher than on the morning of June 12th; at 7 A.M. on
+June 13th it had fallen to 5 feet 8 inches, and at the same time on
+the two following days to 2 feet 7 inches and 6 inches, showing that
+the water had returned nearly to its original level after the lapse of
+two and a half days.
+
+In most of the large rivers, the rise of water was due to the
+formation of partial dams formed by the local elevation of the
+river-beds described above. As the barriers were composed of loose
+sand, they were gradually scoured away and the material was spread
+over the bottom so as to leave the water at a level slightly higher
+than that which it maintained before the earthquake.
+
+
+LANDSLIPS.
+
+The distribution of landslips shows that their formation depends
+almost as much on local conditions as on the violence of the shock.
+The effect of the latter is manifested by their limitation to a
+certain central area. To the east of the North Cachar hills, few, if
+any, were to be seen; but, as far as Kohima, cracks or incipient
+landslips were formed on the hillsides. The Sylhet valley and a line
+to the west of Darjiling form the southern and western boundaries of
+the landslip area, which was therefore not less than 300 miles in
+length from east to west.
+
+Within this area, however, local conditions asserted their
+superiority. Among the more important may be mentioned the
+constitution of the hills and the presence of a thick superficial
+layer of subsoil or rock with an inner bounding surface of weak
+cohesion, the slope of the hillsides, and their height from base to
+crest. Thus, though the epicentral area was situated chiefly to the
+south of the Brahmaputra valley (Fig. 75), the east and west range of
+the landslips was more extensive in the Himalayas on the north side
+than in the Garo and Khasi hills on the south. In many places, the
+steep sides of the Himalayan valleys exist always in a critical
+condition of repose, and the effect of the Indian earthquake was such
+that all along the north side of the Brahmaputra valley, the range is
+scarred by landslips, even to the east of Tezpur.
+
+Again, along the southern edge of the Garo and Khasi hills, landslips
+were unusually prevalent. "Viewed from the deck of a steamer sailing
+up to Sylhet," says Mr. Oldham, "the southern face of these hills
+presented a striking scene. The high sandstone hills facing the plains
+of western Sylhet, usually forest-clad from crest to foot, were
+stripped bare, and the white sandstone shone clear in the sun, in an
+apparently unbroken stretch of about 20 miles in length from east to
+west." At Cherrapunji, also, the deep valleys were so scored that,
+from a distance, there appeared to be more landslip than untouched
+hillside.
+
+But in no part, probably, were landslips more strikingly developed
+than in the small valley of the Mahádeo, which forms an amphitheatre
+about four miles long from east to west, and a mile and a half across,
+lying to the south of the Bálpakrám and Pundengru hills. "Here,"
+remarks Mr. Oldham, "everything combined to favour the formation of
+landslips. The hills were composed of soft sandstone, they were
+steep-sided, high, and narrow from side to side, and consequently were
+doubtless thrown into actual oscillation as a whole; while the range
+of motion of the wave particle was not less than eight inches near the
+edge of the precipices. The result ... has been to produce an
+indescribable scene of desolation. Everywhere the hillsides facing the
+valley have been stripped bare from crest to base, and the seams of
+coal and partings of shale could be seen running in and out of the
+irregularities of the cliffs with a sharpness and distinctness which
+recalled the pictures of the cañons of Colorado. At the bottom of the
+valley was a piled-up heap of _débris_ and broken trees, while the old
+stream had been obliterated and the stream could be seen flowing over
+a sandy bed, which must have been raised many feet above the level of
+the old watercourse."
+
+In the sandstone districts of the area here considered, the landslips
+had some important secondary effects. Along the southern edge of the
+Garo and Khasi hills, great sand-fans spread over the fields, and the
+exposure of the hillsides formerly protected by forest left free scope
+for future denudation. Every stream of any size has in this way
+devastated many square miles of country. Among the hills themselves,
+more sand was brought down than the streams could carry away, and
+everywhere their beds were raised. "Ordinarily, the beds of these
+rivers, which are raging torrents when in flood, consist of a
+succession of deep pools separated by rocky rapids. After the rains of
+1897, it was found that the pools had been filled up, and the rapids
+obliterated by a great deposit of sand, over which the rivers flowed
+in a broad and shallow stream."
+
+A few valleys were for a short time barred across by landslips. In
+one, on the northern foot of the Garo hills, a landslip crossed the
+drainage channel and formed a shallow pond, which was not filled up by
+sand until the end of January 1898. Near Sinya, in the northern Khasi
+hills, an unusually large landslip formed a barrier, of which the
+remains are more than 200 feet above the level of the river-bed.
+Behind this, the water accumulated in a great lake until the beginning
+of September 1897, when the barrier burst and a flood of water rushed
+down the valley.
+
+
+ROTATION OF PILLARS, ETC.
+
+A curious effect of earthquakes strong enough to damage buildings is
+that pillars, monuments, etc., may be fractured and the upper part
+rotated over the lower without being overthrown. Even in Hereford and
+the surrounding villages, several pinnacles and chimney-stacks were
+twisted by the earthquake of 1896. The interest of the phenomenon,
+which has been known, since 1755,[74] is mainly historical, for the
+endeavour to discover its cause was the origin of Mallet's views on
+the dynamics of earthquakes. Partly, also, it lies in the difficulty
+of finding a satisfactory explanation, or rather in deciding which of
+three or four possible explanations is the true one in any particular
+case.
+
+   [Illustration: FIG. 74.--Twisting of monument at Chhatak.
+   (_Oldham._)]
+
+The Indian earthquake offered exceptional opportunities for studying
+the phenomenon in the large number of examples observed and the
+variety of objects rotated. None could be more striking than the
+twisted monument to George Inglis, represented in outline in Fig. 74.
+Chhatak, where this is situated, lies close to the southern boundary
+of the epicentral area. The monument is an obelisk, built of broad
+flat bricks or tiles on a base of 12 feet square, and originally more
+than 60 feet high. It was split by the earthquake into four portions.
+The two upper, about six and nine feet long, were thrown down; while
+the third, 22 feet high, remains standing, but is twisted through an
+angle of 30° with respect to the lowest part, which is unmoved. The
+upper of these two parts had evidently rocked on the lower, as the
+corners and edges were splintered, and below the fracture a slice of
+masonry about 15 inches thick, which was not bonded into the main
+mass, was split off by the pressure on its upper end. The plan of the
+parts still standing is shown in the lower part of Fig. 74.
+
+The possible explanations of the phenomenon are at least three in
+number. According to the first, which was given by Mallet in 1846, the
+adhesion of the twisted portion to its base is not uniform, and the
+resultant resistance to motion is not in the same vertical plane as
+the wave-movement.[75] Some years later, Mallet offered another
+explanation. The body, he imagined, might be tilted on one edge by the
+earthquake, and, while still rocking, a second shock oblique to the
+first might twist it about that edge.[76] In 1880, Professor T. Gray
+suggested that the column might be tilted on one corner and then
+twisted round it by later vibrations of the same shock.[77]
+
+None of these theories, Mr. Oldham argues, can give by itself a
+complete explanation of the phenomena observed in the central district
+of the Indian earthquake; and he therefore favours an extension of the
+second theory, which, though first proposed in 1882,[78] was thought
+out independently and in greater detail by himself. When the focus is
+of considerable dimensions, the shock at neighbouring places is
+constantly varying in direction, owing to the arrival of vibrations
+from different parts of the focus. Thus, instead of the two separate
+shocks required by Mallet's second explanation, we have a number of
+closely successive impulses frequently changing in direction and
+giving rise to what is known in the South of Europe as a vorticose
+shock. And, instead of a single twist of the pillars about one centre
+only, we have a series of small twists round a number of different
+centres, accompanied in consequence by a much smaller displacement of
+the centre of gravity than would have occurred had the same rotation
+been accomplished in one operation.
+
+The theory, it will be seen, accounts for the twisting of the pillar
+without overthrow, and for the splintering of the edges during the
+rocking of the column. It explains why in any district a number of
+similarly placed objects are generally twisted in the same direction.
+Moreover, a low column rocks to and fro more rapidly than a tall one
+similar in form and position, so that, at the instant when a later
+impulse comes from a different direction, two such columns might
+happen to be tilted on opposite edges, and would then be twisted in
+opposite directions. In certain cases, then, as occurred at several
+places during the Indian earthquake, an object may rotate in one
+direction, while others, similar in every respect but size, may be
+twisted in the opposite direction.
+
+
+AFTER-SHOCKS.
+
+_Frequency of After-Shocks._--For some days after the great
+earthquake, the after-shocks by their very frequency and by their wide
+distribution baffled close inquiry. During the first 24 hours,
+hundreds were felt at all points of the epicentral area; indeed, it is
+not too much to say that for several days the ground was never
+actually at rest. At the Bordwar tea-estate, which is traversed by one
+of the great fractures to be described in the next section, the
+surface of a glass of water on a table was for a whole week in a
+constant state of tremor; and at Tura a hanging lamp was kept
+continually swinging for the first three or four days.
+
+Most of these shocks were, of course, very slight; but, interspersed
+among them, were others of greater strength, and a few of considerable
+violence. One, on June 13th, about eight hours after the earthquake,
+was sensible beyond Allahabad--that is, for more than 520 miles from
+the epicentre; and another on the same day was felt in Calcutta,
+distant 255 miles. On June 14th, 22nd, and 29th, and again on August
+2nd and October 9th, shocks were noticed in that city; but, after the
+latter date, the disturbed area of no shock reached to so great a
+distance.
+
+To form any estimate of the total number of after-shocks is
+impossible, even for any one station. At first, lists were kept at
+isolated places, such as Shillong, Maimansingh, Dhubri, and a few
+others. Then, from July 15th, through Mr. Oldham's efforts, the
+records became more numerous until the end of the year, after which
+interest in the subject declined. Mr. Oldham's catalogue closes with
+the year 1898; but the register of a roughly-constructed seismograph,
+erected at Shillong in July 1897, continues to the present day.
+
+Imperfect as all non-instrumental registers must be, they nevertheless
+furnish some idea of the frequency of the after-shocks. Thus, until
+the end of June, 679 shocks were recorded at Rangmahal (North
+Gauhati), 135 at Maimansingh, 89 at Kuch Bihar, and 83 at Kaunia
+(omitting those on June 12th). Again, from August 1st to 15th, 182
+were felt at Goalpara, 151 at Darangiri, 124 at Tura, 105 at Bijni, 94
+at Lakhipur, 94 at Krishnai, 48 at Dhubri, 28 at Rangpur, and 12 at
+Kuch Bihar; while at Borpeta, 113 shocks were reported during the
+first nine days of August. Turning to the registers of longer
+duration, we find that at Maophlang (near Shillong) 1,194 shocks were
+felt by one observer from September 12th, 1897, to October 7th, 1898;
+at the neighbouring station of Mairang, 1,065 from September 7th,
+1897, to December 31st, 1898; and at Tura, in the Garo hills, 1,145
+shocks from July 21st, 1897, to December 31st, 1898. The total number
+of earthquakes registered by the seismograph at Shillong from August
+1897 to the end of 1901 amounts to 1,274, and all of these were
+probably strong enough to arouse the observer from sleep. Outside the
+epicentral area, Mr. Oldham's list includes 88 shocks from June 12th
+to July 15th, about 950 from July 16th to December 31st (the period
+when the after-shocks were most carefully observed), and 296 shocks
+during the year 1898.
+
+_Geographical Distribution of After-Shocks._--When we endeavour to
+compare the lists of after-shocks at different places, we are at once
+met by two serious difficulties,--the imperfection of the records and
+the approximate character of the times of occurrence. Making every
+allowance, however, for these deficiencies, it is evident that very
+few of the shocks felt at any one station were perceptible at its
+neighbours; in other words, that the shocks originated in a large
+number of foci scattered over a very wide area.
+
+For instance, two of the most carefully kept registers of after-shocks
+are those compiled at Maophlang (near Shillong), and at Mairang, only
+11 miles to the north-west. Now, between September 12th and September
+28th, 1897 (both dates inclusive), 92 shocks were felt at Maophlang
+and 83 at Mairang. Of the former, 37 were described as smart, 45
+slight, and 10 feeble; of the latter, 6 as smart, 9 slight, 65 feeble,
+and 3 very feeble. But, of the total number, only 20 were felt at both
+places at recorded times that were not more than fifteen minutes
+apart; 13 being described as smart--one at both places, one at Mairang
+alone, and the remaining 11 at Maophlang alone. When shocks occur so
+frequently, as in these cases, it is inevitable that, even if all were
+independent, some should coincide approximately in time of occurrence.
+It is therefore probable that only one in every eight shocks, and
+possibly only one in every twelve, was felt at both places.
+
+The actual numbers of shocks felt within stated periods at different
+places are perhaps hardly comparable, owing to the obvious
+imperfection of the records and the probably varying standards adopted
+by the reporters. But there can be little doubt that certain districts
+were more subject to after-shocks than others, especially such places
+as North Guahati, Shillong, and neighbouring villages, Tura,
+Darangiri, Goalpara, Bijni, Borpeta, Kaunia, and Rangpur. On the other
+hand, they seem to have been unusually scarce at Dhubri and in the
+district to the north-west, and they became rare at Gauhati long
+before they ceased to be frequent at Borpeta. In the plain to the
+south of the Garo and Khasi hills, they were also uncommon, the
+combined records for Sylhet and Sonamganj for August 1-15 giving only
+20 shocks, and, neither to the east nor to the west of these places,
+is there any sign of greater frequency.
+
+_Sound-Phenomena of After-Shocks._--Many of the after-shocks were
+accompanied by sound, or else consisted of sound-vibrations only; and
+Mr. Oldham notices that such sounds were equally frequent both on the
+rocky ground of the hills and on alluvial plains nearly all the shocks
+that originated under the Borpeta plain being attended by distinctly
+audible rumblings.
+
+During his tour in the epicentral area in the winter of 1897-98, Mr.
+Oldham had many opportunities for observing these earth-sounds. They
+were, he says, close to the lower limit of audibility, less a note
+than a rumble, and very like distant thunder, though sometimes they
+consisted of a rapid succession of short sounds, such as is caused by
+a cart when driven rapidly over a rough pavement. "As a rule, they
+began as a low, almost inaudible rumble, gradually increasing in
+loudness, though to a very varying degree, and then gradually dying
+out after having lasted anything from 5 to 50 seconds. It cannot be
+said that there was any connection between the duration and the
+loudness of the sounds, some of the most prolonged never becoming
+loud, and some of those which lasted a shorter period being as loud as
+ordinary thunder at a distance of two or three miles."
+
+Mr. Oldham records an interesting fact in connection with the
+distribution of the earth-sounds. At Naphak, in the Garo hills and
+about five miles south of Samin, 48 distinct rumbles were heard during
+23 hours on January 21-23, 1898, only seven of them being accompanied
+by a perceptible shock. At Samin, which was visited next, they were
+much less frequent, not more than 8 or 10 a day, and most of them
+attended by tremors. At Damra, a few miles to the north-east, they
+again became frequent; while, in the Chedrang valley, very few were
+heard, and only a small proportion of them were unaccompanied by
+sensible shocks. In the next section, it will be seen that the most
+conspicuous fault-scarps known in the epicentral area pass close by
+Samin and along the Chedrang valley. Thus, though the statement
+perhaps requires further confirmation, it would appear that
+earth-sounds were more common where the surface of the ground had been
+merely bent than where fractures extended right up to the surface.
+
+
+STRUCTURAL CHANGES IN THE EPICENTRAL AREA.
+
+We come now to the important features which assign the Indian
+earthquake to a small class apart from nearly every other shock. Most
+earthquakes are due to movements that are entirely deep-seated. If
+strong enough, they may precipitate landslips or fissure the alluvial
+soil near river-channels. In the Neapolitan, Andalusian, and
+Charleston earthquakes, there were many such effects of the shock
+within the meizoseismal areas. In all three, however, the disturbances
+produced were superficial; no structural change, no fissuring that did
+not die out rapidly downwards, was in any place perceptible. In the
+Riviera earthquake, the seismic sea-waves point to a small
+displacement of the ocean-bed; but it is only in the long fault-scarp
+of the central Japanese plain that we find a rival of the
+mountain-making movements that gave rise to the Indian earthquake.
+
+The boundary of the epicentral area, to the growth of which these
+distortions contributed, is represented by the curve marked A in Fig.
+68, and on a larger scale by the continuous line A in Fig. 75. A great
+part of the district is occupied by a group of hills known by various
+names locally, but which are conveniently included under the general
+term of the Assam range. To avoid the confusion of hill-shading, only
+the boundary of the range is indicated (by the broken line) in the map
+in Fig. 75. The Garo hills form the western part, and the Khasi and
+Jaintia hills the central and western parts, of the range as there
+depicted. They are formed chiefly of crystalline gneissic and granitic
+rocks and some metamorphic schists and quarzite, with cretaceous and
+tertiary rocks of varying thickness along its southern edge.
+
+Three stages have been distinguished in the history of the range.
+During the earliest, an old land-surface was worn down by rain and
+rivers till they were almost incapable of producing any further
+change. Traces of this surface are still visible in the plateau
+character of the mass. It was then elevated, not uniformly, but along
+a series of faults, so that it now consists of a succession of ranges,
+the face of each range being a fault-scarp, and its crest the edge of
+an adjoining plateau sloping away from the summit. With this elevation
+began the third and last stage. The streams were able to work again,
+and deep gorges were cut out of the range, so that in parts its
+original character was nearly effaced. But the retention of that
+character in other districts is of course evidence of the
+comparatively recent date of the final elevation.
+
+   [Illustration: FIG. 75.--Epicentral Area of Indian Earthquake.
+   (_Oldham._)]
+
+Owing to the great size of the epicentre and to the thickness of the
+forests which cover so much of its area, a comparatively small part of
+it could be traversed by Mr. Oldham during his tour in the winter of
+1897-98. The positions of the more important structural changes are
+indicated in Fig. 75. Of these, the fault-scarps are represented by
+continuous straight lines, the Bordwar fracture by the dotted straight
+line, pools and lakes not due to faulting by black ovals, reported
+changes in the aspects of the hills by circles, and the principal
+stations of the revised trigonometrical survey by crosses.
+
+_Fault-Scarps._--The most important fault-scarp is that called by Mr.
+Oldham the Chedrang fault, after the stream which coincides roughly
+with a great part of its course. The longer straight line in Fig. 75
+represents its position and general direction, and the sketch-map in
+Fig. 76 gives the plan of its southern half. From these, it will be
+seen that the fault follows on the whole a nearly straight path from
+south-south-east to north-north-west for not less than twelve miles,
+and that its throw, as indicated by the numbers to the right in Fig.
+76; is very variable, being zero in some places, and in one as much as
+35 feet or more. The upthrow is uniformly on the eastern side of the
+fault.
+
+At its southern end, as mapped in Fig. 76, there is no perceptible
+throw at the surface, but various marks of violence are manifested in
+the fissuring of the hillside and the snapping of small trees. About a
+quarter of a mile from this point, the fault crosses a tributary
+stream, where the throw amounts to two feet, and the same distance
+farther on it meets the Chedrang river, the bed of which it crosses
+many times in its short course.
+
+   [Illustration: FIG. 76.--Plan of Chedrang fault. (_Oldham._)]
+
+Mr. Oldham describes the fault in detail, as observed by him in
+February 1898. Here, it will be sufficient to refer to its more
+important features, and to its effects on the superficial drainage of
+the district. At the spot marked _a_ (Fig. 76) the river, after
+running on the west or down-throw side of the fault for nearly half a
+mile, meets the scarp, and is ponded back by it for about a quarter of
+a mile upstream. For the next half-mile, the river keeps to the
+upthrow side of the fault, the scarp of which blocks the tributary
+streams from the west, forming a number of small pools. At the last of
+these, the total throw is not less than 25 feet. A little farther on,
+the fault crosses the Chedrang and causes the waterfall at _b_, the
+height of which, owing to the fall of dislodged fragments, does not
+exceed nine feet. The fault then runs along the old and now dry bed of
+the river, while the stream itself flows in a depression on the
+down-throw side. About a quarter of a mile below the waterfall, the
+fault crosses the river, and soon after enters a large sheet of water
+at _c_, half a mile long, from 300 to 400 yards wide, and with a
+maximum depth of 18 feet. At first, the pool spreads on both sides of
+the fault, but the inequalities due to the scarp are evidenced by
+soundings. At the point where the fault leaves the pool, its throw is
+reduced to nothing, and it is just here that the water attains its
+greatest depth. To the north the throw increases rather rapidly, to 25
+feet in a quarter of a mile. But the peculiarity of this pool is that
+it is not, like the others mentioned above, dammed back by the
+fault-scarp. There is no barrier at its northern end, where the river
+escapes, except that formed by the gradually increasing throw of the
+fault. The pool is simply due to the reversal of the natural slope of
+the river-bed, caused by the formation of a roll or undulation in the
+ground on the upthrow side of the fault. Its recent origin is evident
+from the number of dead trees and bamboo clumps still standing in the
+water.
+
+For a mile after the fault leaves the pool, its throw varies
+considerably. It rises, as already mentioned, from zero to 25 feet. A
+little farther on, the fault runs up the side of a spur, the throw
+increasing to 31 feet; and, in this part, the violence of the shock
+was shown by the dislodgment of blocks of granite as much as 20 feet
+in diameter, and by the overthrow or destruction of many trees. After
+crossing the spur, the fault returns to the neighbourhood of the
+river, and crosses its bed four times, forming pools (_e_, _g_) or
+waterfalls (_d_, _f_) according as the scarp occurs on the downstream
+or upstream side. The throw of the fault then changes considerably
+within little more than half a mile, from 18 feet to zero and again to
+20 feet, the undulation so formed producing a large pool (_h_)
+entirely on the upthrow side of the fault.
+
+At the point marked _i_ on the map, the river once more crosses the
+fault; but the bottom of the valley is filled with alluvium, and,
+instead of a waterfall, a large sandy delta spreads down the stream.
+The scarp is, however, readily traced on the east side of the river, a
+throw of 32 feet being measured. After this, the alluvium becomes of
+considerable thickness, and the continuation of the fault is marked by
+a short slope, which tilts over the trees when it traverses
+forest-land. Leaving the valley of the Chedrang, the fault crosses an
+open plain, and is followed with some difficulty to the neighbourhood
+of Jhira, where, owing to the thick bed of alluvium, it forms a gentle
+roll or undulation of the surface, crossing the main channel of the
+Krishnai to the north-east of Jhira. On the west side of this barrier
+a large sheet of water, a mile and a half in length, three-quarters of
+a mile wide, and 12 feet in depth, gathered over the village of Jhira.
+"On the east side of the Jhira lake," says Mr. Oldham, "there is ample
+evidence of change of level, for part of the dry land was formerly ...
+perpetually under water, and at one place the remains of an old
+irrigation channel can be seen.... At the northern end of the lake the
+drainage now makes its escape in a broad and shallow sheet of water
+over what was once high land covered with _sal_ forest."
+
+This is the last marked feature due to the Chedrang fault. Beyond the
+north of Jhira the throw rapidly diminishes, and perhaps dies out
+altogether before reaching the low hills lying to the north of that
+village.
+
+In several ways, this fault-scarp differs from that formed with the
+Japanese earthquake of 1891. Throughout its course the down-throw,
+wherever it is perceptible, is invariably to the west; in no place
+could any trace of horizontal shifting be detected; and the plane of
+the fault, when it traversed rock, is practically vertical.
+
+Whether the scarp was formed by the elevation of the rock to the east
+of the fault, or by the depression of that to the west, or by both
+such movements at once, there is no decisive evidence; but there are
+very good reasons for believing the first alternative to be the true
+one. The undulations in the ground which gave rise to the large pools
+at _c_ and _h_ (Fig. 76) occur on the east side of the fault. Also,
+between the outlet of the lake at Jhira and the point where the
+Krishnai rejoins its original channel, the gradient of the river
+approaches that of a mountain stream, although the new bed consists of
+alluvium, and not of rock. Now, the alluvial plain of this district is
+raised so slightly above the sea-level that no subsidence great enough
+to have caused the existing gradient could have occurred without the
+depressed area being flooded with water. Though some movements may
+have taken place on the west side of the fault, it seems clear, then,
+that elevation of the rock on the east side was the predominant, if
+not the sole, cause of the fault-scarp.
+
+As the Chedrang fault has been described somewhat fully, a brief
+reference to the rest will be sufficient The only other known scarp of
+any consequence lies about ten miles to the south of the Chedrang
+fault, and runs by the village of Samin, with an average course from
+E. 30° S. to W. 30° N. Its total length does not exceed 2-1/2 miles.
+The down-throw is uniformly to the north, and the throw, which
+amounts to ten feet near its centre, gradually diminishes to zero at
+either end. Several pools are formed along the course of the
+fault-scarp by the blocking of small streams.
+
+_The Bordwar Fracture._--In the map of the epicentral area (Fig. 75),
+this remarkable fracture is represented by a dotted straight line. It
+is apparently an incipient fault. Though traceable for a distance of
+about seven miles, at no point is there any decisive evidence of
+either vertical or horizontal displacement; and, even if some doubtful
+indications of a change of level should be real, the throw must
+certainly be less than one foot. Yet, in the immediate neighbourhood
+of the fracture, the violence of the shock was extreme. "Trees have
+been overthrown or killed as they stood; a huge mass of rock,
+dislodged from near the crest of the hills, has rolled down the slope,
+scoring the side of the hill. On the opposite side an equally large
+block has been dislodged, and in its downward course cleared a
+straight track down the hill; and on the summit a gap has been cleared
+by the overthrow of trees along the line of fracture." Being only a
+few inches in width where it has rent the solid rock, the fracture was
+difficult to follow in many parts of its course. But, through
+forest-clad land, its track was marked by "a well-defined band of
+about half a mile broad, in which overturned trees are much more
+abundant than on either side, and towards the centre of this band the
+overturned trees are not only more numerous, but many of the smaller
+ones, up to six inches in diameter, have been snapped across by the
+violence of the shock."
+
+_Lakes and Pools not due to Faulting._--A few miles to the south of
+the Chedrang and Samin faults, and also of the Bordwar fracture,
+occurs a group of lakes or pools, represented on the map of the
+epicentral area (Fig. 75) by small black ovals. In the gradual
+increase in depth from either end, they resemble the two large sheets
+of water along the course of the Chedrang fault (_c_ and _h_, Fig.
+76), but they differ from them in having no direct connection with any
+apparent fault.
+
+One of these pools lies in the valley of the Rongtham river, to the
+south of the Samin fault. It seemed, at first sight, to be nothing
+more than an ordinary pool, such as may be seen on any mountain
+stream. On the bottom, and close to the outlet, however, are coarse,
+partially rounded boulders, exactly resembling those farther down the
+river; and, as the old bed was followed up, these became coated with a
+slight deposit of sand and mud, pointing clearly to a change in the
+conditions under which they were formed. The water gradually deepened,
+until trees were met standing in the water, but killed by the recent
+submergence of their roots. The pool is nearly a quarter of a mile
+long, and its greatest depth (12 feet) occurs near the middle, just
+where the former stream, with an average depth of about a foot, was
+crossed by the track from Darangiri. Towards the upper end, the water
+shallows as gradually as it deepens at the other, and ends in a delta
+of boulders brought down by the stream above. As no fault could be
+discovered in the neighbourhood of the pool, it is evident that its
+formation was due to a bend of the river-bed, the maximum change of
+level, taking into account the river-slope, being not less than 24
+feet.
+
+Similar features characterise the other pools that were examined, some
+of which are smaller, and others larger, than that described above.
+One, higher up the valley of the Rongtham, has a length of about 1-1/2
+mile and a maximum depth of 18 feet. Others of the same type, but of
+smaller size, were observed among the Khasi hills, about fifteen miles
+south of the Bordwar fissure; and it is probable that many others
+would have been found in the intermediate district, which Mr. Oldham
+was unable to visit.
+
+_Changes in the Aspects of the Hills._--There are, again, other facts
+of considerable interest which point to changes of level over a wide
+area; the places where they were noticed being indicated by small
+circles in Fig. 75. For instance, from Maophlang, near Shillong, a
+road leads to the neighbouring station of Mairang. Before the
+earthquake, only a short stretch of this road could be seen from the
+former place, as it rounded a spur about three miles away. Now, a much
+longer stretch is visible, and it can also be seen passing round the
+next, and previously hidden, spur. In this district the movements seem
+to have continued with the after-shocks; for, before the earthquake,
+the crest only of a ridge about a mile and a half to the west was
+visible; while, after it, a considerable portion could be seen, and
+much more some months later than immediately after the shock.
+
+Again, from a spot near the southern end of the Chedrang fault, it
+used to be only just possible to see the Brahmaputra over an
+intervening hill; whereas, now, the whole width of the river has come
+into view.
+
+Lastly, at Tura, which is 95 miles west of Maophlang, a battalion of
+military police were accustomed to signal by heliograph with another
+station, Rowmari, 15 miles farther to the west. This, formerly, could
+just be done by means of a ray which grazed a hill between the two
+places; it can now be done quite easily, and, in addition, a broad
+stretch of the plains east of the Brahmaputra is visible from the same
+spot.
+
+_Revision of the Trigonometrical Survey._--The movements described in
+the preceding pages are of course referred to points which may
+themselves have been displaced, and only a revision of the
+trigonometrical survey of the epicentral area and of part of the
+surrounding district could determine their absolute magnitude. During
+the cold weather of 1897-98, some of the triangles were re-measured by
+a member of the trigonometrical survey; but, as the time at his
+disposal was short, they were confined to the eastern part of the
+epicentral area, as the focus at that time was supposed to lie under
+the Khasi hills. The positions of some of these stations are indicated
+by crosses in Fig. 75; and in Fig. 77 the more important triangles are
+shown. In the revised work, all tower stations, consisting of brick
+towers built on alluvium, were omitted, as it could not be assumed
+that they had been undisturbed by displacements of the superficial
+beds.
+
+In re-calculating the lengths of the sides, the side
+Rangsanobo-Taramun Tila was adopted as the initial base, and the
+height of Rangsanobo as the initial height; a choice which later
+experience showed to be unfortunate, for Taramun Tila probably lies
+just outside, and Rangsanobo within, the epicentral area. Of the 16
+sides, whose old and new lengths were compared, only one was found to
+be apparently unchanged, two were shortened by an inch or two, while
+the others were all lengthened by amounts varying from one to eight
+or nine feet, the numbers affixed to the sides in Fig. 77 denoting the
+calculated increases in feet. Assuming the new base-line to be
+unaltered by the earthquake movements, these changes imply the
+following displacements of the principal stations:--Thanjinath 6 feet,
+Mun 4, and Laidera 2, feet to the north; Mopen 5, Dinghei 9, Landau
+Modo 12, and Umter 11, feet to the north-west; and Mosingi 3, and
+Mautherrican 5, feet to the west. At the same time, the height of
+most of the stations was found to be increased with reference to that
+of Rangsanobo: Mun by 2 feet, Thanjinath and Umter by 3, Mosingi by 4,
+Taramun Tila and Laidera by 6, Dinghei by 7, Landau Modo by 17, and
+Mautherrican by 24, feet; while the height of Mopen seems to have been
+diminished by 4 feet. Thus, at first sight, these calculations appear
+to indicate "a general elevation and extension of the hills, such as
+might follow on a bulging upwards of the surface due to the extension
+of a large mass of molten matter underground."
+
+   [Illustration: FIG. 77.--Re-triangulation of Khasi hills.
+   (_Oldham._)]
+
+Unfortunately, as Mr. Oldham shows, a very different, and more
+probable, interpretation may be given of these results; for all the
+calculated changes are rendered uncertain by the choice of the two
+stations which form the ends of the new base-line. One at least may
+have been displaced by the structural movements within the epicentral
+area; and, moreover, the line joining them runs nearly north and
+south. As compression in this direction is to be expected, it is
+probable that this line was shortened; and the assumption that its
+length was unchanged would therefore lead to an apparent expansion of
+all the other sides.
+
+The calculated changes seem to favour this explanation to a great
+extent. The sides joining Mopen, Rangsanobo, and Thanjinath run nearly
+east and west, and are apparently lengthened by 4.9 and 3.4 feet
+respectively; while, of the four sides joining these stations to
+Mosingi and Mun, lying next to the north, two are nearly or quite
+unchanged, and the others increased by 2.3 and 3.2 feet. Again, the
+estimated increase of the Mosingi-Mun line is 4.4 feet; while the four
+sides joining these stations to the next northerly group are
+increased by small amounts--namely, 1.2, 2.6,-0.3, and 2.4 feet. Thus,
+the apparent expansion that should have occurred in these more or less
+northerly sides is lessened, or roughly compensated, probably by a
+compression of the whole region in a meridianal direction.
+
+For a similar reason, the slight general upheaval of the hills
+indicated by the repeated calculations, must be regarded as doubtful,
+for it depends on the assumed fixity of the station of Rangsanobo,
+whereas it is more probable that it was the height of Taramun Tila
+that remained unchanged. Reducing the calculated heights of all the
+other stations by six feet (the assumed rise of the latter), it
+follows that, on the whole, the height of the Khasi hills underwent
+but little change, except at Mautherrican and Landau Modo, and the
+secondary stations of Mairang and Kollong Rock, near Maonoi. The
+apparent elevations of 24, 17, 11, and 15 feet at these places exceed
+the probable error of the observations; and it is worthy of notice
+that all four stations lie close to the edge of fault-scarps, while
+Landau Modo is not far from two of the pools formed by distortion of
+the surface unaccompanied by faulting.
+
+If, then, the revised triangulation of the Khasi hills has failed to
+provide absolute measures of the displacements in the epicentral area,
+it has, nevertheless, proved that important movements, both horizontal
+and vertical, have taken place.
+
+_Distribution of the Structural Changes._--The boundary of the
+epicentral area, as drawn in Figs. 68 and 75, lays no claim to great
+accuracy; but its departure from the true line is probably in no place
+considerable. It must evidently include all the districts where
+marked structural changes occurred, and must therefore extend east of
+Maophlang and west of Tura. Towards the north, these changes have been
+traced to the foot of the Garo hills, and there is some, though not
+very certain, evidence of alterations of level along the course of the
+Brahmaputra. The very large number of after-shocks recorded at Borpeta
+and Bijni also points to an extension of the epicentral area beyond
+these places. To the east, the course of the boundary becomes
+doubtful, but it must pass close to Gauhati and east of Shillong, and
+probably ends a short distance beyond Jaintiapur. The southern
+boundary must coincide nearly with the north edge of the alluvial
+plains of Sylhet, for there is no evidence of its intrusion into the
+plains. On the west side, the epicentral area includes the Garo hills
+and part of the alluvial plain to the west; and, from the large number
+of after-shocks felt at Rangpur and Kaunia, and the great violence of
+the shock at the former, we may infer that both places lie within the
+boundary-line. If, then, there is no great error in the mapping of
+this line, it follows that the epicentre was about 200 miles long from
+east to west, not less than 50, and possibly as much as 100, miles in
+maximum width, and contained an area of at least 6000 square miles.
+
+Near the boundary, the permanent displacements must have been
+comparatively small; but they were certainly marked in the northern
+part of the Assam hills for a distance of 100 miles from east to west.
+At the limits of the latter area, as Mr. Oldham remarks, "the evidence
+points to the changes being of the nature of long, low rolls, the
+change of slope being insufficient to cause any appreciable change in
+the drainage channels. Then comes a zone in which the surface changes
+are more abrupt, the slopes of the stream beds have been altered so as
+to cause conspicuous changes in the nature of the streams, but any
+fracture or faulting which may have taken place has died out before
+the surface was reached. And north of this, close to the edge of the
+hills, the rocks have been fractured and faulted right up to the
+surface."
+
+
+ORIGIN OF THE EARTHQUAKE.
+
+Almost every feature of the great earthquake points to an origin very
+different from that of the others described in this volume. The
+suddenness with which the shock began, its unusual duration, and the
+occurrence of many maxima of intensity, are inconsistent with a simple
+fault-displacement. Again, the excessive velocities of projection at
+Rambrai and elsewhere, the existence of isolated fault-scarps and
+fractures, the local changes of level, the compression indicated by
+the revised trigonometrical survey, the wide area over which these
+structural changes took place, and the numerous distinct centres of
+subsequent activity, all these phenomena demonstrate the intense and
+complex character of the initial disturbances, as well as the
+widespread bodily displacement of the earth's crust within the
+epicentral area. There may, it is conceivable, have been a number of
+foci, nearly or quite detached from one another, and giving rise to a
+group of nearly concurrent shocks. Or--and this is a far more probable
+supposition--there may have been one vast deep-seated centre, from
+which off-shoots ran up towards the surface, each partaking to a
+greater or less degree in the movement within the parent focus.
+
+As Mr. Oldham points out, we have recently become acquainted with a
+structure exactly corresponding to that which is here inferred. The
+great thrust-planes, so typically developed in the Scottish Highlands,
+are only reversed faults which are nearly horizontal instead of being
+highly inclined; and they are accompanied by a number of ordinary
+reversed faults running upwards to the surface. In Fig. 78, the main
+features of a section drawn by the Geological Survey of Scotland are
+reproduced; T, T, representing thrust planes, and _t_, _t_, minor
+thrusts or faults. A great movement along one of the main
+thrust-planes would carry with it dependent slips along many of the
+secondary planes. Direct effects of the former might be invisible at
+the surface, except in the horizontal displacements that would be
+rendered manifest by a renewed trigonometrical survey; whereas the
+latter might or might not reach the surface, giving rise in the one
+case to fissures and fault-scarps, in the other to local changes of
+level, and in both to regions of instability resulting in numerous
+after-shocks.
+
+   [Illustration: FIG. 78.--Diagram of Thrust-planes.]
+
+The enormous dimensions of the parent focus will be obvious from the
+phenomena that have been described above. Mr. Oldham has traced the
+probable form of the epicentre. It may in reality be neither so
+simple nor so symmetrical as is represented in Fig. 75, but there are
+good reasons for thinking that it does not differ sensibly either in
+size or form from that laid down. The part of the thrust-plane over
+which movement took place must therefore have been about 200 miles
+long, not less than 50 miles wide, and between 6000 and 7000 square
+miles in area. With regard to its depth, we have no decisive
+knowledge. It may have been about five miles or less; it can hardly
+have been much greater.
+
+It is a strain on the imagination to try and picture the displacement
+of so huge a mass. We may think, if we will, of a slice of rock three
+or four miles in thickness and large enough to reach from Dover to
+Exeter in one direction and from London to Brighton in the other; not
+slipping intermittently in different places, but giving way almost
+instantaneously throughout its whole extent; crushing all before it,
+both solid rock and earthy ground alike; and, whether by the sudden
+spring of the entire mass or by the jar of its hurtling fragments,
+shattering the strongest work of human hands as easily as the
+frailest. Such a thrust might well be sensible over half a continent,
+and give rise to undulations which, unseen and unfelt, might wend
+their way around the globe.
+
+
+REFERENCES.
+
+  1. AGAMENNONE, G.--"Notizie sui terremoti osservati in Italia
+        durante l'anno 1897 (Terremoto dell' India poco dopo il
+        mezzogiorno del 12 giugno)." _Ital. Sismol. Soc. Boll._, vol.
+        iii., pte. ii., 1897, pp. 249-293.
+
+  2. ---- "Il terremoto dell' India del 12 giugno 1897." _Ibid._, vol.
+        iv., 1898, pp. 33-40.
+
+  3. ---- "Eco in Europa del terremoto indiano del 12 giugno 1897."
+        _Ibid._, vol. iv., 1898, pp. 41-67. (See also the same volume,
+        pp. 167-172.)
+
+  4. BARATTA, M.--"Il grande terremoto indiano del 12 giugno 1897."
+        _Ital. Soc. Geogr. Boll._, vol. x., 1897, fasc. viii.
+
+  5. CANCANI, A.--"I pendoli orizzontali del R. Osservatorio
+        geodinamico di Rocca di Papa, ed il terremoto indiano del 12
+        giugno 1897." _Ital. Sismol. Soc. Boll._, vol. iii., 1897, pp.
+        235-240.
+
+  6. HEATH, T.--"Note on the Calcutta Earthquake (June 12th, 1897) as
+        recorded by the bifilar pendulum at the Edinburgh Royal
+        Observatory." _Edinb. Roy. Soc. Proc._, 1897, pp. 481-488.
+
+  7. OLDHAM, R.D.--"Report on the Great Earthquake of 12th June 1897."
+        _Mems. Geol. Surv. of India_, vol. xxix., 1899, pp. i.-xxx.,
+        1-379, with 44 plates and 3 maps.
+
+  8. ---- "List of After-shocks of the Great Earthquake of 12th June
+        1897." _Ibid._, vol. xxx., pt. i., 1900, pp. 1-102.
+
+  9. ---- "On Tidal Periodicity in the Earthquakes of Assam." _Journ.
+        Asiat. Soc._, vol. lxxi., 1902, pp. 139-153.
+
+FOOTNOTES:
+
+[69] According to some reports, the earthquake was felt in Italy. At
+Livorno, the first movements were registered by seismographs at 11.17
+A.M. (G.M.T.), and tremors were noticed by some persons at rest at
+about 11.15 A.M. At Spinea, a sensible undulatory shock from
+south-east to north-west, and lasting about four seconds, was felt at
+the moment when all the seismographs were set in motion by the Indian
+earthquake. In spite of the great distance, the perception of the
+earthquake in Italy is not impossible, but the records seem to me to
+refer to local tremors rather than to the very slow evanescent
+oscillations of a very distant earthquake.
+
+[70] All the times in this section are referred to Madras mean time,
+which is 5h. 20m. 59.2s. in advance of Greenwich mean time. In the
+next section it will be found convenient to use the latter standard.
+
+[71] It may be useful to give references to works in English in which
+the principal instruments for registering distant earthquakes are
+described. For Cancani's vertical pendulum, see _Brit. Assoc. Rep._,
+1896, pp. 46-47; Darwin's bifilar pendulum, _Brit. Assoc. Rep._, 1893,
+pp. 291-303, and _Nature_, vol. 1., 1894, pp. 246-249; Milne's
+horizontal pendulum, _Seismology_, pp. 58-61; Rebeur-Paschwitz's
+horizontal pendulum, _Brit. Assoc. Rep._, 1893, pp. 303-308.
+
+[72] The beginnings of the second and third phases are shown more
+clearly in the record of the vertical pendulum at Catania, a record,
+however, that will not bear the reduction necessary for these pages.
+
+[73] _Geol. Mag._, vol. x., 1893, pp. 356-360.
+
+[74] _Irish Acad. Trans._, vol. xxi, 1848, p. 52.
+
+[75] _Irish Acad. Trans._, vol. xxi., 1848, pp. 55-57.
+
+[76] _Neapolitan Earthquake of 1857_, vol. i., 1862, pp. 376-378.
+
+[77] _Japan Seismol. Soc. Trans._, vol. i., pt. II., 1880, pp. 33-35.
+
+[78] _Geol. Mag._, vol. ix., 1882, pp. 257-265.
+
+
+
+
+CHAPTER X.
+
+CONCLUSION.
+
+
+In this concluding chapter, I propose to give a summary of the results
+at which we have arrived from the study of recent earthquakes, and
+this can, I think, be done best by describing what may be regarded as
+an average or typical earthquake, though it may be convenient
+occasionally to depart slightly from such a course. Few shocks have
+contributed more to our knowledge than the majority of those described
+in this volume; but, on certain points, we gain additional information
+from the investigation of other earthquakes, and these are referred to
+when necessary for the purpose in view.
+
+
+FORE-SHOCKS.
+
+At the outset, we are met by a question of some interest and great
+practical importance--namely, whether there are any constant signs of
+the coming of great earthquakes by means of which their occurrence
+might be predicted and their disastrous effects mitigated.
+
+Excluding the Ischian earthquakes, which belong to a special class, it
+is evident that there is generally some slight preparation for a great
+earthquake. For a few hours or days beforehand, weak shocks and
+tremors are felt or rumbling noises heard within the future
+meizoseismal area. But, unfortunately, it has not yet been found
+possible to distinguish these disturbances from others of apparently
+the same character which occur alone, so that for the present they
+fail to serve as warnings.
+
+In Japan, where the organisation of earthquake-studies is more
+complete than elsewhere, it is possible that a vague forecast might be
+made, if the distribution of the fore-shocks of the earthquake of 1891
+should prove to be a general feature of all great earthquakes. It was
+at first supposed that this earthquake occurred without preparation of
+any kind; but a closer analysis of the records shows that during the
+previous two years there was a very decided increase in the seismic
+activity of the district, and also that the distribution of the
+epicentres marked out the future fault-scarp, and at the same time
+exhibited a tendency to comparative uniformity over the whole
+fault-region.
+
+For the present, then, the only warning available is that given by the
+preliminary sound, which may precede the strongest vibrations by as
+much as five or ten or even more seconds. Though two or three seconds
+may elapse before its character is recognised, the fore-sound thus
+allows time for many persons to escape from their falling houses. Some
+races, however, are less capable of hearing the sound than others, and
+this may be one reason why Japanese earthquakes are so destructive of
+human life.
+
+
+DISTURBED AREA.
+
+It is usual with some investigators to measure the intensity of an
+earthquake roughly by the extent of its disturbed area. The depth of
+the seismic focus must of course have some influence on the size of
+this area, and this condition is only neglected because we have no
+precise knowledge of the depth in any case. Thus, Mr. Oldham regards
+the Indian earthquake of 1897 as rivalling the Lisbon earthquake of
+1755, which is generally considered to hold the first place, because
+its disturbed area was not certainly exceeded by that of the latter.
+
+That disturbed area is, however, an untrustworthy measure of intensity
+will be evident from the following table, in which the earthquakes
+described in this volume (omitting those of Ischia) are arranged as
+nearly as may be in order of intensity, beginning with the
+strongest:--
+
+    Earthquake.            Disturbed Area
+                           in Sq. Miles.
+
+    Indian                   1,750,000
+    Japanese                   330,000
+    Neapolitan                  39,200
+    Charleston               2,800,000
+    Riviera                    219,000
+    Andalusian                 174,000
+    Hereford                    98,000
+    Inverness                   33,000
+
+Here we see that the Charleston earthquake was perceptible over a
+greater area than the Indian earthquake, while the Neapolitan
+earthquake was inferior to that of Hereford in this respect. The
+explanation of course is that the boundaries of the disturbed areas
+are isoseismal lines corresponding to different degrees of intensity,
+the inhabitants of Great Britain and the United States being evidently
+more sensitive to weak tremors, or more observant, than those of
+Italy, Spain, or Central Asia. The only disturbed areas that are
+bounded by isoseismals of the same intensity are the two last. Very
+roughly, then, we may say that the intensity of the Hereford
+earthquake was three times as great as that of the Inverness
+earthquake.
+
+
+POSITION OF THE EPICENTRE.
+
+One of the first objects in the investigation of an earthquake is to
+determine the position and form of the epicentre. In a few rare cases,
+as in the Japanese and Indian earthquakes, when the fault-scarp is
+left protruding at the surface, only careful mapping is required to
+ascertain both data. But, in the great majority of earthquakes, the
+fault-slip dies out before reaching the surface and the position of
+the epicentre is then inferred by methods depending chiefly on the
+time of occurrence or on the direction or intensity of the shock.
+
+At first sight, methods that involve the time of occurrence at
+different places seem to be of considerable promise. No scientific
+instruments are so widely diffused as clocks and watches; but, on the
+other hand, few are so carelessly adjusted. It is the exception,
+rather than the rule, to find a time-record accurate to the nearest
+minute; and, as small errors in the time may be of consequence,
+methods depending on this element of the earthquake are seldom
+employed. If, however, the number of observations is large for the
+size of the disturbed area, the construction of coseismal lines may
+define approximately the position of the epicentre. In the Hereford
+earthquake of 1896, the centre of the innermost coseismal line (Fig.
+62) is close to the region lying between the two epicentres.
+
+The method of locating the epicentre by means of the intersection of
+two or more lines of direction of the shock was first suggested by
+Michell in 1760,[79] and has been employed by Mallet in investigating
+the Neapolitan earthquake, by Professors Taramelli and Mercalli in
+their studies of the Andalusian and Riviera earthquakes, as well as by
+other seismologists. The diversity of apparent directions at one and
+the same place caused its temporary neglect, until Professor Omori
+showed in 1894 that the mean of a large number of measurements gives a
+trustworthy result (p. 19). His interesting observations should
+reinstate the method to its former place among the more valuable
+instruments at the disposal of the seismologist.
+
+No observations, however, are at present so valuable for the purpose
+in view as those made on the intensity of the shock. For many years,
+it has been the custom to regard the epicentre as coincident with the
+area of greatest damage to buildings; and, when the area is small, the
+assumption cannot be much in error. It is of course merely a rough way
+of obtaining a result that is generally given more accurately by means
+of isoseismal lines; but there are exceptional cases, such as the
+Neapolitan and Ischian earthquakes, when the destruction wrought by
+the earthquake furnishes evidence of the greater value.
+
+A single isoseismal accurately drawn not only gives the position of
+the epicentre with some approach to exactness, but also by the
+direction of its longer axis determines that of the originating fault.
+When two or three such lines can be traced, the relative position
+supplies in addition the hade of the fault (p. 219). The successful
+application of the method requires, it is true, a large number of
+observations, and these cannot as a rule be obtained except in
+districts that are somewhat thickly and uniformly populated, such as
+those surrounding the cities of Hereford and Inverness. In the
+Charleston earthquake, also, the position and form of the epicentres
+were deduced from the trend of isoseismal lines based on the damage to
+railway-lines and various structures within a sparsely inhabited
+meizoseismal area.
+
+In a few cases, of which the Indian earthquake may be regarded as
+typical, a fourth method has recently been found of service. The
+numerous after-shocks which follow a great earthquake originate for
+the most part within the seismic focus of the latter; and, as they
+usually disturb a very small area, it is not difficult to ascertain
+approximately the positions of their epicentres. Some, as in the
+Inverness after-shocks of 1901, result from slips in the very margin
+of the principal focus; but, as a rule, the seat of their activity
+tends to contract towards a central region of the focus. Bearing in
+mind, then, that some of the succeeding shocks originate at and beyond
+the confines of the focus, and that others may be sympathetic shocks
+precipitated by the sudden change of stress, it follows that the
+shifting epicentres of the true after-shocks map out, in part at any
+rate, the epicentral area of the principal earthquake.
+
+
+DEPTH OF THE SEISMIC FOCUS.
+
+It is much to be regretted that we have no satisfactory method of
+determining so interesting an element as the depth of the seismic
+focus. That it amounts to but a few miles at the most is certain from
+the limited areas within which slight shocks are felt or disastrous
+ones exhibit their maximum effects. Nor can we suppose that the rocks
+at very great depths are capable of offering the prolonged resistance
+and sudden collapse under stress that are necessary for the production
+of an earthquake.
+
+The problem is evidently beyond our present powers of solution, and
+its interest is therefore mainly historical. All the known methods are
+vitiated by our ignorance of the refractive powers of the rocks
+traversed by the earth-waves. But, even if this ignorance could be
+replaced by knowledge, most of the methods suggested are open to
+objection. Falb's method, depending on the time-interval between the
+initial epochs of the sound and shock, is of more than doubtful value.
+Dutton's, based on the rate of change of surface-intensity, is
+difficult to apply, and in any case gives only an inferior limit to
+the depth. Time-observations have been employed, especially in New
+Zealand; but the uncertainty in selecting throughout the same phase of
+the movement, and the large errors in the estimated depth resulting
+from small errors in the time-records, are at present most serious
+objections. There remains the method devised by Mallet, and, though he
+claimed for it an exaggerated accuracy, it still, in my opinion, holds
+the field against all its successors. When carefully applied, as it
+has been by Mallet himself, by Johnston-Lavis and Mercalli, we
+probably obtain at least some conception of the depth of the seismic
+focus.
+
+Professor Omori and Mr. K. Hirata have recently[80] lessened the chief
+difficulty in the application of Mallet's method. They have deduced
+the angle of emergence from the vertical and horizontal components of
+the motion as registered by seismographs, instead of from the
+inclination of fissures in damaged walls. In two recent earthquakes
+recorded at Miyako in Japan, they find the angle of emergence to be
+7.2° and 9° respectively, the corresponding depths of the foci being
+5.6 and 9.3 miles. These are probably the most accurate estimates that
+we possess, and it will be noted that they differ little from the mean
+values obtained for the Neapolitan, Andalusian, and Riviera
+earthquakes--namely, 6.6, 7.6, and 10.8 miles.
+
+
+NATURE OF THE SHOCK
+
+In one respect, the earthquakes described above fail to represent the
+progress of modern seismology. They furnish no diagrams made by
+accurately constructed seismographs within their disturbed areas. The
+curve reproduced in Fig. 36, as already pointed out, is no exception
+to this statement. For another reason, the records that were obtained
+in Japan of the earthquake of 1891 are trustworthy for little more
+than the short-period initial vibrations; for, owing to the passage of
+the surface-waves, visible in and near the meizoseismal area, the
+Japanese seismographs registered the tilting of the ground rather than
+the elastic vibrations that traversed the earth's crust.
+
+Notwithstanding this defect, personal impressions of an
+earthquake-shock give a fairly accurate, if incomplete, idea of its
+nature. Nearly all observers placed under favourable conditions agree
+that an earthquake begins with a deep rumbling sound, accompanied,
+after the first second or two, by a faint tremor which gradually, and
+sometimes rapidly, increases in strength until it merges into the
+shock proper, which consists of several or many vibrations of larger
+amplitude and longer period, and during which the attendant sound is
+generally at its loudest; the earthquake dying away, as it began, with
+tremors and a low rumbling sound.
+
+   [Illustration: FIG. 79.--Seismographic Record of Tokio Earthquake
+   of 1894. (_Omori._)]
+
+The vibrations that produce the sensible shock are by no means all
+that are present during an earthquake. The Indian earthquake, for
+instance, seemed to last about three or four minutes at Midnapur; but
+the movements of the bubble of a level showed that the ground
+continued to oscillate for at least five minutes longer (p. 280). Many
+of these unfelt waves are rendered manifest by seismographs, although
+there are still others that elude registration either from the extreme
+shortness or the great length of their periods.
+
+In Fig. 79 is shown the principal part of a diagram obtained at Tokio
+during the Japanese earthquake of June 20th, 1894 (p. 18), the curve
+representing the N.E.-S.W. component of the horizontal motion during
+the first 25 seconds of the record. The instrument employed is one
+specially designed for registering strong earthquakes, and is
+unaffected by very minute tremors. Those which formed the commencement
+of this earthquake lasted for about 10 seconds, as shown by ordinary
+seismographs, and the vibrations had attained a range of a few
+millimetres before they affected the instrument in question. For the
+first 2-1/2 seconds, they occurred at the rate of four or five a
+second. The motion then suddenly became violent, and the ground was
+displaced 37 mm. in one direction, followed by a return movement of 73
+mm., and this again by one of 42 mm., the complete period of the
+oscillation being 1.8 seconds. The succeeding vibrations were of
+smaller amplitude and generally of shorter period for a minute and a
+half, then dying out during the last three minutes as almost
+imperceptible waves with a period of two or more seconds.[81]
+
+Though incomplete in some respects, this diagram illustrates clearly
+the division of the earthquake-motion into three stages--namely, the
+preliminary tremors, the principal portion or most active part of an
+earthquake, and the end-portion or gradually evanescent slow
+undulations. In all three stages, however, both tremors and slow
+undulations may be present; and, as the latter, owing to their long
+period, are more or less insensible to human beings, the ripples of
+the final stage give the impression of a tremulous termination as
+described above. The duration of each stage varies considerably in
+different earthquakes. Thus, in a valuable study of 27 earthquakes
+recorded at Miyako, in Japan, during the years 1896-98, Messrs. Omori
+and Hirata show[82] that the duration of the preliminary stage varies
+from 0 to 26 seconds, with an average of about 10 seconds; that of the
+principal portion from 0.7 to 26 seconds, also with an average of
+about 10 seconds; and that of the end portion from 28 and 105 seconds,
+with an average of about one minute. The total apparent duration,
+however, depends on the instrument employed; one of the earthquakes,
+that of April 23rd, 1898, disturbing the seismograph at Miyako for two
+minutes; while, at Tokio, a horizontal pendulum designed by Professor
+Omori oscillated for at least two hours. The periods of both ripples
+and slow undulations, again, vary from one earthquake to another; but
+it is worthy of notice that the average period of the undulations is
+almost constant in all three stages of the motion, being 1.1, 1.3, and
+1.3 seconds, respectively, for the east-west component of the
+horizontal motion, and 1.0 second throughout for the north-south
+component. For the ripples, the average period is .08 second in the
+preliminary stage, .10 second in the principal portion, and .08 second
+again in the end portion; those of the principal portion being
+slightly larger in amplitude, as well as longer in period, than the
+ripples of the first and third stages.
+
+
+SOUND-PHENOMENA.
+
+Besides the ripples already mentioned, there are others of still
+smaller amplitude and shorter period that are sensible, but as a rule
+only just sensible, to us as sounds. All the known evidence points to
+the extraordinary lowness of the earthquake-sound. According to some
+observers, it seems as if close to their lower limit of audibility;
+while others, however intently they may listen, are unable to hear the
+slightest noise. In other words, the most rapid vibrations present in
+an earthquake do not recur at a rate of much more than about 30 to 50
+per second; or, if they do, they are not strong enough to impress the
+human ear.
+
+To most observers, the sound seems to increase and decrease in
+intensity with the shock, and so gradually and smoothly does this
+change take place that the sound is frequently mistaken for that of an
+underground train approaching the observer's house, passing beneath
+it, and receding in the opposite direction. Some persons, especially
+if situated within the meizoseismal area, hear also loud crashes in
+the midst of the rumbling sound and simultaneously with the strongest
+vibrations. At a moderate distance, say from 30 to 40 miles, the sound
+becomes more harsh and grating while the shock is felt; and, at a
+greater distance, even this change disappears, and nothing is heard
+but an almost monotonous sound like the low roll of distant thunder.
+The explanation of this is that the sound-vibrations are of different
+periods and varying amplitude, and the limiting vibrations tend to
+become inaudible with increasing distance, the lower on account of
+their long period, the higher owing to their small amplitude.
+
+The magnitude of the sound-area depends, even more than that of the
+disturbed area, on the personal equation of the observers. The lower
+limit of audibility varies not only in different individuals, but also
+in different races. In Great Britain, it is doubtful whether an
+earthquake ever occurs unaccompanied by sound; and in the meizoseismal
+area the noise is heard by nearly all observers. With Italians, the
+average lower limit of audibility is higher than with the Anglo-Saxon
+race; slight shocks frequently occur without noticeable sound, but
+with strong ones, the larger number of observers is sure to include
+one or more capable of hearing the rumbling noise. The Japanese are,
+however, seldom affected by the most rapid earthquake-vibrations, and
+the strongest shocks may be unattended by any recorded sound. The
+result is manifest in the size of the sound-area in different
+countries. In the Hereford earthquake, the sound-area contained 70,000
+square miles; in the Neapolitan earthquake, about 3,300 square miles;
+while, in Japanese earthquakes, the sound is rarely heard more than a
+few miles from the epicentre.
+
+Another effect of this personal equation of the observers is that the
+sound-vibrations apparently outrace those of longer period. The
+Italians, for instance, generally hear the sound that precedes the
+shock, and more rarely the weaker sound that follows it. In Japan,
+only the earlier sound-vibrations, if any, seem to be audible. In
+Great Britain, on the contrary, the fore-sound is perceptible to four,
+and the after-sound to three, out of every five observers; and these
+proportions are maintained roughly to considerable distances from the
+epicentre. It follows, therefore, that the sound-vibrations and those
+which constitute the shock must travel with nearly, if not quite, the
+same velocity; and that the greater duration of the sound is due
+either to the prolongation of the initial movement or to the
+overlapping of the principal focus by the sound-focus. Neither
+alternative can be regarded as improbable, but observations made on
+British earthquakes point to the latter explanation as the true one.
+
+It will be sufficient to refer to two phenomena in support of this
+statement. In the first place, the percentage of observers who hear
+the fore-sound varies with the direction from the epicentre. Thus,
+during the Inverness earthquake of 1901, the majority of observers in
+Aberdeenshire regarded the sound as beginning and ending with the
+shock; while, in counties lying more nearly along the course of the
+great fault, the sound was generally heard both before and after the
+shock (p. 253). In this case, then, the initial and concluding sound
+vibrations must have come chiefly from the margins of the seismic
+focus; and those from the margin nearest to an observer would be more
+sensible than those from the farther margin. Again, in slight
+earthquakes, such as the Cornwall earthquake of April 1, 1898,[83] the
+curves of equal sound intensity, while their axes are parallel to
+those of the isoseismal lines, are displaced laterally with respect to
+these curves, owing to the arrival of the strongest sound-vibrations
+from the upper margin of an inclined seismic focus.
+
+When a fault-slip occurs, the displacement is obviously greatest in
+the central region, and dies out gradually towards the margins of the
+focus. The phenomena described above show that the evanescent
+displacement within these margins generate sound-vibrations only; and
+that the greater slip within the central region produces also the more
+important vibrations that compose the shock. As the former are
+perceptible over a limited district, while the latter may be felt
+through half a continent, it is clear that the sound-area should bear
+no fixed relation in point of size to the disturbed area, but should
+be comparatively greater for a slight shock than for a strong one.
+
+
+VELOCITY OF THE EARTH-WAVES.
+
+If we consider only the earthquakes here described, we see at once how
+great is the diversity in the estimated velocity of the earth-waves.
+On the one hand, we have a value as high as 5.2 kms. per sec. for the
+Charleston earthquake, and, at the other end of the scale, a value of
+0.9 km. per sec. for the Hereford earthquake. Between them, and
+equally trustworthy, lie the estimates of 3.0 km. per sec. for the
+Indian earthquake, and 2.1 kms. per sec. for the Japanese earthquake
+and its immediate successors.
+
+It is difficult to account entirely for such discordance. Errors of
+observation may be responsible for a small part of the differences.
+The initial strength of the disturbance appears to have some effect,
+and the nature of the rocks traversed must be a factor of consequence
+when the distances in question are not very great. In the Japanese and
+Hereford earthquakes, all three may have combined to produce the
+divergent results, the distance in these cases being only 275 and 142
+kms. respectively.
+
+In the Indian and Charleston earthquakes, the distances are much
+greater (1944 and 1487 kms.), and the variety of rocks traversed must
+tend to give a truer average. In the former, the result obtained (3.0
+kms. per sec.) agrees so closely with the velocity of the long-period
+undulations of distant earthquakes as to suggest that it was these
+waves that were timed at the stations west of Calcutta and disturbed
+the magnetographs at Bombay.[84]
+
+Omitting, then, the Indian estimate, we find that, for the Japanese
+and Charleston earthquakes, the velocity increases with the distance
+as measured along the surface. To a certain extent, such a result
+might have been expected, had we assumed the earthquake-waves to
+travel along the chords joining the focus to very distant places of
+observation.
+
+The wave-paths that penetrate the earth are straight lines, however,
+only when the conditions that determine the velocity are uniform
+throughout, and such uniformity we have no reason to expect. From what
+we know of the earth's interior, there can, indeed, be little doubt
+that the velocity of earthquake-waves increases with the depth below
+the surface, and that the wave-paths in consequence are curved lines
+with their convexity downwards. It would be out of place to state more
+than the principal result of the recent investigations by Dr. A.
+Schmidt[85] and Prof. P. Rudzki[86] on this subject. These are based on
+the assumptions that the velocity increases with the depth below the
+surface, and that it is always the same at the same depth. From the
+focus of the earthquake, wave-paths diverge in all directions. Those
+which start horizontally curve upwards, and intersect the surface of
+the earth in a circle dividing the whole surface into two areas of very
+unequal size. Within the small area, the surface-velocity is infinite
+at the epicentre, and decreases outwards until it is least on the
+boundary-circle. In the larger region beyond, the surface-velocity
+increases with the distance from the epicentre, until, at the antipodes
+of that point, it is again infinite. But, as the depth of the focus is
+always slight compared with the radius of the earth, the small circular
+area surrounding the epicentre is practically negligible, and we may
+regard the surface-velocity of the waves that traverse the body of the
+earth as a quantity that continually increases with the distance from
+the epicentre.
+
+How fully this interesting theoretical result has been confirmed is
+well shown in Mr. Oldham's recent and very valuable investigation on
+the propagation of earthquake-motion to great distances.[87] A study
+of the records of the Indian earthquake revealed the existence of
+three series of waves, the first two consisting in all probability of
+longitudinal and transversal waves travelling through the body of the
+earth, and the third of undulations spreading over its surface (pp.
+282-285). Extending his inquiries to ten other earthquakes originating
+in six different centres, Mr. Oldham distinguishes the same three
+phases in their movements; the third phase being the most constantly
+recorded, the second less so, while the first phase is the most
+frequently absent. With the exception of a few very divergent records,
+the initial times of these phases and the maximum epoch of the third
+phase are plotted on the accompanying diagram (Fig. 80), in which
+distances from the epicentre in degrees of arc are represented along
+the horizontal line and the time-interval in minutes along the
+perpendicular line. The dots near the two lower curves refer to the
+records of the heavily weighted Italian instruments, and the crosses
+to those of the light horizontal pendulums, which respond somewhat
+irregularly to the motion of the first two phases (p. 282). In the
+third phase, there is less divergence between the indications of the
+two classes of instruments, and dots are used in each case for the
+initial, and crosses for the maximum epoch.
+
+   [Illustration: FIG. 80.--Time-curves of principal epochs of
+   earthquake-waves of distant origin. (_Oldham._)]
+
+Of the smoothed curves drawn between these series of points, those
+marked A, B, and C represent the time-curves of the beginnings of the
+first, second, and third phases respectively, while D is the
+time-curve for the maximum of the third phase.
+
+The concavity of the two lower lines towards the horizontal base-line
+shows that the surface-velocity of the corresponding waves increases
+rapidly with the distance, far more so than would be possible with
+rectilinear motion. The rates at which these waves travel through the
+earth therefore increase with the depth, and the wave-paths must in
+consequence be curved lines convex towards the centre of the earth.
+
+If the time-curves A and B were continued backwards to the origin,
+their inclinations at that point to the horizontal line give the
+initial velocities of the corresponding waves, which prove to be about
+5 and 3 kms. per sec. respectively. Now, according to recent
+experiments made by Mr. H. Nagaoka on the elastic constants of
+rocks,[88] the mean velocity of seven archaean rocks is 5.1 kms. per
+sec. for the longitudinal waves, and 2.8 kms. per sec. for the
+transversal waves--values which agree so closely with those obtained
+for the first two series of earthquake-waves as to leave little doubt
+with regard to their character.
+
+The other time-curves, C and D, corresponding to the initial and
+maximum epochs of the third phase, are practically straight lines.
+Some of the records are slightly discordant for the average curve,
+especially for the initial epoch; but it is often difficult to define
+the commencement of this phase with precision. At any rate, the
+observations show no distinct sign of an increase in the
+surface-velocity of these waves with the distance from the origin. It
+may therefore be concluded that they travel along the surface with
+velocities which are practically constant for each individual
+earthquake, the largest waves at the rate of about 2.9 kms. per sec.,
+and the advance waves with a velocity of about 3.3 kms. per sec.,
+rising occasionally to over 4.0 kms. per sec.
+
+
+STRUCTURAL CHANGES IN THE EPICENTRAL AREA.
+
+Changes of elevation have long been known as accompaniments of great
+earthquakes, though many of the earlier observations and measurements
+left much to be desired in accuracy and completeness. The Japanese
+earthquake of 1891, however, placed the reality of such movements
+beyond doubt, and revealed the existence of a fault-scarp, with a
+height in one place of 18 or 20 feet, and a length of at least 40, if
+not of 70, miles. In the Indian earthquake of 1897, the fault-scarps
+were shorter, though more pronounced in character, the largest known
+(the Chedrang fault) being about 12 miles long, and having a maximum
+throw at the surface of 35 feet. In some other recent earthquakes,
+also, remarkable fault-scarps have been developed. After the great
+shocks felt in Eastern Greece on April 20th and 27th, 1894, a fissure
+was traced for a distance of about 34 miles, running in an
+east-south-east and west-north-west direction through the epicentral
+district, and varying in width from an inch or two to more than three
+yards. That it was a fault, and not an ordinary fissure, was evident
+from its great length, its uniform direction, and its independence of
+geological structure. The throw was generally small, in no place
+exceeding five feet.[89] Again, in British Baluchistan, after the
+severe earthquake of December 20th, 1892, a fresh crack was observed
+in the ground running for several miles in a straight line parallel to
+the axis of the Khojak range. It coincided almost exactly with a line
+of springs, and was clearly produced by a fresh slip along an old line
+of fault, for before the earthquake it had the appearance of an old
+road, and the natives assert that the ground has always cracked along
+this line with every severe shock. In 1892, the change in relative
+height of the two sides of the fault was small, in one place where it
+was measured being only two inches.[90]
+
+But other changes, besides those in a vertical direction, occasionally
+take place; though, owing to their recent discovery, comparatively few
+examples are as yet known. While the throw of the Japanese fault
+varied greatly in amount, and once even in direction, there was also a
+constant shift towards the northwest of the ground on the north-east
+side of the fault, the displacement at one spot being as much as 13
+feet. In the fault-scarp formed in 1894 in Eastern Greece, a similar
+shift took place, though to what extent is unknown. There is,
+moreover, evidence of actual compression of the earth's crust at right
+angles to the fault-line. The Neo valley, traversed by the Japanese
+fault, was apparently narrower after the earthquake than it was
+before, and plots of ground were reduced from 48 to 30 feet in
+length--_i.e._, by nearly 40 per cent. In British Baluchistan, the
+formation of the fissure referred to above was accompanied both by
+compression perpendicular, and by shifting parallel, to the fault. The
+actual displacement in each direction is unknown, but the resultant
+was not less than 27 inches.
+
+There can be no doubt that a fault-scarp is formed in the first place
+with great rapidity. So abrupt, indeed, were the structural
+displacements in the epicentral area of the Indian earthquake, that
+they contributed very materially to the intensity of the shock, giving
+rise to the excessive velocities observed at Rambrai and elsewhere (p.
+273). The growth of the scarp does not, however, always cease with the
+first great earthquake, though it may take place in a contrary sense,
+as in the elevation connected with the Conception earthquake of 1835.
+The principal shock, according to Darwin, was followed during the few
+succeeding days "by some hundred minor ones (though of no
+inconsiderable violence), which seemed to come from the same quarter
+from which the first had proceeded; whilst, on the other hand, the
+level of the ground was certainly not raised by them; but, on the
+contrary, after an interval of some weeks, it stood rather lower than
+it did immediately after the great convulsion."[91]
+
+
+AFTER-SHOCKS.
+
+A series of after-shocks, more or less long, is a constant attendant
+on every great tectonic earthquake, and few are the earthquakes of
+any degree of strength that can be regarded as completely isolated.
+Even in those which visit this country, after-shocks are seldom
+absent. For instance, confining ourselves to the last few years, the
+Pembroke earthquake of 1892 was followed by 8 shocks, the Inverness
+earthquake of 1890 by at least 10, and possibly by 19 shocks, and that
+of the same district in 1901 by 15 well-defined after-shocks in
+addition to many others recorded by one observer. Of 300 Italian
+earthquakes strong enough to cause some damage to buildings, Dr.
+Cancani finds that every one was either preceded or followed, and
+chiefly followed, by its own train of minor shocks.
+
+For some hours, and even for days, after a great earthquake, the
+shocks are so numerous that it is often impossible to keep count of
+them. Many local centres spring into activity in different parts of
+the epicentral area; and, though only the strongest shocks can be
+identified elsewhere, it is clear that as a rule the shocks felt at
+any one station are quite distinct from those observed at another.
+
+The enormous number of after-shocks that follow some earthquakes can
+only be realised when they are subjected to continuous seismographic
+registration; and, even then, countless earth-sounds and the slightest
+tremors must escape detection. The shocks may, indeed, succeed one
+another so rapidly that one begins before another ends, and the result
+is an almost incessant tremulous motion rendered manifest by the
+quivering of water-surfaces or the swinging of chandeliers. Of the
+total number of after-shocks, we may form some idea from recent
+records in Japan. After the Mino-Owari earthquake of 1891, 3,365
+shocks were recorded within little more than two years at Gifu, and
+1,298 at Nagoya, but neither of these figures includes the shocks felt
+within the first few hours. Of the Kumamoto earthquake of July 28th,
+1889, the after-shocks recorded at Kumamoto until the end of 1893
+amount to 922; and those of the Kagoshima earthquake of September 7th,
+1893, recorded at Chiran until the end of January 1894, to 480. During
+the first 30 days, the numbers recorded were 1,746 at Gifu, 340 at
+Kumamoto, and 278 at Chiran; showing, as Professor Omori remarks, that
+the after-shocks diminish in frequency with the size of the disturbed
+areas,[92]--_i.e._, roughly with the initial intensity of the shocks.
+
+Next to absolute number, the rapid decline in general frequency is the
+most marked characteristic of after-shocks. Professor Omori has shown
+that, excluding minor oscillations, it follows the law represented
+geographically by the curves in Fig. 51, and algebraically by the
+equation y = k / (h + x), where _y_ is the frequency at time _x_ and
+_h_ and _k_ are constants for one and the same earthquake. By means of
+this formula, it is possible to estimate roughly the interval of time
+that must elapse before the seismic activity of the central district
+resumes its normal value. For the Mino-Owari earthquake, this proves
+to be about forty years, for the Kumamoto earthquake about seven or
+eight years, and for the Kagoshima earthquake about three or four
+years.
+
+In a recent memoir on Italian after-shocks,[93] Dr. Cancani has urged
+that other factors besides initial intensity determine the duration of
+a seismic period, and prominently among these he places the depth of
+the seismic focus. When the depth is very small, the duration of the
+period is short, not much more than ten days; when the depth is
+moderate, the duration may extend to three months; and, when great, it
+may amount to several years.
+
+The principal law that governs the distribution of after-shocks in
+time may be regarded as well-established. It is otherwise with regard
+to their distribution in space. This has been examined only in the
+cases of the Japanese earthquake of 1891 and the Inverness earthquake
+of 1901. So far as we can judge from the evidence which they furnish,
+after-shocks appear to be most numerous within and near the central
+portion of the seismic focus; though the area of maximum activity is
+subject to continual oscillation. In this region, also, there is
+evidence of a gradual decrease in the depths of the after-shock foci;
+while, near the extremities of the epicentral area, there occur
+districts of slightly greater frequency than elsewhere. With the lapse
+of time, there seems therefore to be a constant extension, both
+upwards and longitudinally, of the area over which the principal
+fault-slip took place.
+
+
+ORIGIN OF EARTHQUAKES.
+
+In the introductory chapter, a brief sketch is given of the different
+causes to which earthquakes are assigned. With those due to rock-falls
+in subterranean channels, we need have little to do. The shocks are
+invariably slight, and the part they play in the shaping of the
+earth's crust is insignificant. Volcanic earthquakes possess a higher
+degree of interest. They represent, no doubt, incipient or
+unsuccessful attempts to produce an eruption. They may be the
+forerunners of a great catastrophe.
+
+Of far higher importance in the history of our globe is the third
+class of earthquakes, including all those connected with the manifold
+changes which the crust has undergone. In the slow annealing process,
+to which it has been subjected from the earliest times, the crust has
+been crumpled and fractured, elevated into the loftiest mountain
+ranges or depressed below the level of the sea. Every sudden yielding
+under stress is the cause of an earthquake. It is chiefly, perhaps
+almost entirely, in the formation of faults that this yielding is
+manifested. The initial fracturing may be the cause of one or many
+shocks, but infinitely the larger number must be referred to the slow
+growth of the fault, the intermittent slips, now in one part, now in
+another, which, after the lapse of ages, culminate in a great
+displacement. Of the length of time occupied in the formation of a
+single fault, we can make no estimate in years. The anticlinal fault
+of Charnwood Forest dates from a pre-carboniferous period. In 1893 it
+had not ceased to grow.[94]
+
+Still less can we conceive, however faintly, the number of elemental
+slips that constitute the history of a single fault. We may think, if
+we please, of the 143 tremors and earth-sounds noted at Comrie in
+Perthshire during the last three months of 1839, of the 306
+earthquakes felt in the Island of Zante during the year 1896, or the
+1,746 shocks recorded at Gifu during thirty days in 1891; but we shall
+be as far as ever from realising the vast number of steps involved in
+the growth of a fault, let alone a mountain-chain.
+
+Yet, all over the land-surface of the globe, the crust is intersected
+by numberless faults, and hardly any portion is there in which some or
+many of these faults are not growing. One country, indeed, such as
+Great Britain, may have reached a condition of comparative stagnancy;
+the fault-slips are few and slight, and earthquakes in consequence are
+rare and generally inconspicuous. In another, like Eastern Japan and
+the adjoining ocean-bed, the movements are frequent, occasionally
+almost incessant, and few years pass without some great convulsion by
+which cities are wrecked and hundreds of human lives are lost. At such
+times, we magnify the rôle of earthquakes, and are in some danger of
+forgetting that, in the formation of a mountain-chain or continent,
+they serve no higher purpose than the creaking of a wheel in the
+complex movements of a great machine.
+
+FOOTNOTES:
+
+[79] _Phil. Trans._, vol. li., pt. ii., 1761, pp. 625-626.
+
+[80] _Journ. Sci. Coll. Imp. Univ._, Tokyo, vol. xi., 1899, pp.
+194-195.
+
+[81] _Journ. Coll. Sci. Imp. Univ._, Tokyo, vol. vii., pt. v., 1894,
+pp. 1-4; _Ital. Sismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.
+
+[82] _Journ. Coll. Sci. Imp. Univ._, Tokyo, vol. xi., 1899, pp.
+161-195.
+
+[83] _Quart. Journ. Geol. Soc._, vol. lvi., 1900, pp. 1-7.
+
+[84] There is no reason why the surface-undulations of the Indian
+earthquake should not have produced a sensible shock even as far as
+Italy. Taking their amplitude in that country at 508 mm. and their
+period at 22 sec. (p. 283), the maximum acceleration would be about 40
+mm. per sec., corresponding to the intensity 2 of the Rossi-Forel
+scale. (_Amer. Journ. Sci._, vol. xxxv., 1888, p. 429.)
+
+[85] _Nature_, vol. lii., 1895, pp. 631-633.
+
+[86] Gerland's _Beiträge zur Geophysik_, vol. iii., pp. 485-518.
+
+[87] _Phil. Trans._, 1900A, pp. 135-174.
+
+[88] _Publ. of Earthq. Inves. Com. in For. Langs._ (Tokyo), No. 4,
+1900, pp. 47-67.
+
+[89] S.A. Papavasiliou, Paris, _Acad. Sci., Compt. Rend._, vol. cxix.,
+1894, pp. 112-114, 380-381.
+
+[90] _Geol. Mag._, vol. x., 1893, pp. 356-360.
+
+[91] _Geol. Soc. Trans._, vol. v., 1840, pp. 618-619.
+
+[92] The disturbed areas of these earthquakes contained, respectively,
+221,000, 39,000, and 30,000 square miles.
+
+[93] _Boll. Sismol. Soc. Ital._, vol. viii., 1902, pp. 17-48.
+
+[94] _Roy. Soc. Proc._, vol. lvii., 1895, pp. 87-95.
+
+
+
+
+INDEX.
+
+
+Acceleration, maximum, of wave-motion in Japanese earthquake, 184, 185;
+  in Indian earthquake, 272
+
+After-shocks, definition, 4;
+  frequency, 198, 256, 296, 344;
+  distribution in space, 200, 203, 298, 326, 345;
+  sound-phenomena, 207, 300;
+  connection with fault-scarps, 300;
+  outlining of epicentre by, 326;
+  origin of, 257;
+  of Neapolitan earthquake, 40;
+  of Ischian earthquakes, 56, 65;
+  of Andalusian earthquake, 97;
+  of Charleston earthquake, 133;
+  of Riviera earthquake, 167;
+  of Japanese earthquake, 198;
+  of Hereford earthquake, 240;
+  of Inverness earthquake, 256;
+  of Indian earthquake, 296;
+  of British earthquakes, 343;
+  of Italian earthquakes, 343;
+  of Japanese earthquakes, 344
+
+Agamennone, G., 93, 94, 101, 319
+
+Alluvium, displacement of, by Indian earthquake, 287
+
+Amplitude of wave-motion, definition, 4;
+  in Neapolitan earthquake, 34;
+  in Japanese earthquake, 185;
+  in Indian earthquake, 270
+
+Andalusian earthquake, preparation for, 75;
+  investigation of, 76;
+  damage caused by, 77;
+  isoseismal lines and disturbed area, 78;
+  the unfelt earthquake, 82;
+  position of epicentre, 84;
+  depth of focus, 85;
+  nature of shock, 87;
+  sound-phenomena, 91;
+  velocity of earth-waves, 92;
+  connection between geological structure and intensity of shock, 95;
+  fissures, 96;
+  landslips, 97;
+  effect on underground water, 97;
+  after-shocks, 97;
+  origin of, 99;
+  bibliography, 101
+
+Animals, effects of earthquakes on, 143
+
+
+Baluchistan earthquake of 1892, 288, 341
+
+Baldacci, L., 70, 73
+
+Baratta, M., 320
+
+Barrois, C., 76
+
+Bergeron, C., 76
+
+Bertelli, T., 175
+
+Bertrand, M., 76
+
+Birds, effects of earthquakes on, 143
+
+Bordwar, crust-fracture at, 309
+
+Bréon, R., 76
+
+Burton, W.K., 214
+
+
+Cancani, A., 281, 282, 320, 343, 345
+
+Castro, M.F. de, 76, 101
+
+Charleston earthquake, investigation of, 102;
+  damage caused by, 103;
+  isoseismal lines and disturbed area, 104;
+  preparation for, 107;
+  nature of shock, 108;
+  double epicentre, 111;
+  origin of double shock, 120;
+  depth of foci, 122;
+  velocity of earth-waves, 126;
+  fissures, 130;
+  sand-craters, 130;
+  effects on human beings, 131;
+  feeling of nausea, 132;
+  after-shocks, 133;
+  origin of, 134;
+  bibliography, 137
+
+Charlon, E., 175
+
+Chedrang, fault-scarp at, 304
+
+Clocks, untrustworthiness of time-records of stopped, 39, 94, 121, 127
+
+Conder, J., 177, 213
+
+Coseismal lines, 227, 324
+
+Covelli, N., 67, 69
+
+
+Damage caused by Neapolitan earthquake, 10, 24;
+  by Ischian earthquakes, 50, 56;
+  by Andalusian earthquake, 77;
+  by Charleston earthquake, 103;
+  by Riviera earthquake, 139;
+  by Japanese earthquake, 181;
+  by Hereford earthquake, 217;
+  by Inverness earthquake, 247
+
+Darwin, H., 281
+
+Daubrée, A., 73
+
+Davison, C, 202-206, 208, 210, 213, 215-261, 295
+
+Death-rate of Neapolitan earthquake, 24;
+  of Ischian earthquakes, 50, 56;
+  of Andalusian earthquake, 77;
+  of Charleston earthquake, 104;
+  of Riviera earthquake, 140;
+  of Japanese earthquake, 182
+
+Denza, F., 155, 175
+
+Depth of seismic focus, methods of determining, 25, 86, 122, 326;
+  of Neapolitan earthquake, 28;
+  of Ischian earthquakes, 54, 61;
+  of Andalusian earthquake, 86;
+  of Charleston earthquake, 122, 125;
+  of Riviera earthquake, 150;
+  of Japanese earthquakes, 328
+
+Derby earthquake of 1903, 236
+
+Direction of shock, 22, 33, 186, 225, 325
+
+Disturbed area, definition of, 3;
+  of Neapolitan earthquake, 10;
+  of Ischian earthquakes, 51, 58;
+  of Andalusian earthquake, 80;
+  of Charleston earthquake, 107;
+  of Riviera earthquake, 145;
+  of Japanese earthquake, 183;
+  of Hereford earthquake, 219;
+  of Inverness earthquake, 249;
+  of Indian earthquake, 265;
+  connection between intensity of shock and, 323
+
+Dolomieu, 11
+
+Du Bois, F., 73
+
+Dutton, C.E., 103-137
+
+Dutton's method of determining depth of seismic focus, 122, 327
+
+
+Earthquake-motion, nature of, 280, 282, 328, 330, 337;
+  propagation of, to great distances, 337
+
+Earth-sound, definition of, 4
+
+Edinburgh, record of Indian earthquake at, 281, 283, 285
+
+Ellis, W., 83
+
+Emergence, angle of, 13
+
+Epicentre, definition of, 3;
+  methods of determining position of, 14, 52, 60, 324;
+  of Neapolitan earthquake, 22, 23;
+  of Ischian earthquakes, 53, 60, 67;
+  of Andalusian earthquake, 84;
+  of Charleston earthquake, 111;
+  of Riviera earthquake, 146;
+  of Hereford earthquake, 224;
+  of Inverness earthquake, 248;
+  of Indian earthquakes, 264, 276, 302
+
+Epomeo, 45, 61, 71
+
+
+Falb's method of determining depth of seismic focus, 86, 327
+
+Fallen pillars, evidence of, 17, 19
+
+Fault, originating, of Hereford earthquake, 219;
+  of Inverness earthquake, 249
+
+Fault-scarp of Japanese earthquake, 189;
+  general appearance, 189;
+  length, 192;
+  throw, 193;
+  horizontal shift, 193;
+  course, 193;
+  swamp formed by it, 194
+
+Fault-scarps of Indian earthquakes, 273, 304;
+  Chedrang fault, 304;
+  Samin fault, 308;
+  of Greek earthquake of 1894, 340, 341;
+  of Baluchistan earthquake of 1893, 341, 342;
+  formation and growth of, 342
+
+Fault-slips, tectonic earthquakes due to, 5, 43, 100, 135, 174, 211,
+  219, 224, 241, 249, 255, 317, 346
+
+Fishes, destruction of, by Riviera earthquake, 162
+
+Fissures, caused by Andalusian earthquake, 96;
+  by Charleston earthquake, 130;
+  by Inverness earthquake, 247;
+  by Indian earthquake, 285
+
+Focus, seismic, definition of, 3
+
+Focus, seismic, depth of, methods of determining, 25, 86, 122, 326;
+  of Neapolitan earthquake, 28;
+  of Ischian earthquakes, 54;
+  of Andalusian earthquake, 86;
+  of Charleston earthquake, 122, 125;
+  of Riviera earthquake, 150;
+  of Japanese earthquakes, 328
+
+Focus, dimensions of seismic, of Hereford earthquake, 224;
+  of Inverness earthquake, 250
+
+Fore-shocks, 321;
+  of Neapolitan earthquake, 40;
+  of Ischian earthquake, 57;
+  of Andalusian earthquake, 76;
+  of Charleston earthquake, 107;
+  of Riviera earthquake, 142;
+  of Japanese earthquake, 201;
+  of Hereford earthquake, 239;
+  of Inverness earthquake, 246
+
+Fouqué, F., 76, 84, 101
+
+Fracture, crust-, at Bordwar, 309
+
+Fractures in buildings, evidence of, 14, 15, 26
+
+Fuchs, C.W.C., 102
+
+
+Galli, I., 82
+
+Geological structure and intensity of shock, connection between, 95,
+  106, 113, 115, 135, 164, 265
+
+Gifu, records of Japanese after-shocks at, 183, 197
+
+Gray, T., 295
+
+Great Glen fault and Inverness earthquakes, connection between, 245
+
+Greek earthquake of 1894, fault-scarp of, 340
+
+
+Hayden, E., 103
+
+Heath, T., 283, 320
+
+Hereford earthquake, investigation of, 215;
+  preparation for, 215, 238;
+  isoseismal lines and disturbed area of, 216;
+  damage caused by, 217, 294;
+  position of originating fault, 219;
+  nature of shock, 220;
+  origin of double series of vibrations, 223;
+  position and dimensions of the two foci, 224;
+  direction of the shock, 225;
+  coseismal lines and velocity of earth-waves, 227;
+  sound-phenomena, 229;
+  isacoustic lines and sound-area, 234;
+  fore-shocks, 238;
+  after-shocks, 240;
+  origin of earthquake, 240;
+  bibliography, 261
+
+Hills, changes in aspect of, after Indian earthquake, 311
+
+Hirata, K., 327, 331
+
+Human beings, effects of Charleston earthquake on, 131
+
+Hypocentre, 3
+
+
+Iberian peninsula, earthquakes of, 75
+
+Indian earthquake, investigation of, 262;
+  isoseismal lines and disturbed area, 264;
+  nature of shock, 266;
+  visible earth-waves, 268;
+  elements of wave-motion, 270;
+  sound-phenomena, 274;
+  velocity of earth-waves, 275;
+  the unfelt earthquake, 280;
+  earth-fissures, 285;
+  displacements of alluvium, 287;
+  sand-vents, 288;
+  rise of river-beds, etc., 290;
+  landslips, 291;
+  rotation of pillars, 293;
+  after-shocks, 296;
+  structural changes in epicentral area, 301, 315;
+  structure of epicentral district, 302;
+  fault-scarps, 304;
+  crust-fractures, 309;
+  lakes and pools not due to faulting, 310;
+  changes in aspects of hills, 311;
+  revision of trigonometrical survey, 312;
+  origin of earthquake, 317;
+  bibliography, 319
+
+Inverness earthquake, preparation for, 246;
+  damage caused by, 247;
+  fissure in ground, 247;
+  isoseismal lines and disturbed area, 247;
+  position of originating fault, 249;
+  nature of shock, 250;
+  sound-phenomena, 253;
+  origin of earthquake, 255;
+  after-shocks and their origin, 256;
+  sympathetic earthquakes, 259;
+  comparison with Japanese earthquake, 260;
+  bibliography, 261.
+
+Investigation, Mallet's methods of, 12, 21
+
+Isacoustic lines, 234;
+  of Hereford earthquake, 235;
+  of Derby earthquake, 236
+
+Ischia, volcanic history of, 45, 70;
+  characteristics of eruptions, 49;
+  seismic history, 49
+
+Ischian earthquake of 1881, investigation of, 50;
+  isoseismal lines and disturbed area, 51;
+  position of epicentre, 52;
+  depth of focus, 54;
+  nature of shock, 55;
+  after-shocks, 56;
+  origin of, 70;
+  bibliography, 73
+
+Ischian earthquake of 1883, investigation of, 56;
+  preparation for, 57;
+  isoseismal lines and disturbed area, 58;
+  position of epicentre, 60;
+  depth of focus, 61;
+  nature of shock, 64;
+  landslips, 64;
+  after-shocks, 65;
+  origin of, 70;
+  bibliography, 73
+
+Ischian earthquakes, characteristics of, 66; origin of, 70
+
+Isoseismal lines, definition of, 3;
+  their use in determining position of epicentre, 219, 249, 325;
+  of Neapolitan earthquake, 9;
+  of Ischian earthquakes, 51, 58;
+  of Andalusian earthquake, 78;
+  of Charleston earthquake, 104;
+  of Riviera earthquake, 143;
+  of Japanese earthquake, 178, 182;
+  of Hereford earthquake, 216;
+  of Inverness earthquake, 247;
+  of Indian earthquake, 264
+
+Issel, A., 139, 163, 164, 166, 175
+
+
+Japanese earthquake of 1887, 18
+
+Japanese earthquake of 1891, investigation of, 177;
+  structure of meizoseismal area, 179;
+  damage caused by, 181;
+  isoseismal lines and disturbed area, 182;
+  nature of shock, 184;
+  the great fault-scarp, 189;
+  minor shocks, 197;
+  distribution of after-shocks in time, 198;
+  preparation for, 201;
+  distribution of after-shocks in space, 203;
+  sound-phenomena of after-shocks, 207;
+  sympathetic earthquakes, 209;
+  origin, of, 211;
+  bibliography, 213
+
+Japanese earthquake of 1894, 18, 329
+
+Johnston-Lavis, H.J., 50-72, 327
+
+
+Kilian, W., 76
+
+Koto, B., 177, 180, 181, 184, 190-196, 209, 212, 213
+
+
+Lakes formed by bending of river-bed during Indian earthquake, 310
+
+Lakes formed by fault-scarp of Japanese earthquake, 194;
+  of Indian earthquake, 305
+
+Landslips caused by Ischian earthquake, 64;
+  by Andalusian earthquake, 97;
+  by Indian earthquake, 291
+
+Lévy, M., 76
+
+Lisbon earthquake of 1755, 75, 82
+
+
+McGee, W.J., 134
+
+Macpherson, J., 101
+
+Magnetographs, earthquakes recorded by, 82, 157, 160, 189, 277, 282
+
+Mallet, R., 7-44, 85, 102, 124, 150, 294-296, 325
+
+Mallet's method of determining depth of focus, 25, 327
+
+Masato, H., 178, 213
+
+Mascart, E., 159, 160
+
+May Hill anticlinal and Hereford earthquake, connection between, 242
+
+Meizoseismal area, definition of, 3;
+  of Andalusian earthquake, 99;
+  of Japanese earthquake, 179
+
+Mercalli, G., 11, 57, 58, 60, 61, 63, 67, 70-73, 76, 80, 84, 85, 88,
+  90, 101, 138-175, 325, 327
+
+Michell, J., 325
+
+Milne, J., 35, 177, 181, 182, 186, 189, 200, 213, 281, 283
+
+Minor shocks of Neapolitan earthquake, 40;
+  of Japanese earthquake, 197
+
+Mountain ranges, effect of, on intensity of shock, 95, 106
+
+Moureaux, T., 161
+
+
+Nagaoka, H., 177, 214, 339
+
+Nagoya, records of Japanese after-shocks at, 183, 197
+
+Nature of shock, Neapolitan earthquake, 30;
+  Ischian earthquakes, 55, 64;
+  Andalusian earthquake, 87;
+  Charleston earthquake, 108;
+  Riviera earthquake, 150;
+  Japanese earthquake, 184;
+  Hereford earthquake, 220;
+  Inverness earthquake, 250;
+  Indian earthquake, 266
+
+Nausea, feeling of, caused by Charleston earthquake, 132
+
+Neapolitan earthquake, investigation of, 7, 12;
+  isoseismal lines and disturbed area, 9;
+  damage caused by, 10;
+  position of epicentre, 14;
+  depth of focus, 25;
+  nature of shock, 30;
+  sound-phenomena, 37;
+  velocity of earth-waves, 39;
+  minor shocks, 40;
+  origin, 41;
+  bibliography, 44
+
+Ness, Loch, connection between Inverness earthquakes and formation of,
+  255, 257, 261
+
+Nogués, A.F., 101
+
+
+Oddone, E., 175
+
+Offret, A., 76, 158, 159, 175
+
+Oglialoro, A., 73
+
+Oldham, R.D., 262-320, 337, 340
+
+Omori, F., 19, 20, 177, 183-186, 188, 197-199, 207, 214, 262, 325, 327,
+  329, 331
+
+Origin of earthquakes, 2, 5, 345;
+  of Neapolitan earthquake, 41;
+  of Ischian earthquakes, 70;
+  of Andalusian earthquake, 101;
+  of Charleston earthquake, 134;
+  of Riviera earthquakes, 174;
+  of Japanese earthquake, 211;
+  of Hereford earthquake, 240;
+  of Inverness earthquake, 255;
+  of Indian earthquake, 317
+
+Overturned bodies, maximum acceleration deduced from, 184, 272
+
+
+Palmieri, L., 57, 72, 73
+
+Periodicity of Japanese after-shocks, 199
+
+Perrey, A., 7
+
+Potenza, evidence of damaged church at, 15, 26
+
+Prediction of earthquakes, possible, 322
+
+Preparation for earthquakes, 40, 57, 76, 107, 142, 201, 238, 246, 321
+
+
+Rails, flexure of, by Charleston earthquake, 112;
+  by Japanese earthquake, 182;
+  by Indian earthquake, 288
+
+Railway-tunnels, observations of Riviera earthquake in, 166
+
+Rebeur-Paschwitz, E. von, 281
+
+River-beds, rise of, caused by Indian earthquake, 290
+
+Riviera earthquake, investigation, 138;
+  damage caused by, 139;
+  preparation for, 142;
+  isoseismal lines and disturbed area, 143;
+  position of epicentre, 146;
+  depth of principal focus, 149;
+  nature of shock, 150;
+  sound-phenomena, 156;
+  the unfelt earthquake, 157;
+  effects of earthquake at sea, 162;
+  destruction of fishes, 162;
+  seismic sea-waves, 163;
+  connection between geological structure and intensity of shock, 164;
+  observations in railway-tunnels, 166;
+  after-shocks, 167;
+  recent movements in the Riviera, 170;
+  seismic history of the Riviera, 171;
+  origin of, 171;
+  bibliography, 175
+
+Rocca di Papa, record of Indian earthquake at, 281, 282, 285
+
+Rossi, M.S. de, 57, 74, 82, 101, 175
+
+Rossi-Forel scale of seismic intensity, 104, 216, 247
+
+Rotation of pillars, caused by Hereford earthquake, 294;
+  by Indian earthquake, 293;
+  explanation of, 295
+
+Rudzki, P., 336
+
+Rumi, Prof., 169
+
+
+Samin, fault-scarp at, 308
+
+Sand-craters caused by Charleston earthquake, 130;
+  by Indian earthquake, 288
+
+Schmidt, A., 336
+
+Seismic sea-waves of Riviera earthquake, 142, 163
+
+Seismic vertical, 12, 29, 62
+
+Seismographic records of Riviera earthquake, 154;
+  of Japanese earthquake of 1894, 329
+
+Sekiya, S., 18, 19
+
+Serpieri, A., 74
+
+Shillong, nature of Indian earthquake at, 266
+
+Sloan, E., 103, 117-119, 134, 135
+
+Sound-area, definition of, 3;
+  of Neapolitan earthquake, 38;
+  of Andalusian earthquake, 92;
+  of Hereford earthquake, 234;
+  of Inverness earthquake, 252;
+  of Indian earthquake, 275
+
+Sound-phenomena, nature of sound, 38, 229, 252, 332;
+  inaudibility to some observers, 231, 274; its cause, 233;
+  isacoustic lines, 234-236;
+  variations in nature of sound throughout sound-area, 237;
+  time-relation of sound and shock, 238, 253;
+  origin of earthquake-sounds, 334;
+  sound-phenomena of Neapolitan earthquake, 37;
+  of Andalusian earthquake, 91;
+  of Charleston earthquake, 133;
+  of Riviera earthquake, 156;
+  of Japanese after-shocks, 207;
+  of Hereford earthquake, 229;
+  of Inverness earthquake, 252;
+  of Indian earthquake, 274
+
+Structural changes, distribution of, in Indian earthquake, 315
+
+Subsultory shock, 5
+
+Sympathetic earthquakes of Japanese earthquake, 209;
+  of Inverness earthquake, 259
+
+
+Tanakadate, A., 177, 214
+
+Taramelli, T., 76, 84, 85, 88, 90, 101, 138, 150, 165, 175, 325
+
+Tectonic earthquakes, 5
+
+Thrust-plane, Indian earthquake due to movement along, 318
+
+Time-curve of Indian earthquake, 278;
+  of principal epochs of earthquake-waves of distant origin, 338
+
+Time-records, general inaccuracy of, 324
+
+Time-relations of sound and shock in Hereford earthquake, 238;
+  in Inverness earthquake, 253
+
+Trigonometrical survey, revised, of Khasi hills after Indian
+  earthquake, 312;
+  interpretation of results, 314
+
+Twin earthquakes, origin of, 32, 89, 120, 153, 174, 223;
+  Neapolitan earthquake, 31;
+  Andalusian earthquake, 87;
+  Charleston earthquake, 108;
+  Riviera earthquake, 149, 150;
+  Hereford earthquake, 221
+
+
+Undulatory shock, 5
+
+Unfelt earth-waves, Andalusian earthquake, 82;
+  Riviera earthquake, 157;
+  Indian earthquake, 280
+
+Uzielli, G., 143, 176
+
+
+Velocity, maximum, of wave-motion, in Neapolitan earthquake, 35;
+  in Indian earthquake, 272
+
+Velocity of earth-waves, methods of determining, 39, 93, 127, 229;
+  variation with depth, 336;
+  form of wave-paths, 336;
+  velocity of different phases, 339;
+  of Neapolitan earthquake, 39;
+  of Andalusian earthquake, 92;
+  of Charleston earthquake, 126;
+  of Japanese earthquakes, 188;
+  of Hereford earthquake, 229;
+  of Indian earthquake, 275, 279, 284
+
+Visible earth-waves in Charleston earthquake, 110;
+  in Japanese earthquake, 186;
+  in Indian earthquake, 268
+
+Volcanic earthquakes, 5, 70
+
+Vorticose shock, 5
+
+
+Water, effect of Andalusian earthquake on underground, 97
+
+Waterfalls caused by fault-scarps of Indian earthquake, 305
+
+Wave-path, 13
+
+West, C.D., 272
+
+Woolhope anticlinal and Hereford earthquake, connection between, 241
+
+
+       *       *       *       *       *
+
+    +-----------------------------------------------------------+
+    | Typographical errors corrected in text:                   |
+    |                                                           |
+    | Page  54: Casamenello replaced with Casamenella           |
+    | Page 117: 'Captain Dutton' replaced with 'Major Dutton'   |
+    | Page 119: 'Capt. Dutton' replaced with 'Major Dutton'     |
+    | Page 315: Rangsonobo replaced with Rangsanobo             |
+    | Page 336: 'per sec. per sec.' replaced with 'per sec.'    |
+    | Page 337: negligeable replaced with negligible            |
+    |                                                           |
+    +-----------------------------------------------------------+
+
+       *       *       *       *       *
+
+
+
+
+
+
+End of Project Gutenberg's A Study of Recent Earthquakes, by Charles Davison
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+<pre>
+
+Project Gutenberg's A Study of Recent Earthquakes, by Charles Davison
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Study of Recent Earthquakes
+
+Author: Charles Davison
+
+Release Date: April 12, 2008 [EBook #25062]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A STUDY OF RECENT EARTHQUAKES ***
+
+
+
+
+Produced by Jeannie Howse, Henry Craig and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+</pre>
+
+
+<br />
+<hr />
+<br />
+
+<div class="tr">
+<p class="cen" style="font-weight: bold;">Transcriber's Note:</p>
+<br />
+<p class="noin">Inconsistent hyphenation in the original document has been preserved.</p>
+<p class="noin" style="text-align: left;">Obvious typographical errors have been corrected.
+For a complete list, please see the <span style="white-space: nowrap;"><a href="#TN">end of this document</a>.</span></p>
+<p class="noin">Some unicode characters may not show in all browsers, hover your mouse over the missing character to see the meaning. <span class="uni" title="like this">Like this.</span></p>
+<p class="noin">Click on the images to see a larger version.</p>
+</div>
+
+<br />
+<br />
+<br />
+<br />
+<hr />
+<br />
+
+<h1>A STUDY OF<br />
+RECENT EARTHQUAKES.</h1>
+
+<br />
+<br />
+<br />
+
+<h3>BY</h3>
+<h2>CHARLES DAVISON, Sc.D., F.G.S.</h2>
+
+<br />
+
+<h5>AUTHOR OF<br />
+"THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896."</h5>
+
+<br />
+<br />
+<br />
+
+<h4>WITH 80 ILLUSTRATIONS</h4>
+
+<br />
+<br />
+<br />
+
+<h4>London and Newcastle-on-Tyne:<br />
+THE WALTER SCOTT PUBLISHING CO., LTD.<br />
+1905</h4>
+
+<br />
+<hr />
+<br />
+<br />
+<span class='pagenum'><a name="Page_v" id="Page_v">[v]</a></span><br />
+<br />
+
+<h3>PREFACE.</h3>
+<br />
+
+<p>The present volume differs from a text-book of seismology in giving
+brief, though detailed, accounts of individual earthquakes rather than
+a discussion of the phenomena and distribution of earthquakes in
+general. At the close of his <i>Les Tremblements de Terre</i>, Professor
+Fouqu&eacute; has devoted a few chapters to some of the principal earthquakes
+between 1854 and 1887; and there are also the well-known chapters in
+Lyell's <i>Principles of Geology</i> dealing with earthquakes of a still
+earlier date. With these exceptions, there is no other work covering
+the same ground; and he who wishes to study any particular earthquake
+can only do so by reading long reports or series of papers written
+perhaps in several different languages. The object of this volume is
+to save him this trouble, and to present to him the facts that seem
+most worthy of his attention.</p>
+
+<p>The chapter on the Japanese earthquake is reprinted, with a few slight
+additions, from a paper published in the <i>Geographical Journal</i>, and I
+am indebted to the editor, not only for the necessary permission, but
+also for his courtesy in furnishing <span class='pagenum'><a name="Page_vi" id="Page_vi">[vi]</a></span>me with <i>clich&eacute;s</i> of the blocks
+which illustrated the original paper. The editor of <i>Knowledge</i> has
+also allowed me to use a paper which appeared four years ago as the
+foundation of the ninth chapter in this book.</p>
+
+<p class="right">CHARLES DAVISON.</p>
+
+<p class="noin" style="padding-left: 2em;"><span class="sc">Birmingham,</span><br />
+<i>January, 1905.</i></p>
+
+<br />
+<br />
+<br />
+<br />
+<a name="toc" id="toc"></a><hr />
+<span class='pagenum'><a name="Page_vii" id="Page_vii">[vii]</a></span>
+<br />
+
+<h3>CONTENTS.</h3>
+<br />
+
+<div class="centered">
+<table border="0" cellpadding="2" cellspacing="0" width="70%" summary="Table of Contents">
+  <tr>
+  <td class="tdl" width="90%">&nbsp;</td>
+  <td class="tdr" width="10%" style="font-size: 80%;">PAGE</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER I.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_I">Introduction</a></td>
+  <td class="tdr">1</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER II.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_II">The Neapolitan Earthquake of December 16th, 1857</a></td>
+  <td class="tdr">7</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER III.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_III">The Ischian Earthquakes of March 4th, 1881, and July 28th, 1883</a></td>
+  <td class="tdr">45</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER IV.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_IV">The Andalusian Earthquake of December 25th, 1884</a></td>
+  <td class="tdr">75</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER V.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_V">The Charleston Earthquake of August 31st, 1886</a></td>
+  <td class="tdr">102</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER VI.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_VI">The Riviera Earthquake of February 23rd, 1887</a></td>
+  <td class="tdr">138</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2"><span class='pagenum'><a name="Page_viii" id="Page_viii">[viii]</a></span>CHAPTER VII.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_VII">The Japanese Earthquake of October 28th, 1891</a></td>
+  <td class="tdr">177</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER VIII.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_VIII">The Hereford Earthquake of December 17th, 1896, and the Inverness Earthquake of September 18th, 1901</a></td>
+  <td class="tdr">215</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER IX.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_IX">The Indian Earthquake of June 12th, 1897</a></td>
+  <td class="tdr">262</td>
+  </tr>
+  <tr>
+  <td class="tdc" colspan="2">CHAPTER X.</td>
+  </tr>
+  <tr>
+  <td class="tdlsc"><a href="#CHAPTER_X">Conclusion</a></td>
+  <td class="tdr">321</td>
+  </tr>
+  <tr>
+  <td class="tdlsc" style="padding-top: 1em;"><a href="#INDEX">Index</a></td>
+  <td class="tdr" style="padding-top: 1em;">349</td>
+  </tr>
+</table>
+</div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="toi" id="toi"></a><hr />
+<span class='pagenum'><a name="Page_ix" id="Page_ix">[ix]</a></span><br />
+
+<h3>LIST OF ILLUSTRATIONS.</h3>
+<br />
+
+<div class="centered">
+<table border="0" cellpadding="2" cellspacing="0" width="80%" summary="List of Illustrations">
+  <tr>
+  <td class="tdr1" width="7%" style="font-size: 80%;">FIG.</td>
+  <td class="tdl" width="83%">&nbsp;</td>
+  <td class="tdr" width="10%" style="font-size: 80%;">PAGE</td>
+  </tr>
+  <tr>
+  <td class="tdr1">1.</td>
+  <td class="tdl"><a href="#Fig1">Diagram to illustrate Simple Harmonic Motion</a></td>
+  <td class="tdr">4</td>
+  </tr>
+  <tr>
+  <td class="tdr1">2.</td>
+  <td class="tdl"><a href="#Fig2">Isoseismal Lines of the Neapolitan Earthquake</a></td>
+  <td class="tdr">9</td>
+  </tr>
+  <tr>
+  <td class="tdr1">3.</td>
+  <td class="tdl"><a href="#Fig3">Diagram to illustrate Wave-path and Angle of Emergence</a></td>
+  <td class="tdr">12</td>
+  </tr>
+  <tr>
+  <td class="tdr1">4.</td>
+  <td class="tdl"><a href="#Fig4">Diagram to illustrate Mallet's Method of determining
+   Position of Epicentre</a></td>
+  <td class="tdr">14</td>
+  </tr>
+  <tr>
+  <td class="tdr1">5.</td>
+  <td class="tdl"><a href="#Fig5">Plan of Cathedral Church at Potenza</a></td>
+  <td class="tdr">16</td>
+  </tr>
+  <tr>
+  <td class="tdr1">6.</td>
+  <td class="tdl"><a href="#Fig6">Fallen Gate-pillars near Saponara</a></td>
+  <td class="tdr">17</td>
+  </tr>
+  <tr>
+  <td class="tdr1">7.</td>
+  <td class="tdl"><a href="#Fig7">Model to illustrate the Motion of an Earth-particle
+   during an Earthquake</a></td>
+  <td class="tdr">19</td>
+  </tr>
+  <tr>
+  <td class="tdr1">8.</td>
+  <td class="tdl"><a href="#Fig8">Plan of Directions of Fall of Overturned Stone-lamps
+   at Tokio during the Earthquake of 1894</a></td>
+  <td class="tdr">19</td>
+  </tr>
+  <tr>
+  <td class="tdr1">9.</td>
+  <td class="tdl"><a href="#Fig9">Meizoseismal Area of Neapolitan Earthquake</a></td>
+  <td class="tdr">22</td>
+  </tr>
+  <tr>
+  <td class="tdr1">10.</td>
+  <td class="tdl"><a href="#Fig10">Distribution of Death-rate within Meizoseismal Area
+    of Neapolitan Earthquake</a></td>
+  <td class="tdr">24</td>
+  </tr>
+  <tr>
+  <td class="tdr1">11.</td>
+  <td class="tdl"><a href="#Fig11">Diagram to illustrate Mallet's Method of determining
+    Depth of Seismic Focus</a></td>
+  <td class="tdr">26</td>
+  </tr>
+  <tr>
+  <td class="tdr1">12.</td>
+  <td class="tdl"><a href="#Fig12">Vertical Section of Cathedral Church at Potenza</a></td>
+  <td class="tdr">27</td>
+  </tr>
+  <tr>
+  <td class="tdr1">13.</td>
+  <td class="tdl"><a href="#Fig13">Diagram of Wave-paths at Seismic Vertical of Neapolitan
+    Earthquake</a></td>
+  <td class="tdr">29</td>
+  </tr>
+  <tr>
+  <td class="tdr1">14.</td>
+  <td class="tdl"><a href="#Fig14">Geological Sketch-map of Ischia</a></td>
+  <td class="tdr">47</td>
+  </tr>
+  <tr>
+  <td class="tdr1">15.</td>
+  <td class="tdl"><a href="#Fig15">Isoseismal Lines of Ischian Earthquake of 1881</a></td>
+  <td class="tdr">51</td>
+  </tr>
+  <tr>
+  <td class="tdr1">16.</td>
+  <td class="tdl"><a href="#Fig16">Isoseismal Lines of Ischian Earthquake of 1883</a></td>
+  <td class="tdr">59</td>
+  </tr>
+  <tr>
+  <td class="tdr1">17.</td>
+  <td class="tdl"><a href="#Fig17">Diagram of Wave-paths at Seismic Vertical of Ischian
+    Earthquake of 1883</a></td>
+  <td class="tdr">62</td>
+  </tr>
+  <tr>
+  <td class="tdr1"><span class='pagenum'><a name="Page_x" id="Page_x">[x]</a></span>18.</td>
+  <td class="tdl"><a href="#Fig18">Diagram showing connection between Depth of Focus
+    and Rate of Decline in Intensity</a></td>
+  <td class="tdr">68</td>
+  </tr>
+  <tr>
+  <td class="tdr1">19.</td>
+  <td class="tdl"><a href="#Fig19">Isoseismal Lines of Andalusian Earthquake according
+    to Taramelli and Mercalli</a></td>
+  <td class="tdr">79</td>
+  </tr>
+  <tr>
+  <td class="tdr1">20.</td>
+  <td class="tdl"><a href="#Fig20">Isoseismal Lines of Andalusian Earthquake according
+    to Fouqu&eacute;, etc.</a></td>
+  <td class="tdr">81</td>
+  </tr>
+  <tr>
+  <td class="tdr1">21.</td>
+  <td class="tdl"><a href="#Fig21">Magnetograph Records of Andalusian Earthquake at Lisbon</a></td>
+  <td class="tdr">83</td>
+  </tr>
+  <tr>
+  <td class="tdr1">22.</td>
+  <td class="tdl"><a href="#Fig22">Nature of Shock of Andalusian Earthquake</a></td>
+  <td class="tdr">88</td>
+  </tr>
+  <tr>
+  <td class="tdr1">23.</td>
+  <td class="tdl"><a href="#Fig23">Diagram to illustrate Variation in Nature of Shock</a></td>
+  <td class="tdr">89</td>
+  </tr>
+  <tr>
+  <td class="tdr1">24.</td>
+  <td class="tdl"><a href="#Fig24">Structure of Meizoseismal Area of Andalusian Earthquake</a></td>
+  <td class="tdr">100</td>
+  </tr>
+  <tr>
+  <td class="tdr1">25.</td>
+  <td class="tdl"><a href="#Fig25">Isoseismal Lines of Charleston Earthquake</a></td>
+  <td class="tdr">106</td>
+  </tr>
+  <tr>
+  <td class="tdr1">26.</td>
+  <td class="tdl"><a href="#Fig26">Curve of Intensity at Charleston</a></td>
+  <td class="tdr">110</td>
+  </tr>
+  <tr>
+  <td class="tdr1">27.</td>
+  <td class="tdl"><a href="#Fig27">Flexure of Rails at Jedburgh</a></td>
+  <td class="tdr">113</td>
+  </tr>
+  <tr>
+  <td class="tdr1">28.</td>
+  <td class="tdl"><a href="#Fig28">Epicentral Isoseismal Lines of Charleston Earthquake
+    according to Sloan</a></td>
+  <td class="tdr">118</td>
+  </tr>
+  <tr>
+  <td class="tdr1">29.</td>
+  <td class="tdl"><a href="#Fig29">Epicentral Isoseismal Lines of Charleston Earthquake
+    according to Dutton</a></td>
+  <td class="tdr">119</td>
+  </tr>
+  <tr>
+  <td class="tdr1">30.</td>
+  <td class="tdl"><a href="#Fig30">Planes of Oscillation of Stopped Pendulum Clocks at
+    Charleston</a></td>
+  <td class="tdr">121</td>
+  </tr>
+  <tr>
+  <td class="tdr1">31.</td>
+  <td class="tdl"><a href="#Fig31">Diagram to illustrate Dutton's Method of determining
+    Depth of Seismic Focus</a></td>
+  <td class="tdr">124</td>
+  </tr>
+  <tr>
+  <td class="tdr1">32.</td>
+  <td class="tdl"><a href="#Fig32">Diagram to explain Origin of Regions of Defective
+    Intensity</a></td>
+  <td class="tdr">136</td>
+  </tr>
+  <tr>
+  <td class="tdr1">33.</td>
+  <td class="tdl"><a href="#Fig33">Isoseismal Lines of Riviera Earthquake</a></td>
+  <td class="tdr">144</td>
+  </tr>
+  <tr>
+  <td class="tdr1">34.</td>
+  <td class="tdl"><a href="#Fig34">Meizoseismal Area of Riviera Earthquake</a></td>
+  <td class="tdr">148</td>
+  </tr>
+  <tr>
+  <td class="tdr1">35.</td>
+  <td class="tdl"><a href="#Fig35">Nature of Shock of Riviera Earthquake</a></td>
+  <td class="tdr">152</td>
+  </tr>
+  <tr>
+  <td class="tdr1">36.</td>
+  <td class="tdl"><a href="#Fig36">Seismographic Record at Moncalieri</a></td>
+  <td class="tdr">155</td>
+  </tr>
+  <tr>
+  <td class="tdr1">37.</td>
+  <td class="tdl"><a href="#Fig37">Distribution of Observatories at which Magnetographs
+    were disturbed by Riviera Earthquake</a></td>
+  <td class="tdr">158</td>
+  </tr>
+  <tr>
+  <td class="tdr1">38.</td>
+  <td class="tdl"><a href="#Fig38">Record of Tide-gauge at Nice</a></td>
+  <td class="tdr">163</td>
+  </tr>
+  <tr>
+  <td class="tdr1">39.</td>
+  <td class="tdl"><a href="#Fig39">Record of Tide-gauge at Genoa</a></td>
+  <td class="tdr">164</td>
+  </tr>
+  <tr>
+  <td class="tdr1">40.</td>
+  <td class="tdl"><a href="#Fig40">Distribution of Seismic Activity in the Riviera</a></td>
+  <td class="tdr">172</td>
+  </tr>
+  <tr>
+  <td class="tdr1">41.</td>
+  <td class="tdl"><a href="#Fig41">Isoseismal Lines of Japanese Earthquake</a></td>
+  <td class="tdr">178</td>
+  </tr>
+  <tr>
+  <td class="tdr1"><span class='pagenum'><a name="Page_xi" id="Page_xi">[xi]</a></span>42.</td>
+  <td class="tdl"><a href="#Fig42">Structure of Meizoseismal Area of Japanese Earthquake</a></td>
+  <td class="tdr">180</td>
+  </tr>
+  <tr>
+  <td class="tdr1">43.</td>
+  <td class="tdl"><a href="#Fig43">Plan of Directions of Fall of Overturned Bodies at
+    Nagoya</a></td>
+  <td class="tdr">187</td>
+  </tr>
+  <tr>
+  <td class="tdr1">44.</td>
+  <td class="tdl"><a href="#Fig44">Map of Mean Directions of Shock and Isoseismal Lines
+    in Central District of Japanese Earthquake</a></td>
+  <td class="tdr">188</td>
+  </tr>
+  <tr>
+  <td class="tdr1">45.</td>
+  <td class="tdl"><a href="#Fig45">Meizoseismal Area of Japanese Earthquake</a></td>
+  <td class="tdr">190</td>
+  </tr>
+  <tr>
+  <td class="tdr1">46.</td>
+  <td class="tdl"><a href="#Fig46">Fault-scarp near Fujitani</a></td>
+  <td class="tdr">191</td>
+  </tr>
+  <tr>
+  <td class="tdr1">47.</td>
+  <td class="tdl"><a href="#Fig47">Fault-scarp at Midori</a></td>
+  <td class="tdr">192</td>
+  </tr>
+  <tr>
+  <td class="tdr1">48.</td>
+  <td class="tdl"><a href="#Fig48">Displacement of Field-divisions by the Fault near
+    Nishi-Katabira</a></td>
+  <td class="tdr">193</td>
+  </tr>
+  <tr>
+  <td class="tdr1">49.</td>
+  <td class="tdl"><a href="#Fig49">Map of Swamp formed by Stoppage of River Toba by
+    Fault-scarp</a></td>
+  <td class="tdr">194</td>
+  </tr>
+  <tr>
+  <td class="tdr1">50.</td>
+  <td class="tdl"><a href="#Fig50">Shifting of Trees by Fault at Um&eacute;hara </a></td>
+  <td class="tdr">195</td>
+  </tr>
+  <tr>
+  <td class="tdr1">51.</td>
+  <td class="tdl"><a href="#Fig51">Daily Frequency of After-shocks at Gifu and Nagoya</a></td>
+  <td class="tdr">196</td>
+  </tr>
+  <tr>
+  <td class="tdr1">52.</td>
+  <td class="tdl"><a href="#Fig52">Monthly Frequency of After-shocks at Gifu</a></td>
+  <td class="tdr">197</td>
+  </tr>
+  <tr>
+  <td class="tdr1">53.</td>
+  <td class="tdl"><a href="#Fig53">Distribution of Preliminary Shocks in Space</a></td>
+  <td class="tdr">202</td>
+  </tr>
+  <tr>
+  <td class="tdr1">54.</td>
+  <td class="tdl"><a href="#Fig54">Distribution of After-shocks in Space, Nov.-Dec. 1891</a></td>
+  <td class="tdr">203</td>
+  </tr>
+  <tr>
+  <td class="tdr1">55.</td>
+  <td class="tdl"><a href="#Fig55">Distribution of After-shocks in Space, Jan.-Feb. 1892</a></td>
+  <td class="tdr">204</td>
+  </tr>
+  <tr>
+  <td class="tdr1">56.</td>
+  <td class="tdl"><a href="#Fig56">Distribution of After-shocks in Space, March-April 1892</a></td>
+  <td class="tdr">205</td>
+  </tr>
+  <tr>
+  <td class="tdr1">57.</td>
+  <td class="tdl"><a href="#Fig57">Distribution of After-shocks in Space, May-June 1892</a></td>
+  <td class="tdr">206</td>
+  </tr>
+  <tr>
+  <td class="tdr1">58.</td>
+  <td class="tdl"><a href="#Fig58">Distribution of Audible After-shocks in Space, Nov.
+    1891-Dec. 1892</a></td>
+  <td class="tdr">208</td>
+  </tr>
+  <tr>
+  <td class="tdr1">59.</td>
+  <td class="tdl"><a href="#Fig59">Map of Adjoining Regions in which Seismic Activity
+    was affected by Japanese Earthquake</a></td>
+  <td class="tdr">210</td>
+  </tr>
+  <tr>
+  <td class="tdr1">60.</td>
+  <td class="tdl"><a href="#Fig60">Isoseismal and Isacoustic Lines of Hereford Earthquake</a></td>
+  <td class="tdr">216</td>
+  </tr>
+  <tr>
+  <td class="tdr1">61.</td>
+  <td class="tdl"><a href="#Fig61">Nature of Shock of Hereford Earthquake</a></td>
+  <td class="tdr">222</td>
+  </tr>
+  <tr>
+  <td class="tdr1">62.</td>
+  <td class="tdl"><a href="#Fig62">Coseismal Lines of Hereford Earthquake</a></td>
+  <td class="tdr">228</td>
+  </tr>
+  <tr>
+  <td class="tdr1">63.</td>
+  <td class="tdl"><a href="#Fig63">Map of Minor Shocks of Hereford Earthquake</a></td>
+  <td class="tdr">239</td>
+  </tr>
+  <tr>
+  <td class="tdr1">64.</td>
+  <td class="tdl"><a href="#Fig64">Geology of Meizoseismal Area of Hereford Earthquake</a></td>
+  <td class="tdr">241</td>
+  </tr>
+  <tr>
+  <td class="tdr1">65.</td>
+  <td class="tdl"><a href="#Fig65">Isoseismal Lines of Inverness Earthquake</a></td>
+  <td class="tdr">248</td>
+  </tr>
+  <tr>
+  <td class="tdr1"><span class='pagenum'><a name="Page_xii" id="Page_xii">[xii]</a></span>66.</td>
+  <td class="tdl"><a href="#Fig66">Diagram to illustrate supposed Fault-displacement
+    causing Inverness Earthquake</a></td>
+  <td class="tdr">256</td>
+  </tr>
+  <tr>
+  <td class="tdr1">67.</td>
+  <td class="tdl"><a href="#Fig67">Map of Epicentres of After-shocks of Inverness
+    Earthquake</a></td>
+  <td class="tdr">258</td>
+  </tr>
+  <tr>
+  <td class="tdr1">68.</td>
+  <td class="tdl"><a href="#Fig68">Isoseismal Lines of Indian Earthquake</a></td>
+  <td class="tdr">263</td>
+  </tr>
+  <tr>
+  <td class="tdr1">69.</td>
+  <td class="tdl"><a href="#Fig69">Section of Tombs in Cemetery at Cherrapunji</a></td>
+  <td class="tdr">270</td>
+  </tr>
+  <tr>
+  <td class="tdr1">70.</td>
+  <td class="tdl"><a href="#Fig70">Time-curve of Indian Earthquake</a></td>
+  <td class="tdr">278</td>
+  </tr>
+  <tr>
+  <td class="tdr1">71.</td>
+  <td class="tdl"><a href="#Fig71">Seismographic Record of Indian Earthquake at Rocca
+    di Papa</a></td>
+  <td class="tdr">282</td>
+  </tr>
+  <tr>
+  <td class="tdr1">72.</td>
+  <td class="tdl"><a href="#Fig72">Seismographic Record of Indian Earthquake at Edinburgh</a></td>
+  <td class="tdr">283</td>
+  </tr>
+  <tr>
+  <td class="tdr1">73.</td>
+  <td class="tdl"><a href="#Fig73">Displacement of Alluvium at Foot of a Hill</a></td>
+  <td class="tdr">287</td>
+  </tr>
+  <tr>
+  <td class="tdr1">74.</td>
+  <td class="tdl"><a href="#Fig74">Twisting of Monument at Chhatak</a></td>
+  <td class="tdr">294</td>
+  </tr>
+  <tr>
+  <td class="tdr1">75.</td>
+  <td class="tdl"><a href="#Fig75">Epicentral Area of Indian Earthquake</a></td>
+  <td class="tdr">303</td>
+  </tr>
+  <tr>
+  <td class="tdr1">76.</td>
+  <td class="tdl"><a href="#Fig76">Plan of Chedrang Fault</a></td>
+  <td class="tdr">305</td>
+  </tr>
+  <tr>
+  <td class="tdr1">77.</td>
+  <td class="tdl"><a href="#Fig77">Re-triangulation of Khasi Hills</a></td>
+  <td class="tdr">313</td>
+  </tr>
+  <tr>
+  <td class="tdr1">78.</td>
+  <td class="tdl"><a href="#Fig78">Diagram of Thrust-planes</a></td>
+  <td class="tdr">318</td>
+  </tr>
+  <tr>
+  <td class="tdr1">79.</td>
+  <td class="tdl"><a href="#Fig79">Seismographic Record of Tokio Earthquake of 1894</a></td>
+  <td class="tdr">329</td>
+  </tr>
+  <tr>
+  <td class="tdr1">80.</td>
+  <td class="tdl"><a href="#Fig80">Time-curves of Principal Epochs of Earthquake-waves of
+    Distant Origin</a></td>
+  <td class="tdr">338</td>
+  </tr>
+</table>
+</div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_I" id="CHAPTER_I"></a><hr />
+<span class='pagenum'><a name="Page_1" id="Page_1">[1]</a></span><br />
+
+<h1>A STUDY OF <br />RECENT EARTHQUAKES.</h1>
+<br />
+
+<h3>CHAPTER I.</h3>
+
+<h4>INTRODUCTION.</h4>
+<br />
+
+<p>I propose in this book to describe a few of the more important
+earthquakes that have occurred during the last half century. In
+judging of importance, the standard which I have adopted is not that
+of intensity only, but rather of the scientific value of the results
+that have been achieved by the study of the shocks. Even with this
+reservation, the number of earthquakes that might be included is
+considerable; and I have therefore selected those which seem to
+illustrate best the different methods of investigation employed by
+seismologists, or which are of special interest owing to the unusual
+character of their phenomena or to the light cast by them on the
+nature and origin of earthquakes in general.</p>
+
+<p>Thus, the Neapolitan earthquake possesses interest from a historical
+point of view; it is the first earthquake in the study of which modern
+scientific methods were employed. The Ischian earthquakes are
+described as examples of those connected with <span class='pagenum'><a name="Page_2" id="Page_2">[2]</a></span>volcanic action; the
+Andalusian earthquake is chiefly remarkable for the recognition of the
+unfelt earth-waves; that of Charleston for the detection of the double
+epicentre and the calculation of the velocity with which the
+vibrations travelled. In the Riviera earthquake are combined the
+principal features of the last two shocks with several phenomena of
+miscellaneous interest, especially those connected with its submarine
+foci. The Japanese earthquake is distinguished from others by its
+extraordinary fault-scarp and the very numerous shocks that followed
+it. The Hereford earthquake is a typical example of a twin earthquake,
+and provided many observations on the sound phenomena; while the
+Inverness earthquakes are important on account of their connection
+with the growth of a well-known fault. The great Indian earthquake
+owns few, if any, rivals within historical times, whether we consider
+the intensity of the disturbance or the diversity and interest of the
+phenomena displayed by it&mdash;the widespread changes in the earth's
+crust, both superficial and deep-seated, and the tracking of the
+unfelt pulsations completely round the globe.</p>
+
+<br />
+
+<h4>TERMS AND DEFINITIONS.</h4>
+
+<p>Some terms are of such frequent use in describing earthquakes that it
+will be convenient to group them here for reference, others more
+rarely employed being introduced as they are required.</p>
+
+<p>An earthquake is caused by a sudden displacement of the material which
+composes the earth's interior. The displacement gives rise to series
+of waves, which are propagated outwards in all directions, and which,
+<span class='pagenum'><a name="Page_3" id="Page_3">[3]</a></span>when they reach the surface, produce the sensations known to us as
+those of an earthquake.</p>
+
+<p>The region within which the displacement occurs is sometimes called
+the <i>hypocentre</i>, but more frequently the <i>seismic focus</i> or simply
+the <i>focus</i>. The portion of the earth's surface which is vertically
+above the seismic focus is called the <i>epicentre</i>. The focus and
+epicentre are often spoken of for convenience as if they were points,
+and they may then be regarded as the centres of the region and area in
+which the intensity was greatest. This is not quite accurate, but to
+attempt a more exact definition would at present be out of place.</p>
+
+<p>An <i>isoseismal line</i> is a curve which passes through all points at
+which the intensity of the shock was the same. It is but rarely that
+the absolute intensity at any point of an isoseismal line can be
+ascertained, and only one example is given in this volume. As a rule,
+the intensity of a shock is determined by reference to the degrees of
+different arbitrary scales. These will be quoted when required.</p>
+
+<div class="imgl" style="width: 25%;"><a name="Fig1" id="Fig1"></a>
+<a href="images/fig01.png">
+<img border="0" src="images/fig01.png" width="90%" alt="Diagram to illustrate simple harmonic motion." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 1.&mdash;Diagram to illustrate simple harmonic motion.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>In every strong earthquake there is a central district which differs
+in a marked manner from that outside in the far greater strength and
+complexity of the phenomena. As this district includes the epicentre,
+it is sometimes referred to as the <i>epicentral area</i>, but the term
+<i>meizoseismal area</i> is more appropriate, and will be employed
+accordingly.</p>
+
+<p>The district over which an earthquake is perceptible to human beings
+without instrumental aid is its <i>disturbed area</i>. In like manner, that
+over which the earthquake-sound is heard is the <i>sound-area</i>.</p>
+
+<p>A great earthquake never occurs alone. It is merely the most prominent
+member of a group of <span class='pagenum'><a name="Page_4" id="Page_4">[4]</a></span>shocks of greater or less intensity, and is
+known as the <i>principal shock</i> or <i>earthquake</i>, while the others are
+called <i>minor</i> or <i>accessory shocks</i>, and <i>fore-shocks</i> or
+<i>after-shocks</i> according as they occur before or after the principal
+earthquake. When the sound only is heard, without an accompanying
+tremor being anywhere perceptible, it is more accurately called an
+<i>earth-sound</i>, but is frequently for convenience numbered among the
+minor shocks.</p>
+
+<p>The movement of the ground during a vibration of the simplest
+character (known as simple harmonic motion) is represented in Fig. 1.
+The pointer of the recording seismograph is here supposed to oscillate
+along a line at right angles to AB, and the smoked paper or glass on
+which the record is made to travel to the left. The distance MP of the
+crest P of any wave from the line AB represents the <i>amplitude</i> of the
+vibration, the sum of the distances MP and NQ its <i>range</i>, and the
+length AB the <i>period</i> of the vibration. From the amplitude and period
+we can calculate, in the case of simple harmonic motion, both the
+<i>maximum velocity</i> and <i>maximum acceleration</i> of the vibrating
+particles of the ground.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></p>
+
+<p>A few terms describing the nature of the shock are <span class='pagenum'><a name="Page_5" id="Page_5">[5]</a></span>also in common use
+among Italians and Spaniards. An <i>undulatory</i> shock consists of one or
+several waves, the movement to and fro being along a nearly horizontal
+line; a <i>subsultory</i> shock of movements in a nearly vertical
+direction; while a <i>vorticose</i> shock consists of undulatory or
+subsultory movements crossing one another in different directions.</p>
+
+<br style="clear: both;" />
+
+<h4>ORIGIN OF EARTHQUAKES.</h4>
+
+<p>Earthquakes are grouped, according to their origin, into three
+classes. The first consists of slight local shocks, caused by the fall
+of rock in underground passages; the second of <i>volcanic</i> earthquakes,
+also local in character, but often of considerable intensity near the
+centre of the disturbed area; while in the third class we have
+<i>tectonic</i> earthquakes, or those directly connected with the shaping
+of the earth's crust, which vary in strength from the weakest
+perceptible tremor to the most destructive and widely felt shock. Of
+the earthquakes described in this volume, the Ischian earthquakes
+belong to the second class, and all the others to the third.</p>
+
+<p>That tectonic earthquakes are closely connected with the formation of
+faults seems now established beyond doubt. They occur far from all
+traces of recent volcanic action. Their isoseismal lines are elongated
+in directions parallel to known faults, and this is sometimes the case
+in one and the same district with faults that occur at right angles to
+one another. Indeed, when several isoseismals are carefully drawn, it
+is possible from their form and relative position to predict the
+position of the originating fault.<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> The <span class='pagenum'><a name="Page_6" id="Page_6">[6]</a></span>initial formation and
+further spreading of the rent may be the cause of a few earthquakes,
+but by far the larger number are due to the subsequent growth of the
+fault. The relative displacement of the rocks adjoining the fault,
+which may amount to thousands of feet, occasionally even to miles, is
+the result, not of one great movement, but of innumerable slips taking
+place in different parts of the fault and spread over vast ages of
+time. With every fault-slip, intense friction is suddenly brought into
+action by the rubbing of one mass of rock against the other; and,
+according to the modern view, it is this friction that gives rise to
+the earthquake waves.</p>
+
+<p>In most earthquakes, the slip takes place at a considerable depth,
+perhaps not less than one or several miles, and the vertical slip is
+so small that it dies out before reaching the surface. But, in a few
+violent earthquakes, such as the Japanese and Indian earthquakes
+described in this volume, the slip is continued up to the surface and
+is left visible there as a small cliff or fault-scarp. In these cases,
+the sudden spring of the crust may increase and complicate the effects
+of the vibratory shock.</p>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> If <i>a</i> is the amplitude of the vibration and T its
+period, the maximum velocity is 2&pi;<i>a</i> &divide; T and the maximum
+acceleration 4&pi;&sup2;<i>a</i> &divide; T&sup2;.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> See Chapter VIII., on the Hereford and Inverness
+earthquakes.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_II" id="CHAPTER_II"></a><hr />
+<span class='pagenum'><a name="Page_7" id="Page_7">[7]</a></span><br />
+
+<h3>CHAPTER II.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857.</h4>
+<br />
+
+<p>Half a century ago, seismology was in its infancy. On the Continent,
+Alexis Perrey of Dijon was compiling his earthquake catalogues with
+unfailing enthusiasm and industry. In 1846, Robert Mallet applied the
+laws of wave-motion in solids, as they were then known, to the
+phenomena of earthquakes; and his memoir on the Dynamics of
+Earthquakes<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> may be regarded as the foundation-stone of the new
+science. During the next twelve years he contributed his well-known
+Reports to the British Association,<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> and prepared a series of
+instructions for the observation and study of earthquake-shocks.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a>
+The latter, it is worth noting, contains an outline, but hardly more
+than an outline, of the methods of investigation which he developed
+and employed eight years afterwards in studying the Neapolitan
+earthquake.</p>
+
+<p>The history of Mallet's preparation for his great work is somewhat
+strange. No one else at that time possessed so full a knowledge of
+earthquake phenomena. It was, however, a knowledge that had little,
+<span class='pagenum'><a name="Page_8" id="Page_8">[8]</a></span>if any, foundation in actual experience; for, when he was awakened by
+the British earthquake of November 9th, 1852, he failed to recognise
+its seismic character. Although this shock disturbed an area of about
+75,000 square miles and was felt in all four parts of the kingdom, the
+paucity of observations and the absence of durable records combined in
+preventing the successful application of his new modes of study.<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a>
+Nevertheless, with confidence unshaken in their power, he awaited the
+occurrence of a more violent shock, but five years had to pass before
+his opportunity came towards the close of 1857.</p>
+
+<p>So destructive was the Neapolitan earthquake of this year (Mallet
+ranks it third among European earthquakes in extent and severity),
+that nearly a week elapsed before any news of it reached the outer
+world. Without further loss of time, he applied for and obtained a
+grant of money from the Council of the Royal Society, and proceeded
+early in the following February to what was then the kingdom of
+Naples. Armed with letters of authority to different officials, he
+visited the chief towns and villages in the meizoseismal area; and, in
+spite of unfavourable weather and the difficulties of travelling in a
+country so recently devastated, he completed his examination in little
+more than two months. It was a task, surely, that would have baffled
+any but the most enthusiastic investigator or one unspurred by the
+feeling that he possessed the key to one of the most obscure of
+Nature's problems.</p>
+
+<p>Mallet's confidence in the accuracy of his methods was almost
+unbounded. His great report was published four years later; but he
+seems to have <span class='pagenum'><a name="Page_9" id="Page_9">[9]</a></span>regarded it almost as a text-book of "observational
+seismology" and the results of his Neapolitan work as mere
+illustrations. His successors, however, have transposed the order of
+importance, and rank his two large volumes as the model, if not the
+inspirer, of many of our more recent earthquake monographs.</p>
+
+<div class="img"><a name="Fig2" id="Fig2"></a>
+<a href="images/fig02.png">
+<img border="0" src="images/fig02.png" width="50%" alt="Isoseismal Lines of the Neapolitan Earthquake of 1857." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 2.&mdash;Isoseismal Lines of the Neapolitan Earthquake of 1857. (<i>Mallet.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>The position of the meizoseismal area, to which Mallet devoted most of
+his time, is indicated by the small oval area marked 1 in Fig. 2,
+represented on a larger scale in Fig. 9. It is 40 miles long and 23
+miles <span class='pagenum'><a name="Page_10" id="Page_10">[10]</a></span>wide,<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a> and contains 950 square miles. Within this area, the
+loss of life was great and most of the towns were absolutely
+prostrated.</p>
+
+<p>The next isoseismal, No. 2, which is also shown more clearly in Fig.
+9, bounds the area in which the loss of life was still great and many
+persons were wounded, while large portions of the towns within it were
+thrown down. Its length is 65 miles, width 47 miles, and area 2,240
+square miles. The third isoseismal includes a district in which
+buildings were only occasionally thrown down, though none escaped some
+slight damage, and in which practically no loss of life occurred. This
+curve is 103 miles long, 82 miles wide, and includes 6,615 square
+miles. Lastly, the fourth isoseismal marks the boundary of the
+disturbed area, which is 250 miles long, 210 miles wide, and contains
+not more than 39,200 square miles; an amount that must be regarded as
+strangely small, and hardly justifying Mallet's estimate of the
+Neapolitan earthquake as the third among European earthquakes in
+extent as well as in seventy.</p>
+
+<br />
+
+<h4>DAMAGE CAUSED BY THE EARTHQUAKE.</h4>
+
+<p>As regards destruction to life and property, however, the Neapolitan
+earthquake owns but few European rivals. Less favourable conditions
+for withstanding a great shock are seldom, indeed, to be found than
+those possessed by the medi&aelig;val towns and villages of the meizoseismal
+area. In buildings of every class, the walls are very thick and
+consist as a rule of a coarse, short-bedded, ill-laid rubble masonry,
+without <span class='pagenum'><a name="Page_11" id="Page_11">[11]</a></span>thorough bonding and connected by mortar of slender cohesion.
+The floors are made of planks coated with a layer of concrete from six
+to eight inches thick, the whole weighing from sixty to a hundred
+pounds per square foot. Only a little less heavy are the roofs, which
+are covered with thick tiles secured, except at the ridges, by their
+own weight alone. Thus, for the most part, the walls, floors, and
+roofs are extremely massive, while the connections of all to
+themselves and to each other are loose and imperfect.</p>
+
+<p>Again, the towns, for greater security from attacks in early times,
+are generally perched upon the summits and steep flanks of hills,
+especially of the lower spurs that skirt the great mountain ranges;
+and the rocking of the hill-sites, in Mallet's opinion, greatly
+aggravated the natural effects of the shock. The streets, moreover,
+are steep and narrow, sometimes only five feet, and not often more
+than fifteen feet, in width; and the houses, when shaken down, fell
+against one another and upon those beneath them. As Dolomieu said of
+the great earthquake in 1783, "the ground was shaken down like ashes
+or sand laid upon a table."</p>
+
+<p>Of the total amount of damage, not even the roughest estimate can be
+made. The official returns are clearly, and no doubt purposely,
+deficient, and obstacles were placed in Mallet's way when he
+endeavoured to ascertain the numbers of persons killed and wounded.
+Taking only the towns into account, he calculated that, out of a total
+population of 207,000, the number of persons killed was 9,589, and of
+wounded 1,343.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a> A few towns were marked by an <span class='pagenum'><a name="Page_12" id="Page_12">[12]</a></span>excessively high
+death-rate. Thus, at Montemurro, 5000 out of 7002 persons were killed
+and 500 wounded; at Saponara, 2000 out of 4010 were killed; and, at
+Polla, more than 2000 out of a population of less than 7000.</p>
+
+<br />
+
+<h4>GENERAL OBJECTS OF INVESTIGATION.</h4>
+
+<p>The principal objects of Mallet's investigation were to determine the
+position of the epicentre and the depth of the seismic focus. If, in
+Fig. 3, F represents the seismic focus (here, for convenience,
+supposed to be a point), the vertical line FE will cut the surface of
+the earth in the epicentre E.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> The dotted lines represent circles
+drawn on the surface of the earth with E as centre and passing through
+the places P and Q.</p>
+
+<div class="img"><a name="Fig3" id="Fig3"></a>
+<a href="images/fig03.png">
+<img border="0" src="images/fig03.png" width="50%" alt="Diagram to illustrate wave-path and angle of emergence." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 3.&mdash;Diagram to illustrate wave-path and angle of emergence.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_13" id="Page_13">[13]</a></span>When the impulse causing the earthquake takes place at the focus, two
+elastic waves spread outwards from it in all directions through the
+earth's crust. The first wave which reaches a point P consists of
+longitudinal vibrations, that is, the particle of rock at P moves in a
+closed curve with its longer axis in the direction FP. Mallet supposes
+this curve to be so elongated that it is practically a straight line
+coincident in direction with FP. In the second or transversal wave,
+the vibration of the particle at P takes place in a plane at right
+angles to FP. These vibrations Mallet, for his main purpose, neglects.</p>
+
+<p>Returning to the longitudinal wave, Mallet calls the line FP the
+<i>wave-path</i> at P. The direction EP gives the azimuth of the wave-path,
+or its direction along the surface of the earth. The angle LPA, or
+EPF, he defines as the <i>angle of emergence</i> at the point P. If Q be
+farther from E than P, the angle EQF is less than the angle EPF, or
+the angle of emergence diminishes as the distance from the epicentre
+increases. At the epicentre, the angle of emergence is a right-angle;
+at a great distance from the epicentre, it is nearly zero.</p>
+
+<p>Mallet argued that the direction of the wave-path FPA, or its
+equivalents, the horizontal direction EPL and the angle of emergence
+EPF, should be discoverable from the effects of the shock at P. The
+cracks in damaged buildings, he urged, would be at right angles to the
+wave-path FPA; overturned monuments or gate-pillars should fall along
+the line EPL, either towards or from the epicentre according to their
+conditions of support; loose or slightly attached bodies, such as the
+stone balls surmounting gate-pillars, should be projected nearly in
+the direction of <span class='pagenum'><a name="Page_14" id="Page_14">[14]</a></span>the wave-path FPA, and their subsequent positions,
+supposing the balls not to have rolled, should give the horizontal
+direction EPL of the wave-path, and might, in some circumstances,
+determine the angle of emergence and the velocity with which they were
+projected. I shall return to details later on. For the present, it is
+clear that, in the destruction wrought by the earthquake, Mallet
+expected to find the materials most valuable for his purpose. Indeed,
+so obvious did this mode of examination appear to him, that he could
+not conceal his surprise at the blindness of his predecessors. They
+seem, he says, "to have been perfectly unconscious that in the
+fractured walls and overthrown objects scattered in all directions
+beneath their eyes, they had the most precious data for determining
+the velocities and directions of the shocks that produced them."</p>
+
+<br />
+
+<h4>POSITION OF THE EPICENTRE.</h4>
+
+<p><i>Mallet's Method of Determining the Position of the Epicentre.</i>&mdash;In
+many cases the examination of a damaged building or of an overthrown
+body served more than one purpose, providing materials for
+ascertaining the depth of the seismic focus as well as the position of
+the epicentre. For the present, however, it will be convenient to
+consider alone the method by which the latter object was to be
+attained.</p>
+
+<div class="imgl" style="width: 40%;"><a name="Fig4" id="Fig4"></a>
+<a href="images/fig04.png">
+<img border="0" src="images/fig04.png" width="95%" alt="Diagram to illustrate Mallet's method of determining position of epicentre." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 4.&mdash;Diagram to illustrate Mallet's method of determining position of epicentre.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Nothing could be simpler than the principle of the <span class='pagenum'><a name="Page_15" id="Page_15">[15]</a></span>method proposed.
+The horizontal direction PL of the wave-path at any place P (Fig. 4),
+when produced backwards, must pass through the epicentre E; and the
+intersection of the directions at two places, P and Q, must therefore
+give the position of the epicentre. In practice, it is of course
+impossible to determine the direction with very great accuracy, and
+Mallet therefore found it necessary to make several measurements in
+every place, and to visit all the more important towns within and near
+the meizoseismal area.</p>
+
+<p>In a ruined town there are many objects from which the direction may
+be ascertained, the most important of all, according to Mallet, being
+fissures in walls that are fractured but not overthrown. He regarded
+such fissures, indeed, as "the sheet-anchor, as respects direction of
+wave-path, to the seismologist in the field," and at least three out
+of every four of his determinations of the direction were made by
+their means. If the buildings are detached and large, simple and
+symmetrical in form, well built and not too much injured, the fissures
+in the walls should, he argued, occur along lines at right angles to
+the wave-path, whether that path be parallel or inclined to the
+principal axis of the building. Cracks in the floors and ceilings
+should also be similarly directed, and provide evidence which Mallet
+regarded as only second in value to that given by the walls.</p>
+
+<div class="img" style="clear: both;"><a name="Fig5" id="Fig5"></a>
+<a href="images/fig05.png">
+<img border="0" src="images/fig05.png" width="90%" alt="Plan of Cathedral Church at Potenza." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 5.&mdash;Plan of Cathedral Church at Potenza. (<i>Mallet.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>No building showed the different kinds of evidence on which Mallet
+relied as clearly as the cathedral church at Potenza, the plan of
+which is given in Fig. 5, and the vertical section along its axis in
+Fig. 12. This is a modern work, nearly 200 feet long, with its axis
+directed east and west. The walls are composed<span class='pagenum'><a name="Page_16" id="Page_16">[16]</a></span> <span class='pagenum'><a name="Page_17" id="Page_17">[17]</a></span>of fairly good rubble
+masonry and brick; and the arches in the nave and transepts, the
+semi-cylindrical roof and the central dome are made of brick. The
+fissures represented in both diagrams were drawn to scale by the
+cathedral architect before Mallet's arrival, and, as the work of an
+unbiassed observer, are of special value. Most of those in the roof,
+it will be seen, were transverse to the axial line of the church; but
+there were others parallel to this line, one in particular running
+right along the soffit of the nave and chancel. There were also
+numerous small fissures in the dome, due to local structural causes
+and therefore of varying direction, and a large portion of the dome
+slipped westward, leaving open fissures of seven to eight inches in
+width. The mean direction of the wave-path, as deduced from nine sets
+of fissures, none of which differs more than four degrees from the
+mean, is W. 2&frac12;&deg; S. and E. 2&frac12;&deg; N., which corresponds precisely
+with the direction of throw on the displaced portion of the dome. The
+great east and west fissures in the arch of the nave and chancel
+Mallet attributed to a second shock, of the existence of which there
+is ample evidence.</p>
+
+<div class="imgr" style="width: 40%;"><a name="Fig6" id="Fig6"></a>
+<a href="images/fig06.png">
+<img border="0" src="images/fig06.png" width="95%" alt="Fallen gate-pillars near Saponara." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 6.&mdash;Fallen gate-pillars near Saponara. (<i>Mallet.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Next to fissures, Mallet made most use of overthrown objects, such as
+the two gate piers near Saponara, represented in Fig. 6. They were
+made of rubble ashlar masonry, three feet square and seven feet in
+height. Both were fractured clean off at the <span class='pagenum'><a name="Page_18" id="Page_18">[18]</a></span>level of the ground, the
+mortar being poor, and fell in directions that were accurately
+parallel, indicating a wave-path towards S. 39&frac12;&deg;E. A few
+observations were also made on projected stones, fissures in nearly
+level ground, and the swinging of lamps and chandeliers; but their
+value was small, except as corroboration of the more important
+evidence afforded by fissures in the walls and roofs of buildings.</p>
+
+<p><i>Remarks on Mallets Method.</i>&mdash;It would have been more difficult in
+Mallet's day than it is now, to offer objections to his method of
+determining the position of the epicentre. The focus, as he was well
+aware, could not be a point, and, at places near the epicentre (the
+very places where most of his observations were made), there must be
+rapid changes of direction due to the arrival of vibrations from
+different parts of the focus. He records the occurrence of the
+so-called vorticose shocks at several places, though he attributes
+them to another cause. Perhaps the best known example of such a shock
+is that which has been so well illustrated by the late Professor
+Sekiya's model of the motion of an earth-particle during the Japanese
+earthquake of January 15th, 1887. The motion in this case was so
+complicated that the model was, for simplicity, made in three parts,
+the first of which alone is represented in Fig. 7.<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> It is clear
+that in such an earthquake, Mallet's method would utterly fail in
+giving definite results.</p>
+
+<p>While this shock was one of great complexity, another Japanese
+earthquake, that of June 20th, 1894, was unusually simple in
+character. The movement at Tokio consisted of one very prominent
+oscillation <span class='pagenum'><a name="Page_19" id="Page_19">[19]</a></span>with a total range of 73 mm. or 2.9 inches in the
+direction S. 70&deg; W.; the vibrations which preceded and followed it
+being comparatively small. Most, if not all, of the damage caused by
+the earthquake must have been due to this great oscillation; and yet
+the cylindrical stone-lamps so common in Japanese gardens were found
+by Professor Omori to have fallen in many different directions. Taking
+only those which had circular bases, twenty-nine were overthrown in
+directions between north and east, sixteen between east and south,
+eighty-one between south and west, and fourteen between west and
+north.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a> Fig. 8 represents Professor Omori's results graphically,
+<span class='pagenum'><a name="Page_20" id="Page_20">[20]</a></span>the line drawn from <span style="font-size: 115%;">O</span> to any point being proportional to the number of
+lamps which fell in directions between 7&frac12;&deg; on either side of the
+line.</p>
+
+<div class="img"><a name="Fig7" id="Fig7"></a>
+<a href="images/fig07.png">
+<img border="0" src="images/fig07.png" width="70%" alt="Model to illustrate the motion of an earth-particle during an earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 7.&mdash;Model to illustrate the motion of an earth-particle during an earthquake. (<i>Sekiya.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="img"><a name="Fig8" id="Fig8"></a>
+<a href="images/fig08.png">
+<img border="0" src="images/fig08.png" width="70%" alt="Plan of directions of fall of overturned stone-lamps at Tokio during the earthquake of 1894." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 8.&mdash;Plan of directions of fall of overturned stone-lamps at Tokio during the earthquake of 1894.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>It will be seen from this figure that most of the stone lamps fell in
+directions between west and south-west, and it is remarkable that the
+mean direction of fall is S. 70&deg; W.,<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> which is exactly the same as
+that of the great oscillation. Somewhat similar results were obtained
+by this able seismologist at different places affected by the great
+Japanese earthquake of 1891 (Figs. 43 and 44), and the study of the
+apparent directions observed during the Hereford earthquake of 1896
+leads to the same conclusion.</p>
+
+<p>It thus appears that an isolated observation may give a result very
+different from the true direction. Indeed, if we may judge from
+Professor Omori's measurements in 1894, the chance that a single
+direction may be within five degrees of the mean direction is about 1
+in 9. But, on the other hand, it is equally clear from these and other
+observations that the mean of a large number of measurements will give
+a result that agrees very closely with the true direction.</p>
+
+<p>One other point may be alluded to before leaving Professor Omori's
+interesting observations. It would seem, from the list that he gives,
+that he exercised no selection in his measurements, but continued
+measuring the direction of every fallen lamp indifferently until he
+had obtained sufficient records for his purpose. Now, if the number of
+fallen lamps at his disposal had been small, say 12 <span class='pagenum'><a name="Page_21" id="Page_21">[21]</a></span>instead of 144,
+the mean observed direction would probably have differed from the
+direction given from the seismograph.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a> But, on the other hand, a
+preliminary survey without any actual measurements would have revealed
+at once the predominant direction of overthrow, and a fairly accurate
+result might have been obtained by neglecting discordant directions
+and taking the mean of those only which appeared to agree with the
+mentally determined average.</p>
+
+<p>This, indeed, appears to have been the course followed, more or less
+unconsciously, by Mallet in his Neapolitan work. "When the observer,"
+he says, "first enters upon one of those earthquake-shaken towns, he
+finds himself in the midst of utter confusion. The eye is bewildered
+by 'a city become an heap.' He wanders over masses of dislocated stone
+and mortar, with timbers half buried, prostrate, or standing stark up
+against the light, and is appalled by spectacles of desolation....
+Houses seem to have been precipitated to the ground in every direction
+of azimuth. There seems no governing law, nor any indication of a
+prevailing direction of overturning force. It is only by first gaining
+some commanding point, whence a general view over the whole field of
+ruin can be had, and observing its places of greatest and least
+destruction, and then by patient examination, compass in hand, of many
+details of overthrow, house by house and street by street, analysing
+each detail and comparing the results, as to the direction of force,
+that must have produced each particular fall, with those previously
+<span class='pagenum'><a name="Page_22" id="Page_22">[22]</a></span>observed and compared, that we at length perceive, once for all, that
+this apparent confusion is but superficial."</p>
+
+<div class="img"><a name="Fig9" id="Fig9"></a>
+<a href="images/fig09.png">
+<img border="0" src="images/fig09.png" width="80%" alt="Meizoseismal area of Neapolitan earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 9.&mdash;Meizoseismal area of Neapolitan earthquake. (<i>Mallet.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Mallet's Determination of the Epicentre.</i>&mdash;Within the third
+isoseismal line Mallet made altogether 177 measurements of the
+direction of the wave-path at 78 places. These are plotted on his
+great map <span class='pagenum'><a name="Page_23" id="Page_23">[23]</a></span>of the earthquake; but, owing to the small scale of Fig. 9,
+it is only possible to represent, by means of short lines, the mean or
+most trustworthy direction at each place.<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a> Producing these
+directions backwards, he found that those at sixteen places passed
+within five hundred yards of a point which is practically coincident
+with the village of Caggiano; those at sixteen other places passed
+within one geographical mile (1.153 statute miles) of this point; the
+directions at sixteen more places within two and a half geographical
+miles; while those at twelve places passed through points not more
+than five geographical miles from Caggiano. As the direction of the
+shock at places near the epicentre must have been influenced by the
+mere size of the focus, this approximate coincidence is certainly
+remarkable, and there can be little doubt, I think, that the
+epicentre, or, at any rate, <i>an</i> epicentre must have been situated not
+far from the position assigned to it by Mallet's laborious
+observations.</p>
+
+<p><i>Existence of Two Epicentres.</i>&mdash;It is difficult, however, to realise
+that the impulse at the focus corresponding to Mallet's epicentre was
+the origin of all the destruction of life and property that occurred.
+The position of the epicentre close to the north-west boundary of the
+meizoseismal area, the extraordinary extension of that area towards
+the south-east, and especially the great loss of life at Montemurro
+and the adjoining towns, can hardly be accounted for in this manner.
+Mallet himself recognised that these facts required explanation, and
+he suggested that the situation and character of the <span class='pagenum'><a name="Page_24" id="Page_24">[24]</a></span>towns were in
+part responsible for their ruin, and the physical structure of the
+country for the course of the isoseismal lines. But the comparative
+escape of places much nearer Caggiano, and the wide extent of the
+meizoseismal area, embracing many towns and villages of varied
+character and site and many different surface-features, point
+unmistakably to a different explanation.</p>
+
+<div class="img"><a name="Fig10" id="Fig10"></a>
+<a href="images/fig10.png">
+<img border="0" src="images/fig10.png" width="60%" alt="Distribution of death-rate within meizoseismal area of Neapolitan earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 10.&mdash;Distribution of death-rate within meizoseismal area of Neapolitan earthquake.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>One clue to the solution of the problem is afforded by the seismic
+death-rate of the damaged towns. From a table given by Mallet (vol.
+ii. pp. 162-163), we know the population before the earthquake of the
+different communes in the province of Basilicata, and the loss of life
+in each due to the shock; and from these figures we can find the
+percentage of deaths at nearly every place of importance. As will be
+seen from Fig. 10, it varies from seventy-one at <span class='pagenum'><a name="Page_25" id="Page_25">[25]</a></span>Montemurro and fifty
+at Saponara down to less than one at all the places marked to which
+figures are not attached. There is thus a group of places, with its
+centre near Montemurro, where the loss of life far exceeded that in
+the surrounding country; and also a slightly less-marked group, with
+its centre near Polla, in the north-west of the meizoseismal area;
+while in the intermediate region the death-rate was invariably small.
+Too much stress should not be laid upon the exact figures, for there
+were no doubt local conditions that affected the death-roll. But it
+seems clear that one focus was situated not far from Montemurro; while
+the north-westerly group of places, combined with Mallet's
+observations on the direction, point to a second focus near Polla,
+about twenty-four miles to the north-west. It will be seen in a later
+section that the observations on the nature of the shock also imply
+the existence of a double focus.</p>
+
+<br />
+
+<h4>DEPTH OF THE SEISMIC FOCUS.</h4>
+
+<p><i>Mallet's Method of Determining the Depth of the Focus.</i>&mdash;In
+ascertaining the position of the epicentre, Mallet's work was
+remarkable only for the novelty of the method employed by him; but, in
+his attempt to calculate the depth of the seismic focus, he was
+breaking new ground. That the depth must be comparatively small had
+already been recognised, and was indeed obvious from the limited area
+disturbed by nearly every earthquake. No one, however, had tried to
+estimate the depth in miles; and it is impossible not to sympathise
+with Mallet while he accumulated his observations with feverish
+activity and subjected them to the first rough examination <span class='pagenum'><a name="Page_26" id="Page_26">[26]</a></span>even if
+one cannot share his confidence that he had succeeded in measuring the
+depth "in miles and yards with the certainty that belongs to an
+ordinary geodetic operation."</p>
+
+<div class="imgl" style="width: 50%;"><a name="Fig11" id="Fig11"></a>
+<a href="images/fig11.png">
+<img border="0" src="images/fig11.png" width="90%" alt="Diagram to illustrate Mallet's method of determining depth of seismic focus." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 11.&mdash;Diagram to illustrate Mallet's method of determining depth of seismic focus.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The method employed by him for the purpose is no less simple
+theoretically than that used for locating the epicentre. If the
+position of the latter (E) is known, one accurate measurement of the
+angle of emergence EPF, at any other point P would be sufficient to
+fix the depth of some point within the focus F (Fig. 11). Here, again,
+Mallet relied chiefly on fissures in walls that were fractured but not
+overthrown. In detail, these fissures are nearly always jagged or
+serrated, for they tend to follow the lines of joints rather than
+break through the solid stone, though they sometimes traverse bricks
+and mortar alike. But the general course of the fissures, he urged,
+would be at right angles to the wave-path, and their inclination to
+the vertical should be equal to the angle of emergence.</p>
+
+<p>In obtaining measurements of this angle, the buildings to be chosen
+are those of large size, with few windows or other apertures, and with
+walls made of brick or small short-bedded stones. The cathedral-church
+at Potenza perhaps satisfies these conditions more closely than any
+other structure examined by Mallet. The plan of the fissures in the
+walls and roof has been given in Fig. 5, and Fig. 12 represents the
+fissures In the vertical section along the axial line and looking
+north, as drawn by the cathedral architect. From these fissures Mallet
+calculated the mean angle of emergence at Potenza to be 23&deg; 7'. The
+distance of Potenza from Caggiano being seventeen miles, and the
+height of the former being 2,580 feet, the depth of the focus
+resulting from this observation alone would be 6&frac34; miles below the
+level of the sea.</p>
+
+<div class="img" style="clear: both;"><a name="Fig12" id="Fig12"></a><span class='pagenum'><a name="Page_27" id="Page_27">[27]</a></span>
+<a href="images/fig12.png">
+<img border="0" src="images/fig12.png" width="90%" alt="Vertical section of Cathedral Church at Potenza." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 12.&mdash;Vertical section of Cathedral Church at Potenza. (<i>Mallet.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_28" id="Page_28">[28]</a></span><i>Objection to Mallet's Method.</i>&mdash;The weakest point in Mallet's method
+is probably his assumption that the wave-paths are straight lines
+extending outward from the focus. Even if the depth of the focus is
+not more than a few miles, the waves must traverse rocks of varying
+density and elasticity, and, at every bounding surface, they must
+undergo refraction. If the rocks are so constituted that the velocity
+of the earth-waves in them increases with the depth, then the
+wave-paths must be bent continually outwards from the vertical, so
+that the angle of emergence at the surface may be considerably less
+than it would have been with a constant velocity throughout. In this
+case, the actual depth will be greater, perhaps much greater, than the
+calculated depth. For instance, if the angle of emergence at Potenza
+were diminished only 5&deg; by refraction, the calculated depth of the
+focus would be too small by 1&frac34; miles.</p>
+
+<div class="imgr" style="width: 25%;"><a name="Fig13" id="Fig13"></a>
+<a href="images/fig13.png">
+<img border="0" src="images/fig13.png" width="65%" alt="Diagram of wave-paths at seismic vertical of Neopolitan earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 13.&mdash;Diagram of wave-paths at seismic vertical of Neopolitan earthquake. (<i>Mallet.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Mallet's Estimate of the Depth of the Focus.</i>&mdash;Mallet measured the
+angle of emergence at twenty-six places, the mean angle (<i>i.e.</i> the
+mean of the greatest and least observed angles) varying from 72&deg; at
+Vietri di Potenza and 70&deg; degrees at Pertosa, which are about two
+miles from the calculated epicentre, to 11&frac12;&deg; at Salerno, distant
+about 40 miles. Fig. 13 reproduces part of the diagram on which he
+plotted the mean <span class='pagenum'><a name="Page_29" id="Page_29">[29]</a></span>angle of emergence at different places. The
+horizontal line represents the level of the sea, and the vertical line
+one passing through the epicentre and focus, called by Mallet the
+"seismic vertical." The lines on the left-hand side represent the
+commencing wave-paths (assumed straight) to the observing stations
+situated to the westward of the meridian through the epicentre, those
+on the right-hand side corresponding to places to the eastward of the
+same meridian. Small horizontal marks are added to indicate the depth
+in miles below the level of the sea.</p>
+
+<p>It will be seen, from this diagram, that all the wave-paths start from
+the seismic vertical at depths between three and nine miles; but the
+points of departure are clustered thickly within a portion, the length
+of which is about 3&frac12; miles and the mean depth about 6&frac12; miles. So
+great was Mallet's confidence in these calculations that he assigns
+the diverging origin of the wave-paths to different points of the
+focus, and thus concludes that, while the mean depth of the focus was
+about 6&frac12; miles, its dimensions in a vertical direction did not
+exceed 3&frac12; miles.</p>
+
+<p><span class='pagenum'><a name="Page_30" id="Page_30">[30]</a></span>How far Mallet's results should be accepted as correct, it is
+difficult to say in our ignorance of the constitution of the earth's
+interior. There can be no doubt that the focus was of considerable
+size, and that, in consequence, the wave-paths would diverge from
+different points of it. But that each wave-path should actually
+intersect the focus, and so enable its magnitude to be determined,
+would surely involve an approach to some law connecting the direction
+of a wave-path with the depth of its own origin, and no such law seems
+to be ascertainable. Nor can the limitation of these apparent origins
+between certain depths be held to argue that the focus, or any part of
+it, was equally confined, for the wave-paths would to a great extent
+be similarly refracted. I fear that the only conclusions that we can
+with safety draw from Mallet's admirable work are that his figures
+indicate the order of magnitude both of the vertical dimensions and of
+the mean depth of the focus.</p>
+
+<br style="clear: both;" />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>It is not easy to form any precise image of the earthquake as it
+appeared to the terrified witnesses within the meizoseismal area. To
+minds unbalanced by the suddenness of the shock and by the crash of
+falling houses, actuated too by the intense need of safety, the mere
+succession of events must have presented but little interest. The
+interval of two months that elapsed between the occurrence of the
+earthquake and its investigation was also unfavourable to the
+collection of accurate accounts from a wonder-loving people. Only one
+feature, therefore, stands <span class='pagenum'><a name="Page_31" id="Page_31">[31]</a></span>out clearly in the few records given by
+Mallet&mdash;namely, the division of the shock into two distinct parts.</p>
+
+<p>In the central district, this division is perhaps less apparent than
+elsewhere. At Polla, for instance, which lies close to the north-west
+epicentre, the first warning was given by a rushing sound; almost
+instantly, and while it was yet heard, came a strong subsultory or
+up-and-down movement, succeeded after a few seconds, but without any
+interval, by an undulatory motion. At Potenza, which is not far from
+the same epicentre but a few miles outside the meizoseismal area, the
+separation was more pronounced. According to one observer, the first
+movement was from west to east; and, within a second or two
+afterwards, there was a less violent shock in a transverse direction,
+followed immediately by a shaking in all directions, called by the
+Italians vorticose. Naples lies sixty-nine miles from the north-west
+epicentre, and here more accurate observations could be made. Dr.
+Lardner, well known fifty years ago as a writer of scientific works,
+describes the first movement felt there as "a short, jarring,
+horizontal oscillation, that made all doors and windows rattle, and
+the floors and furniture creak. This ceased, and after an interval
+that seemed but a few seconds was renewed with greater violence, and,
+he thought, with a distinctly undulatory movement, 'like that in the
+cabin of a small vessel in a very short chopping sea.'"</p>
+
+<p>In five other earthquakes studied in this volume, the separation of
+the shock into two parts was a well-marked phenomenon. In the
+Neapolitan earthquake, the separation was so distinct that Mallet took
+some <span class='pagenum'><a name="Page_32" id="Page_32">[32]</a></span>pains to account for its origin. He regarded it in every case as
+due to the reflection or refraction of the earth-waves by underlying
+rocks, though he does not explain why the reflected or refracted wave
+should be more intense than that transmitted directly. I shall refer
+to the subject in greater detail when describing the Andalusian,
+Charleston, Riviera, and Hereford earthquakes. For the present, it may
+be sufficient to urge that the double shock cannot have been due to
+the separation of the original waves by underground reflection or
+refraction, for then the second part should have been generally the
+weaker; nor to the succession of longitudinal and transverse waves,
+for, in that case, every earthquake-shock should be duplicated. The
+only remaining supposition is that there was a second impulse
+occurring either in the same or in a different focus.</p>
+
+<p>Which alternative should be adopted, the evidence on the nature of the
+shock is too scanty to determine. The defect is, however, supplemented
+by Mallet's observations on the direction of motion; for, at many
+places within and near the meizoseismal area, he met with the clearest
+signs of a double direction. Sometimes this was apparent to the senses
+of the observer; in other cases, damaged buildings presented two sets
+of fissures. At La Sala and near Padula, the first movement was
+roughly east and west, the second north and south. At Moliterno, there
+was evidence of a subordinate shock at right angles to the chief one;
+in the neighbourhood of Tramutola, its direction was from about E. 30&deg;
+S. In these and other cases, Mallet saw the effects of
+earthquake-echoes; but the underground reflection of earth-waves would
+give rise to the second part of the shock, not the first as <span class='pagenum'><a name="Page_33" id="Page_33">[33]</a></span>at La
+Sala and Padula. Moreover, the secondary directions, though they are
+seldom recorded accurately, point nearly to an epicentre not far from
+Montemurro. The observations on the nature and direction of the double
+shock thus confirm the conclusion, derived from the distribution of
+the seismic death-rate, that there were two detached foci, one near
+Polla and the other near Montemurro.</p>
+
+<p>This seems to be the best explanation of the facts recorded by Mallet.
+There is, however, a possible difficulty that should not be
+overlooked&mdash;namely, the apparently slight influence of the Montemurro
+focus on the mean direction of the shock (Fig. 9). At a few places, of
+course, the mean direction passes through both epicentres; at some
+others, as we have seen, one of the two observed directions points
+towards the Montemurro epicentre. It is not impossible, also, that
+Mallet, after the first few days' work, may occasionally have quite
+unconsciously selected and measured those fissures from the maze
+presented to him which agreed most closely with his early impressions
+obtained from the neighbourhood of Polla. But, for places nearer Polla
+than Montemurro (and these form the majority of those visited by
+Mallet), the probable explanation of the difficulty is that the
+Montemurro focus was not so deep as the Polla focus. This, as will
+appear more fully in the next chapter, would account for the
+comparatively great intensity in the immediate neighbourhood of
+Montemurro and for its rapid decline outwards; and it receives some
+support from an isolated reference by Mallet to two angles of
+emergence at Padula, one of 25&deg; from the north, and the other of 8&deg; or
+10&deg; in the perpendicular walls.</p>
+
+<span class='pagenum'><a name="Page_34" id="Page_34">[34]</a></span><br />
+
+<h4>ELEMENTS OF THE WAVE-MOTION.</h4>
+
+<p>The elements of the wave-motion, as mentioned in the introductory
+chapter, are four in number, namely, the period, amplitude, maximum
+velocity, and maximum acceleration. If any two of these are known for
+each vibration&mdash;and the first two are now given by every accurately
+constructed seismograph&mdash;the others can be determined if the
+vibrations follow the law of simple harmonic motion.<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a></p>
+
+<p><i>Amplitude.</i>&mdash;To ascertain the amplitude, Mallet had to rely chiefly
+on the fissures made in very inelastic walls. If the parts into which
+such a wall are fractured are free to move, and yet, being inelastic,
+obliged to remain in the farthest position to which they are carried
+by the wave, the distance traversed by the centre of gravity of one of
+the displaced parts should give a "rude approximate measure" of the
+horizontal amplitude of the earth-wave. At Certosa, near Padula, he
+thus found the amplitude to be about 4 inches, at Sarconi about 4&frac34;
+inches, and at Tramutola about 4&frac12; inches. From somewhat similar
+evidence, the amplitude at Polla appears to have been about 2&frac12; or 3
+inches; and, from the oscillation of a suspended clock or watch on a
+rough wall, about 3&frac12; inches at La Sala and 1&frac34; inches at
+Barielle. With the exception of Barielle, these places lie nearly on a
+straight line passing through Mallet's epicentre, and he gives the
+following table, showing an increase in amplitude with the distance
+from the epicentre:&mdash;</p>
+
+<span class='pagenum'><a name="Page_35" id="Page_35">[35]</a></span><br />
+
+<table border="0" cellpadding="2" cellspacing="0" width="60%" summary="Increase in Amplitude">
+  <tr>
+  <td class="tdl" width="40%" style="white-space: nowrap;">&nbsp;</td>
+  <td class="tdc2" width="12%">Polla.</td>
+  <td class="tdc2" width="12%">La Sala.</td>
+  <td class="tdc2" width="12%">Certosa.</td>
+  <td class="tdc2" width="12%">Tramutola.</td>
+  <td class="tdc2" width="12%">Sarconi.</td>
+  </tr>
+  <tr>
+  <td class="tdl" style="white-space: nowrap;">Distance in miles</td>
+  <td class="tdc2">4.0</td>
+  <td class="tdc2">13.4</td>
+  <td class="tdc2">19.0</td>
+  <td class="tdc2">23.8</td>
+  <td class="tdc2">30.8</td>
+  </tr>
+  <tr>
+  <td class="tdl" style="white-space: nowrap;">Amplitude in inches</td>
+  <td class="tdc2">2&frac12;</td>
+  <td class="tdc2">3&frac12;</td>
+  <td class="tdc2">4</td>
+  <td class="tdc2">4&frac12;</td>
+  <td class="tdc2">4&frac34;</td>
+  </tr>
+</table>
+
+<br />
+
+<p class="noin">The existence of the Montemurro focus must, however, complicate any
+relation that may connect these two quantities.</p>
+
+<p><i>Maximum Velocity.</i>&mdash;The means at Mallet's disposal for determining
+the maximum velocity were more numerous than those available for the
+amplitude. From the dimensions of a fallen column of regular form we
+should be able, he remarks, to find an inferior limit to the value of
+the maximum velocity; while a superior limit at the same place may be
+obtained from some other regular solid which escaped being overthrown.
+If a loose body is projected by the shock at a place where the angle
+of emergence is known, the horizontal and vertical distances traversed
+by the centre of gravity will give the velocity of projection. Or, if
+two such bodies are projected at one place, the same measures for each
+will as a rule give both the angle of emergence and the velocity of
+projection. A third method depends on the fissuring of walls,
+supposing that we know the force per unit surface which, when suddenly
+applied, is just sufficient to produce fracture. Sometimes more than
+one method must be applied to the same object. The two gate-pillars
+near Saponara (illustrated in Fig. 6) for example required a
+horizontal velocity of 5.48 feet per second to fracture them, and an
+additional velocity of 5.14 feet per second to overthrow them.</p>
+
+<p>The well-known seismologist, Professor Milne, urges very forcibly that
+measurements obtained from the projection or fall of columns are
+unreliable, for the earlier tremors might cause the columns to rock,
+and <span class='pagenum'><a name="Page_36" id="Page_36">[36]</a></span>their overthrow need not therefore measure accurately the maximum
+velocity of the critical vibration.<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a> There can be no doubt that
+Mallet was alive to this difficulty, though he may not have
+appreciated it at its full value. Thus, at the Certosa de St. Lorenzo,
+a monastery near Padula, a vase projected from the summit of a slender
+gate-pier implied a velocity of 21&frac34; feet per second; and the excess
+of about 8&frac14; feet per second above the velocity determined by other
+means is attributed by him to the oscillation of the pier itself. How
+far this source of error enters into other observations it is
+impossible to say; but it is worth noticing how closely the velocities
+obtained by different methods agree with one another. Thus, from
+projection only, we have velocities of 11.5 feet per second at the
+Certosa, 11.8 at Moliterno and Monticchio, 14.8 at Tramutola, and 9.8
+feet per second at Sarconi; from overthrow alone, 11.0 feet per second
+at Viscolione, near Saponara, and 11.6 at Barielle; from overthrow and
+projection, 13.2 feet per second at Polla and 12.9 at Padula; from
+fracture and overthrow, 12.3 feet per second at Potenza and 15.6 at
+Saponara. The comparatively high values at Tramutola and Saponara,
+Mallet imagined might be due to the oscillation of the hills on which
+these towns are built. He therefore omits them in calculating the mean
+maximum velocity, which he finds to be twelve feet per second, a
+velocity less than that with which a man reaches the ground when he
+jumps off a table.</p>
+
+<p>With the same omissions, Mallet gives the following table, showing a
+general decrease in the maximum velocity as the distance from his
+epicentre increases:&mdash;</p>
+
+<span class='pagenum'><a name="Page_37" id="Page_37">[37]</a></span><br />
+
+<table border="0" cellpadding="2" cellspacing="0" width="70%" summary="Decrease in the Maximum Velocity">
+  <tr>
+  <td class="tdl" width="40%" style="white-space: nowrap;">&nbsp;</td>
+  <td class="tdc2" width="10%">Polla.</td>
+  <td class="tdc2" width="10%">Padula.</td>
+  <td class="tdc2" width="10%">Certosa.</td>
+  <td class="tdc2" width="10%">Moliterno.</td>
+  <td class="tdc2" width="10%">Sarconi.</td>
+  <td class="tdc2" width="10%">Viscolione.</td>
+  </tr>
+  <tr>
+  <td class="tdl" style="white-space: nowrap;">Distance in miles</td>
+  <td class="tdc2">4.0</td>
+  <td class="tdc2">19.0</td>
+  <td class="tdc2">19.0</td>
+  <td class="tdc2">29.4</td>
+  <td class="tdc2">30.0</td>
+  <td class="tdc2">30.8</td>
+  </tr>
+  <tr>
+  <td class="tdl" style="white-space: nowrap;">Max. vel. in ft. per sec.</td>
+  <td class="tdc2">13.2</td>
+  <td class="tdc2">12.9</td>
+  <td class="tdc2">11.5</td>
+  <td class="tdc2">11.8</td>
+  <td class="tdc2">11.0</td>
+  <td class="tdc2">9.8</td>
+  </tr>
+</table>
+
+<p>On the north side of the epicentre we have:&mdash;</p>
+
+<table border="0" cellpadding="2" cellspacing="0" width="70%" summary="Decrease in the Maximum Velocity">
+  <tr>
+  <td class="tdl" width="40%" style="white-space: nowrap;">&nbsp;</td>
+  <td class="tdc2" width="10%">Potenza.</td>
+  <td class="tdc2" width="10%">Monticchio.</td>
+  <td class="tdc2" width="10%">Barielle.</td>
+  <td class="tdc2" width="30%">&nbsp;</td>
+  </tr>
+  <tr>
+  <td class="tdl" style="white-space: nowrap;">Distance in miles</td>
+  <td class="tdc2">17.3</td>
+  <td class="tdc2">27.1</td>
+  <td class="tdc2">28.2</td>
+  <td class="tdc2">&nbsp;</td>
+  </tr>
+  <tr>
+  <td class="tdl" style="white-space: nowrap;">Max. vel. in ft. per sec.</td>
+  <td class="tdc2">12.3</td>
+  <td class="tdc2">11.8</td>
+  <td class="tdc2">11.6</td>
+  <td class="tdc2">&nbsp;</td>
+  </tr>
+</table>
+
+<p class="noin">It is not impossible that the high calculated velocities at Tramutola
+and Saponara were partly or entirely due to the impulse from the
+Montemurro focus.</p>
+
+<p>If we take 4 inches for the amplitude of the largest variation, and 12
+feet per second for the maximum velocity, and assume the motion to
+have been of a simple harmonic character, the period of a complete
+vibration would be less than one-fifth of a second.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a> Now, we know
+from seismographic records that this is roughly the period of the
+small tremors that form the commencement of an earthquake-shock, while
+the period of the largest vibrations may amount to as much as one or
+two seconds. We may therefore conclude either that the assumption of
+simple harmonic motion is incorrect, or that the maximum velocity is
+too great, or more probably perhaps that the amplitude is too
+small.<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a></p>
+
+<br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p>Mallet was one of the first seismologists to realise the significance
+of the earthquake-sound; and he attended closely to the subject,
+though finding the <span class='pagenum'><a name="Page_38" id="Page_38">[38]</a></span>sound even more elusive of precise observation
+than the shock.</p>
+
+<p>The chief result obtained by him was the comparative smallness of the
+area over which the sound was heard. He estimates it at little more
+than 3,300 square miles, or about one-twelfth of that over which the
+shock was felt. It extends north and south from Melfi to Lagonegro,
+and east and west from Monte Peloso to Duchessa and Senerchia. The
+sound was thus confined to the region in which the shock attained its
+most destructive character.</p>
+
+<p>Towards the north and south ends of the sound-area all observers
+described the sound as a low, grating, heavy, sighing rush, lasting
+from twenty to sixty seconds, some adding that it was also of a
+rumbling nature. Near the centre and the east and west boundaries, the
+sound was distinctly more rumbling; it was shorter in duration, and
+began and ended more abruptly.</p>
+
+<p>The earthquake, Mallet remarks, "began everywhere with tremors; the
+sounds generally arrived at the same time; the apparent direction of
+movement of the tremulous oscillations appeared rapidly to change, and
+still more rapidly to increase in amplitude; then the great <i>shove</i> of
+the destructive shock arrived, in some places rather before, in some a
+little after, the moment of loudest sound, and it died away suddenly
+(<i>i.e.</i>, with extreme rapidity) into tremors again, but differing in
+direction from that of the great shock itself."<a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a></p>
+
+<p>The earthquake-sound will be described more fully in the chapter
+dealing with the Hereford earthquake <span class='pagenum'><a name="Page_39" id="Page_39">[39]</a></span>of 1896, in which it will be
+found that the phenomena recorded by Mallet are equally characteristic
+of the slighter shocks felt in this country.</p>
+
+<br />
+
+<h4>VELOCITY OF THE EARTH-WAVES.</h4>
+
+<p>In 1857 little was known about the velocity of earthquake-waves.
+Experiments had been made by Mallet himself in 1849 in the
+neighbourhood of Dublin. These gave 825 feet per second for the
+velocity in dense wet sand, 1,306 feet per second in discontinuous
+granite, and 1,665 feet per second in more solid granite.<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a> The only
+earthquake for which the velocity had been calculated was the Rhenish
+earthquake of 1846, the value ascertained by Schmidt being 1,376
+French feet, or 1,466 English feet, per second.</p>
+
+<p>The accurate public measurement of time, which, as Mallet remarks, is
+one of the surest indications of advancing civilisation, was, however,
+unknown in the kingdom of Naples; and his attempt was therefore
+fettered by the rarity of precise estimates of the time of occurrence.
+Throughout the whole disturbed area only six good records could be
+obtained, and three of these (at Vietri di Potenza, Atella, and
+Naples) were derived from stopped clocks, witnesses of rather doubtful
+value. At Montefermo and Barielle the time was at once read from a
+watch, and at Melfi from an accurate pocket chronometer. The times
+given vary from 9h. 59m. 16s. <span class="fakesc">P.M.</span> (Naples mean time) at
+Vietri di Potenza to 10h. 7m. 44s. at Naples. Allowing for the
+supposed change of direction by refraction at <span class='pagenum'><a name="Page_40" id="Page_40">[40]</a></span>the Monte St. Angelo
+range on the way to Naples, Mallet finds the mean surface velocity to
+be 787 feet per second. Omitting the Naples record, and taking account
+of the calculated depth of the focus, the mean velocity becomes 804
+feet per second.</p>
+
+<br />
+
+<h4>MINOR SHOCKS.</h4>
+
+<p>A great earthquake rarely, if ever, occurs without some preparation in
+the form of a marked increase of seismic activity. Perrey records
+several shocks during the two years 1856-57 that were felt at places
+as far apart as Naples, Melfi, and Cosenza. On December 7th, 1857, a
+slight shock, with a report from beneath like the explosion of a mine,
+was felt at Potenza. Then came the great earthquake on December 16th,
+at about 10 <span class="fakesc">P.M.</span></p>
+
+<p>This was followed by numerous after-shocks&mdash;how numerous it is
+impossible to say, for the records are of the scantiest description.
+For some hours the ground within the meizoseismal area is said to have
+trembled almost incessantly. At Potenza many slight shocks, both
+vertical and horizontal, were felt during the night, and for a month
+or more they were so frequent as to render enumeration difficult.
+Mallet's last record is dated March 23rd, 1858, when four slight
+shocks were felt at La Sala and Potenza, but occasional tremors were
+reported to him until May 1859.</p>
+
+<p>The most important of all these after-shocks was one felt about an
+hour after the principal earthquake. Everywhere far less powerful, it
+was yet strong enough to shake down many buildings at Polla that had
+been shattered by the great shock. <span class='pagenum'><a name="Page_41" id="Page_41">[41]</a></span>Towards the south at Moliterno,
+and towards the north at Oliveto and Barielle, it evidently attracted
+very little attention. So far as can be judged from the evidence given
+by Mallet, the disturbed area seems to have been approximately of the
+same form and dimensions as the meizoseismal area, and elongated in
+the same direction, but concentric with the north-west focus.</p>
+
+<p>On the other hand, if we may rely on too brief evidence, several
+after-shocks recorded only at Montemurro, Saponara, Viggiano, or
+Lagonegro, were probably connected with the south-east or Montemurro
+focus.</p>
+
+<br />
+
+<h4>ORIGIN OF THE EARTHQUAKE.</h4>
+
+<p>Mallet's theories have suffered perhaps more than any other part of
+his work from the recent growth of our knowledge. From a historical
+point of view, some reference to his explanation of the origin of the
+Neapolitan earthquake seems desirable, and his own conscientious work
+demands it. On the other hand, his conclusions are, for the present at
+any rate, superseded, and it will therefore be sufficient to describe
+them briefly.</p>
+
+<p>Most of the wave-paths, as we have seen, pass within three miles of a
+point almost coincident with the village of Caggiano. Of the
+remainder, six traverse a spot about two miles farther to the
+south-west, and three cross another about two miles farther to the
+north-east. Neglecting other points of intersection, but taking
+account of the observed emergences at Vietri di Potenza, Auletta,
+Polla, etc., Mallet infers that the horizontal section of the focus
+<span class='pagenum'><a name="Page_42" id="Page_42">[42]</a></span>was a curve (indicated by the dotted line in Fig. 9) not less than ten
+miles in length, and passing from near Balvano on the north, close to
+Vietri di Potenza, Caggiano, and Pertosa, to a point about two and a
+half miles west of Polla. Again, he remarks, the observed emergences
+at places near the epicentre indicate that the vertical section of the
+seismic focus was either more or less curved, or more probably a
+surface inclined towards the south-east. He concludes, therefore, that
+the seismic focus was a curved fissure, 10 miles long and 3&frac12; miles
+in height, and with its centre at a depth of 6&frac12; miles below the
+level of the sea.</p>
+
+<p>The production of this great fissure, accompanied, perhaps by the
+injection into it of steam at high pressure, was regarded by Mallet as
+the cause of the principal earthquake. He imagines that the rent would
+start at or near the central point of the focus and then extend
+rapidly outwards in all directions. In the initial stage, vibrations
+of very small amplitude would alone be transmitted, and these would
+give rise to the early sounds and tremors. As the rending proceeded,
+the vibrations would increase in strength up to a certain point when
+they produced the shock itself. After this, they would decrease; and,
+in the final stage, would give place to the small vibrations
+corresponding to the sounds and tremors that marked the close of the
+earthquake.</p>
+
+<p>The rush of steam at high pressure into the focus Mallet does not seem
+to have considered essential, though he evidently regarded it as
+possible, indeed probable; and he suggests that it may have been in
+part the cause of the earthquake which occurred an hour later. Though
+feeling sceptical as to the <span class='pagenum'><a name="Page_43" id="Page_43">[43]</a></span>existence of any general law of increase
+of underground temperature, he assumes it, for the sake of
+illustration, to be 1&deg; F. for every 60 feet of descent. This would
+give a temperature of 339&deg; F. at the upper limit of the focus, 643&deg; F.
+at its central point, and 884&deg; F. at its lower margin. If the focus
+were filled with steam at each of these temperatures, the
+corresponding pressures on its walls would be 8, 149, and 684
+atmospheres, respectively. As the steam may be supposed to be admitted
+suddenly and to be unlimited in supply, Mallet infers that it might
+exist at the tension due to the highest of these temperatures, in
+which case it would be capable of lifting a column of limestone 8,550
+feet in height (or about one-half the depth of the upper margin of the
+focus), and would exert a pressure on the walls of the focus of 4.58
+tons per square inch, or of more than 640,528 millions of tons upon
+its whole surface.</p>
+
+<p>So many pages have already been given to this interesting earthquake
+that I must sketch still more briefly my own view as to its origin.
+There were, I believe, two distinct foci with their centres about
+twenty-four miles apart along a north-west and south-east line, and it
+was to this arrangement that the elongation of the meizoseismal area
+was chiefly, though not entirely, due. The evidence is insufficient to
+determine whether the earthquake was caused by fault-slipping; it is
+in no way opposed to this view, but if the Neapolitan earthquake stood
+alone, we should hardly be justified in drawing any further inference.
+Relying, however, on knowledge obtained from the study of more recent
+shocks, it seems to me probable that the two foci formed parts of one
+<span class='pagenum'><a name="Page_44" id="Page_44">[44]</a></span>fault with a general north-west and south-east direction. The slip
+causing the first part of the double shock apparently took place
+within the south-east focus, and was followed after a few seconds by
+one within the north-west focus, greater in amount as well as more
+deeply seated. In consequence of these displacements there were local
+increases of stress, causing numerous small slips within or near both
+principal foci; and, if we may judge from some slight shocks felt at
+La Sala, accompanied also by other minor slips in the intermediate
+region of the fault.</p>
+
+<br />
+
+<h4>REFERENCE.</h4>
+
+<div class="block"><p class="hang"><span class="sc">Mallet, R.</span>&mdash;<i>The Great Neapolitan Earthquake of 1857: The
+First Principles of Observational Seismology</i>, etc. 2 vols
+1862.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> <i>Irish Acad. Trans.</i>, vol. xxi., 1848, pp. 51-105 (read
+Feb. 9, 1846).</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> <i>Brit. Assoc. Reports</i>, 1850, pp. 1-87; 1851, pp.
+272-330; 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> <i>A Manual of Scientific Enquiry</i>, edited by Sir J.F.W.
+Herschel, 1849, pp. 196-223.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> <i>Irish Acad. Trans.</i>, vol. xxii., 1855, pp. 397-410.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> The linear dimensions of the isoseismal lines are
+obtained by measurements from Mallet's maps. The areas are given by
+him in geographical square miles.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> Mallet, by some accident, omitted the losses at Polla and
+neighbouring towns from this estimate. Mercalli (<i>Geologia d'Italia</i>,
+pte. 3, p. 324) gives the number of killed as more than 12,300.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> Mallet does not make use of the term <i>epicentre</i>; he
+speaks of the line FE as the <i>seismic vertical</i>. The modern and
+accepted terms are used above.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> <i>Japan Seismol. Soc. Trans.</i>, vol. xi., 1887, pp.
+175-177.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> <i>Ital. Seismol. Soc. Boll.</i>, vol. ii., 1896, pp.
+180-188.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> Professor Omori gives the mean direction as S. 71&deg; W.,
+but this was obtained from observation on lamps with square, as well
+as with circular bases.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> Twelve measurements chosen at random from Professor
+Omori's list gave a mean direction of S. 78&deg; W.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> When the accuracy of all the observations seemed equally
+probable, he adopted the mean of the two extremes as the true
+direction.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> If <i>a</i> be the amplitude of a simple harmonic vibration,
+<i>T</i> its complete period, <i>v</i> its maximum velocity, and <i>f</i> its maximum
+acceleration, we have <i>v</i> = 2&pi;a &divide; <i>T</i> and&nbsp; <i>f</i> = 4&pi;&sup2;<i>a</i> &divide; <i>T</i>&sup2;</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> <i>Earthquakes and other Earth Movements</i>, pp. 81-82.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> Obtained from the formula: <i>T</i> = 2&pi;<i>a</i> &divide; <i>v</i> = 2&pi;<i>x</i> <span class="uni" title="one third">&#8531;</span> &divide; 12</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> If we take the maximum velocity to be 12 feet per
+second, and the period to be one second, the amplitude would be about
+11&frac12; inches.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> Vol. ii., p. 299. The punctuation of the original is not
+followed in the above extract.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> <i>British Association Report</i>, 1851, pp. 272-320.</p></div>
+
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_III" id="CHAPTER_III"></a><hr />
+<span class='pagenum'><a name="Page_45" id="Page_45">[45]</a></span><br />
+
+<h3>CHAPTER III.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY 28TH, 1883.</h4>
+<br />
+
+<p>Separated from Italy by a distance of not more than six miles, Ischia
+and the intermediate island of Procida strictly form part of the
+Phlegr&aelig;an Fields, the well-known volcanic district to the north of
+Naples. Ischia, the larger of the two islands, is six miles long from
+east to west, and five miles from north to south, and contains an area
+of twenty-six square miles. In 1881, the total population was 22,170,
+that of Casamicciola, the largest town, being 3,963.</p>
+
+<br />
+
+<h4>VOLCANIC HISTORY OF ISCHIA.</h4>
+
+<p>The central feature of Ischia is the great crater of Epomeo (<i>a</i>, Fig.
+14). On the south side, and partly also on the east, the crater-wall
+has been broken down and removed; the portion remaining is about 1&frac12;
+mile in diameter from east to west, and reaches a height of 2,600 feet
+above the sea-level. All the upper part of the mountain is composed of
+a pumiceous tufa, rich in sanidine and of a characteristic greenish
+colour. At two points, to the west near Forio and to the north between
+Lacco and Casamicciola, this tufa is seen reaching down to the sea;
+but, in all other parts, it is covered by streams of trachitic lava,
+by more recent tufas, or by a deposit of marly appearance, <span class='pagenum'><a name="Page_46" id="Page_46">[46]</a></span>which is
+regarded by Fuchs as resulting from the decomposition of the Epomean
+tufa.</p>
+
+<p>There are two distinct periods in the geological history of Ischia.
+The first, a submarine period, probably began with the dawn of the
+quaternary epoch, for all the marine fossils of the island belong to
+existing species. About this time, Epomeo seems to have originated in
+eruptions occurring in a sea at least 1,700 feet in depth&mdash;eruptions
+that preceded the formation of Monte Somma and were either
+contemporaneous or alternating with those that gave rise to the oldest
+trachitic tufas of the Phlegr&aelig;an Fields. The destruction of the south
+wall may have occurred much later through some great eruptive
+paroxysm, but more probably, as Professor Mercalli suggests, through
+early marine erosion and subsequent subaerial denudation. To the
+submarine period must also be assigned the formation of the trachitic
+masses which compose Monti Trippiti, Vetta, and Garofoli (<i>b</i>, <i>c</i>,
+<i>d</i>, Fig. 14), on the east side of Epomeo; and, in part only, those of
+Monte Campagnano and Monte Vezza (<i>f</i>, <i>g</i>).</p>
+
+<p>At or near the close of the elevation, many violent eruptions occurred
+on the south-west of Epomeo, during which was formed the south-west
+corner of the island, including Monte Imperatore and Capo Sant' Angelo
+(<i>h</i>, <i>i</i>).</p>
+
+<p>In the second or terrestrial period, when the island had practically
+attained its present altitude, the eruptive activity was almost
+confined to the eastern and northern flanks of Epomeo. At the
+beginning Monte Lo Toppo (<i>j</i>) was formed by a lateral eruption. In
+the north-west corner of the island, Monte Marecocco and Monte Zale
+(<i>k</i> and <i>l</i>) owe their origin to a gigantic flow of sanidinic
+trachite,<span class='pagenum'><a name="Page_47" id="Page_47">[47]</a></span> <span class='pagenum'><a name="Page_48" id="Page_48">[48]</a></span>issuing probably from the depression which now exists
+between them. Lastly, towards the north-east, are the recent lateral
+craters of Rotaro, Montagnone, Bagno, and Cremate (<i>m</i>, <i>n</i>, <i>p</i>,
+<i>s</i>), the first two being the most regular and best preserved in the
+island.</p>
+
+<div class="img"><a name="Fig14" id="Fig14"></a>
+<a href="images/fig14.png">
+<img border="0" src="images/fig14.png" width="60%" alt="Geological sketch-map of Ischia." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 14.&mdash;Geological sketch-map of Ischia. (<i>Mercalli.</i>)<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a><span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The earliest eruption of the historic, or rather human, period appears
+to have taken place from Montagnone, and probably also at about the
+same time from the secondary crater of Porto d'Ischia (<i>u</i>), about the
+beginning of the eleventh century <span class="fakesc">B.C.</span> The eruptions of
+Marecocco and Zale are referred to about <span class="fakesc">B.C.</span> 470; and those
+of Rotaro and Tabor (<i>q</i>) to between the years 400 and 352
+<span class="fakesc">B.C.</span> Another eruption is said to have occurred in
+<span class="fakesc">B.C.</span> 89, but the site of it is unknown; and three others are
+recorded on doubtful authority about the years <span class="fakesc">A.D.</span> 79-81,
+138-161, and 284-305. The last outburst of all took place after the
+series of earthquakes in 1302 from a new crater, that of Cremate
+(<i>s</i>), which opened on the north-east flank of Epomeo, and from which
+a stream of lava, called the Arso (<i>t</i>), flowed down rapidly and,
+after a course of two miles, reached the sea.</p>
+
+<p>After the first eruptions to which it owed its origin, the central
+crater of Epomeo apparently remained inactive. All the later eruptions
+occurred either on the external flanks of the mountain or on radial
+fractures of the cone.<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a> Trippiti, Lo Toppo, Montagnone and the Lago
+del Bagno (<i>b</i>, <i>j</i>, <i>n</i>, <i>p</i>) lie in one line, Vetta and Cremate
+(<i>c</i>, <i>s</i>) on another, and Garofoli and Vatoliere (<i>d</i>, <i>e</i>) on a
+third, all passing through a point near the town of Fontana, which
+occupies the centre of the old crater of Epomeo.</p>
+
+<p><span class='pagenum'><a name="Page_49" id="Page_49">[49]</a></span>Professor Mercalli points out that the lateral eruptions of Epomeo
+differ in one respect from those of Etna and Vesuvius. In these
+volcanoes the lava ascends to a considerable height in the central
+chimney, and by its own weight rends open the flanks of the cone. In
+Epomeo, it appears to traverse lateral passages at some depth, perhaps
+far below the level of the sea, and to rend the mountain by means of
+the elastic force of the aqueous vapour, etc., which it contains. It
+will be seen how important is the bearing of this difference on the
+occurrence of the Ischian earthquakes.</p>
+
+<p>The eruptions that have taken place during the last three thousand
+years agree in several particulars. They either occurred suddenly, or,
+at any rate, were not preceded by a stage of moderate Strombolian
+activity; they were always accompanied by violent earthquakes; and all
+succeeded intervals of long repose. As the eruption of 1302 happened
+after at least a thousand years of rest, the lapse of six more
+centuries does not justify us in concluding that Epomeo is at last
+extinct.</p>
+
+<p>We seem, on the contrary, to be drawing near another epoch of
+activity. During the four and a half centuries that followed the
+eruption of 1302, we have no record of Ischian earthquakes.<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a> Then,
+suddenly, on the night of July 28-29, 1762, Casamicciola was visited
+by sixty-two shocks, some of which were very strong and damaged
+buildings. On March 18th, 1796, another severe shock took place, but
+destructive only in the neighbourhood of Casamicciola, where seven
+persons were killed. On February 2nd, 1828, the area of damage, though
+concentric with the <span class='pagenum'><a name="Page_50" id="Page_50">[50]</a></span>former, enlarged its boundaries; 30 persons were
+killed and 50 wounded. On March 6th, 1841, and during the night of
+August 15-16, 1867, further shocks injured houses at Casamicciola, but
+without causing any loss of life. Slight tremors occurred at various
+dates in 1874, 1875, 1879, and 1880, leading up to the disastrous
+earthquakes here described, those of March 4th, 1881, when 127 persons
+were killed, and July 28th, 1883, which resulted in the death of 2,313
+persons and the wounding of many others.</p>
+
+<br />
+
+<h4>EARTHQUAKE OF MARCH 4TH, 1881.</h4>
+
+<p>The Ischian earthquakes have been fortunate in their investigators. In
+the spring of 1881, Dr. H.J. Johnston-Lavis, the chronicler for many
+years of Vesuvian phenomena, was residing in Naples. Impressed by a
+recent perusal of Mallet's report on the Neapolitan earthquake, and
+wishing to test the value of the methods explained in the last
+chapter, he crossed over to Ischia on March 5th; and to his unwearied
+inquiries extending over more than three weeks and lasting from
+thirteen to sixteen hours a day, we are indebted for most of what we
+know about the earthquake of 1881.</p>
+
+<p>On March 4th, at 1.5 <span class="fakesc">P.M.</span>, the great shock occurred abruptly,
+without any warning tremors. Its effects were aggravated by the faulty
+construction of the houses. The walls are of great thickness, loosely
+put together, and connected by mortar of the poorest quality. The
+chimneys and roofs also are massive, and the rafters are so slightly
+inserted in the walls that they were drawn out with the rocking of the
+houses. In such cases, the destruction was often so complete that no
+fissures were left available for measurement.</p>
+
+<span class='pagenum'><a name="Page_51" id="Page_51">[51]</a></span><br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>The isoseismal lines as drawn by Dr. Johnston-Lavis are represented by
+the curves in Fig. 15. The isoseismal marked 1 bounds the area of
+complete destruction; it is about 1 mile long from east to west, 2/3
+of a mile broad, and contains an area of not more than half a square
+mile. The next isoseismal (2) marks the area of partial, but still
+serious, destruction; this is nearly 2 miles long from east to west,
+1&frac14; miles broad, and 2 square miles in area. Within the isoseismal
+3, buildings were more or less slightly damaged. The course of this
+curve is somewhat doubtful, but, as <span class='pagenum'><a name="Page_52" id="Page_52">[52]</a></span>drawn, it is about 3 miles long,
+2 miles wide, and 5 square miles in area.</p>
+
+<div class="img"><a name="Fig15" id="Fig15"></a>
+<a href="images/fig15.png">
+<img border="0" src="images/fig15.png" width="60%" alt="Isoseismal lines of the Ischian earthquake of 1881." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 15.&mdash;Isoseismal lines of the Ischian earthquake of 1881. (<i>Johnston-Lavis.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Outside the last curve, the shock diminished rapidly in intensity. At
+Monte Tabor and Bagno, it was very slight; in the town of Ischia, only
+about half the people were conscious of any movement; and at Capella,
+a small village to the south, it was not felt at all. Again, the shock
+was perceptible, though only faintly, in the neighbourhood of
+Campagnano, at Serrara to the south of Epomeo, and at Panza near the
+south-west corner of the island. On the other hand, at Fontana, which
+occupies approximately the centre of the crater of Epomeo, there were
+evidences of a distinctly stronger shock. No house actually fell, and
+side walls were but little injured; but the roofs, which are of great
+weight, suffered considerable injury.</p>
+
+<p>In the adjacent island of Procida, the shock was felt distinctly by
+many people, and by some, though slightly, at Monte di Procida,
+Misenum, and Bacoli, on the coast of Italy. No record whatever was
+given by the seismographs in the university of Naples and the
+observatory on Vesuvius. We have of course no means of estimating the
+exact size of the disturbed area, but in this respect, disastrous as
+the earthquake was in the neighbourhood of Casamicciola, it was
+clearly inferior to all but the very weakest earthquakes felt in the
+British Islands.</p>
+
+<br />
+
+<h4>POSITION OF THE EPICENTRE.</h4>
+
+<p>In determining the position of the epicentre, Mallet's method was
+closely followed. Fissures in buildings were used for the most part,
+in two out of <span class='pagenum'><a name="Page_53" id="Page_53">[53]</a></span>every three cases; and occasional measurements were
+made from objects overthrown, projected, or shifted, and also from the
+personal experiences of observers. The attempt to apply the method
+was, however, fraught with difficulties. The heterogeneous structure
+of the island was no doubt responsible for many divergent azimuths;
+the irregularity of the buildings both in form and material and their
+variety of site furnished other sources of error; even the smallness
+of the area was a disadvantage in lessening the number of trustworthy
+records.</p>
+
+<p>Measurements were made at 55 places altogether, but in most cases they
+were the results of isolated observations, not the means of several at
+each place. On this account, I have not reproduced in Fig. 15 the
+azimuths shown in Dr. Johnston-Lavis's map of the earthquake. A large
+number of them clearly converge towards an area lying to the west of
+Casamicciola; and, from their arrangement, Dr. Johnston-Lavis
+concludes, though the evidence does not seem to me quite strong enough
+for the purpose, that they emanated from a fracture running from a
+little west of north to a little east of south.</p>
+
+<p>This conclusion is, however, justified by other evidence. In the
+centre of the injured district, Dr. Johnston-Lavis has traced a
+meizoseismal band, in which the shock must have been nearly or quite
+vertical. "The damage inflicted on buildings included within this band
+was," he says, "very characteristic of the nature of the shock; the
+walls having received but slight injury, whilst almost every floor and
+ceiling had been totally destroyed. In fact," he adds, "many houses
+would have required no other repairs than the replacing of the
+divisions between <span class='pagenum'><a name="Page_54" id="Page_54">[54]</a></span>the different storeys." The shaded central area in
+Fig. 15 represents this band, passing in a nearly north and south
+direction from a point midway between Campo and the upper part of
+Lacco on the north, through the west part of Casamenella and Campo, to
+a point near Frasso on the south; the length of the band being thus
+about two-thirds of a mile.</p>
+
+<p>If the central line of this band is produced towards the south, as
+indicated by the dotted line, it grazes the west side of Fontana,
+where, as we have seen, there was a second meizoseismal area, much
+smaller than the other and surrounded by a district in which houses
+were almost uninjured. That the shock in this town was vertical or
+nearly so, is shown by the nature of the damage (p. 52) and also by
+the testimony of the inhabitants. I will give Dr. Johnston-Lavis's
+explanation of this detached meizoseismal area when discussing the
+origin of the Ischian earthquakes; but the evidence seems to me to
+favour either the existence of two distinct foci or, more probably
+perhaps, the extension of the fissure to the south with an increased
+impulse beneath the centre of Epomeo.</p>
+
+<br />
+
+<h4>DEPTH OF THE SEISMIC FOCUS.</h4>
+
+<p>At nine places, Dr. Johnston-Lavis was able to make measurements of
+the angle of emergence, in every case from fissures in buildings, and
+therefore liable to sources of error already referred to. On the other
+hand, owing to the small depth of the focus, there would probably be
+less general refraction of the wave-paths than in the Neapolitan
+earthquake. The depths indicated by these observations vary between
+<span class='pagenum'><a name="Page_55" id="Page_55">[55]</a></span>about 615 and 2,885 feet, a difference that is no greater than might
+be expected, as the size of the focus was no doubt comparable with
+that of the district in which observations were made. The mean depth
+Dr. Johnston-Lavis finds to be about 1,700 feet, or a little less than
+one-third of a mile.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>The limited depth of the focus is also evident from the nature of the
+shock. It was only within the actual meizoseismal band that the shock
+was subsultory or vertical throughout; at a short distance from the
+epicentre, the movement was both subsultory and undulatory; while near
+the third isoseismal, and in most of the region outside, the movement
+was entirely undulatory or lateral. An observer at Perrone (which lies
+1-2/3 miles east of the epicentre) gives the following account of the
+shock:&mdash;"I was standing on my balcony (this faces Casamicciola)
+admiring the scene ... when I felt the house rock, feeling at the same
+time as if something was rolling along beneath the ground. This
+movement was accompanied by a sound like this, Boob, boob&mdash;boob&mdash; &mdash;
+boob&mdash; &mdash; &mdash; boob&mdash; &mdash; &mdash; &mdash; boob. Both noise and movement seemed to
+come from Casamicciola.... In a few seconds, in the distance over the
+town arose a terrific cloud of white dust, so that I imagined the town
+on fire.... I felt hardly any, if any, subsultory movement, but as I
+leant upon the balcony rails, I was alternately pressed against them
+and then drawn away."</p>
+
+<p>At Fontana, however, the undulatory shock was replaced by a vertical
+one. This was the universal experience, though one or two persons felt
+a slight <span class='pagenum'><a name="Page_56" id="Page_56">[56]</a></span>lateral movement immediately after. At Valle (near Barano)
+and Piejo, both places about a mile from Fontana, the vertical
+component was also perceptible.</p>
+
+<br />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p>The after-shocks were few and of slight intensity. Dr. Johnston-Lavis
+gives the following dates: March 7th, 12.5 <span class="fakesc">A.M.</span> and midday;
+March 11-12, 15-16, 17-18, 27 (?), April 5th and 6th, and July 18th,
+8.30 <span class="fakesc">P.M.</span> The only shock of the series marked as strong
+occurred at midnight on March 15-16 at Casamicciola. The last of all,
+that of July 18th, consisted of a rumble and slight shock, and was
+most perceptible at Fango.</p>
+
+<br />
+
+<h4>EARTHQUAKE OF JULY 28TH, 1883.</h4>
+
+<p>Undeterred by the experience of 1881 or by the warnings of
+seismologists, Casamicciola was rebuilt, only to suffer more complete
+disaster. On July 28th, 1883, at 9.25 <span class="fakesc">P.M.</span>, occurred the most
+destructive earthquake of which we have any record in Ischia. The
+shock lasted about fifteen seconds, and before it was over clouds of
+dust were rising above the ruins of Casamicciola, Lacco, and Forio;
+1,200 houses were destroyed, 2,313 persons were killed, nearly 1,800
+in Casamicciola alone, and more than 800 seriously wounded. "No better
+idea," says Dr. Johnston-Lavis, "of the absolute destruction of
+buildings could be conceived than what was actually realised at
+Casamicciola and Campo. Looking, on the following Monday, over the
+field of destruction, I could discover (with few exceptions) the
+wall-stumps only remaining."</p>
+
+<p><span class='pagenum'><a name="Page_57" id="Page_57">[57]</a></span>Dr. Johnston-Lavis again spent about three weeks in the island,
+examining the effects of the new shock with equal zeal and wider
+experience. His monograph is now our chief work of reference on
+Ischian earthquakes. Inquiries were also made by several Italian
+seismologists, among others by Professor M.S. de Rossi, the organiser
+of earthquake-studies in the peninsula; by Professor L. Palmieri, the
+founder of the Vesuvian observatory; and especially by Professor G.
+Mercalli, whose valuable memoir supplements the report of Dr.
+Johnston-Lavis in some important particulars.</p>
+
+<br />
+
+<h4>PREPARATORY SIGNS.</h4>
+
+<p>The interval between July 18th, 1881, when the last shock of that year
+was felt, and July 28th, 1883, was one of almost complete quiescence.
+Early in March 1882, a few slight shocks were noticed at Casamicciola.
+On July 24th, 1883, a watch hanging from a nail in a wall was seen to
+swing at 6 <span class="fakesc">A.M.</span> and 9 <span class="fakesc">A.M.</span>, and, on the same
+morning, at about 8.30, a slight shock, accompanied by a rumbling
+sound, was felt at Casamicciola. Again, on the 28th, about a quarter
+of an hour before the great shock, one observer at Casamicciola states
+that an underground noise was heard, and that some persons in
+consequence left their houses.</p>
+
+<p>Many assertions have been made with regard to variations witnessed a
+day or two before the shock in the hot springs, such as an increase of
+flow or temperature and changes in their volume and purity. Fumaroles
+are alleged to have burst out with violence, and even flames to have
+been seen. The statements, <span class='pagenum'><a name="Page_58" id="Page_58">[58]</a></span>though widely quoted, can hardly be said
+to rest on satisfactory evidence. On the other hand, Dr.
+Johnston-Lavis arrived in the island within twenty-four hours after
+the shock, and, before another day had elapsed, he had examined most
+of the places where the phenomena were said to have occurred, but
+could find no remarkable change nor any signs of such having taken
+place. It is also known, as he remarks, that the temperature of the
+Ischian springs and fumaroles sometimes varies considerably without
+any earthquake following, that of the water of Gurgitello occasionally
+changing by as much as 30&deg; or 40&deg;. We may therefore, I think, conclude
+that, except for one or two shocks and underground noises too slight
+to cause general alarm, there were no decisive heralds of the great
+earthquake.</p>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>The curves in Fig. 16 represent the isoseismal lines as drawn by Dr.
+Johnston-Lavis. As in the earthquake of 1881, they bound respectively
+the areas of complete destruction, partial destruction and slight
+damage to buildings, the course of the outer line being to a great
+extent conjectural owing to the small extent of land traversed by it.
+The first isoseismal is about 2&frac12; miles long, 1&frac12; miles broad, and
+3 square miles in area; the second about 4 miles long, 3&frac12; miles
+broad, and 11 square miles in area; and the third about 6&frac12; miles
+long, 6 miles broad, and 30 square miles in area. The curve drawn by
+Professor Mercalli (Fig. 14) coincides nearly with the second of these
+lines.</p>
+
+<p>At Fontana, the damage exceeded that in the <span class='pagenum'><a name="Page_59" id="Page_59">[59]</a></span>surrounding country,
+though the difference was of course less marked than on the previous
+occasion.</p>
+
+<div class="img"><a name="Fig16" id="Fig16"></a>
+<a href="images/fig16.png">
+<img border="0" src="images/fig16.png" width="80%" alt="Isoseismal lines of the Ischian earthquake of 1883." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 16.&mdash;Isoseismal lines of the Ischian earthquake of 1883.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Outside Ischia, the shock was felt distinctly in all the island of
+Procida and in Vivara; on the mainland, <span class='pagenum'><a name="Page_60" id="Page_60">[60]</a></span>by some as far as Pozzuoli
+and by several persons in Naples, which is twenty miles from
+Casamicciola. The seismograph at the university of this city
+registered two small shocks, the first at 9.10 <span class="fakesc">P.M.</span>, and the
+second and stronger at 9.25 <span class="fakesc">P.M.</span>; and De Rossi states that at
+about 9.30 <span class="fakesc">P.M.</span> the seismographs at Ceccano, Velletri, and
+Rome recorded a shock consisting of very slow undulations. There are
+again no materials for estimating the size of the disturbed area, but
+there can be no doubt that it was much less than that of a moderately
+strong British earthquake.</p>
+
+<br />
+
+<h4>POSITION OF THE EPICENTRE.</h4>
+
+<p>Owing to the limited size of the disturbed area, time-observations,
+even had they been available, would not have sufficed to determine the
+position of the epicentre, and both Dr. Johnston-Lavis and Professor
+Mercalli therefore had recourse to Mallet's method, the former relying
+chiefly, as before, on fissures in damaged buildings, and the latter
+on the overthrow or displacement of columns and other objects.</p>
+
+<p>Dr. Johnston-Lavis measured the azimuth of the wave-paths at
+sixty-five places, and at about one-third of these was able to make
+two or more observations. The azimuths converge towards the same
+region as in 1881, but the area covered by their intersections is
+larger. The meizoseismal band of maximum vertical destruction
+indicated by shading in Fig. 16 is also of the same form and slightly
+greater extent, reaching from the upper part of Lacco to a little
+south of Frasso, and being therefore nearly a mile in length. The
+centre of maximum impulse was in the same position as in 1881, or
+possibly a little more to the south.</p>
+
+<p><span class='pagenum'><a name="Page_61" id="Page_61">[61]</a></span>Professor Mercalli's observations were made at forty-eight places, and
+in only six cases were they the same as those used by his predecessor.
+He also notices that most of the azimuths converge towards
+Casamenella, and intersect within an elongated area. This area runs in
+the same direction as Dr. Johnston-Lavis's meizoseismal band, but is
+less elongated, and situated a short distance farther to the south,
+though on the whole the agreement between the two areas is remarkably
+close.</p>
+
+<p>There was again apparently a second epicentre at Fontana. In this
+town, according to Dr. Johnston-Lavis, there were two distinct types
+of damage. As in 1881, there was evidence of a vertical blow, the only
+one that absolutely ruined houses; but, in addition, there was another
+independent set of fissures, quite as widely distributed as the
+others, though evidently caused by a less violent movement. These
+indicated a wave-path with a low angle of emergence coming from
+between north and north-north-west, or almost exactly in the line of
+meizoseismal band. To the south of Fontana, however, there is a group
+of places, including Panza, Serrara, Barano, etc., where the azimuths
+diverged rather widely from the epicentre at Casamenella. These
+azimuths are twelve in number, and it is worthy of notice that they
+all intersected the crater of Epomeo, while half of them passed within
+a few hundred yards of Fontana.</p>
+
+<br />
+
+<h4>DEPTH OF THE SEISMIC FOCUS.</h4>
+
+<p>Measurements of the angle of emergence were made by Dr. Johnston-Lavis
+at twenty-four places, and in <span class='pagenum'><a name="Page_62" id="Page_62">[62]</a></span>every case from fissured walls. The
+greater part of the diagram on which his results are depicted is
+reproduced in Fig. 17. The horizontal line, as in Fig. 13, represents
+the level of the sea, the longer vertical line one passing through the
+epicentre, and the shorter another through Fontana. The short lines
+on <span class='pagenum'><a name="Page_63" id="Page_63">[63]</a></span>the left of the former show the incipient wave-paths to places
+lying east of the epicentre; those on the right, with one exception,
+represent the wave-paths to places west of the same meridian. Small
+horizontal marks are inserted on the vertical lines to show the depth
+in tenths of a mile below the level of the sea.</p>
+
+<div class="img"><a name="Fig17" id="Fig17"></a>
+<a href="images/fig17.png">
+<img border="0" src="images/fig17.png" width="60%" alt="Diagram of wave-paths at seismic vertical of Ischian earthquake of 1883." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 17.&mdash;Diagram of wave-paths at seismic vertical of Ischian earthquake of 1883. (<i>Johnston-Lavis.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The six angles of emergence that would give the greatest depth below
+the epicentre were all measured at places in the south of the island
+close to the line joining Panza and Barano, and it will be noticed
+that five of these apparent depths are much greater than those
+obtained from the other wave-paths. Excluding these observations, the
+remaining eighteen give depths ranging from about 450 to about 3,350
+feet, and a mean depth of 1,730 feet,<a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a> or nearly one-third of a
+mile, that is, almost exactly the same as the mean depth found from
+the earthquake of 1881.</p>
+
+<p>The six exceptional angles of emergence come from the district of
+divergent azimuths to the south of Epomeo. Three of the corresponding
+azimuths pass within one-quarter of a mile from the centre of Fontana,
+and none of the other three more than three-quarters of a mile from
+the same point. Though disbelieving in a subsidiary focus below this
+town, Dr. Johnston-Lavis has calculated its mean depth, supposing it
+to exist, and found it to be about 1,560 feet below the sea level, a
+result which is remarkably close to the calculated mean depth of the
+focus near Casamenella.</p>
+
+<span class='pagenum'><a name="Page_64" id="Page_64">[64]</a></span><br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>In the meizoseismal band, preliminary tremor and rumbling sound were
+alike absent. So sudden, indeed, was the onset of the earthquake, that
+the survivors generally found themselves beneath the ruins of their
+houses before they were conscious of any shock. The destruction,
+practically instantaneous, was wrought by four or five vertical blows,
+so powerful that, according to some observers, Casamicciola seemed to
+jump into the air. Then followed undulations, not noticed by all, that
+appeared to come from every direction. The shock lasted altogether
+fifteen seconds or more,<a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a> and was accompanied by a rumbling noise,
+in the midst of which were detonations as of thunder or of great blows
+given upon an empty barrel.</p>
+
+<p>In the immediate neighbourhood of the meizoseismal area, at Perrone,
+Pennella, and Lower Lacco, the subsultory movement was still the more
+prominent; but, farther away, as at Panza, Testacchio, Barano, Ischia,
+and Bagno, the subsultory motion was followed by distinctly horizontal
+undulations, while outside the island of Ischia only slow undulatory
+movements were perceptible.</p>
+
+<br />
+
+<h4>LANDSLIPS.</h4>
+
+<p>The dotted areas in Fig. 16 indicate the sites of the only landslips
+of importance that were precipitated by the earthquake of 1883. Two of
+these occurred <span class='pagenum'><a name="Page_65" id="Page_65">[65]</a></span>on the north slope of Epomeo, and the third on the
+west flank of Monte Rotaro. The materials of the Epomean landslips had
+evidently been separated for some time by shallow fissures from the
+adjoining rock, for the surfaces of the fissures were discoloured by
+fumarolic action. Immediately after the earthquake a cloud of dust was
+seen to rise from the spots; the masses, already detached laterally,
+were merely set in motion by the shock; and they continued to slide
+down during the following days either through the action of the
+after-shocks or of the heavy rains that followed.</p>
+
+<p>All over the island, however, fissures and minor landslips occurred.
+At two places on the north coast the steep cliffs of incoherent tufa
+were so much damaged that, according to Dr. Johnston-Lavis, "large
+quantities of their materials were thrown into the sea. The water then
+sorted out the pieces of pumice, which in many cases were of very
+large size, and were seen floating about in the neighbourhood for some
+days," giving rise to the supposition that a submarine eruption had
+taken place to the north of the island.</p>
+
+<br />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p>The after-shocks in 1883 were much more numerous than in 1881. Between
+9.25 <span class="fakesc">P.M.</span> on July 28th and noon on August 3rd, twenty-one
+slight shocks were recorded at Casamicciola. At 2.15 <span class="fakesc">P.M.</span> on
+August 3rd, a violent shock occurred that caused further damage at
+Forio, and even at places so far from the epicentre as Fiaiano,
+Barano, and Fontana, and increased the displacements of the landslips
+on <span class='pagenum'><a name="Page_66" id="Page_66">[66]</a></span>Epomeo. This shock was also registered at the observatory on
+Vesuvius.</p>
+
+<p>After this the shocks became less frequent and slighter, twelve being
+felt at Casamicciola during the remainder of the year, and six in the
+first half of 1884. Several shocks and rumbling noises were also
+observed in other parts of the island. Among them may be mentioned
+noises heard at Fontana on August 12th and 15th, and a slight shock at
+the same place on August 17th; also on September 4th, at 10.30 and
+10.40 <span class="fakesc">A.M.</span>, slight shocks at Barano, Serrara, and Forio. On
+March 27th, 1884, at 2.7 <span class="fakesc">P.M.</span>, another strong shock occurred;
+strongest at Serrara, where the shock was subsultory and accompanied
+by noise; and less strong, though still subsultory, at Ciglio, Panza,
+Forio, Fiaiano, and Casamicciola, and very slight at Ischia. The
+series seems to have ended during the following summer, with a slight
+shock at Casamicciola on July 21st, and a stronger one on July 23rd,
+felt from Casamicciola on the north to Serrara on the south.</p>
+
+<p>Most of the after-shocks must have originated in the neighbourhood of
+Casamicciola, but it is worthy of notice that more than one centre was
+in action. Several were recorded at Ischia only. Others, as mentioned
+above, affected chiefly the south part of the island, and especially
+the small towns of Serrara and Fontana.</p>
+
+<br />
+
+<h4>CHARACTERISTICS OF ISCHIAN EARTHQUAKES.</h4>
+
+<p>After the eruption of 1302, there succeeded a period of comparative
+repose in Ischia. The revival of activity dates from 1762, and, since
+that year, there <span class='pagenum'><a name="Page_67" id="Page_67">[67]</a></span>have been four great earthquakes, namely, those of
+1796, 1828, 1881, and 1883. In every respect but that of increasing
+intensity, these earthquakes were apparently identical; each, as
+Professor Mercalli says, was merely a replica on a different scale of
+those that preceded it. The principal features in which they resemble
+one another, and differ from the average tectonic earthquake, are the
+coincidence of the epicentres, the small depth of the foci, and the
+sudden onset of the principal shock.</p>
+
+<p>1. <i>Coincidence of Epicentres.</i>&mdash;In Fig. 14, which is copied from
+Professor Mercalli's map, are shown the areas in which buildings were
+seriously damaged by these four earthquakes. The curves for 1796,
+1828, and 1881 are approximately concentric. In 1796, the shock was
+disastrous only to the west of Casamicciola; in 1828, according to
+Covelli, "the ground most injured was not precisely the region of
+Casamicciola, but that which lies between the district called Fango
+and that known as Casamenella, situated to the west of Casamicciola,
+and a short distance from it."<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a> The epicentres may have varied
+slightly in size, but, in position, it is clear that all four were
+nearly or quite coincident. The meizoseismal bands in 1881 and 1883
+were also similar in form and elongated in the same direction.</p>
+
+<p>In the last two earthquakes there was, as we have seen, very distinct
+evidence of a secondary meizoseismal area surrounding Fontana, and it
+is remarkable that this was also noticeable in the earthquake of 1828.
+"Besides the centre of vibration in the district of Fango," says
+Covelli, "another less powerful centre <span class='pagenum'><a name="Page_68" id="Page_68">[68]</a></span>showed itself in the locality
+of Fontana; this made itself felt more heavily than in surrounding
+localities; as if another centre of movement had taken place from that
+part, independent of the former."</p>
+
+<p>2. <i>Small Depth of the Foci.</i>&mdash;Mallet's method, as noted above, cannot
+be trusted to yield accurate estimates of the focal depth, or to
+indicate more than its order of magnitude. But it is remarkable that
+the depths calculated by Dr. Johnston-Lavis for the last two
+earthquakes are both only a little less than a third of a mile, and it
+is probable that the actual depth did not differ very greatly from
+this amount. The nature of the shock, vertical or nearly so close to
+the epicentre and horizontal at a short distance from it, is merely
+personal testimony of the same character as fissures in masonry, and
+of course points to the same result.</p>
+
+<div class="imgl" style="width: 45%;"><a name="Fig18" id="Fig18"></a>
+<a href="images/fig18.png">
+<img border="0" src="images/fig18.png" width="90%" alt="Diagram showing connection between depth of focus and rate of decline in intensity." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 18.&mdash;Diagram showing connection between depth of focus and rate of decline in intensity.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>But the most conclusive evidence on which we have to rely is the
+extraordinary intensity of the shock at the centre of a very small
+distributed area. In Great Britain, an earthquake felt over a district
+of equal size would hardly at the centre exceed the trembling produced
+in a station platform by a passing train. The curves in Fig. 18 show
+how the rate of decline in intensity depends on the depth of the
+focus. They are drawn on the supposition that the intensity at any
+point on the surface varies inversely as the square of its distance
+from the focus; the curves <i>a</i>, <i>b</i>, <i>c</i> corresponding to foci
+situated at depths <span class='pagenum'><a name="Page_69" id="Page_69">[69]</a></span>of one-third of a mile, one mile, and two miles
+respectively, and the figures below the horizontal line denoting the
+distance in miles from the epicentre. Thus, the rapid decline of
+intensity from the epicentre outwards shows that, in each of the four
+great Ischian earthquakes, the depths of the focus must have been very
+small.</p>
+
+<p>3. <i>Suddenness of the Shocks.</i>&mdash;In 1796, we have no record of
+preparatory shocks, but the evidence is scanty; in 1828 and 1881, none
+are mentioned; in 1883, one or two tremors and underground noises,
+possibly of seismic origin, gave warning to a few. Fore-shocks, for
+all practical purposes, were conspicuous by their absence.</p>
+
+<p>Still more remarkable is the sudden advent of the great shocks. There
+were no preliminary tremors or rumbling sound, no animals showed signs
+of uneasiness and no birds fluttered screaming from trees or ground.
+The shock of 1828, says Covelli, "was announced by three powerful
+blows coming almost vertically, from below upwards;" and the same
+words apply equally well to the earthquakes of 1881 and 1883. The
+destruction of houses in every case was practically instantaneous, and
+coincident with the first vibration.</p>
+
+<p>In all respects, tectonic earthquakes differ widely from the Ischian
+shocks. The epicentres of successive earthquakes are rarely
+coincident, but show a distinct tendency to migration along certain
+lines; the decline in intensity outwards from the epicentre is nearly
+always very gradual, and therefore indicative of a comparatively
+deep-seated focus; they are almost invariably preceded either by a
+series of slight shocks and rumbling sounds, or, in an unstable
+<span class='pagenum'><a name="Page_70" id="Page_70">[70]</a></span>district, by a marked increase in their frequency. Distinctions, so
+great as these are, evidently remove the Ischian shocks from the
+category of tectonic earthquakes.</p>
+
+<br />
+
+<h4>ORIGIN OF THE ISCHIAN EARTHQUAKES.</h4>
+
+<p>On the other hand, the Ischian earthquakes possess several features
+which connect them closely with true volcanic earthquakes.</p>
+
+<p>1. They originate beneath the northern slope of Epomeo&mdash;a volcano that
+we have no reason to consider absolutely extinct, but rather as one
+subject to eruptions at long intervals of time&mdash;in a region as yet
+unoccupied by parasitic craters, but having the same relation to the
+central cone of Epomeo as those in which the recent craters of Monte
+Rotaro, Montagnone and Cremate are situated.</p>
+
+<p>2. In both the earthquakes of 1881 and 1883, the epicentre is an
+elongated band, the axis of which, if produced, would pass through the
+centre of the old crater of Epomeo. Along the line of this band, occur
+the fumaroles of Monte Cito and Ignazio Verde and the thermal springs
+of the Rita and Capitello. These facts, as Professor Mercalli
+suggests, lead us to believe that the foci of the earthquakes coincide
+with a radial fracture of the volcano, the course of which, as traced
+by him, is represented by the continuous line in Fig. 14.<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a></p>
+
+<p>3. Except in their relations with actual eruptions, <span class='pagenum'><a name="Page_71" id="Page_71">[71]</a></span>the Ischian
+earthquakes resemble closely the true volcanic earthquakes which from
+time to time shake the flanks of Etna. These are marked by great
+intensity of the shock at the centre of a comparatively small
+disturbed area, epicentres often elongated radially to the cone,
+frequent repetition with similar characters in the same districts; and
+as a rule they precede by a short interval, but sometimes accompany or
+follow, volcanic eruptions.<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a></p>
+
+<p>Two other phenomena may be referred to as probably indicating some
+connection between Ischian earthquakes and the structure and history
+of Epomeo.</p>
+
+<p>We have seen that, in the three earthquakes of 1828, 1881, and 1883,
+there is distinct evidence of a second meizoseismal area at Fontana,
+within which the shock was mainly subsultory. Dr. Johnston-Lavis,
+though recognising the possibility of the existence of two epicentres,
+prefers another explanation.<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a> But the wide extension of the
+southern boundary of the area of destruction in 1883, and the
+limitation of several of the after-shocks to the south <span class='pagenum'><a name="Page_72" id="Page_72">[72]</a></span>of the island,
+seem to me to favour the existence of a second focus beneath the
+crater of Epomeo, though, it may be, not entirely detached from the
+chief focus beneath Casamenella.</p>
+
+<p>Again, as Professor Mercalli remarks, all historic eruptions on the
+flanks of Epomeo were accompanied by very violent earthquakes; while,
+previously to 1302, only one disastrous earthquake, so far as known,
+occurred in the island without being attended by an eruption. It
+should be noticed also that the principal shocks during the recent
+revival of activity (<i>i.e.</i>, since 1762) show a continual increase in
+intensity, whether this be measured by the damage to buildings, the
+loss of life, or the extent of the area of destruction (Fig. 14).</p>
+
+<p>It therefore seems legitimate to conclude that, in the recent Ischian
+earthquakes, we have merely so many unsuccessful attempts to force a
+new volcanic eruption. The passages once existing through Epomeo and
+its parasitic craters having become blocked, the highly heated magma
+beneath is compelled to find a new outlet. Its tension slowly
+increasing, the crust above is at last rent, or an incipient rent is
+enlarged, the fluid rock is injected almost instantaneously with great
+force into the open fissure, and its sudden arrest by the containing
+walls is the ultimate cause of an earthquake. With the expansion of
+the magma, its tension is at once correspondingly reduced, and some
+time must elapse before it can again reach the critical point at which
+a further rupture, resulting in a second shock, takes place.<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a></p>
+
+<p><span class='pagenum'><a name="Page_73" id="Page_73">[73]</a></span>Thus, with each great Ischian earthquake, we are, I believe, advancing
+a step nearer the time, which may be close at hand or may be very
+remote, when the fracture will at last reach the surface, and above
+the site of Casamenella a new parasitic cone will rise, from which, as
+from Cremate in 1302, a stream of lava may flow down towards the sea.</p>
+
+<br />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Baldacci, L.</span>&mdash;"Alcune osservazioni sul terremoto
+avvenuto all' Isola d'Ischia il 28 luglio 1883." <i>Ital. Com.
+Geol. Boll.</i>, vol. xiv., 1883, pp. 157-166.</p>
+
+<p class="hang">2. <span class="sc">Daubr&eacute;e, A.</span>&mdash;"Rapport sur le tremblement de terre
+ressenti &agrave; Ischia le 28 juillet, 1883; causes probables des
+tremblements de terre." Paris, <i>Acad. Sci.</i>, <i>Compt. Rend.</i>,
+vol. xcvii., 1883, pp. 768-778.</p>
+
+<p class="hang">3. <span class="sc">Du Bois, F.</span>&mdash;"The Earthquakes of Ischia." <i>Japan Seism.
+Soc. Trans.</i>, vol. vii., pt i., 1883-84, pp. 16-42.</p>
+
+<p class="hang">4. &mdash;&mdash; "Farther Notes on the Earthquakes of Ischia." <i>Ibid.</i>, vol.
+viii., 1885, pp. 95-99.</p>
+
+<p class="hang">5. <span class="sc">Johnston-Lavis, H.J.</span>&mdash;<i>Monograph of the Earthquakes of
+Ischia</i> (1885).</p>
+
+<p class="hang">6. <span class="sc">Mercalli, G.</span>&mdash;<i>Vulcani e fenomeni vulcanici in Italia</i>
+(vol. iii. of <i>Geologia d'Italia</i>, by G. Negri, A. Stoppani,
+and G. Mercalli), 1883, pp. 46-50, 331-332.</p>
+
+<p class="hang">7. &mdash;&mdash; <i>L'Isola d'Ischia ed il terremoto del 28 luglio 1883</i>
+(Milano, 1884).</p>
+
+<p class="hang">8. <span class="sc">Palmieri, L., E A. Oglialoro.</span>&mdash;"Sul terremoto dell'
+Isola d'Ischia della sera del 28 luglio 1883." Napoli, <i>R.
+Accad. Atti</i>, vol. i., 1884, pp. 1-28.</p>
+
+<p class="hang"><span class='pagenum'><a name="Page_74" id="Page_74">[74]</a></span>9.<span class="sc"> Rossi, M.S. de.</span>&mdash;"Il terremoto di Casamicciola del 4
+marzo 1881." <i>Bull. del Vulc. Ital.</i>, anno viii., 1881, pp.
+5-12. (In the same volume are brief notices by different
+writers on pp. 22, 38-42, 52-53, 67-68, 70-74.)</p>
+
+<p class="hang">10. &mdash;&mdash; "Raccolta di fatti, relazioni, bibliografie sul terremoto
+di Casamicciola del 28 luglio 1883, con brevi osservazioni."
+<i>Bull. del Vulc. Ital.</i>, anno xi., 1884, pp. 65-172.</p>
+
+<p class="hang">11. &mdash;&mdash; "Intorno all' odierna fase dei terremoti in Italia e
+segnatamente sul terremoto in Casamicciola del 4 Marzo 1881."
+<i>Ital. Soc. Geogr. Boll.</i>, 1881.</p>
+
+<p class="hang">12. <span class="sc">Serpieri, A.</span>&mdash;"Sul terremoto d'Ischia il 28 luglio
+1883." <i>Scritti di Sismologia</i>, Pte. ii., pp. 207-216.</p>
+
+<p class="hang">13. &mdash;&mdash; "Sul terremoto dell' Isola d'Ischia il 28 luglio 1883."
+<i>Ibid.</i>, pp. 217-232.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> The shaded areas indicate the principal trachytic
+masses, the broken lines represent the boundaries of the craters that
+are still recognisable, and the dotted lines the boundaries of the
+areas within which buildings were damaged by the earthquakes of 1796,
+1828, 1881, and 1883 (according to Mercalli). The continuous curved
+line shows the position of the radial fracture with which the
+earthquakes were probably connected. The trachytic masses and craters
+are denoted by the following tables:&mdash;</p>
+
+
+<div class="centered">
+<table border="0" cellpadding="2" cellspacing="0" width="60%" summary="">
+  <tr>
+  <td class="tdl" width="60%"><i>a.</i> Epomeo.</td>
+  <td class="tdl" width="40%"><i>k.</i> Marecocco.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>b.</i> Trippiti.</td>
+  <td class="tdl"><i>l.</i> Zale.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>c.</i> Vetta.</td>
+  <td class="tdl"><i>m.</i> Rotaro.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>d.</i> Garofoli.</td>
+  <td class="tdl"><i>n.</i> Montagnone.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>e.</i> Vatoliere.</td>
+  <td class="tdl"><i>p.</i> Bagno.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>f.</i> Campagnano.</td>
+  <td class="tdl"><i>q.</i> Tabor.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>g.</i> Vezza.</td>
+  <td class="tdl"><i>r.</i> P. Castiglione.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>h.</i> Imperatore.</td>
+  <td class="tdl"><i>s.</i> Cremate.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>i.</i> C. St. Angelo.</td>
+  <td class="tdl"><i>t.</i> Arso.</td>
+  </tr>
+  <tr>
+  <td class="tdl"><i>j.</i> Lo Toppo.</td>
+  <td class="tdl"><i>u.</i> Porto d'Ischia.</td>
+  </tr>
+</table>
+</div>
+</div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> It is possible that Monte Campagnano may form an
+exception to this statement.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> Shocks were felt in the island in 1559 and 1659, but one
+at least was of external origin.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> Prof. Mercalli, from the five estimates of the angle of
+emergence which he considered most reliable, found the mean depth to
+be about 3,280 feet.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> Professor de Rossi estimated the mean duration as not
+much exceeding ten seconds. Dr. Johnston-Lavis, on the other hand,
+considers the general estimate of fifteen seconds as far too low. In
+one case, at Casamicciola, he ranks it as high as thirty-one seconds.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> Quoted from the useful translation of Covelli's memoir
+given by Dr. Johnston-Lavis.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> Baldacci supposes that the thermal springs and fumaroles
+of Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie
+along a tangential fracture starting from Forio and passing by
+Casamicciola to near Punta di Castiglione. Mercalli, however, argues
+forcibly against this inference.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> Professor Mercalli adds, as a fourth point of contact
+between Ischian earthquakes and volcanic phenomena, the changes in the
+fumaroles and hot springs which preceded or accompanied or followed
+the earthquakes of 1828, 1881, and 1883.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> "Fontana," he says, "occupies the centre of the great
+crater of Epomeo..., and therefore lies immediately over the ancient
+chimney, which in all probability is filled by an old plug of
+consolidated trachyte, which must descend to the igneous reservoir.
+Any mass of igneous matter, that might determine the further rupture
+of a collateral fissure, would result in the conduction of any changes
+of pressure or vibrations, along the column of highly elastic
+trachyte; whilst the same earth-waves would be annulled or absorbed by
+the inelastic tufas surrounding it, so that the blow would be struck
+perpendicularly to the surface, and in a small area with well defined
+limits. The undulatory sensations, after the principal local shock,
+were those that arrived from the great centre of impulse beneath
+Casamenella."</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> The above paragraph is a summary of the reasoning stated
+with admirable clearness by Dr. Johnston-Lavis. It should be mentioned
+that the late Professor Palmieri, relying on the extremely limited
+disturbed area, dissented from this view; but his difficulty is met by
+supposing the focus to be small as well as shallow, a supposition that
+is supported by the shortness of the meizoseismal band, as well as by
+the elongation of the isoseismal lines in the direction perpendicular
+to this band.</p></div>
+
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_IV" id="CHAPTER_IV"></a><hr />
+<span class='pagenum'><a name="Page_75" id="Page_75">[75]</a></span><br />
+
+<h3>CHAPTER IV.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884.</h4>
+<br />
+
+<p>In most countries the principal seismic districts are of limited
+extent. Thus, in central Japan, the east coast is frequently visited
+by earthquakes, while the west coast is relatively undisturbed. Of the
+earthquakes felt in the kingdom of Greece during the years 1893-98, 63
+per cent. were observed in Zante, and were for the most part confined
+to that island. In the interior of the Iberian peninsula&mdash;in Leon and
+in New and Old Castile&mdash;destructive earthquakes are practically
+unknown; while the littoral regions of central and southern Portugal,
+Andalusia, and Catalonia are noted for their disastrous shocks.</p>
+
+<p>During the eighteenth century seismic activity was chiefly
+concentrated in Portugal, and culminated in the great Lisbon
+earthquake of 1755. In the following century the seat of disturbance
+was transferred from the west to the south of the peninsula; Portugal
+remained throughout in comparative repose, while Almeria experienced
+destructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and
+1864, leading up to the Andalusian earthquakes of 1884-85, described
+in the present chapter.</p>
+
+<p>The preparation for the principal earthquake of <span class='pagenum'><a name="Page_76" id="Page_76">[76]</a></span>December 25th, 1884,
+was unusually indistinct. For a day or two before, shocks were felt
+here and there in Andalusia, but so weak were they that they passed
+almost unperceived. During the night of December 24-25, one slight
+shock was noticed at Colme&ntilde;ar (Fig. 19) and another at Zafarraya. On
+the 25th, a faint movement of the ground was noticed at Malaga, and a
+few weak tremors at Periana; and shortly after came the great shock at
+about 8.50 <span class="fakesc">P.M.</span> mean time of Malaga, or about 9.8
+<span class="fakesc">P.M.</span> Greenwich mean time.</p>
+
+<p>This earthquake was investigated by no fewer than three official
+committees. The first in the field was nominated by the Spanish
+Government on January 7th, 1885, and consisted of four members, the
+President being Se&ntilde;or M.F. de Castro, the director of the Geological
+Survey of Spain. The report of this commission was presented to the
+Minister of Agriculture, etc., on March 12th. Early in February a
+French Commission, appointed by the Academy of Sciences, proceeded to
+the scene of the disaster. With Professor F. Fouqu&eacute; as chief, and MM.
+L&eacute;vy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Br&eacute;on as
+members, this committee resolved itself after a time into one for
+studying the geology of the central area; and, of their voluminous
+report of more than 700 quarto pages (published in 1889), only 55 are
+immediately concerned with the earthquake. At the beginning of April,
+Professors Taramelli and Mercalli, sent by the Italian Government,
+arrived in Andalusia; and their memoir, read a few months later before
+the Reale Accademia dei Lincei, forms by far the most valuable
+contribution to our knowledge of the earthquake.</p>
+
+<span class='pagenum'><a name="Page_77" id="Page_77">[77]</a></span><br />
+
+<h4>DAMAGE CAUSED BY THE EARTHQUAKE.</h4>
+
+<p>The meizoseismal area (see Figs. 19 and 20) lies in a mountainous
+district, almost equidistant from the cities of Malaga and Granada. In
+this area, which contains nearly 900 square miles, the shock was
+disastrous to all but well-built houses. Whole villages were
+overthrown. In the surrounding zone many buildings escaped serious
+damage, and only a few were completely destroyed. It is estimated by
+the Spanish Commission that, in the province of Granada, 3,342 houses
+were totally, and 2,138 partially, ruined; in the province of Malaga,
+1,057 houses were totally, and 4,178 partially, ruined; while in the
+two provinces together 6,463 houses were damaged; making a total of
+17,178 buildings more or less seriously injured.</p>
+
+<p>As usual in the South of Europe, bad construction and narrow streets
+were largely responsible for the loss of property, houses that were
+regularly built and made of good materials being only slightly
+injured. But, in this case, the great slope of the ground, the bad
+quality of the foundations, and the nature of the underlying rocks
+were contributing factors. Many buildings also had been damaged by
+previous shocks, and their ruin was only completed by the earthquake
+of 1884.</p>
+
+<p>The total loss of life is variously estimated. According to the
+Spanish Commission, 690 persons were killed and 1,426 wounded in the
+province of Granada, while 55 were killed and 59 wounded in that of
+Malaga, making a total of 745 persons killed and 1,485 wounded. The
+Italian seismologists, having additional materials at their disposal,
+raise <span class='pagenum'><a name="Page_78" id="Page_78">[78]</a></span>the total figures to 750 persons killed and 1,554 severely
+wounded. Careful inquiries were also made on this subject by the
+conductors of the newspaper <i>El Defensor de Granada</i>. In Granada
+alone, they reckon that 828 persons were killed and 1,164 wounded.</p>
+
+<p>From the table given in the Italian report, it appears that 330
+persons were killed at Alhama, 118 at Arenas del Rey, 102 at
+Albu&ntilde;uelas, 77 at Ventas de Zafarraya, and 40 at Periana; the
+percentage of mortality being 9 at Arenas del Rey, about the same at
+Ventas de Zafarraya, and 3 or 4 at Alhama, Albu&ntilde;uelas and Periana.
+Comparing these latter figures with the death rates of 71 per cent. at
+Montemurro, caused by the Neapolitan earthquake, and of about 45 per
+cent. at Casamicciola, by the Ischian earthquake of 1883, it will be
+seen that the loss of life during the Andalusian earthquake was
+comparatively small&mdash;an exemption which is attributed by the Italian
+commissioners to the absence of inhabited places from the immediate
+neighbourhood of the epicentre, and to the fact that the destructive
+vibrations occurred towards the end of the shock, thus allowing
+opportunity for escape.</p>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>Fig. 19 shows the principal isoseismal lines as drawn by the Italian
+commissioners. The meizoseismal area, which included all places at
+which the shock was disastrous, is bounded by an ellipse (marked 1 on
+the map) 40 miles long from east to west, 28 miles wide, and about 886
+square miles in area. The next isoseismal (2) includes the places in
+<span class='pagenum'><a name="Page_79" id="Page_79">[79]</a></span>which some buildings were ruined, but not as a rule completely, and in
+which there was no loss of life. Its bounding line is also elliptical,
+the longer axis being about 71 miles long and running nearly east and
+west. Towards the south this zone is interrupted by the sea. It will
+be noticed that these isoseismals are not concentric, the second
+extending <span class='pagenum'><a name="Page_80" id="Page_80">[80]</a></span>much farther to the west and south-west than in the
+opposite direction. A third isoseismal (not shown in the map) encloses
+the district in which the shock was "very strong," or just capable of
+producing cracks in the walls of houses. It is similar in form to the
+second isoseismal, reaching as far as Estepone to the south-west,
+Osuna, Cordova, and Seville to the west, Jaen to the north, while
+towards the east it stops short of Almeria.</p>
+
+<div class="img"><a name="Fig19" id="Fig19"></a>
+<a href="images/fig19.png">
+<img border="0" src="images/fig19.png" width="80%" alt="Isoseismal lines of Andalusian earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 19.&mdash;Isoseismal lines of Andalusian earthquake. (<i>Taramelli and Mercalli.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The French Commission have also published a map of the earthquake,
+and, though the work of an experienced seismologist like Professor
+Mercalli is probably more trustworthy, it is interesting to compare
+his isoseismal lines with those obtained by his French colleagues,
+which are reproduced in Fig. 20. The curves in this figure are drawn
+so as to include the places that were, respectively, ruined, seriously
+damaged, and slightly damaged, by the shock. They should therefore
+correspond with the lines in Fig. 19. It will be seen that they differ
+considerably in form, but at the same time they present certain points
+of agreement, such as the east and west elongation of the meizoseismal
+area, and the great extension of the two outer isoseismals towards the
+west and south-west The greatest difference is to be found in the
+eastern portion of the third isoseismal, which, according to the
+Italians, extends beyond the limits included in Fig. 20, and,
+according to the French, is bayed back by the great masses of the
+Sierra Nevada.</p>
+
+<p>Outside Andalusia the earthquake was sensibly felt to the north as far
+as Madrid and Segovia, to the west at Huelva, C&aacute;rceres and Lisbon, and
+to the east at Valencia and Murcia. Towards the <span class='pagenum'><a name="Page_81" id="Page_81">[81]</a></span>south, the greater
+part of the disturbed area was cut off by the Mediterranean, and there
+are no records forthcoming from the opposite coast of Africa. The
+total area disturbed by the earthquake is roughly estimated by the
+French Commission at about 154,000 square miles, and by the Italian
+Commission at about 174,000 square miles; but, as the shock was
+<span class='pagenum'><a name="Page_82" id="Page_82">[82]</a></span>strong enough to stop clocks and ring bells at Madrid, it is evident
+that even the greater of these values is too small.</p>
+
+<div class="img"><a name="Fig20" id="Fig20"></a>
+<a href="images/fig20.png">
+<img border="0" src="images/fig20.png" width="80%" alt="Isoseismal lines of Andalusian earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 20.&mdash;Isoseismal lines of Andalusian earthquake. (<i>Fouqu&eacute;, etc.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<br />
+
+<h4>THE UNFELT EARTHQUAKE.</h4>
+
+<div class="imgr" style="width: 20%;"><a name="Fig21" id="Fig21"></a>
+<a href="images/fig21.png">
+<img border="0" src="images/fig21.png" width="80%" alt="Magnetograph records of Andalusian earthquake at Lisbon." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 21.&mdash;Magnetograph records of Andalusian earthquake at Lisbon. (<i>Fouqu&eacute;, etc.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Far beyond the limits of the disturbed area, however, the long slow
+waves sped over the surface, disturbing magnetographs and other
+delicate instruments. More than a century before, the great Lisbon
+earthquake of 1755 had caused oscillations in Scottish lakes, and on
+other occasions the effects of remote earthquakes had been witnessed
+at isolated places. But, in 1884, the concurrent registration of the
+Andalusian earth-waves at distant observatories attracted general
+attention, and in part suggested the world-wide network of
+seismological stations, the foundation of which was laid before
+another decade had passed.</p>
+
+<p>In Italy, probable records of the earthquake were obtained at two
+observatories, but, owing to the approximate times given, their
+connection with it is not established. At Velletri, near Rome,
+Professor Galli's seismodynamograph registered a very slight movement
+at 10 <span class="fakesc">P.M.</span>, and at Rome itself Professor de Rossi found a
+tromometer making unusual oscillations at 10.15 <span class="fakesc">P.M.</span><a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a></p>
+
+<p>The most interesting records, however, are those furnished by the
+magnetographs at Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
+<span class='pagenum'><a name="Page_83" id="Page_83">[83]</a></span>Wilhelmshaven. At Lisbon, the records are extremely clear. The curves
+of the declination, horizontal force and vertical force magnets, as
+seen in Fig. 21, are abruptly broken at 8.33 <span class="fakesc">P.M.</span> (Lisbon
+time, or 9h. 9m. 45s., G.M.T.). The disturbances, which are greatest
+on the declination curve and least on the vertical force curve, lasted
+in all three for about 12 minutes, and are quite distinct from the
+ordinary magnetic perturbations. At Parc Saint-Maur, the magnetographs
+seem to be ill-adapted to act as seismographs, for only a slight mark
+was discovered on a re-examination of the curves, beginning at 9.24
+<span class="fakesc">P.M.</span> (Paris time, or 9h. 14m. 39s., G.M.T.) At Greenwich, Mr.
+W. Ellis writes, there was "a small simultaneous disturbance of the
+declination and horizontal force magnets, occurring at 9h. 15m....
+Both magnets were at this time set into slight vibration, the extent
+of vibration in the case of declination being about 2' of arc, and in
+horizontal force equivalent to .001 of the whole horizontal force
+nearly." Of the three instruments at Wilhelmshaven, only one showed
+any movement at the time of the earthquake. The declination magnet was
+undisturbed, the horizontal force curve was accidentally interrupted,
+but the vertical force curve indicated a very perceptible shock.
+Beginning at 9.52 <span class="fakesc">P.M.</span> (Wilhelmshaven mean time, or 9h. 29m.
+29s., G.M.T.), the curve was broken for four minutes, for the rapid
+swinging of the needle could not be registered until the motion
+<span class='pagenum'><a name="Page_84" id="Page_84">[84]</a></span>became fainter. Further disturbances also occurred at 9.59, 10, 10.2,
+and 10.5 <span class="fakesc">P.M.</span><a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a></p>
+
+<br style="clear: both;" />
+
+<h4>POSITION OF THE EPICENTRE.</h4>
+
+<p>The innermost isoseismal being too large, and the time-records too
+inaccurate, to give the position of the epicentre, both Commissions
+resorted to observations of the direction, Professor Fouqu&eacute; and his
+colleagues depending chiefly on the oscillation of hanging lamps, and
+Professors Taramelli and Mercalli on the fall or displacement of
+statues and other objects, and all avoiding as far as possible the
+evidence of fissures in buildings.</p>
+
+<p>The Italian observers point out that, among the divergent directions
+visible at any place, there is generally one more distinctly marked
+than the others, and this, they consider, corresponds to the movement
+coming almost directly from the centre of disturbance. Plotting these
+directions (36 in number), they find that they converge as a rule
+within the triangle formed by joining Ventas de Zafarraya, Alhama, and
+Jatar, while a large number of them traverse the elliptical area,
+whose boundary is represented by the dotted line in Fig. 19. This area
+is about 9 miles long and 2&frac12; miles wide, its longer axis runs
+nearly <span class='pagenum'><a name="Page_85" id="Page_85">[85]</a></span>east and west, and its centre coincides with the western focus
+of the ellipse which forms the boundary of the meizoseismal area. It
+lies, moreover, close to Ventas de Zafarraya and Arenas del Rey, the
+two places where the seismic death-rate was highest, while its major
+axis almost coincides with the line joining them.</p>
+
+<p>The evidence of hanging lamps collected by the French Commission was
+more consistent than that of the fallen objects. At every place, the
+plane in which the lamps oscillated was nearly constant, the
+deviations being generally attributable to irregularities in the mode
+of suspension. The azimuths again intersect within an elliptical area,
+which, according to the Commission, differs little from the central
+region of the earthquake (Fig. 20). It Is clear, however, from the map
+accompanying the French report, that the majority converge towards a
+narrow band extending east and west from near Arenas del Rey to near
+Ventas de Zafarraya, and therefore agreeing closely with the
+epicentral area as determined by Professors Taramelli and
+Mercalli.<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a></p>
+
+<br />
+
+<h4>DEPTH OF THE SEISMIC FOCUS.</h4>
+
+<p>If the depth of the seismic focus amounts to several miles, one of the
+most serious objections to Mallet's method lies in the varying
+refractive power of the different strata traversed by the earth-waves
+(p. 28). <span class='pagenum'><a name="Page_86" id="Page_86">[86]</a></span>At present we have no way of meeting this objection, and all
+calculations of the depth of the focus are therefore more or less
+doubtful. A difficulty in practice has also been urged, depending on
+the widely differing inclinations of the fractures at any place; but
+the Italian observers found that the errors from this source were
+greatly reduced by avoiding all fissures in poorly-built houses, or
+which start from windows or other apertures, and selecting only those
+which occur in homogeneous walls directed towards the epicentre. The
+best angles of emergence thus measured by them are thirteen in number,
+all made at places lying within 5 and 23 miles from the centre of the
+epicentral area, and, with two exceptions, inside the meizoseismal
+zone (Fig. 19). The depths corresponding to the different wave-paths
+vary from 5.3 to 23.0 miles, the mean depth of the focus given by all
+thirteen observations being 7.6 miles.</p>
+
+<p>The only estimate made by the French Commission&mdash;and it is one that
+they rightly regarded with considerable doubt&mdash;was based on a method
+devised by Falb. As the sound generally precedes the shock, Falb
+assumes that it travels with a greater velocity. If the velocities of
+both series of waves are known, and if they start at the same instant
+and from the same region, the interval that elapses between the
+arrivals of the sound and shock should give the distance traversed by
+them and consequently the depth of the focus. It is unnecessary to
+mention more than two of the serious objections to this method. The
+duration of the preliminary sound should increase rapidly with the
+distance from the focus, and of this there is not the slightest
+evidence. Moreover, the sound-vibrations that are first heard do not
+<span class='pagenum'><a name="Page_87" id="Page_87">[87]</a></span>necessarily come from the same part of the focus as those which cause
+the shock, but, as will be seen in Chapter VIII., probably from its
+nearer lateral margin. The French Commission, finding the average
+duration of the fore-sound near the epicentre to be 5 seconds,
+estimate the depth of the focus at about 7 miles&mdash;a result which
+agrees remarkably with that obtained from the angles of emergence, but
+which is not, on that account, entitled to credit.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>In the nature of the shock, there was a singular uniformity throughout
+the whole disturbed area, the chief variation noticed being evidently
+dependent on the observer's distance from the epicentre.</p>
+
+<p>For instance, in the meizoseismal area (Fig. 19), at Ventas de
+Zafarraya, a loud sound like thunder was first heard, and before it
+ceased there came a violent subsultory movement preceded by a very
+brief oscillation, then a pause of one or two seconds, and lastly a
+more intense and longer series of undulations, the whole movement
+lasting 12 seconds. At Cacin, three phases were distinguished, the
+first a slight undulatory movement coincident with the sound, followed
+immediately by the subsultory motion, a pause, and stronger
+undulations, the total duration being 15 seconds. The variations
+noticeable in this zone seem to have been apparent only, sensitive
+observers perceiving a tremulous motion before the vertical
+vibrations, and in the pause between them and the concluding
+undulations. In both phases, the intensity increased to a maximum and
+then gradually decreased. The movement at Ventas de Zafarraya <span class='pagenum'><a name="Page_88" id="Page_88">[88]</a></span>and
+Cacin is represented by Professors Taramelli and Mercalli by the
+curves <i>a</i> and <i>b</i> in Fig. 22.</p>
+
+<p>In the second zone (Fig. 19), the same two phases were universally
+observed, but the subsultory movement was less pronounced or the
+movement was partly subsultory and partly undulatory, and occasionally
+both phases are described as undulatory. The motion near Malaga is
+represented by the curve <i>c</i> in Fig. 22.</p>
+
+<div class="imgl" style="width: 45%;"><a name="Fig22" id="Fig22"></a>
+<a href="images/fig22.png">
+<img border="0" src="images/fig22.png" width="90%" alt="Nature of shock of Andalusian earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 22.&mdash;Nature of shock of Andalusian earthquake. (<i>Taramelli and Mercalli.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Outside the ruinous zone, the first phase rapidly lost what remained
+of its subsultory form, and the pause between the two parts was
+noticeably longer than near the epicentre. Thus, at Seville and
+Cordova, two shocks were felt, separated by an interval of some
+seconds; the second according to some observers at Seville,
+terminating with vertical tremors. At Madrid, also, the two parts were
+perceived, the interval between them being 3 or 4 seconds in length;
+but, as a rule, outside Andalusia, only a single undulatory shock was
+felt, without any preliminary sound.</p>
+
+<p>That the changes observed in the shock were merely an effect of less
+or greater distance, will be obvious from Fig. 23, in which the
+intensity at any moment is that represented by the distance of the
+corresponding point on the curve from the different <span class='pagenum'><a name="Page_89" id="Page_89">[89]</a></span>base-lines, the
+base-line <i>a</i> corresponding to a place near the epicentre, and <i>b</i>,
+<i>c</i>, <i>d</i>, etc., to places at gradually increasing distances. Thus, at
+a place corresponding to the base-line <i>b</i>, the intensity of the
+tremors during the intervening pause (represented by the short line
+PN) was so slight that they frequently escaped notice, while the
+preliminary tremors observed by some near the epicentre were
+altogether imperceptible. At the places corresponding to the
+base-lines <i>c</i>, <i>d</i>, <i>e</i>, <i>f</i>, the duration of the whole shock and of
+each part gradually diminished, while the interval between the two
+parts increased owing to the gradual extinction of the final
+vibrations of the first part and of the initial vibrations of the
+second. At the farthest of these places (<i>f</i>) the first part was so
+weak that it sometimes passed unobserved. Lastly, at a place
+corresponding to the base-line <i>g</i>, the first part was imperceptible
+to all observers, and the shock consisted of a single series of
+horizontal undulations.</p>
+
+<div class="img"><a name="Fig23" id="Fig23"></a>
+<a href="images/fig23.png">
+<img border="0" src="images/fig23.png" width="70%" alt="Diagram to illustrate variation in nature of shock of Andalusian earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 23.&mdash;Diagram to illustrate variation in nature of shock of Andalusian earthquake.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Origin of the Double Shock.</i>&mdash;If the double shock were observed at
+only a few places, we should naturally look for some local explanation
+of the peculiarity. The second shock, for instance, might be a
+subterranean echo, the earth-waves being reflected <span class='pagenum'><a name="Page_90" id="Page_90">[90]</a></span>at the bounding
+surface of two different kinds of rock. In the case of the Andalusian
+earthquake, such an explanation is precluded by the almost universal
+observation of the double shock, the greater intensity of the second
+part, and the longer period of its vibrations.</p>
+
+<p>The Italian observers, who paid considerable attention to the double
+shock, give a more general explanation. They regard the two parts of
+the shock as corresponding in the main to longitudinal and transversal
+waves starting simultaneously from the same focus (see p. 13). The
+former vibrations would be vertical at the epicentre and would
+gradually become horizontal in spreading outwards; the latter would be
+horizontal at the epicentre and at a distance from it (<i>e.g.</i> at
+Seville) nearly vertical. Also, as the longitudinal waves travel more
+rapidly than others, the interval between the two parts of the shock
+would increase with the distance from the origin. Owing again, to the
+large size of the focus, the first part of the shock would at no place
+be instantaneous, and its later vibrations might coalesce with the
+earlier transverse vibrations, so that, within and near the
+meizoseismal area, the second part of the shock might be stronger than
+the first. A similar result might be produced in the same district if
+the transverse vibrations coincided with reflected longitudinal
+vibrations, and Professors Taramelli and Mercalli think that such
+reflection would occur from the old crystalline rocks of the Sierra de
+Almijara and possibly also from the calcareous and crystalline rocks
+to the south-west of Cartama.</p>
+
+<p>Satisfactory as it seems to be in some respects, this explanation is
+open to serious objections, of which I <span class='pagenum'><a name="Page_91" id="Page_91">[91]</a></span>will mention only two. The
+first is that, though the pause between the two parts of the shock
+does increase with the distance, it does not increase rapidly enough;
+at Seville, it should be two or three minutes, instead of "some
+seconds" in length. A more fatal objection, however, is that, if the
+explanation were correct, every earthquake-shock should consist of two
+parts, and this is only the case with a small minority.</p>
+
+<p>On the other hand, if the velocities of the waves composing each part
+were the same, the slight increase in the length of the interval is
+readily accounted for, as we have seen, by the gradual extinction of
+its weak terminal vibrations. But in any case, the long interval that
+elapsed between the beginnings of the two parts at a place so near the
+epicentre as Ventas de Zafarraya, shows that each part was due to a
+distinct impulse; and, judging from the directions of the respective
+movements, it would seem that the focus of the first impulse was
+situated at a greater depth than the focus of the second. Whether the
+epicentres corresponding to the two foci were coincident or more or
+less separate is not clear from the nature of the shock; but it is
+probable that they were nearly or quite detached, and that a second
+epicentre was situated near the eastern focus of the ellipse bounding
+the meizoseismal area.</p>
+
+<br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p>In the Neapolitan earthquake, the sound was only heard in a district
+of about 3,300 square miles immediately surrounding the epicentres,
+while the whole area disturbed by the shock was not less than <span class='pagenum'><a name="Page_92" id="Page_92">[92]</a></span>39,000
+square miles. A similar limitation was noticed in the Andalusian
+earthquake. According to the Spanish Commission, the sound was heard
+at only one place (Cordova) outside the provinces of Granada and
+Malaga; and its audibility was a rule confined to the area within
+which buildings were damaged by the shock. It was compared at
+different places to the noise of a passing train or a carriage heavily
+laden running on a paved road, of distant thunder, a great storm, or
+the discharge of heavy guns.</p>
+
+<p>At every place where the sound was heard, it distinctly preceded the
+shock, frequently allowing time for escape from houses that were
+afterwards ruined. Its duration within the meizoseismal area was on an
+average about five or six seconds, rarely perhaps did it exceed ten
+seconds. At some places in the same area, it overlapped the beginning
+of the shock, but generally it was separated from the latter by a very
+short interval, estimated at a second. From this precedence of the
+sound, the Italian Commission conclude that the sound-waves travelled
+more rapidly than those which formed the shock, an inference that
+depends on the assumption that both waves started simultaneously from
+within precisely the same focal limits. A different explanation, not
+based on these assumptions, will be considered more fully in Chapter
+VIII, dealing with the recent earthquakes of Hereford and Inverness.</p>
+
+<br />
+
+<h4>VELOCITY OF THE EARTH-WAVES.</h4>
+
+<p>If, in a highly-civilised country, the time-records of an earthquake
+vary within wide limits, it is not surprising that those given for the
+Andalusian <span class='pagenum'><a name="Page_93" id="Page_93">[93]</a></span>earthquake should be wholly untrustworthy. Even the clocks
+in public buildings and railway stations differed by as much as 25
+minutes in their indications. An interesting observation is, however,
+described in the French report and is worth repeating, though it does
+not lead to any accurate result. At the time of the principal shock,
+two telegraph-clerks were in communication, one at Malaga and the
+other at Velez-Malaga. The latter, surprised by the shock, suddenly
+stopped his message; and, about six seconds later, the arrival of the
+earth-waves at Malaga explained the interruption to his colleague. As,
+according to the French report, Velez-Malaga is 9 kms. (or about 5&frac12;
+miles) nearer than Malaga to the mean epicentral point, it follows
+that the velocity of the earth-waves must have been about 1.5 kms., or
+nearly a mile, per second.<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a></p>
+
+<p>The only observations of any real value in determining the velocity
+are those given by the stopped clock at the observatory of San
+Fernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur,
+Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon,
+Greenwich, and Wilhelmshaven at 9.18, 9.19, 9.25, and 9.29
+<span class="fakesc">P.M.</span> respectively (Paris mean time) and the mean epicentral
+point as coinciding with Alhama, the French Commission estimates
+roughly the mean surface-velocity between Cadiz and Lisbon at 3.6 kms.
+per second, between Cadiz and Greenwich at 4.5 kms. per second,
+between Cadiz and Wilhelmshaven at 3.1 kms. per second, and between
+Greenwich and Wilhelmshaven at 1.6 kms. per <span class='pagenum'><a name="Page_94" id="Page_94">[94]</a></span>second. Dr. Agamennone,
+however, notices that the distances from Alhama are not correctly
+measured, and substitutes for the above figures 4.83, 3.43, 2.82, and
+1.75 kms. per second respectively.</p>
+
+<p>These results apparently show a decrease in the velocity with the
+outward spread of the earth-waves, but, as Dr. Agamennone again points
+out, a comparatively small error in the time at Cadiz would neutralise
+the apparent decrease. It is not to be supposed that the astronomical
+clock at this observatory was wrong by more than a second or two, but
+the behaviour of clocks during an earthquake is so irregular&mdash;some
+stopping at once, others staggering on for some seconds before
+arrest&mdash;that the Cadiz time may differ from the true time by several
+seconds.</p>
+
+<p>Besides this possible error, there is also considerable uncertainty in
+the records from the magnetic observatories, owing to the slow rate at
+which the photographic paper travels. At Parc Saint-Maur this rate is
+only 10 mm. per hour, and at the other observatories about 15 mm. per
+hour. Allowing, therefore, for an error of half-a-minute in the
+time-record at Cadiz, of one minute in those of Lisbon, Greenwich, and
+Wilhelmshaven, and of two minutes in that at Parc Saint-Maur, and
+taking the mean epicentral point as determined by the Italian
+observers, Dr. Agamennone, applying the method of least squares, finds
+the probable value of the velocity of propagation to be 3.15 kms. (or
+nearly 2 miles) per second, with a possible error of .19 kms. per
+second. This result agrees closely with the value found for the long
+slow undulations of more recent earthquakes.</p>
+
+<span class='pagenum'><a name="Page_95" id="Page_95">[95]</a></span><br />
+
+<h4>MISCELLANEOUS PHENOMENA.</h4>
+
+<p><i>Connection between Geological Structure and the Intensity of the
+Shock.</i>&mdash;While a great part of the injury to buildings must be
+attributed to their faulty construction, the connection between the
+nature of the underlying rock and the amount of damage was very
+clearly marked. Other conditions being the same, houses built on
+alluvial ground suffered most of all; and the destruction was also
+great in those standing on soft sedimentary rocks such as clays and
+friable limestones. On the other hand, when compact limestones or
+ancient schists formed the foundation-rock, the amount of damage was
+conspicuously less than in other cases.</p>
+
+<p>The members of both the French and the Italian Commissions agree in
+ascribing the peculiar form and relative positions of the isoseismal
+lines to geological conditions. To the east of the epicentre, the
+schists and crystalline limestones form a deep, uniform, and compact
+mass; while, to the west, the old crystalline rocks are covered by
+jurassic, cretaceous, and eocene formations, constituting a less
+homogeneous and less elastic mass, in which the intensity of the shock
+would fade off much more rapidly, with the result that the epicentre
+occupies the western focus of the elliptical boundary of the
+meizoseismal area (Fig. 19).<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a></p>
+
+<p>That mountain-ranges have an important influence on the form of
+isoseismal lines is evident from both maps (Figs. 19 and 20), but
+especially from that <span class='pagenum'><a name="Page_96" id="Page_96">[96]</a></span>published by the French Commission (Fig. 20).
+The resistance offered by the Sierra Nevada to the propagation of the
+earth-waves is shown in the former map by the approximation of the
+first and second isoseismals at the east end, and in the latter by the
+great bay in the third isoseismal line. Whichever interpretation of
+the evidence is the more accurate, the action of the mountainous mass
+is clearly to lessen rapidly the intensity of the shock&mdash;an effect
+which is probably due to the abrupt changes in the direction and
+nature of the strata encountered normally by the earth-waves. On the
+opposite side of the epicentre, the waves meet the Sierra de Ronda
+obliquely. In traversing this range, the shock lost a great part of
+its strength, while it continued to be felt severely along its eastern
+foot, thus giving rise to the south-westerly extension of the third
+isoseismal in Fig. 20, and, though to a less extent, that of the
+second in Fig. 19.</p>
+
+<p><i>Fissures, Landslips, etc.</i>&mdash;The earthquake resulted in many
+superficial changes, such as fissures, landslips, and derangement of
+the underground water-system&mdash;all changes of the same order as the
+destruction of buildings&mdash;but, so far as known, in no fault-scarps or
+other external evidence of deep-seated movements.</p>
+
+<p>Some of the fissures were of great length. One of the most remarkable
+occurred at Guevejar, a village built on the south-west slope of the
+Sierra de Cogollos. It was in the form of a horse-shoe, and was about
+two miles long, from ten to fifty feet wide, and of great depth. In
+its neighbourhood, innumerable small cracks appeared, some
+<span class='pagenum'><a name="Page_97" id="Page_97">[97]</a></span>perpendicular and others parallel to the great fissure. The ground
+within, a bed of clay resting on limestone, also slid down towards the
+river. Houses near the centre of the fissured tract were shifted as
+much as thirty yards within the first month, and others near its
+extremity about ten feet; while the accumulation of the material at
+the south end of the fissure resulted in the formation of a small
+lake, of about 250 to 350 square yards in area and about 30 feet deep.
+All streams within the fissured zone disappeared, and the spring,
+which provided the drinking-water of the village, ceased to flow.</p>
+
+<p>The underground water-system was generally affected throughout the
+central area. In some places, mineral springs disappeared; in others,
+new springs broke out or old ones flowed more abundantly. At Alhama,
+the increased flow was accompanied by a permanent rise in temperature
+from 47&deg; to 50&deg; C., and by a marked change in character.</p>
+
+<br />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p>Frequent after-shocks are a characteristic of the earthquakes of
+Southern Spain. After the Cordova earthquake of 1170, they continued
+for at least three years. The Murcian earthquake of 1828 was followed
+by 300 minor shocks during the next twenty-four hours, and for more
+than a year slight tremors were often felt. For some time after the
+great earthquake of 1884, the movements of the ground were extremely
+numerous in the immediate neighbourhood of the epicentre, farther away
+they were rarer and of less intensity, and outside the area of damaged
+buildings they were nearly absent.</p>
+
+<p><span class='pagenum'><a name="Page_98" id="Page_98">[98]</a></span>Thus, during the night of December 25-26, 110 after-shocks were
+counted at Jatar, from 14 to 17 at Alcaucin, Ventas de Huelma, Motril,
+Cacin, Durcal, Malaga, etc.; about 11 at La Mala and Albu&ntilde;uelas; 9 at
+Velez-Malaga and Lenteje; and from 5 to 7 at Frigiliana, Riogordo, and
+Cartama. The strongest of these shocks occurred at 2.20 <span class="fakesc">A.M.</span>,
+and, though none was violent, several helped to complete the ruin of
+many houses that had been damaged by the principal shock.</p>
+
+<p>From this time, after-shocks occurred almost daily until the end of
+May, after which they became much less frequent. According to the list
+given in the Italian report, which closes at the end of January 1886,
+237 shocks were felt, 23 up to the end of December, 30 in January
+1885, 25 in February, 27 in March, 46 in April, and 43 in May. In June
+1885, only three are recorded, and the average number during each of
+the following seven months lies between five and six. This list,
+however, does not include the very weak shocks,<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a> for nearly all
+those contained in it were felt as far as Malaga or its neighbourhood.</p>
+
+<p>The shocks varied considerably in intensity as well as in frequency,
+five of them being much more violent than the rest. One that occurred
+on December 30th was felt strongly in all the damaged area, two others
+on January 3rd and 5th caused fresh injury to buildings, a fourth, on
+February 27th, disturbed an area bounded roughly by the second
+<span class='pagenum'><a name="Page_99" id="Page_99">[99]</a></span>isoseismal of the principal earthquake (Fig. 19), while the fifth and
+strongest, that of April 11th, was felt over a large part of the zone
+beyond.</p>
+
+<p>At places within and near the meizoseismal area, earth-sounds were
+sometimes heard without any sensible shock; occasionally, also,
+tremors were felt with no attendant sound; but, as a rule, the shocks
+were accompanied by sound, and in every such case, as in the principal
+earthquake, the sound preceded the shock, or at most was partly
+contemporaneous with it.</p>
+
+<p>Several of the after-shocks resembled the principal earthquake in
+their division into two parts separated by an interval of rest or
+weaker movement from half a second to a second in length, though the
+whole duration of the shock itself in no case exceeded five or six
+seconds. Occasionally, the likeness was still closer, in the
+succession of sound, subsultory motion and concluding horizontal
+undulations.</p>
+
+<br />
+
+<h4>GEOLOGY OF THE MEIZOSEISMAL AREA AND ORIGIN OF THE EARTHQUAKES.</h4>
+
+<p>The meizoseismal area and surrounding zones lie in the midst of the
+mountainous region that separates the basin of the Guadalquiver from
+that of the Mediterranean, the essential structure of which, according
+to the geologists of the French Commission, is outlined in Fig. 24. In
+this sketch-map, the lightly-shaded bands correspond to an upper
+series of crystalline schists, and the cross-shaded bands to the lower
+series of mica-schists and dolomites that form the anticlinal folds of
+the Sierra de Ronda, the Sierra de Mijas, and the Sierra Tejeda.</p>
+
+<p><span class='pagenum'><a name="Page_100" id="Page_100">[100]</a></span>In addition to the faulting and intense folding in the direction of
+their strikes, these rocks are also intersected by three nearly
+parallel transverse faults of post-Triassic age, which, aided by
+subsequent denudation, have cut up the whole range into a number of
+distinct sierras. They are represented by the broken lines in Fig. 24.</p>
+
+<div class="img"><a name="Fig24" id="Fig24"></a>
+<a href="images/fig24.png">
+<img border="0" src="images/fig24.png" width="80%" alt="Structure of meizoseismal area of Andalusian earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 24.&mdash;Structure of meizoseismal area of Andalusian earthquake. (<i>Fouqu&eacute;, etc.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>One of these faults, that which passes near Motril, traverses the
+meizoseismal area, whose boundary, as laid down by the French
+Commission, is indicated by the dotted line on the sketch-map.<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a> In
+the neighbourhood of Zafarraya, the fault intersects the broken
+anticlinal fold of the Sierra Tejeda, and the epicentre is thus
+situated in one of the most disturbed tracts of the whole region. The
+evidence, both seismic and <span class='pagenum'><a name="Page_101" id="Page_101">[101]</a></span>geological, is insufficient to support any
+precise view as to the origin of the earthquake, but there can be
+little doubt that it was closely connected with movements along one or
+more of the system of faults that intersect not far from Zafarraya.</p>
+
+<br />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Agamennone, G.</span>&mdash;"Alcune considerazioni sui different
+metodi fino ad oggi adoperati nel calcolare la velocit&agrave; di
+propagazione del terremoto andaluso del 25 dicembre 1884."
+Roma, <i>R. Accad. Lincei, Rend.</i>, vol. iii., 1894, pp.
+303-310.</p>
+
+<p class="hang">2. &mdash;&mdash; "Velocit&agrave; superficiale di propagazione delle onde sismiche
+in occasione della grande scossa di terremoto dell' Andalusia
+del 25 dicembre 1884." <i>Ibid.</i>, vol. iii., 1894, pp. 317-325.</p>
+
+<p class="hang">3. <span class="sc">Castro, M.F.</span> de.&mdash;<i>Terremotos de Andaluc&iacute;a: Informe de
+la comision nombrada para su estudio dando cuenta del estado
+de los trabajos en 7 de marzo de 1885.</i> (Madrid, 1885; 107
+pp.)</p>
+
+<p class="hang">4. <span class="sc">Fouqu&eacute;, F.</span>, etc.&mdash;"Mission d'Andalousie: &Eacute;tudes
+relatives au tremblement de terre du 25 d&eacute;cembre 1884, et &agrave; la
+constitution g&eacute;ologique du sol &eacute;branl&eacute; par les secousses."
+Paris, <i>Acad. Sci. M&eacute;m.</i>, vol. xxx., pp. 1-772.</p>
+
+<p class="hang">5. <span class="sc">Macpherson, J.</span>&mdash;"Tremblements de terre en Espagne."
+Paris, <i>Acad. Sci., Compt. Rend.</i>, vol. c., 1885, pp. 397-399.</p>
+
+<p class="hang">6. <span class="sc">Nogu&eacute;s, A.F.</span>&mdash;"Ph&eacute;nom&egrave;nes g&eacute;ologiques produits par les
+tremblements de terre de l'Andalousie, du 25 d&eacute;cembre 1884 au
+16 janvier 1885." <i>Ibid.</i>, pp. 253-256.</p>
+
+<p class="hang">7. <span class="sc">Rossi, M.S.</span> de.&mdash;"Gli odierni terremoti di Spagna ed il
+loro eco in Italia." <i>Bull. Vulc. Ital.</i>, anno xii., 1885, pp.
+17-31.</p>
+
+<p class="hang">8. <span class="sc">Taramelli, T.</span>, and <span class="sc">G. Mercalli</span>.&mdash;"I terremoti
+Andalusi cominciati il 25 dicembre 1884." Roma, <i>R. Accad.
+Lincei, Mem.</i>, vol: iii., 1885, pp. 116-222.</p>
+
+<p class="hang">9. Paris, <i>Acad. Sci., Compt. Rend.</i>, vol. c., 1885, pp. 24-27,
+136-138, 196-197, 256-257, 598-601, 1113-1120, 1436 (the last
+three by F. Fouqu&eacute;).</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> These times correspond to about 9.10 and 9.25
+<span class="fakesc">P.M.</span>, Greenwich mean time. The earthquake stopped a clock at
+the Royal Observatory of San Fernando (Cadiz), at 8h. 43m. 54.5s. mean
+local time, corresponding to 9h. 8m. 44s., G.M.T.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> The earthquake is also said to have been registered at
+the observatory of Moncalieri, near Turin, but I have not been able to
+ascertain the time of occurrence. A movement felt at about 10.20
+<span class="fakesc">P.M.</span> at Ramsbury, in Wiltshire, was attributed to the
+earthquake, though the time is about an hour too late. On December
+26th, an astronomical clock was stopped at Brussels and its pillar
+displaced; and, on the evening of the same day, the large telescope at
+the observatory was also found to have been shifted. These effects, it
+is suggested, were caused by the Andalusian earthquake, but the
+connection between them seems to me very doubtful.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> The French observers have also applied a method
+depending on the time of occurrence of the shock. Joining places where
+the recorded times were the same, they notice that the perpendicular
+bisectors of these lines intersect within an area which agrees
+practically with that determined by the azimuths. The inaccuracy of
+the time-records must, however, lessen the significance of this
+result.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> Dr. Agamennone points out that, according to the Italian
+report, the difference in distance is 22 kms. (or 13&frac34; miles),
+leading to a velocity of about 3.6 kms., or 2.3 miles per second.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> It should be remembered that it is not improbable that
+there were two detached epicentres, coinciding roughly with the two
+foci of this curve.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> Only eight are recorded during the night of December
+25-26. On several occasions during April and May 1885, groups of
+slight shocks were felt; but as their individual times are not given,
+they are regarded as equivalent to one shock each in the above
+totals.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> The boundary, as drawn in this figure, differs slightly
+from that given in Fig. 20.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_V" id="CHAPTER_V"></a><hr />
+<span class='pagenum'><a name="Page_102" id="Page_102">[102]</a></span><br />
+
+<h3>CHAPTER V.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886.</h4>
+<br />
+
+<p>The Charleston earthquake stands alone among the great earthquakes
+described in this volume, and indeed among nearly all great
+earthquakes, in visiting a region where seismic disturbances were
+almost unknown. Calabria and Ischia, the Riviera and Andalusia, Assam
+and the provinces of Mino and Owari in Japan, are all regions where
+earthquake-shocks are more or less frequent and occasionally of
+destructive violence. But, from the foundation of Charleston in 1680
+until 1886, that is, for more than two centuries, it is probably not
+too much to say that few counties in Great Britain were so free from
+earthquakes as the State of South Carolina.<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a></p>
+
+<p>The practical isolation of the earthquake of 1886 left its trace on
+the character of the investigation. Not only were the observers
+untrained, but the <span class='pagenum'><a name="Page_103" id="Page_103">[103]</a></span>investigators themselves were unprepared. For
+instance, the scale of intensity used in drawing the isoseismal lines
+was not adopted until after the first letters of inquiry were issued.
+On the other hand, nothing could exceed the energy and ability with
+which the epicentral tracts were examined by Mr. Earle Sloan and the
+collection of time-records made by Mr. Everett Hayden. To them, and to
+Major C.E. Dutton, whose valuable monograph supersedes all other
+accounts, we are indebted for the two chief additions to our knowledge
+resulting from the study of the Charleston earthquake. These are the
+determination of the double epicentre, and the measurement of the
+velocity with which the earth-waves travelled.</p>
+
+<br />
+
+<h4>DAMAGE CAUSED BY THE EARTHQUAKE.</h4>
+
+<p>The land-area disturbed by the earthquake and the isoseismal lines are
+shown in Fig. 25, the small black oval area (which Includes
+Charleston) being that within which the greatest damage to buildings
+occurred. The chief part of the epicentre, however, lies from 12 to 15
+miles to the west and north-west of Charleston, in a forest-clad
+district, containing only two villages and various scattered houses.</p>
+
+<p>The city of Charleston, whose population between 1880 and 1891
+increased from fifty to fifty-five thousand, is built on a peninsula
+between the Cooper River on the east and the Ashley River on the
+south-west. Originally, this was an irregular tract of comparatively
+high and dry land, intersected by numerous creeks, which, as the city
+grew, were filled up to the general level of the higher ground. It is
+on this <span class='pagenum'><a name="Page_104" id="Page_104">[104]</a></span>"made land" as a rule that the more serious damage to
+buildings occurred.</p>
+
+<p>At 9.51 <span class="fakesc">P.M.</span> (standard time of the 75th meridian), the great
+earthquake occurred, and, one minute later, there was left hardly a
+building in the city that was not injured more or less seriously. "The
+destruction," as Major Dutton remarks, "was not of that sweeping and
+unmitigated order which has befallen other cities, and in which every
+structure built of material other than wood has been levelled
+completely to the earth in a chaos of broken rubble, beams, tiles, and
+planking, or left in a condition practically no better." The number of
+houses entirely demolished was not great, but several hundred lost a
+large part of their walls, and many were condemned as unsafe and
+afterwards pulled down. A board of inspectors, appointed to
+investigate the condition of the houses, reported that not one hundred
+out of fourteen thousand chimneys examined by them escaped damage, and
+that 95 per cent. of those injured were broken off at the roof. The
+total cost of the necessary repairs, it was estimated, would amount to
+about one million pounds.</p>
+
+<p>According to the official records, 27 persons were killed in
+Charleston during the earthquake, but, by cold, exposure, etc., this
+number was brought up to not less than 83. The number of persons
+wounded was never ascertained.</p>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>In drawing the isoseismal lines (represented by the continuous curves
+in Fig. 25), Major Dutton made use of the well-known Rossi-Forel scale
+of seismic <span class='pagenum'><a name="Page_105" id="Page_105">[105]</a></span>intensity, a translation of which is given below.<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a> The
+curves range from the highest degree, 10, corresponding to disastrous
+effects on buildings, down to the lowest but one, 2, which would be
+applied to a shock felt only by a small number of persons at rest. It
+is evident, I think, that these lines cannot be regarded as drawn with
+great accuracy. The number of records (nearly 4000, from about 1,600
+places), great as it is, is hardly sufficient for the purpose; and
+many were collected from newspapers. The circulars of inquiry also
+contained no distinct questions corresponding to the different degrees
+of the scale employed, and therefore it is not always certain that the
+intensity recorded was the maximum observed. But, if the curves might
+have varied in detail with a larger and more accurate series of
+observations, they must represent in their main features the
+distribution of seismic intensity throughout the disturbed area. One
+<span class='pagenum'><a name="Page_106" id="Page_106">[106]</a></span>point of importance is the partial earthquake-shadow in the region of
+the Appalachian Mountains shown by the southward incurving of the
+isoseismals 4, 5, and 6, and especially by the first two of these
+lines. Another is the close grouping of the isoseismals in the State
+of Mississippi, illustrating a rapid fading of <span class='pagenum'><a name="Page_107" id="Page_107">[107]</a></span>intensity as the
+earth-waves crossed the unconsolidated materials of the Mississippi
+delta.</p>
+
+<div class="img"><a name="Fig25" id="Fig25"></a>
+<a href="images/fig25.png">
+<img border="0" src="images/fig25.png" width="50%" alt="Isoseismal lines of Charleston earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 25.&mdash;Isoseismal lines of Charleston earthquake. (<i>Dutton, etc.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Owing to the short distance between the epicentre and the sea-coast,
+it is impossible to make more than a rough estimate of the extent of
+the disturbed area. Even when the boundary lies on land, it traverses
+some districts which are thinly populated and others where the
+inhabitants are unobservant, and unlikely to notice the slow
+oscillations which were alone perceptible at great distances. The
+shock was, however, felt at Boston (800 miles from the epicentre), La
+Crosse on the upper Mississippi (950 miles to the north-west), at
+several places in Cuba (between 700 and 710 miles), and in Bermuda
+(950 miles). To the south, the limits are unknown, there being no
+report from Yucatan, the nearest point of which is distant about 930
+miles. If we assume the disturbed area to have a mean radius of 950
+miles, then it must have covered no less than 2,800,000 square miles.
+And, that this estimate is not excessive, will be evident from the
+fact that the land-area disturbed (including parts of the great lakes
+and inlets in the sea-coast) amounted to about 920,000 square miles.</p>
+
+<br />
+
+<h4>PREPARATION FOR THE EARTHQUAKE.</h4>
+
+<p>The preparation for the earthquake seems to have begun about three
+months before. During June, and even earlier, slight but undoubted
+tremors are said to have been felt in Charleston, but no record of
+them was kept until about 8 <span class="fakesc">A.M.</span> on August 27th, when a
+decided earthquake occurred at Summerville, a village twenty-two miles
+to the north-west. The shock and sound were simultaneous, the shock a
+single jolt or <span class='pagenum'><a name="Page_108" id="Page_108">[108]</a></span>heavy jar, the sound loud and sudden; they were such
+as might have been caused by the firing of a heavy cannon or the
+explosion of a boiler or blast of gunpowder. At 4.45 <span class="fakesc">A.M.</span> on
+August 28th, the shock and sound were repeated, only more strongly,
+the former being distinctly felt as far as Charleston. During that day
+and the next, there were several other shocks at Summerville, and then
+rest and quiet succeeded until the evening of August 31st.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>At 9.51 <span class="fakesc">P.M.</span> (to take one of the best descriptions), the
+attention of an observer in Charleston was "vaguely attracted by a
+sound that seemed to come from the office below, and was supposed for
+a moment to be caused by the rapid rolling of a heavy body, as an iron
+safe or a heavily-laden truck, over the floor. Accompanying the sound
+there was a perceptible tremor of the building, not more marked,
+however, than would be caused by the passage of a car or dray along
+the street. For perhaps two or three seconds the occurrence excited no
+surprise or comment. Then by swift degrees, or all at once&mdash;it is
+difficult to say which&mdash;the sound deepened in volume, the tremor
+became more decided, the ear caught the rattle of window-sashes,
+gas-fixtures, and other movable objects; the men in the office ...
+glanced hurriedly at each other and sprang to their feet.... And then
+all was bewilderment and confusion.</p>
+
+<p>"The long roll deepened and spread into an awful roar, that seemed to
+pervade at once the troubled earth and the still air above and around.
+The tremor was now a rude, rapid quiver, that agitated the whole
+<span class='pagenum'><a name="Page_109" id="Page_109">[109]</a></span>lofty, strong-walled building as though it were being shaken&mdash;shaken
+by the hand of an immeasurable power, with intent to tear its joints
+asunder and scatter its stones and bricks abroad....</p>
+
+<p>"There was no intermission in the vibration.... From the first to the
+last it was a continuous jar, adding force with every moment, and, as
+it approached and reached the climax of its manifestation, it seemed
+for a few terrible seconds that no work of human hands could possibly
+survive the shocks. The floors were heaving under-foot, the
+surrounding walls and partitions visibly swayed to and fro, the crash
+of falling masses of stone and brick and mortar was heard overhead and
+without....</p>
+
+<p>"For a second or two it seemed that the worst had passed, and that the
+violent motion was subsiding. It increased again and became as severe
+as before. None expected to escape. A sudden rush was simultaneously
+made to endeavor to attain the open-air and fly to a place of safety;
+but, before the door was reached all stopped short, as by a common
+impulse, feeling that hope was vain&mdash;that it was only a question of
+death within the building or without, of being buried beneath the
+sinking roof or crushed by the falling walls. The uproar slowly died
+away in seeming distance. The earth was still, and oh! the blessed
+relief of that stillness."</p>
+
+<p>If somewhat sensational in form, this report gives an extremely vivid
+and generally accurate account of the great shock. Other observers in
+Charleston concur in dividing the movement into five phases. The
+preliminary tremors and murmuring sound lasted about twelve seconds,
+and, although they increased in strength, they were succeeded somewhat
+suddenly <span class='pagenum'><a name="Page_110" id="Page_110">[110]</a></span>by the violent oscillations of the second phase, followed by
+a third phase of much less intensity and a fourth of stronger
+oscillations, these three phases lasting altogether about fifty
+seconds. The fifth phase, in which the tremors died out rather
+rapidly, continued about eight seconds; so that the total duration of
+the earthquake was not less than seventy seconds. The variation of the
+intensity with the time is represented roughly by the curve in Fig.
+26.</p>
+
+<div class="img"><a name="Fig26" id="Fig26"></a>
+<a href="images/fig26.png">
+<img border="0" src="images/fig26.png" width="80%" alt="Curve of intensity at Charleston." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 26.&mdash;Curve of intensity at Charleston. (<i>Dutton.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>At Charleston, there were thus two decided maxima of intensity, nearly
+equal in strength, though the first seems to have been slightly more
+powerful than the second. As in the Andalusian earthquake, the
+intervening tremors were imperceptible at a distance from the
+epicentre, and the earthquake appeared in the form of two distinct
+shocks, separated by an interval the average duration of which was
+estimated at slightly less than half a minute. At most places, the
+first shock is described as the stronger, but the difference in
+intensity of the two parts could not have been great, for both were
+noticed at several places more than 600 miles from the epicentre.</p>
+
+<p><i>Visible Earth-Waves.</i>&mdash;Many persons in the meizoseismal area assert
+that they saw waves moving along the surface of the ground. At
+Charleston, according to an observer who was facing a street-lamp at
+the time, "the progress of the waves as they passed the <span class='pagenum'><a name="Page_111" id="Page_111">[111]</a></span>house, going
+towards the south-east, was plainly observed, although they travelled
+with incomparable swiftness. The shadow of each moving ridge cast from
+the gas-light was distinctly seen. The waves were not in long rollers,
+but had rather the appearance of 'ground-swells' in deep water," the
+height of which from crest to trough he estimated at not less than two
+feet. In the words of another observer, "The vibrations increased
+rapidly and the ground began to undulate like the sea. The street was
+well lighted, having three gas-lamps within a distance of 200 feet,
+and I could see the earth waves as they passed as distinctly as I have
+a thousand times seen the waves roll along Sullivan's Island beach.
+The first wave came from the south-west, and as I attempted to make my
+way ... I was borne irresistibly across from the south side to the
+north side of the street. The waves seemed then to come from both the
+south-west and north-west, and crossed the street diagonally,
+intersecting each other, and lifting me up and letting me down as if I
+were standing on a chop sea. I could see perfectly, and made careful
+observations, and I estimate that the waves were at least two feet in
+height."</p>
+
+<br />
+
+<h4>THE DOUBLE EPICENTRE.</h4>
+
+<p>For seismological purposes, it is unfortunate that the epicentral
+district should be one containing so few buildings and other objects
+that could preserve the effects of the shock. It is for the most part
+a barren, forest-clad region, in places swampy, with occasional
+scattered houses. But it is crossed by three lines of railway
+diverging from Charleston, <span class='pagenum'><a name="Page_112" id="Page_112">[112]</a></span>and the damage which they suffered
+supplements to some extent the defects arising from the scarcity of
+buildings. These railway lines are the South Carolina, the
+North-Eastern, and the Charleston and Savannah, denoted by the letters
+A, B, and C, respectively, in Figs. 28 and 29.<a name="FNanchor_40_40" id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a> It will be
+convenient to follow Major Dutton, and trace the variation of
+intensity exhibited along each line.</p>
+
+<p>For six miles along the South Carolina Railway (A) the damage to the
+line, though indicative of a strong shock, was of little consequence.
+In the first half of this distance no repairs were required, but at
+3-2/3 miles the rails were bent and the joints between them opened; at
+5 miles, the fish-plates were torn from their fastenings and the
+joints between the rails opened seven inches; and at nearly 6 miles
+the joints were again opened, and the road-bed depressed six inches.
+After this point, deflections of the line and elevations and
+depressions of the road-bed were no longer rare. Near the 9-mile
+point, the intensity of the shock seemed to increase most rapidly;
+lateral displacements of the line became more frequent as well as
+greater in amount. The distortions of the lines were probably greatest
+between 10 and 11 miles; here they were often displaced laterally,
+sometimes depressed or elevated, and occasionally twisted into
+S-shaped curves, while many hundred yards of the track were shoved
+bodily towards the south-east. "The buckling always took place when
+this lateral shoving encountered a rigid obstacle, usually a long
+rigid trestle. At the <span class='pagenum'><a name="Page_113" id="Page_113">[113]</a></span>north-western end of the trestle the
+accumulation of rails resulted in a sharp kink. Corresponding
+extensions of the track by the opening of the joints and shearing of
+the fish-plate bolts occurred some distance to the north-westward." At
+11&frac12; miles, the lines were again stretched and the joints opened by
+about seven inches; but, from this point for more than four miles, the
+sharp kinks revealing a sliding of the track were entirely absent,
+though there were still long slight flexures in the lines and changes
+of level in the road-bed. The railway in this section traverses a
+district which is partly a swamp and partly a rice-field; and thus it
+may be, as Major Dutton suggests, that the ground was less fitted to
+preserve the effects of the shock.<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a> At about 18 miles, the line
+reaches higher and firmer ground; and, from here to Summerville
+(21-2/3 miles), there were many sinuous flexures. For six miles
+farther, violent distortions of the rails ceased to occur, the rate of
+decrease in intensity being most marked near the 23-mile point. The
+last flexure occurred at Jedburgh (27&frac12; miles) at the south end of a
+long, heavy trestle (Fig. 27).</p>
+
+<div class="imgr" style="width: 50%;"><a name="Fig27" id="Fig27"></a>
+<a href="images/fig27.png">
+<img border="0" src="images/fig27.png" width="90%" alt="Flexure of rails at Jedburgh." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 27.&mdash;Flexure of rails at Jedburgh. (<i>Dutton.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>There is thus a certain symmetry in the damage to this line with
+respect to a point about 15 or 16 miles from the Charleston terminus.
+The changes of intensity are most rapid at distances of about 9 and 23
+miles from the terminus. Also, on the <span class='pagenum'><a name="Page_114" id="Page_114">[114]</a></span>south-east side of the 16-mile
+point, the longitudinal displacements of the line are always to the
+south-east; on the other side, always to the north-west. Major Dutton
+therefore infers that the epicentre must be on a line drawn nearly
+through the 16-mile point at right angles to the railway.</p>
+
+<p>Somewhat similar changes were noted along the North-Eastern Railway
+(B), the Charleston terminus of which is about three-quarters of a
+mile to the south-east of that of the South Carolina Railway. Slight
+flexures in the line occurred at distances of 1&frac12; and 4 miles from
+the terminus, and at about 6 miles the road-bed was depressed, in one
+part by as much as 22 inches. At about 6<span class="uni" title="one third">&#8531;</span> miles, the joints between
+the rails were opened 14 inches, and there were slight sinuous
+flexures in the line near the 7-mile and 8-mile points. The
+indications of great intensity then rapidly increased, the rate of
+change being greatest near the 9-mile point. Here, there was a long
+lateral flexure with a shift of 4 inches eastward. Half-a-mile
+farther, the fish-plates were broken and the rails parted 8&frac12;
+inches. A little beyond the 10-mile point, an embankment 15 feet high
+was pushed 4&frac12; feet eastward along a chord of 150 feet. At the
+12-mile point and beyond, fish-plates were broken, lines were bent and
+the joints opened; the road-bed was cut by a series of cracks, one of
+which was 21 inches wide, while the beginning of a long trestle was
+shifted 8<span class="uni" title="one third">&#8531;</span> feet to the west. From 12&frac12; to 14&frac12; miles, several
+buildings were damaged or destroyed by a movement which was clearly
+more vertical than horizontal. Near the 16-mile point, the ground was
+fissured and thrown into ridges, the rails being similarly bent in a
+vertical plane. Soon after this, the line reaches a <span class='pagenum'><a name="Page_115" id="Page_115">[115]</a></span>broad, sandy
+tract, and, though the thickness of the sand is probably not much more
+than 40 feet in any place, the disturbances diminish almost at once,
+and, for a distance of more than two miles, there was little damage
+done to the line. At Mount Holly Station (18 miles), the intensity was
+so slight that the houses suffered no injury more serious than the
+loss of chimneys. Half-a-mile farther, the ground becomes less sandy,
+and the effects of the shock more distinct. The lines were bent in
+places for about a quarter of a mile, after which they again pass into
+the sandy area with a decrease of damage, the last flexure being near
+the 21-mile point. The rate of change of intensity in this part of the
+line appears to have been greatest at a distance of about 19&frac12; miles
+from the terminus, but the exact distance is obviously somewhat
+uncertain.</p>
+
+<p>There is again a rough symmetry in the damage to the line, the central
+point being about 14 miles from the Charleston terminus. A line drawn
+through this point at right angles to the North-Eastern Railway (or
+rather to that part of it between the 9-mile and 19&frac12;-mile points)
+should pass through the epicentre. It meets the corresponding line for
+the South Carolina Railway in a point which is indicated in Figs. 27
+and 28 by a small circle (W). Houses and other buildings are rare in
+the surrounding district; but, as the intensity of the shock
+diminished outwards in all directions, this point must mark
+approximately the position of the epicentre. As it is close to the
+Woodstock Station on the South Carolina Railway, it is called by Major
+Dutton the Woodstock epicentre.</p>
+
+<p>The Charleston and Savannah Railway (C) uses the same lines as the
+North-Eastern for the first seven <span class='pagenum'><a name="Page_116" id="Page_116">[116]</a></span>miles from Charleston, and then
+turns off in a south-westerly direction. For 4&frac12; miles from the
+junction the signs of disturbance were few and unimportant. The
+railway then crosses the Ashley River, the banks of which slid towards
+one another and jammed the drawbridge; but for four miles farther
+there was no serious damage done to the lines. At about 16&frac12; miles
+the effects of the shock became rapidly more apparent. For nearly
+1&frac12; mile the entire railroad was deflected into an irregular curve,
+the displacement being greatest at the bridge, where it crosses the
+Stono River. Here, it was as much as 37 inches to the south. After
+Rantowles Station (18 miles), there were many displacements, both
+lateral and vertical. At 18&frac12; miles, a long southward deflection
+began, the amount of which reached 25 inches at the 19-mile point, 50
+inches half-a-mile farther on, and was still greater at 20-2/3 miles.
+For two miles more, sinuous flexures were continuous, but, at the
+22-2/3-mile point, they rapidly disappeared, the railroad passing on
+to higher and firmer ground. Between 25 and 27 miles, there were
+occasional slight flexures in the line or depressions of the railroad;
+but, after the 27&frac14;-mile point, they seldom occur, and, when they
+do, are of little consequence.</p>
+
+<p>Some of the effects described in the last paragraph may, as Major
+Dutton suggests, be due to the varying nature of the surface-rocks. It
+is important to notice, however, that disturbances of the lines were
+exceedingly rare in the section that lies nearest to the Woodstock
+epicentre, and that they increase in violence for some distance from
+that region, the maximum intensity being reached a mile or two to the
+west of Rantowles Station. This points clearly <span class='pagenum'><a name="Page_117" id="Page_117">[117]</a></span>to the existence of a
+second focus. Unfortunately, there are very few houses or other
+objects in the neighbourhood, and the position of the corresponding
+epicentre cannot be determined accurately. Major Dutton places it in
+the position indicated by a small circle (R), and calls it the
+Rantowles epicentre from its vicinity to the station of that name.</p>
+
+<p>If the meizoseismal area had been a thickly populated one, the
+evidence of ruined and damaged houses would have provided materials
+for the construction of isoseismal lines surrounding the two
+epicentres. It is difficult, as it is, to gauge the equality of the
+effects on objects so different as railway-lines and buildings; and
+the isoseismals shown in Figs. 28 and 29 can therefore lay no claim to
+accuracy.</p>
+
+<p>Fig. 28 shows the epicentral isoseismals as they are drawn by Mr.
+Earle Sloan. They do not correspond to the degrees of any definite
+scale of seismic intensity; but they may be taken as representing the
+impressions of a very careful observer, who traversed the district
+immediately after the occurrence of the earthquake, and who, when
+drawing these lines, was biassed by no preconceived theory.</p>
+
+<p>Major Dutton, relying chiefly on Mr. Sloan's written notes, interprets
+the evidence differently, and obtains the series of curves shown in
+Fig. 29. In this case, also, the isoseismals correspond to no
+expressed standard of intensity. They are intended merely to represent
+the forms of the curves, and, by their less or greater distance apart,
+the more or less rapid rate at which the intensity varied.</p>
+
+<p>The chief difference between the two maps concerns the form of the
+Woodstock isoseismals. Major Dutton draws them approximately
+circular, <span class='pagenum'><a name="Page_118" id="Page_118">[118]</a></span>leaving the map blank towards the north, where hardly any
+evidence was forthcoming. Mr. Sloan attributes the scantiness of
+effects here to a diminution of intensity, and makes the lines curve
+<span class='pagenum'><a name="Page_119" id="Page_119">[119]</a></span>in towards the epicentre. They certainly must do so in crossing the
+North-Eastern Railway; and the somewhat southerly trend of Mr. Sloan's
+curves to the east of this railway seems to me to furnish the better
+representation of the distinctly greater intensity in that region.</p>
+
+<div class="img"><a name="Fig28" id="Fig28"></a>
+<a href="images/fig28.png">
+<img border="0" src="images/fig28.png" width="50%" alt="Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 28.&mdash;Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (<i>Dutton.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="img"><a name="Fig29" id="Fig29"></a>
+<a href="images/fig29.png">
+<img border="0" src="images/fig29.png" width="50%" alt="Epicentral isoseismal lines of Charleston earthquake according to Major Dutton." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 29.&mdash;Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (<i>Dutton.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_120" id="Page_120">[120]</a></span>More important, however, than this divergence of opinion is the
+agreement in one respect between the two sets of curves. Both show a
+marked expansion around the points known as the Woodstock and
+Rantowles epicentres, especially about the former, and a contraction
+in the intermediate region. The evidence of these isoseismals
+therefore confirms that of the damaged railway lines, and establishes
+Major Dutton's inference that there were two distinct foci, the
+epicentres of which were about thirteen miles apart.</p>
+
+<br />
+
+<h4>ORIGIN OF THE DOUBLE SHOCK.</h4>
+
+<p>In the last chapter, it was shown that the double shock of the
+Andalusian earthquake could be due only to two distinct impulses
+taking place either within the same focus or, more probably, in two
+detached foci. Similar reasoning applies to the Charleston earthquake.
+The double maximum or double shock was observed in no less than
+fourteen States. Moreover, the interval between the two maxima at
+Charleston appears from Fig. 26 to have been about 34 seconds in
+length. Thus, the duplication of the shock cannot have been a merely
+local phenomenon, nor can it have resulted from the separation into
+two parts of the earth-waves proceeding from a single disturbance.
+Each maximum must therefore be connected with a distinct impulse.</p>
+
+<p>Combining this inference with Major Dutton's discovery of the double
+focus, no doubt can remain as to the origin of the repeated shock. It
+is clear, also, that the impulse at the Woodstock focus was the
+stronger of the two; for the isoseismals spread out <span class='pagenum'><a name="Page_121" id="Page_121">[121]</a></span>more widely round
+the corresponding epicentre, and there was no rapid decline of
+intensity from that point, such as might be associated with a weaker
+disturbance within a shallow focus.</p>
+
+<div class="imgr" style="width: 40%;"><a name="Fig30" id="Fig30"></a>
+<a href="images/fig30.png">
+<img border="0" src="images/fig30.png" width="90%" alt="Planes of oscillation of stopped pendulum clocks at Charleston." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 30.&mdash;Planes of oscillation of stopped pendulum clocks at Charleston.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Again, since the earlier part of the shock is almost uniformly
+described as the stronger, it follows that the Woodstock focus was the
+first in action. A curious fact recorded by Major Dutton supports this
+inference. In Charleston, four clocks were stopped by the shock, the
+errors of which at the time were certainly less than eight or nine
+seconds. The planes in which their pendulums oscillated are shown by
+the lines lettered A, B, C, and D in Fig. 30, the broken lines W and R
+representing respectively the directions from Charleston of the
+Woodstock and Rantowles epicentres. Clock A stopped at 9h. 51m. 0s., B
+at 9h. 51m. 15s., C at 9h. 51m. 16s., and D (which had been reset to
+the second earlier in the day) at 9h. 51m. 48s. Now, if the plane of
+oscillation coincided nearly with the direction of the shock, the only
+effect would be a temporary change in the period of oscillation; but
+if it was at right angles to the direction of the shock, the clock
+might be stopped by the point of the pendulum catching behind the
+graduated arc in front of which it oscillated. The planes of the first
+three clocks, it will be seen, were approximately at right angles to
+the direction of the Woodstock epicentre, and B and C were indeed
+stopped in the <span class='pagenum'><a name="Page_122" id="Page_122">[122]</a></span>manner just described; while the plane of shock D was
+nearly perpendicular to the direction of the Rantowles epicentre. As
+the pendulums of B and C might make a few staggering oscillations
+before their final arrest, Major Dutton assigns 9h. 51m. 12s. as the
+epoch of the first maximum at Charleston; and, as the interval between
+the two maxima was about 34 seconds, this would give about 9h. 51m.
+46s. for the epoch of the second maximum&mdash;a time which agrees very
+closely with that given by clock D. Thus, clocks A, B, and C must have
+been stopped by the Woodstock vibrations, and clock D about
+half-a-minute later by those coming from the Rantowles focus.</p>
+
+<br style="clear: both;" />
+
+<h4>DEPTH OF THE SEISMIC FOCI.</h4>
+
+<p>Two methods of estimating the depth of the seismic focus have been
+described in the preceding pages&mdash;namely, Mallet's, depending on the
+angle of emergence, and Falb's, based on the interval between the
+initial epochs of the sound and shock. To these, Major Dutton adds a
+third method, in which he relies on the rate at which the intensity of
+the shock varies with the distance from the epicentre.</p>
+
+<p><i>Dutton's Method of determining the Depth of the Focus.</i>&mdash;If the
+seismic focus is either a point or a sphere, and the initial impulse
+equal in all directions, and if the intensity of the shock diminishes
+inversely as the square of the distance from the focus, then the
+continuous curve in Fig. 31 will represent the variation of intensity
+along a line passing through the epicentre E. The form of the curve on
+these assumptions does not depend in any way on the <span class='pagenum'><a name="Page_123" id="Page_123">[123]</a></span>initial intensity
+of the impulse; it is governed solely by the depth of the focus. The
+deeper the focus, the flatter becomes the curve, as we have seen in
+discussing the Ischian earthquakes (p. 68). In all directions from the
+epicentre, the intensity at first diminishes slowly; but the rate of
+change of intensity with the distance soon becomes more rapid, until
+it is a maximum at the points C, C; after which it again diminishes
+and dies out very slowly when the distance becomes great. It will be
+evident from Fig. 18 that the deeper the focus the greater also is the
+distance EC of the points where the intensity of the shock changes
+most rapidly. It may be easily shown, indeed, that this distance
+always bears to the depth of the focus the constant ratio of 1 to
+&#8730;<span style="border-top: solid 1px">3</span>, or about 1 to 1.73.<a name="FNanchor_42_42" id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a></p>
+
+<p>Now, if a series of isoseismals could be drawn corresponding to
+intensities which differ by constant amounts, we should have a series
+of circles like those surrounding the Woodstock epicentre in Fig. 29,
+the distance between successive lines at first decreasing gradually
+until it is a minimum at the dotted circle and afterwards gradually
+increasing. This dotted circle is obviously that which passes through
+all <span class='pagenum'><a name="Page_124" id="Page_124">[124]</a></span>points where the intensity of the shock changes most rapidly.
+Major Dutton calls it the <i>index-circle</i> and, when its radius is
+known, the depth of the focus is at once obtained by multiplying the
+radius by 1.73.</p>
+
+<p>In 1858, Mallet proposed a method which bears some resemblance to the
+above,<a name="FNanchor_43_43" id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a> but depending only on the intensity of the longitudinal
+waves. Major Dutton claims for his method that the effects of the
+longitudinal and transverse waves are not separated, that it takes
+account of the "total energy irrespective of direction or kind of
+vibration."</p>
+
+<div class="img"><a name="Fig31" id="Fig31"></a>
+<a href="images/fig31.png">
+<img border="0" src="images/fig31.png" width="50%" alt="Diagram to illustrate Dutton's method of determining depth of seismic focus." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 31.&mdash;Diagram to illustrate Dutton's method of determining depth of seismic focus.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Objections to Dutton's Method.</i>&mdash;I have described this method
+somewhat fully, though it seems to me open to more serious objections
+than Mallet's first method which it is intended to replace.</p>
+
+<p>We have, in the first place, no reason for supposing that the focus is
+either a point or a sphere, or that the initial impulse is uniform in
+all directions. If <span class='pagenum'><a name="Page_125" id="Page_125">[125]</a></span>the earthquake were caused by fault-slipping, both
+assumptions would be untrue, and it is for those who employ the method
+to prove their validity.</p>
+
+<p>But of greater consequence is the fact that, if the method were
+correct, all earthquakes originating at the same depth must have
+index-circles of equal radii. If the depth of the focus were, say, ten
+miles, then the index-circle must have a radius of about six miles,
+whether the initial disturbance be of extreme violence or so weak that
+it is not felt at the surface at all, much less so far as six miles
+from the epicentre. The law of the inverse square is of course only
+true for a perfectly elastic and continuous medium, and the real curve
+of intensity is not that of the continuous line in Fig. 31, but
+something of the form represented by the dotted line. In this case,
+the rate of change of intensity is greatest at some point C', nearer
+than C to the epicentre, and the application of Major Dutton's rule
+would give a point F', nearer the surface than F, for the focus. Thus,
+assuming that the method can be applied in practice&mdash;and the test
+involved is one so delicate that it would be difficult to apply except
+with refined measurements&mdash;then all that we can assert is that the
+calculated depth is certainly less than the true depth.</p>
+
+<p><i>Dutton's Estimate of the Depth of the Seismic Foci.</i>&mdash;In applying the
+method, the chief difficulty is to obtain a series of isoseismal lines
+corresponding to equidistant degrees of intensity. As already pointed
+out, those given in Fig. 29 are merely diagrammatic; but the
+index-circle of the Woodstock focus, represented by the dotted line,
+is made to pass through the places where the rate of change of
+intensity was <span class='pagenum'><a name="Page_126" id="Page_126">[126]</a></span>found to be greatest. The radius of this circle being
+very nearly seven miles, it follows that the resulting depth of the
+Woodstock focal point would be about twelve miles. Major Dutton
+regards this estimate as probably correct within two miles.</p>
+
+<p>In the neighbourhood of the Rantowles epicentre, the isoseismals in
+both Figs. 28 and 29 are elongated in form. The <i>index-circuit</i>, as it
+would be called in such a case, cannot be drawn completely, but its
+radius parallel to the shorter axis of the curves is about 4&frac12;
+miles, and the resulting depth of the Rantowles focal point would be
+nearly eight miles.</p>
+
+<br />
+
+<h4>VELOCITY OF THE EARTH-WAVES.</h4>
+
+<p>The recognition of the double epicentre is, from a geological point of
+view, the most important fact established by the investigation of the
+Charleston earthquake. But of equal interest, from a physical point of
+view, is the estimate of the velocity of the earth-waves, which is
+probably more accurate than that determined for any previous shock.
+Owing to the existence of the standard time system in the United
+States, the exact time is transmitted once a day to every town and
+village within reach of a telegraph line; and the effect of small
+errors in the observations is considerably lessened by the great
+distance traversed by the earth-waves, sixty good reports coming from
+places more than 500 miles from the epicentre, and ten from places
+more than 800 miles distant.</p>
+
+<p>The total number of time-records collected is 316, but of these 130
+had to be rejected, either because they were obviously too early or
+too late, or <span class='pagenum'><a name="Page_127" id="Page_127">[127]</a></span>because they were only given to the nearest
+five-minutes' interval. There remain 186 observations which are
+divided by Major Dutton into four classes according to their probable
+value.</p>
+
+<p>In an earthquake of such great duration (about 70 seconds at
+Charleston), it is necessary in the first place to select some special
+phase of the movement as that to which the records mainly refer, and
+then to determine as accurately as possible the time of occurrence of
+this phase at the origin.</p>
+
+<p>There can be little doubt as to which phase should be chosen. The
+shock began with a series of tremors, which passed somewhat abruptly
+into the oscillations that formed the first and stronger maximum.
+These were clearly felt all over the disturbed area, and, as the
+beginning of the first maximum at places near the epicentre and the
+beginning of the shock at distant stations were probably due to the
+same vibrations, this particular phase may be fairly selected as that
+to which the time-measurements refer.</p>
+
+<p>The time of this phase at the origin can only be ascertained from the
+time at which it reached Charleston, and our knowledge of this depends
+chiefly on the evidence of stopped clocks. How unreliable this may be
+is well known. Clocks may indeed be stopped at almost any phase of the
+movement; and, whenever stopped clocks can be compared with really
+good personal observations, they almost invariably show a later time.
+At Charleston three good clocks were stopped by the vibrations from
+the Woodstock focus, two of them being in close agreement (p. 121);
+and, allowing for a few oscillations before their final arrest, Major
+Dutton places the time of arrival of the selected phase at <span class='pagenum'><a name="Page_128" id="Page_128">[128]</a></span>Charleston
+at 9h. 51m. 12s. <span class="fakesc">P.M.</span> The evidence of these clocks is also
+supported by that of other observations, which show that 9.51 was
+certainly the nearest minute to the time of arrival, and favour a
+somewhat later instant much more strongly than one a little earlier.</p>
+
+<p>Now, the distance of Charleston from the Woodstock epicentre is
+sixteen miles, and from the corresponding focus (with the calculated
+value of its depth) twenty miles. A first estimate of the velocity
+gives a value of a little more than three miles a second, and the time
+at the Woodstock focus may therefore be taken as 9h. 51m. 6s. with a
+probable error of a few seconds.<a name="FNanchor_44_44" id="FNanchor_44_44"></a><a href="#Footnote_44_44" class="fnanchor">[44]</a></p>
+
+<p>Proceeding to the observations at a distance, we find them, even after
+all rejections, to be very different in value. They were therefore
+divided into groups consisting of observations which are as nearly as
+possible homogeneous.</p>
+
+<p>The first group contains five records from places between 452 and 645
+miles from the Woodstock epicentre. They give the time to within 15
+seconds, obtained from an accurately kept clock, or from a clock or
+watch that was compared with such within a few hours of the
+earthquake. The resulting velocity is 3.236 &plusmn; .105 miles
+(or 5205 &plusmn; 168 meters) per second.<a name="FNanchor_45_45" id="FNanchor_45_45"></a><a href="#Footnote_45_45" class="fnanchor">[45]</a></p>
+
+<p><span class='pagenum'><a name="Page_129" id="Page_129">[129]</a></span>In the second group there are eleven observations (between distances
+of 438 and 770 miles) which satisfy the same conditions as those in
+the first group, except that the time is only given to the nearest
+minute or half-minute. The velocity obtained from them is 3.226 &plusmn;
+.147 miles (or 5192 &plusmn; 236 metres) per second.</p>
+
+<p>The third group included all but the above records and those obtained
+from stopped clocks. They are 125 in number (between distances of 80
+and 924 miles), but it is uncertain whether they correspond to the
+selected phase of the movement, and the errors of the clocks and
+watches used were unknown. They give a mean velocity of 3.013 &plusmn;
+.027 miles (or 4848 &plusmn; 43 metres) per second.</p>
+
+<p>In the fourth group, we have the evidence of 45 stopped clocks (at
+places between 20 and 855 miles), which apparently give a velocity of
+2.638 &plusmn; .105 miles (or 4245 &plusmn; .168 metres) per
+second. At six places, however, the times indicated by stopped clocks
+can be compared with good personal observations; and these show that
+the time of traverse from the origin obtained from the former is on an
+average 1.28 times the time of traverse obtained from the latter. If a
+similar correction be made for all the stopped clocks, the corrected
+velocity of the earth-waves would be from 3.17 to 3.23 miles (or 5100
+to 5200 metres) per second.</p>
+
+<p>In obtaining the mean value of the velocity from all the observations,
+those of the fourth group are omitted, and the weights of the first
+three groups are <span class='pagenum'><a name="Page_130" id="Page_130">[130]</a></span>taken inversely as the squares of the probable
+errors&mdash;that is, as 2: 1: 4. The resulting mean velocity is 3.221 &plusmn;
+.050 miles (or 5184 &plusmn; 80 metres) per second; and,
+though it does not follow that all other estimates are erroneous (for
+the velocity may vary with the strength of the earthquake and with
+other conditions), it is probable that this result is more nearly
+accurate than any other previously obtained.</p>
+
+<br />
+
+<h4>MISCELLANEOUS PHENOMENA.</h4>
+
+<p><i>Fissures and Sand-Craters.</i>&mdash;In point of size, there was nothing
+remarkable about the fissures in the ground produced by the Charleston
+earthquake. The largest were only a few hundred yards long, and,
+except near the river-banks, they rarely exceeded an inch in width.
+They seem, however, to have been unusually abundant; for, within an
+area of nearly 600 square miles surrounding the two epicentres, they
+were almost universal, and over a much wider area they still occurred
+in great numbers, though with somewhat less continuity.</p>
+
+<p>From many of these fissures water was ejected, carrying with it large
+quantities of sand and silt, and so abundantly that every stream-bed,
+even though generally dry in summer, was flooded. By the passage of
+the water, some part of the fissures was often enlarged into a round
+hole of considerable size, ending in a craterlet at the surface.
+Certain belts within the fissured area contained large numbers of
+these craterlets, of all sizes up to twenty feet or more in diameter.
+One near Ten-Mile Hill was twenty-one feet across. In this district,
+they were apparently larger and more numerous than elsewhere; many
+<span class='pagenum'><a name="Page_131" id="Page_131">[131]</a></span>acres of ground being covered with sand, which, close to the orifices,
+was two feet or more in depth.</p>
+
+<p>Here and there, the water was ejected with considerable violence, as
+was manifest from the heights to which it spurted. The testimony of
+witnesses on this point is of course doubtful, for the earthquake
+occurred after nightfall, but in a few places the branches and leaves
+of trees overhanging the orifices were smirched with sand and mud up
+to a height of fifteen or twenty feet.</p>
+
+<p><i>Effects on Human Beings.</i>&mdash;It is interesting to notice the behaviour
+of different races under the influence of a violent earthquake, and
+perhaps no greater contrast could be observed than between the
+calmness exhibited by the Japanese in the presence of disaster and the
+wild fear merging into helpless panic that characterised the
+residents, and especially the negroes, of Charleston. "As we dashed
+down the stairway," says a writer already quoted (p. 108), "and out
+into the street, from every quarter arose the shrieks, the cries of
+pain and fear, the prayers and wailings of terrified women and
+children, commingled with the hoarse shouts of excited men.... On
+every side were hurrying forms of men and women, bareheaded, partially
+dressed, some almost nude, and all nearly crazed with fear and
+excitement.... A few steps away, under the gas-lamp, a woman lies
+prone and motionless on the pavement, with upturned face and
+outstretched limbs, and the crowd which has now gathered in the street
+passes her by, none pausing to see whether she is alive or dead ... no
+one knows which way to turn, or where to offer aid; many voices are
+speaking at once, but few heed what is said."</p>
+
+<p><span class='pagenum'><a name="Page_132" id="Page_132">[132]</a></span>Between the selfish rush for safety here described and the calm
+interest of the most distant observers, Major Dutton records nearly
+every possible variety of mental effects, the actions resulting from
+which may be roughly classified as follows:</p>
+
+<p>A. No persons leave their rooms.</p>
+
+<p>B. Some persons leave their houses.</p>
+
+<p>C. Most persons run into the streets, which are full of excited
+people.</p>
+
+<p>D. People rush wildly for open spaces and remain all night
+out-of-doors.</p>
+
+<p>In the map of the isoseismal lines (Fig. 25), the dotted curves bound
+the areas in which the effects corresponding to the three highest
+degrees of the above scale were observed. The curve for the first
+degree (A) coincides of course with the isoseismal line of intensity
+2.</p>
+
+<p>It will be seen that there is a certain rough correspondence between
+these curves and the isoseismal lines. The curve D and the isoseismal
+8 are close together; in other words, people thought it wiser to camp
+out-of-doors for the night if the shock was strong enough to damage
+buildings slightly. The curve C and the isoseismal 6 are similarly
+connected; that is, if the movement made pictures swing, etc., people
+rushed into the streets. On the whole, the curve B and the isoseismal
+3 roughly coincide, or, if the shock was not strong enough to make
+doors and windows rattle, some persons left their houses and public
+meetings were dispersed.</p>
+
+<p><i>Feeling of Nausea.</i>&mdash;A feeling of nausea was experienced by many
+persons at the time of the earthquake, somewhat rarely it appears in
+the neighbourhood of the epicentre and even outside the isoseismal <span class='pagenum'><a name="Page_133" id="Page_133">[133]</a></span>7,
+but more frequently beyond these limits, and perceptible as far as the
+broken line in Fig. 25. The most distant places at which it was
+noticed are Blue Mountain Creek (New York) and Dubuque (Iowa), which
+are respectively 823 and 886 miles from Charleston.</p>
+
+<br />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p>As Summerville lies six miles to the north-west of the Woodstock
+epicentre and Charleston 16 miles to the south-east, it is probable
+that many of the after-shocks were unfelt and a still greater number
+unrecorded. In Charleston, seven shocks, all much slighter than the
+principal shock, were felt during the night of August 31&mdash;September 1,
+and thirty before the end of September. Of these, the shock of
+September 3rd, at 11 <span class="fakesc">P.M.</span>, was the strongest, but those which
+occurred on September 1st, 2nd, 21st, and 27th were also described as
+severe, and the remainder as moderate or slight. For weeks after the
+great shock, curious sensations were distinctly perceptible during the
+still hours of the night "as though the crust of the earth were
+resting on a gelatinous mass in constant motion." The last shock felt
+in Charleston seems to have been one recorded on March 18th, 1887.</p>
+
+<p>At Summerville, many shocks occurred that were scarcely perceptible in
+Charleston, and those noticed in both places were usually stronger,
+and the motion more nearly vertical, at Summerville. "The peculiar
+characteristic of all of them was the deep, solemn, powerful boom,
+like the report of a heavy cannon, usually accompanied by a quick,
+short jar. Sometimes it was prolonged into a heavy roar or rumble, <span class='pagenum'><a name="Page_134" id="Page_134">[134]</a></span>as
+if many reports were delivered in a volley. The number of them was
+never recorded." On September 3rd, Mr. W.J. McGee, of the United
+States Geological Survey, arrived at Summerville. During the evening
+of that day, detonations were heard at intervals, averaging perhaps
+half-an-hour, accompanied occasionally by very slight spasmodic
+tremors of an instant's duration. They were much like peals of thunder
+at a distance of half-a-mile or more, though rather more muffled. "It
+was my impression," Mr. McGee remarks, "that the sound was sometimes
+about as grave as the ear can perceive, resembling somewhat the
+tremulous roar sometimes accompanying combustion in locomotives."
+These sounds continued, but with diminishing frequency, throughout the
+remainder of the year and as late as July 1st, 1887.</p>
+
+<br />
+
+<h4>ORIGIN OF THE EARTHQUAKE.</h4>
+
+<p>Major Dutton's valuable monograph is a record of the
+earthquake-phenomena. He offers no theory as to the cause of the
+shock, and is therefore in no way responsible for the account given in
+the remaining part of this chapter.</p>
+
+<p>That there were two seismic foci he has shown, I think, conclusively;
+and my object is now to trace out briefly the probable nature of the
+movements that produced the double shock.</p>
+
+<p>Referring to Figs. 28 and 29, it will be seen that, according to both
+Mr. Sloan and Major Dutton, the isoseismals surrounding the Rantowles
+epicentre are distorted along a line which runs from a few degrees
+east of north to a few degrees west of south. Their oval form is in
+all probability connected with a <span class='pagenum'><a name="Page_135" id="Page_135">[135]</a></span>focus elongated in about the same
+direction. Near the Woodstock epicentre, the isoseismals are drawn
+differently in the two maps, and in neither case do they offer any
+sure guide as to the form of the seismic focus. If, however, we follow
+Mr. Sloan's interpretation of the evidence, and suppose the earthquake
+to have been fault-formed, then it is probable that in this region the
+fault bends round slightly towards the east.</p>
+
+<p>The only other evidence on this point is that afforded by the regions
+of defective intensity, real or apparent, along the three
+railway-lines diverging from Charleston. One of these occurred near
+Mount Holly Station on the North-Eastern Railway (B, Figs. 28 and 29),
+another for four miles starting from the 11&frac12;-mile point on the
+South Carolina Railway (A), and a third along the Charleston and
+Savannah Railway (C) over a distance of four miles from the Ashley
+River. In the first two cases, Major Dutton suggests that the less
+amount of damage was due to the nature of the soil traversed by the
+railway; but it is on the softer ground that the effects of an
+earthquake-shock are generally the more disastrous. On the whole, it
+seems to me probable that the three tracts referred to are really
+regions of less intensity, and it is worthy of notice that they lie
+along a nearly straight line.</p>
+
+<p>To show the bearing of these remarks, let CD (Fig. 32) represent the
+section of a fault and EF that of the surface of the earth, and
+suppose the rock-mass A to slip slightly and suddenly downwards. Then
+the particles of A at the surface of the fault will, by impulsive
+friction, be drawn sharply upwards, and those of B correspondingly
+downwards; so that the <span class='pagenum'><a name="Page_136" id="Page_136">[136]</a></span>earth-waves in the two rock-masses will start
+in opposite phases of vibration. Along the line of fault, every
+particle of rock, being urged upwards and downwards almost equally,
+will remain practically at rest. Thus, regions of defective intensity
+may arise from partial interference by the spreading of either
+earth-wave in the adjoining rock-mass.</p>
+
+<div class="imgl" style="width: 30%;"><a name="Fig32" id="Fig32"></a>
+<a href="images/fig32.png">
+<img border="0" src="images/fig32.png" width="90%" alt="Diagram to explain origin of regions of defective intensity." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 32.&mdash;Diagram to explain origin of regions of defective intensity.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>If this be the correct explanation, the path of the originating fault
+may be taken as that indicated by the broken line in Fig. 28, a line
+which is nearly parallel to the chief branches of the isoseismal
+curves.<a name="FNanchor_46_46" id="FNanchor_46_46"></a><a href="#Footnote_46_46" class="fnanchor">[46]</a> As both epicentres lie on the west side of this line, the
+fault must hade or slope in this direction. The distortion of the
+Woodstock isoseismals towards the north-west confirms the latter
+inference, for the intensity of the shock is greater on the side
+towards which the fault hades.</p>
+
+<p>From the comparative absence of earthquakes in South Carolina, we may
+infer that the fault is one subject to displacements at wide intervals
+of time. The gradually increasing stress along its surface was
+relieved at one or two points in or near the Woodstock focus on August
+27th and 28th, and perhaps during the preceding months. But the first
+great slip <span class='pagenum'><a name="Page_137" id="Page_137">[137]</a></span>took place suddenly in that focus, and spread gradually
+southwards&mdash;for there was no interruption in the movement&mdash;until about
+half-a-minute later it reached the Rantowles focus, where the second
+great slip occurred. Eight or ten minutes afterwards there was another
+slip&mdash;in what part of the fault is uncertain&mdash;and this was followed at
+irregular intervals by many small movements gradually diminishing in
+frequency and in focal area. Within a year from the first disturbance,
+the fault-system attained once more its usual condition of rest.</p>
+
+<br style="clear: both;" />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Dutton, C.E.</span>&mdash;"The Charleston Earthquake of August
+31st, 1886." <i>Amer. Geol. Survey, Ninth Annual Report</i>, pp.
+209-528.</p>
+
+<p class="hang">2. <i>Nature</i>, vol. xxxv., 1887, pp. 31-33; vol. lxiii., 1901, pp.
+165-166.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> The authorities for this statement are Mallet's
+Catalogue of Recorded Earthquakes (<i>Brit. Assoc. Rep.</i>, 1852, pp.
+1-176; 1853, pp. 117-212; 1854, pp. 1-326), which closes with the year
+1842, and Fuchs' <i>Statistik der Erdbeben von 1865-1885</i>. According to
+Mallet, there was an earthquake in S. Carolina in November 1776, and
+the New Madrid earthquake of December 16th, 1811, was felt at
+Charleston. Fuchs records two earthquakes at Charleston on May 12th,
+1870, and December 12th, 1876; and two in S. Carolina on December 12th
+and 13th, 1879.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> 1. Recorded by a single seismograph, or by some
+seismographs of the same pattern, but not by several seismographs of
+different kinds, the shock felt by an experienced observer.</p>
+
+<p class="noin">2. Recorded by seismographs of different kinds; felt by a small number
+of persons at rest.</p>
+
+<p class="noin">3. Felt by several persons at rest; strong enough for the duration or
+direction to be appreciable.</p>
+
+<p class="noin">4. Felt by several persons in motion; disturbance of movable objects,
+doors, windows; creaking of floors.</p>
+
+<p class="noin">5. Felt generally by every one; disturbance of furniture and beds;
+ringing of some bells.</p>
+
+<p class="noin">6. General awaking of those asleep; general ringing of bells;
+oscillation of chandeliers, stopping of clocks; visible disturbance of
+trees and shrubs; some startled persons leave their dwellings.</p>
+
+<p class="noin">7. Overthrow of movable objects, fall of plaster, ringing of church
+bells, general panic, without damage to buildings.</p>
+
+<p class="noin">8. Fall of chimneys, cracks in the walls of buildings.</p>
+
+<p class="noin">9. Partial or total destruction of some buildings.</p>
+
+<p class="noin">10. Great disasters, ruins, disturbance of strata, fissures in the
+earth's crust, rock-falls from mountains.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_40_40" id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> In order to simplify these figures, the rivers, most of
+the inlets, and other details are omitted. Small figures are added
+along the railway lines to denote the distance in miles from the
+stations in Charleston.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_41_41" id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> If this were so, the decrease in intensity would be only
+apparent; but it may have been real, and a possible explanation on
+this supposition is given later on (p. 135).</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_42_42" id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> If <i>c</i> be the depth of the focus, <i>a</i> the intensity at
+unit distance from the focus, and <i>y</i> the intensity on the surface at
+distance <i>x</i> from the epicentre, then</p>
+
+<p class="pad"><i>y</i> = <i>a</i> / (<i>c</i>&sup2; + <i>x</i>&sup2;)</p>
+
+<p class="noin">The inclination of the curve at any point is given by</p>
+
+<p class="pad"><i>dy</i> / <i>dx</i> = -2<i>ax</i> / (<i>c</i>&sup2; + <i>x</i>&sup2;)&sup2;,</p>
+
+<p class="noin">and this is a maximum when</p>
+
+<p class="pad"><i>d</i>&sup2;y / <i>dx</i>&sup2; or (3<i>x</i>&sup2; - <i>c</i>&sup2;) / (<i>c</i>&sup2; + <i>x</i>&sup2;)&sup3;</p>
+
+<p class="noin">is zero, which is satisfied when <i>c</i> = <i>x</i> &#8730;<span style="border-top: solid 1px;">3</span></p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_43_43" id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> <i>British Association Report</i>, 1858, pp. 101-103.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_44_44" id="Footnote_44_44"></a><a href="#FNanchor_44_44"><span class="label">[44]</span></a> The above time would have to be increased by one second
+if the depth of the focus were very small, and diminished by one
+second if it were as great as 23 miles; the difference in either case
+being less than the probable error.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_45_45" id="Footnote_45_45"></a><a href="#FNanchor_45_45"><span class="label">[45]</span></a> The method employed is as follows: Let t<sub>0</sub> be the
+computed time (9h. 51m. 6s.) at the focus, <i>x</i> seconds the error in
+this estimate, <i>t</i> the reported time at a given place, <i>D</i> its
+distance from the focus in miles, and <i>y</i> the number of seconds
+required to travel one mile; then, assuming that <i>y</i> does not vary
+with the distance, we have <i>x</i> + <i>Dy</i> = <i>t</i> + t<sub>0</sub>. An equation
+of this form is obtained from each observation, and the method of
+least squares is then applied to determine the most probable values of
+<i>x</i> and <i>y</i>.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_46_46" id="Footnote_46_46"></a><a href="#FNanchor_46_46"><span class="label">[46]</span></a> This seems to me the more probable course. It is
+possible, however, that the fault-line may pass from Mount Holly
+Station to the east of the Woodstock epicentre as shown in Fig. 28,
+and then to the west of the Rantowles epicentre, the fault changing
+its direction of hade in the intermediate district.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_VI" id="CHAPTER_VI"></a><hr />
+<span class='pagenum'><a name="Page_138" id="Page_138">[138]</a></span><br />
+
+<h3>CHAPTER VI.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887.</h4>
+<br />
+
+<p>Few earthquakes have aroused a more widespread interest than those
+which struck the thronged cities of the Riviera on February 23rd,
+1887. The first and greatest of the shocks occurred at about 6.20
+<span class="fakesc">A.M.</span>, the second nine minutes later, and the third,
+intermediate in strength, at about 8.51 <span class="fakesc">A.M.</span><a name="FNanchor_47_47" id="FNanchor_47_47"></a><a href="#Footnote_47_47" class="fnanchor">[47]</a> All three
+shocks were of destructive violence, the damage wrought by them
+extending along the coast and for a short distance inland from Nice to
+beyond Savona. Most of the injury to property and nearly all the loss
+of life were, however, concentrated on the eastern side of the
+frontier; and it therefore fell to the lot of the Italian Government
+to provide for the scientific investigation of the earthquakes, as
+well as to meet the wants of those deprived of home and support.
+Professors Taramelli and Mercalli, who two years before had studied
+the earthquakes in Andalusia, were again nominated, the former to
+examine the geology of the central regions, and the latter to report
+on the seismic phenomena. Their joint memoir forms one of the most
+complete accounts that we possess of any earthquake, and is the chief
+authority for the description given in this chapter. Another valuable
+monograph <span class='pagenum'><a name="Page_139" id="Page_139">[139]</a></span>is that prepared by Professor A. Issel, of Genoa, who
+received an independent appointment from the same Ministry. A third
+official commission was also sent to estimate the amount of damage
+caused by the earthquakes in the Italian towns and villages. In
+France, the destruction of property was much less serious, and
+attention was confined chiefly to the records of the shock provided by
+magnetographs and other instruments in distant observatories. In
+Switzerland, the effects remarked were merely those due to the
+evanescent vibrations of a remote earthquake; but many interesting
+records were collected by the permanent seismological commission
+established in that country.</p>
+
+<br />
+
+<h4>DAMAGE CAUSED BY THE EARTHQUAKES.</h4>
+
+<p>Owing to variations in the nature, foundation, and site of buildings,
+there is always great diversity in the destructive effects of an
+earthquake. In one and the same town, most of the houses may be razed
+to the ground, while in their midst may be found some that are
+shattered but still standing, and others perhaps that are practically
+unharmed. The stronger after-shocks often complete the ruin of the
+partially damaged houses; though in such cases the real loss is as a
+rule comparatively small.</p>
+
+<p>The close succession of the two strong after-shocks of February 23rd
+made it impossible as a rule to separate their effects from those due
+to the first shock; but it has been roughly estimated that about
+one-quarter of the total damage was caused by the two after-shocks
+together. To them also <span class='pagenum'><a name="Page_140" id="Page_140">[140]</a></span>must be referred in part the comparatively
+small number of wounded, many persons buried beneath the ruins having
+no doubt perished from subsequent falls before they could be
+extricated.</p>
+
+<p>Taking all three shocks together, the total loss to property,
+according to Professor Mercalli, must be valued at about 22 million
+francs in Italy alone. For the province of the Alpes Maritimes in
+France, full details are wanting, but the loss there cannot fall far
+short of three million francs. The total amount of damage must
+therefore be placed at about a million pounds. From the figures given
+by the official commissions, it appears that the earthquakes were most
+disastrous at Diano Marina and Diano Castello; while other places,
+such as Oneglia, Bussana, Baiardo, Pompeiana, and Vallecrosia,
+suffered only a little less severely. At Mentone about 155 houses, and
+at Nice about 61 houses, were rendered uninhabitable, and many others
+were badly injured.</p>
+
+<p>In Italy, 633 persons were killed, 432 seriously wounded, and 104
+slightly wounded; in France, 7 persons were killed and 30 seriously
+wounded, the number of persons slightly wounded being unknown. The
+majority of the deaths occurred in two or three places. Thus, at Diano
+Marina, 190 persons were killed and 102 wounded; at Baiardo, 220 were
+killed and 60 wounded; at Bussana, there were 53 killed and 27
+wounded. The death-rates were, however, comparatively small, amounting
+for the above places to not more than 8&frac12;, 14, and 6&frac12; per cent.,
+respectively; figures which only slightly exceed those obtained for
+places in the meizoseismal area of the Andalusian earthquake.</p>
+
+<p><span class='pagenum'><a name="Page_141" id="Page_141">[141]</a></span>Though the damage can hardly be regarded as excessive, it was
+nevertheless largely due to the peculiar architecture prevalent in the
+Riviera. Arches in the walls are common even in the upper storeys,
+and, in Oneglia and Diano Marina, if not also in other places, the
+floors are nearly always brick arches abutting against the walls and
+without other lateral support. Professor Mercalli believes that, in
+private houses, more than 90 per cent. of the dead bodies were crushed
+beneath these fallen arches. The height of the buildings is also great
+in proportion to the foundation and to the thickness of the walls; and
+the main walls are interrupted by numerous apertures, from the corners
+of which nearly all the fissures sprang. In some of the coast towns,
+the houses are built of rounded stones gathered from the beach, or of
+rubble with stones of all shapes and sizes, bound by cement of the
+poorest quality. Lastly, as much of the damage due to previous
+earthquakes had been badly repaired, it is evident that the
+destructiveness of the Riviera earthquakes must to a great extent be
+referred to preventable causes.</p>
+
+<p>The occurrence of the principal shock shortly after six on the morning
+of Ash Wednesday must also have increased the death-rate; for many
+persons, after a night of amusement, had lain down for a short time
+and were sleeping heavily; while others had already risen and were
+collected in the churches; the circumstances in either case rendering
+escape more difficult.</p>
+
+<p>Taking account, however, of this accidental increase in the number of
+victims, Professor Mercalli considers that the earthquake of 1887 was
+the most disastrous <span class='pagenum'><a name="Page_142" id="Page_142">[142]</a></span>of all those which have visited either the
+Riviera or northern Italy in the last three centuries; though, during
+the nineteenth century, there were three Italian earthquakes of far
+greater destructive power, but all confined to the southern part of
+the peninsula&mdash;namely, the Neapolitan earthquakes of 1805 and 1857,
+and the Ischian earthquake of 1883.</p>
+
+<br />
+
+<h4>PREPARATION FOR THE EARTHQUAKES.</h4>
+
+<p>It is difficult, as usual, to specify the exact moment when the first
+earthquake of the 1887 series took place; but it is evident that the
+preparation for the great shock was very brief. At Oneglia, it is
+alleged that faint shocks and sounds were observed many times, chiefly
+at night, during the month preceding February 23rd; though they were
+not at the time supposed to be of seismic origin. A slight shock is
+also reported from Diano at about midnight on February 21-22.</p>
+
+<p>The first undoubted shock occurred on February 22nd, at about 8.30
+<span class="fakesc">P.M.</span>, or ten hours before the principal earthquake. Though
+very slight, it was felt throughout the Riviera and in part of
+Piedmont. Another shock, also weak, took place at about 11
+<span class="fakesc">P.M.</span>; and a third, sensible only in the eastern part of the
+Ligurian Apennines, on February 23rd, at about 1 <span class="fakesc">A.M.</span>; at
+which time the tide-gauge at Genoa recorded some abnormal
+oscillations. An hour later, a more important, though by no means a
+strong, shock occurred; this was perceptible all over the Riviera, in
+Piedmont, and in Corsica; in other words, it disturbed a region
+agreeing closely with the central area of the disastrous shock. At
+<span class='pagenum'><a name="Page_143" id="Page_143">[143]</a></span>about 5 <span class="fakesc">A.M.</span>, a fifth shock, somewhat weaker than the
+preceding, was felt over the same area, concurrently, or nearly so,
+with another abnormal oscillation of the tide-gauge at Genoa; while a
+sixth shock was noticed at several places a few minutes before the
+great shock.</p>
+
+<p>During the night of February 22-23, nervous persons in many towns and
+villages were agitated without apparent reason. Birds and animals,
+more sensitive than human beings to faint tremors, were more
+distinctly affected, especially for some minutes before the
+earthquake. Horses refused food, were restless or tried to escape from
+their stables, dogs howled, birds flew about and uttered cries of
+alarm. As these symptoms were noticed at more than one hundred and
+thirty places within the Italian part of the central area, there can
+be little doubt that they were caused by microseismic movements for
+the most part insensible to man.</p>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>The only complete map of the isoseismal lines is that drawn by
+Professor Mercalli.<a name="FNanchor_48_48" id="FNanchor_48_48"></a><a href="#Footnote_48_48" class="fnanchor">[48]</a> In this map, reproduced in Fig. 33, the
+continuous curves represent the principal isoseismal lines; the dotted
+curves define the disturbed areas of two of the stronger after-shocks.</p>
+
+<p>The meizoseismal area, bounded by the curve marked 1 in Fig. 33, is
+also shown on a larger scale in Fig. 34. At the places denoted by
+small circles in the latter figure, the principal shock was
+<span class='pagenum'><a name="Page_144" id="Page_144">[144]</a></span>"disastrous," some of the houses in each being either totally or
+partially ruined. At those marked by a small cross, the shock was
+"almost ruinous"; in other words, numerous houses were damaged, but in
+no case was the injury of a serious character. The meizoseismal area
+is thus a narrow band, skirting the Riviera coast from Mentone to
+Albissola, a distance of 106 miles, and extending inland for not more
+than from nine to twelve miles. The greatest intensity, corresponding
+to <span class='pagenum'><a name="Page_145" id="Page_145">[145]</a></span>the ruin of many houses with considerable loss of life, was
+reached at only a few places between Bussano and Diano Marina, all
+lying within a littoral band about twenty miles in length and three to
+three and a half miles in width. If, however, the epicentre had lain
+on land, the area would have been much greater, Professor Mercalli
+estimates about four times greater, than its actual amount.</p>
+
+<div class="img"><a name="Fig33" id="Fig33"></a>
+<a href="images/fig33.png">
+<img border="0" src="images/fig33.png" width="65%" alt="Isoseismal lines of the Riviera earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 33.&mdash;Isoseismal lines of the Riviera earthquake. (<i>Mercalli.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The curve marked 2 (Fig. 33) bounds the "almost ruinous" zone; its
+expansion towards the north and contraction towards the west,
+north-west, and east, being its most noteworthy features. The next
+zone, that of slight damage, is contained between the isoseismals 2
+and 3, the latter curve probably grazing the north end of Corsica.
+Beyond this lies the "strong" zone, in which the shock was generally
+felt without causing any damage to buildings. Its boundary (marked 4)
+passes near Marseilles, Como, and Parma, and includes nearly the whole
+of Corsica; towards the north-west, in the valley of Aosta, it curves
+in towards the isoseismal 3.</p>
+
+<p>In the outermost zone of all the shock was "slight," and towards the
+margin was only just perceptible. The boundary, which of course
+defines that of the disturbed area, reaches as far north as Basle and
+Dijon, to Perpignan on the west, Trento, Venice, and Pordenone on the
+east, and to the south as far as Tivoli (near Rome) and the northern
+end of Sardinia. In eastern Switzerland, it shows a marked curve
+inwards; possibly, as Professor Mercalli suggests, from the vibrations
+having to cross the northern Apennines in a direction nearly at right
+angles to their axis. Except for this bay, <span class='pagenum'><a name="Page_146" id="Page_146">[146]</a></span>however, the curve differs
+little from a circle, the centre of which lies in the sea, a little to
+the south of Oneglia, close to the position assigned by other evidence
+to the epicentre. The radius of this circle being about 264 miles, it
+follows that the disturbed area must have contained about 219,000
+square miles&mdash;by no means a large amount for so strong an earthquake.</p>
+
+<br />
+
+<h4>POSITION OF THE EPICENTRES.</h4>
+
+<p>It is evident, from the form of the meizoseismal area shown in Fig.
+33, that a mere fringe of it lies upon land, and that the epicentre
+must be situated some distance out at sea. Other facts may be
+mentioned which point to the same conclusion. There were, for
+instance, no purely vertical movements observed, even in the districts
+where the damage done by the shock was greatest. Nor were any large
+landslips to be seen in those areas; there were no lasting changes in
+the underground water-system; and in general, as Professor Mercalli
+remarks, all the superficial distortions of the ground which are so
+characteristic of the epicentral area of a great earthquake were
+conspicuous by their absence. There is evidence, again, of some
+disturbance of the sea-bed in the death and flight of fishes from
+great depths and in the seismic sea-waves recorded by the tide-gauges
+at Genoa and Nice. These phenomena will be described in a later
+section, but reference should be made here to an interesting
+observation at Oneglia on the occurrence of some of the stronger
+after-shocks. Persons on the coast, it is said, saw the sea curling
+and moving, and immediately afterwards the shock was felt.</p>
+
+<p><span class='pagenum'><a name="Page_147" id="Page_147">[147]</a></span>In determining the position of the epicentre, Professor Mercalli had
+recourse as usual to observations on the direction of the shock,
+especially those derived from the oscillation of lamps or other
+suspended objects, the projection or fall of bodies free to move,
+fractures, etc., in damaged houses, and the stopping of pendulum
+clocks. Such observations were made at 120 places&mdash;72 in the western
+Riviera and the Alpes Maritimes, and 48 at Piedmont, Lombardy, and
+Tuscany.</p>
+
+<p>At many of these places the movement was extremely complicated. In
+nearly all parts of the area most strongly shaken, for instance, the
+direction of the shock changed more than once; and it was therefore
+necessary to select whenever possible the principal direction of the
+shock at each place. In some towns, such as Oneglia, Mentone, Antibes,
+Cuneo, etc., the shock had two dominant directions, and these appeared
+to be sensibly at right angles to one another; an inclination which,
+as Professor Mercalli suggests, may be due in part to the
+approximation of the real directions to those of the principal walls
+of the houses in which the observations were made.</p>
+
+<p>Most of the lines of direction, when plotted on the map, converge
+towards an area lying between the meridians of Oneglia and San Remo,
+and between nine and fifteen miles from the coast. For places near the
+epicentre, the most trustworthy, in Mercalli's opinion, are those made
+at Oneglia, Mentone, Taggia, Bordighera, Castel Vittorio, Nice, and
+Genoa; and the points in which these lines Intersect one another are
+Indicated by small crosses on the map of the meizoseismal area (Fig.
+34). All of them lie at sea at distances between six and fifteen miles
+to the <span class='pagenum'><a name="Page_148" id="Page_148">[148]</a></span>south of Oneglia. The most probable position of the principal
+epicentre is that marked by the small circle A, which is situated
+about fifteen miles south of Oneglia.</p>
+
+<div class="img"><a name="Fig34" id="Fig34"></a>
+<a href="images/fig34.png">
+<img border="0" src="images/fig34.png" width="80%" alt="Meizoseismal area of the Riviera earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 34.&mdash;Meizoseismal area of the Riviera earthquake. (<i>Taramelli and Mercalli.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_149" id="Page_149">[149]</a></span>There are, however, several lines of direction which can have no
+connection with this epicentre. Besides the east and west lines at
+Nice, Mentone, and Antibes, there are others at the same places which
+run north and south or nearly so. Professor Mercalli believes that
+they were due to vibrations coming from a second focus lying to the
+south of Nice, and there are also several lines of direction at more
+distant places which converge towards the neighbourhood of the
+corresponding epicentre.</p>
+
+<p>This conclusion receives unexpected support from some of the best
+time-records. At the railway-stations of Loano and Pietra Ligure, the
+times of occurrence were given as 6h. 20m. 5s. and 6h. 20m.
+respectively&mdash;estimates which are probably accurate to within a few
+seconds; for, at the moment of the shock, the officer who brought the
+exact time along the railway-line from Genoa was at Loana, and had
+just passed through Pietra Ligure. On the other hand, the estimates
+for Mentone and Nice&mdash;namely, 6h. 18m. 35s. and 6h. 19m. 43s., if not
+equally exact, cannot err by many seconds, certainly not by so much as
+one minute. Since the distances of Loana and Pietra Ligure from the
+principal epicentre are 31 and 32 miles, and those of Mentone and Nice
+28 and 37 miles, it is therefore clear that the vibrations which
+arrived first at Nice and Mentone must have come from a local focus,
+where the impulse preceded that at the principal focus by several
+seconds.</p>
+
+<br />
+
+<h4>DEPTH OF THE PRINCIPAL FOCUS.</h4>
+
+<p>Inaccurate as are all the methods of determining the depth of focus,
+it seems probable, as Professor <span class='pagenum'><a name="Page_150" id="Page_150">[150]</a></span>Issel argues, that the principal
+Riviera focus was situated at a considerable distance from the
+surface. In no part of the meizoseismal area was the shock a really
+violent one; yet its intensity must have faded very slowly outwards,
+for it was strong enough to stop clocks at places in Switzerland and
+elsewhere not less than 250 miles from the origin.</p>
+
+<p>Professor Mercalli regards Mallet's method with greater favour than
+most seismologists. He points to the gradual increase in the angle of
+emergence from the outer zones disturbed by the Riviera earthquake
+towards the meizoseismal area, where several good observations were
+made from fissures in walls parallel to the dominant direction of the
+shock. The angles of emergence which he considers as most trustworthy
+are those of 35&deg; at Taggia, 40&deg; at Oneglia, and about 30&deg; at
+Bordighera. The corresponding depths for the focus are 10.4, 10.4, and
+11.6 miles, giving an average of about 10&frac34; miles.</p>
+
+<p>There are no similar observations forthcoming for the depth of the
+secondary focus near Nice and Mentone; but Professor Mercalli observes
+that it must have been shallower than the other, for the vertical
+component of the vibrations from this focus was much less sensible
+than that of the motion coming from the principal focus.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p><i>The Double Shock.</i>&mdash;In the valuable collection of records made by
+Professors Taramelli and Mercalli there appears at first sight to be
+the utmost diversity in the evidence with regard to the nature of the
+shock. Thus, in the province of P. Maurizio alone, <span class='pagenum'><a name="Page_151" id="Page_151">[151]</a></span>the shock was
+described as subsultory first and then undulatory or vorticose at 25
+places, undulatory and then subsultory at 22, undulatory and then
+subsultory and again undulatory or vorticose at 13, and subsultory
+first, then undulatory, and finally subsultory and vorticose at two
+places. It is clear that the shock was of considerable duration, not
+less than half-a-minute as a rule, and that there were several phases
+in the movement; and it would seem that one or more of these phases
+may have passed unnoticed owing to the alarm occasioned by the shock,
+and to the fact that most of the observers were asleep when the
+earthquake began. Defects of memory must also have an influence not to
+be neglected, for, even with the simple shocks felt in the British
+Isles, persons in the same or neighbouring places differ greatly in
+their testimony.</p>
+
+<p>But, if we confine ourselves to the accounts of careful persons alone,
+the discrepancies to a large extent disappear. Indeed, all over the
+ruinous area (Fig. 33) the shock maintained a nearly uniform
+character. At Oneglia, for instance, there were two well-marked
+phases, the first of which began with a brief subsultory movement,
+followed by more horizontal undulations of longer period; a pause,
+lasting but for an instant, was succeeded by vibrations which, though
+not vertical, were highly inclined to the horizon; they continued
+throughout the second phase, but, towards the end, new undulations
+were superposed, and these, coming from different directions, resulted
+in an apparently vorticose movement. Professor Mercalli represents the
+motion diagrammatically by the curve <i>a</i> in Fig. 35. At Diano Marina,
+as will be seen from the curve <i>b</i>, the shock <span class='pagenum'><a name="Page_152" id="Page_152">[152]</a></span>again consisted of two
+phases, each beginning with a few subsultory vibrations and ending
+with horizontal undulations of much longer period. In the first phase,
+the undulations were marked by a dominant direction, but, towards the
+close of the second phase, there was no determinate direction, and the
+impression was again that of a vorticose shock. At Savona, the
+movement, which is represented by the curve <i>c</i>, must have lasted from
+twenty-five to thirty seconds. It also consisted of two phases, with
+subsultory vibrations and undulations in the same order; and it was
+noticed that the second part of the shock was much stronger than the
+first. According to some observers, the concluding movements were
+vorticose.</p>
+
+<div class="img"><a name="Fig35" id="Fig35"></a>
+<a href="images/fig35.png">
+<img border="0" src="images/fig35.png" width="60%" alt="Nature of shock of Riviera earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 35.&mdash;Nature of shock of Riviera earthquake. (<i>Taramelli and Mercalli.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>In the zone surrounding the ruinous area, the vertical component of
+the motion was observed to diminish with the intensity; but, in other
+respects as well as in duration, the shock retained the same general
+form. At Genoa, Turin, Acqui, Alessandria, Antibes, and other places,
+two distinct phases were <span class='pagenum'><a name="Page_153" id="Page_153">[153]</a></span>perceived, occasionally separated by a brief
+pause, the first being invariably the weaker. At some places, the
+observers speak of two shocks at about 6.20 <span class="fakesc">A.M.</span>, separated
+by an interval of a few seconds; and this division was noticeable as
+far as Sal&ograve; on the shore of Lake Garda and Vicenza in Venetia. Only in
+Switzerland and other districts near the boundary of the disturbed
+area did the weaker part of the shock become insensible, the other
+consisting of horizontal oscillations, remarkable for their slowness
+and regularity, and lasting for as much as twenty or thirty seconds.</p>
+
+<p>We may thus conclude, with Professor Mercalli, that the earthquake
+resulted from the almost immediate succession of two distinct shocks,
+in each of which the nearly vertical vibrations were more marked at
+the beginning, while the slower undulations predominated towards the
+close, those of the second phase generally becoming vorticose through
+the superposition of movements coming from different directions. The
+second part of the shock in all of the more carefully written accounts
+is described as the stronger, especially as regards the subsultory
+vibrations in the meizoseismal area; except in the immediate
+neighbourhood of Nice, where the second phase was generally regarded
+as the weaker, or at any rate as not stronger than the first.</p>
+
+<p><i>Origin of the Double Shock.</i>&mdash;These observations show, not only that
+the principal earthquake consisted of two distinct shocks, but also
+that the shocks originated in different foci. For, if the vibrations
+of both had started from one focus, the second shock would have been
+everywhere the stronger; instead of which there was a small area near
+Nice where the <span class='pagenum'><a name="Page_154" id="Page_154">[154]</a></span>intensity of the first was the greater. This points
+clearly to the existence of another focus situated not far from Nice;
+and it is evident that the greater intensity of the first part in that
+district was due solely to the proximity of this focus, for, still
+farther to the west, at Antibes, the second part was again the
+stronger.</p>
+
+<p>There is thus a striking agreement in the inferences drawn from
+observations on the direction, time of occurrence, and nature of the
+shock. In the face of such concurring testimony, little doubt can
+remain as to the existence of two foci, one to the south of Oneglia
+and the other to the south of Nice, the initial impulse at the latter
+being decidedly the weaker, and preceding that at the eastern focus by
+an interval of some seconds, long enough at any rate for the resulting
+vibrations to reach the Oneglia focus and to spread beyond it before
+the vibrations from that focus started on their outward journey.</p>
+
+<p><i>Seismographic Records.</i>&mdash;In 1887, the Riviera and the districts
+adjoining it were unprovided with accurately constructed seismographs.
+The observatories at Alessandria, Milan, Monza, Parma, Florence, and
+other places in Italy contained seismoscopes and other pendulums, and
+these all registered the fact that an earthquake had occurred, and in
+many cases traced a series of elliptical or elongated curves. A record
+of the shock was also given by a Cecchi seismograph at Perpignan in
+France, but the distance from the epicentre was too great to allow
+details to be shown. The most valuable record was that obtained from a
+Cecchi seismograph at the observatory of Moncalieri, near Turin, about
+ninety miles north of the principal epicentre.</p>
+
+<p><span class='pagenum'><a name="Page_155" id="Page_155">[155]</a></span>In this seismograph, the pendulums are provided with pointers, the
+tips of which touch vertical sheets of paper attached to the sides of
+an upright rectangular box. When an earthquake occurs, this box is
+made to descend slowly with a uniform velocity, while the moving
+pointers trace curves upon the smoked paper. The north-and-south
+component of the horizontal motion is inscribed on the sheet of paper
+facing west, and the east-and-west component on the paper facing
+south.</p>
+
+<div class="img"><a name="Fig36" id="Fig36"></a>
+<a href="images/fig36.png">
+<img border="0" src="images/fig36.png" width="90%" alt="Seismographic record of the Riviera earthquake at Moncalieri." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 36.&mdash;Seismographic record of the Riviera earthquake at Moncalieri. (<i>Denza.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>During the principal Riviera earthquake, the former pendulum furnished
+an indistinct record, while the other traced the diagram reproduced in
+Fig. 36. The movement, as here represented, began at about 6h. 21m.
+50s. <span class="fakesc">A.M.</span> (mean time of Rome) with a series of small tremors,
+which lasted for about twelve seconds. Then followed some large
+oscillations, always in a nearly east-and-west direction, which at 6h.
+22m. 21s. gave place to a second series of tremors similar to those at
+the beginning of the shock, but of greater amplitude. These continued
+for at least twelve seconds, at the end of which time the motion of
+the smoked paper ceased. The total duration of the movement at
+Moncalieri cannot therefore have been less than forty-three seconds.</p>
+
+<p><span class='pagenum'><a name="Page_156" id="Page_156">[156]</a></span>Interesting as this record is, it is doubtful how far it represents
+accurately the movement of the ground. The Moncalieri instrument was
+erected before the modern type of seismograph was designed, in which
+some part remains steady, or very nearly steady, during the
+complicated movements of the ground that take place in an earthquake.
+It will be noticed that the curve in Fig. 36 shows no sign of the
+division of the shock into two distinct parts, and this may perhaps be
+due to the swinging of the pendulum itself; in which case, the curve
+described by the pointer would be the resultant of the oscillations of
+the ground and the proper motion of the pendulum.</p>
+
+<br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p>The sounds that preceded and accompanied the Riviera earthquake have
+attracted but little study, although they seem to have been widely
+observed. No attempt was made to define the limits of the area over
+which they were audible; but Professor Mercalli states that in the two
+outer zones (Fig. 33) the sound generally passed unobserved. It was,
+however, heard near Piacenza in Lombardy and Reggio in Emilia, places
+which are about 115 and 140 miles from the principal epicentre.</p>
+
+<p>In the area in which the shock was most violent, the sound resembled
+that of trains and vehicles in motion; while, outside this area it
+generally appeared to be like the hissing of a violent wind. In only a
+few places was it compared to detonations, the crashes of artillery or
+distant thunder. Some observers describe the sound as appearing at
+first <span class='pagenum'><a name="Page_157" id="Page_157">[157]</a></span>as if a strong wind were rising, and then as the roaring of a
+heavy railway-train passing.</p>
+
+<p>Nearly all the observers, who were awake at the beginning of the
+earthquake, agree in asserting that the sound distinctly preceded any
+movement of the ground. From this, as in the case of the Andalusian
+earthquake, Professor Mercalli infers that the sound-vibrations
+travelled with the greater velocity; but, as will be shown in Chapter
+VIII., the general precedence of the sound admits of another and more
+probable explanation.</p>
+
+<br />
+
+<h4>THE UNFELT EARTHQUAKE.</h4>
+
+<p>If the Andalusian earthquake first drew general attention to the
+distant spread of unfelt earth-waves, the Riviera earthquake showed
+that this was no isolated phenomenon. We know now that the propagation
+of such waves is only limited by the surface of the earth, but in 1887
+some doubt was felt at first as to the nature of the disturbance,
+whether it was magnetic or mechanical in its origin.</p>
+
+<p>In 1884, the only observatories at which magnetographs were disturbed
+were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
+Wilhelmshaven. In 1887, the magnetographs registered the Riviera
+earthquake at these and several other observatories, the distribution
+of which is shown in Fig. 37. In this sketch-map, the position of the
+principal epicentre is represented by the small cross, while the
+nearly circular line shows the boundary of the disturbed area.</p>
+
+<div class="img"><a name="Fig37" id="Fig37"></a>
+<a href="images/fig37.png">
+<img border="0" src="images/fig37.png" width="75%" alt="Distribution of observatories at which magnetographs were disturbed by the Riviera earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 37.&mdash;Distribution of observatories at which magnetographs were disturbed by the Riviera earthquake.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Three of the observatories, those of Nice, Lyons, <span class='pagenum'><a name="Page_158" id="Page_158">[158]</a></span>and Perpignan, lie
+inside this area. At Nice (which is thirty-seven miles from the
+principal epicentre), M. Perrotin states that the magnetograph curves
+show nothing of any interest, except a notable magnetic perturbation
+on the vertical force curve, the time of which, however, is not
+stated.<a name="FNanchor_49_49" id="FNanchor_49_49"></a><a href="#Footnote_49_49" class="fnanchor">[49]</a> At Lyons (211 miles), the declination, horizontal force
+and vertical force, magnets were all disturbed at 6h. 25m. 47s.
+<span class="fakesc">A.M.</span>, and Perpignan (264 miles), all three magnets, but
+especially those for the <span class='pagenum'><a name="Page_159" id="Page_159">[159]</a></span>declination and horizontal force, were set
+abruptly oscillating at 6h. 25m. 20s.</p>
+
+<p>Elsewhere in France, the disturbances were noticed at the
+observatories of Parc Saint-Maur and Montsouris, near Paris (about 447
+miles), and at Nantes (538 miles). At Parc Saint-Maur, all three
+curves show a very clear trace of the earthquake at 6h. 25m. 35s., the
+oscillations lasting several minutes, and at Montsouris they also
+began at the same time. At Nantes, the perturbations were so slight
+that they escaped notice on a first examination.</p>
+
+<p>In Austria, disturbances were observed at Pola (295 miles) and Vienna
+(506 miles), beginning at 6h. 28m. 35s. and 6h. 30m. 35s.,
+respectively. They reached Brussels (522 miles) at 6h. 29m. 27s., and
+Utrecht (600 miles) at 6h. 28m. 38s.<a name="FNanchor_50_50" id="FNanchor_50_50"></a><a href="#Footnote_50_50" class="fnanchor">[50]</a> At Wilhelmshaven (690 miles),
+only the vertical force magnet was affected, the oscillations
+beginning at 6h. 30m. 35s., and lasting for fourteen minutes. At 6h.
+27m. 55s., the declination and horizontal force magnets of Greenwich
+observatory (642 miles) were set vibrating, but no similar
+disturbances were revealed by the vertical force curve or by the two
+earth-current registers. At Kew (652 miles), the horizontal force
+magnetograph was moved by the earthquake at about 6h. 29m. 55s. The
+curves at Stonyhurst and Falmouth show no sign of any disturbance, nor
+do those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles),
+however, the three curves indicate disturbances at 6h. 32m. 35s., but
+so feeble are they that they would have escaped discovery if the
+occurrence of the earthquake had been unknown.</p>
+
+<p><span class='pagenum'><a name="Page_160" id="Page_160">[160]</a></span>The effects registered on the magnetograms are quite different from
+those which correspond to ordinary magnetic perturbations; but they
+are not unlike those produced by the action of the momentary currents
+which are used for making the hour-marks, except that the
+earthquake-oscillations lasted several minutes (see Fig. 21). In each
+case, then, the magnetic bars must have received a succession of
+several or many impulses.</p>
+
+<p>Now, the effect of these impulses on each magnet must depend on the
+relations which exist between the period of oscillation of the magnet,
+the rate of damping of such oscillations, and the interval between the
+successive impulses. Also, the apparent commencement of the phenomena
+may be delayed if two impulses of contrary sense should follow one
+another before the bar is perceptibly displaced. It is therefore to be
+expected, as M. Mascart points out, that the disturbances of the three
+instruments need not be of the same order of magnitude, that with
+different forms of apparatus the effects may be very variable, and
+that the deflection of one instrument may precede that of another at
+one and the same place.</p>
+
+<p>In all the magnetographs, the record is made on photographic paper,
+which travels so slowly that the time of a movement can only be
+ascertained to the nearest minute. As the disturbances on the French
+curves were apparently almost simultaneous, and as no two of the
+others differed in time of occurrence by more than five minutes, there
+is thus some colour for M. Mascart's contention that the magnetic
+apparatus registered, not the movements of the ground, but the passage
+of electric currents <span class='pagenum'><a name="Page_161" id="Page_161">[161]</a></span>produced in the ground at a certain epoch of the
+earthquake.<a name="FNanchor_51_51" id="FNanchor_51_51"></a><a href="#Footnote_51_51" class="fnanchor">[51]</a></p>
+
+<p>On the other hand, it is important to notice that, in the central part
+of the disturbed area, at Nice, two, if not all three, of the
+magnetographs were unaffected at the time of the earthquake.</p>
+
+<p>At first sight, this fact seems equally opposed to a mechanical
+explanation of the disturbance. But, when the vibrations are very
+rapid, as they are in the neighbourhood of the epicentre, the magnetic
+bars, owing to their mode of suspension, have not sufficient time to
+be sensibly deflected in the brief interval between successive phases
+of the impulse. The magnetograms of the Montsouris observatory show,
+for instance, hardly any perceptible trace of disturbance during the
+passage of railway trains along two adjacent lines. The farther,
+however, the earth-waves travel from the origin, the longer becomes
+the period of their vibrations. In Switzerland, they were remarkable
+for their slowness, even to the unaided senses. Thus, at places more
+or less remote from the Riviera, the magnets would receive impulses at
+intervals approximating to their own periods of vibration, and they
+would then oscillate freely for some time.</p>
+
+<p>Again, notwithstanding some variations, it will be <span class='pagenum'><a name="Page_162" id="Page_162">[162]</a></span>remarked that on
+the whole the retardation of the initial epoch of the disturbances
+increases with the distance from the epicentre. It thus seems clear, I
+think, that the cause of the disturbances must be sought in the shock
+itself; although their initial epochs at different places are too
+roughly defined for ascertaining the velocity with which the
+earth-waves travelled.</p>
+
+<br />
+
+<h4>EFFECTS OF THE EARTHQUAKE AT SEA.</h4>
+
+<p>The Riviera earthquake, owing to its submarine origin, was marked by
+certain phenomena that were absent from the other earthquakes
+described in this volume.</p>
+
+<p><i>Nature of the Earthquake at Sea.</i>&mdash;At the time of the earthquake,
+several vessels were close to the epicentral area. One, about three
+miles off Diano Marina, was shaken twice at about 6.20 <span class="fakesc">A.M.</span>,
+and so violently that it seemed as if the masts would be broken off.
+Another, about ten miles south of P. Maurizio, also experienced two
+shocks, a few minutes apart, as if each time it had struck the bottom.
+These observations are chiefly interesting in showing that the double
+shock was felt at sea as well as on land. As transverse vibrations are
+not propagated through water, it follows that the second part of the
+shock cannot, as some maintain, have been composed of transverse
+vibrations.</p>
+
+<p><i>Destruction of Fishes.</i>&mdash;During the days immediately following the
+earthquake, a large number of deep-sea fishes were found dead or
+half-dead either in shallow water or stranded on the beach, especially
+in the neighbourhood of Nice. Among them were numerous specimens,
+mostly dead and floating, of <span class='pagenum'><a name="Page_163" id="Page_163">[163]</a></span><i>Alepocephalus rostratus</i>, a typical
+deep-sea form, several of <i>Pomatomus telescopium</i>, <i>Scopelus
+elongatus</i>, and <i>S. humboldti</i>, and many of <i>Dentex macrophthalmus</i>
+and <i>Spinax niger</i>. The death and flight of these fishes must have
+been due to a sudden shock, almost like that caused by the explosion
+of dynamite, and communicated simultaneously to the whole surface of
+their bodies.</p>
+
+<div class="imgr" style="width: 40%;"><a name="Fig38" id="Fig38"></a>
+<a href="images/fig38.png">
+<img border="0" src="images/fig38.png" width="80%" alt="Record of tide-gauge at Nice." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 38.&mdash;Record of tide-gauge at Nice. (<i>Issel.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Seismic Sea-Waves.</i>&mdash;Immediately after the earthquake, the sea
+retired a short distance, variously estimated at from ten to thirty
+metres, laying bare some rocks that were usually immersed. At P.
+Maurizio, the surface was lowered by a little more than a metre; and
+after a few minutes it rose to nearly a metre above its original
+level, returning to it after a series of continually-decreasing
+oscillations. At San Remo, a fall of about the same amount took place,
+the sea returning after five minutes, and a ship anchored in the
+harbour broke from her moorings. Again, at Antibes, the sea was
+suddenly lowered by about a metre, so that ships afloat in the harbour
+were aground for some instants, and then returned with some
+impetuosity to its original level.</p>
+
+<p>The evidence of eye-witnesses is confirmed by that of the tide-gauges
+at Nice and Genoa, the curves of which are reproduced in Figs. 38 and
+39. At Nice, the first arrest of the curve in its usual course
+occurred <span class='pagenum'><a name="Page_164" id="Page_164">[164]</a></span>at 6.30 <span class="fakesc">A.M.</span>;<a name="FNanchor_52_52" id="FNanchor_52_52"></a><a href="#Footnote_52_52" class="fnanchor">[52]</a> the sea-level sank somewhat
+abruptly, and after a few marked oscillations gradually returned to
+its normal position at 7.50 <span class="fakesc">A.M.</span> At Genoa, the shock caused
+the writing-pen of the tide-gauge to dent the paper on which the
+record is made, and soon afterwards the curve shows a series of
+irregular oscillations, about eight taking place every hour, and
+gradually decreasing until they ceased to be perceptible about two
+hours after the principal earthquake.</p>
+
+<div class="img" style="clear: both;"><a name="Fig39" id="Fig39"></a>
+<a href="images/fig39.png">
+<img border="0" src="images/fig39.png" width="80%" alt="Record of tide-gauge at Genoa." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 39.&mdash;Record of tide-gauge at Genoa. (<i>Issel.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<br />
+
+<h4>MISCELLANEOUS PHENOMENA.</h4>
+
+<p><i>Connection between Geological Structure and the Intensity of the
+Shock.</i>&mdash;As with the Andalusian earthquake, faulty construction and
+defective materials were responsible for much of the damage caused by
+the Riviera earthquake. But, if we may judge from the sharp local
+variations in its amount, the nature of the surface-rocks must have
+exerted a still more potent influence. At Cervo, for example, the
+injury to property amounted to less than &pound;3 per head of the
+population; at Diano Marina, only two or three <span class='pagenum'><a name="Page_165" id="Page_165">[165]</a></span>miles to the west, it
+rose to &pound;22 per head. The death-rate at Cervo was about one-tenth, and
+at Diano Marina about 8&frac12; per cent. Again, at Mentone, the damage
+must have been considerable, for about 155 houses were rendered
+uninhabitable; while Monte Carlo, only a few miles farther west,
+escaped almost unharmed. Now, Mentone and Diano Marina are for the
+most part built on clay or alluvial deposits, and Monte Carlo on a
+foundation of limestone.</p>
+
+<p>Even within the limits of a single town, variations no less striking
+were perceptible. In Mentone, the greatest damage occurred to houses
+of two storeys built on alluvial soil in the low-lying parts near the
+sea and in the valleys. The effect of the foundation in this part was
+well shown in the case of two equally well-built houses not more than
+300 yards apart. One in the valley, with doubtful foundations, was
+very much shattered; the other, built on rock, was uninjured. The
+large hotels, especially those on high ground, suffered least, few of
+them having their main walls seriously damaged. These buildings rise
+to heights of from four to six storeys, and of necessity have a firm
+and solid foundation.</p>
+
+<p>Professors Taramelli and Mercalli have made a careful study of the
+subject of this section. The general conclusions at which they arrive
+are that the intensity of the shock was greatest at places built on
+pliocene conglomerates, beds of clay superposed on compact old rocks,
+patches of alluvium, miocene formations of some thickness formed of
+repeated alternations of strata of incoherent marls and limestones or
+compact sandstones, beds of chalk, or somewhat rotten dolomite.</p>
+
+<p>The shock was also more destructive on the <span class='pagenum'><a name="Page_166" id="Page_166">[166]</a></span>summits of isolated hills
+and ridges and on the steep slopes of mountains. The influence of the
+form of the ground was, however, subordinate to that exerted by the
+nature of the subsoil. Thus, at Mentone, as we have seen, and also at
+Nice and Genoa, houses built on rock in elevated positions suffered
+much less than those situated on the plains below that are composed of
+sand and recent alluvium.</p>
+
+<p><i>Observations of the Earthquake in Railway-Tunnels.</i>&mdash;Observations
+made in mines at various times and places have proved that an
+earthquake is felt less strongly in deep workings, if felt at all,
+than on the surface of the ground. In the railway-tunnels of the
+Riviera, as Professor Issel has shown, the same result was established
+during the earthquake of 1887.</p>
+
+<p>On the line which runs northward from Genoa to Piedmont, a tunnel more
+than five miles in length pierces the hilly ground between Ponterosso
+and Ronco, the greatest thickness of rock above being about a thousand
+feet. At the time of the earthquake, the tunnel was not everywhere
+opened out to its full width, and men were at work in different
+sections. Outside, the shock was strong enough to damage buildings.
+Inside, at about 200 yards from the south end, only a feeble shock was
+felt; at 1,350 and 1,625 yards, some bricks were seen to fall from the
+facing, but the shock was not otherwise perceived, and only a few
+yards farther nothing unusual was noticed by the men at work.</p>
+
+<p>Again, in an unfinished tunnel, about three-quarters of a mile long,
+between the harbour of Genoa and the eastern railway-station, the
+vibrations were very slightly felt. Even in the tunnels traversed by
+the coast railway from <span class='pagenum'><a name="Page_167" id="Page_167">[167]</a></span>Genoa to Nice&mdash;that is, in those situated
+within the meizoseismal area&mdash;the shock was either very weak or not
+felt at all, and not one of the tunnels suffered the slightest injury.</p>
+
+<p>To men at work inside a long tunnel, the conditions for observing
+earthquakes are somewhat imperfect, but these facts, nevertheless,
+bring out very clearly the inferior intensity of the shock at some
+depth below the surface.</p>
+
+<br />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p>While the unfelt earth-waves of the great earthquake were still
+wending their way over the zone that surrounds the disturbed area, the
+central regions were again shaken, at 6.29 <span class="fakesc">A.M.</span>, by a shock
+strong enough to produce fresh ruins in the stricken towns along the
+coast. Nearly two and a half hours of quiet followed, broken only by a
+few subterranean rumblings in the central part of the meizoseismal
+area. Then, at 8.51 <span class="fakesc">A.M.</span>, occurred another shock, short and
+sharp, and inferior in strength only to the principal earthquake. Both
+of these after-shocks were felt in Western Switzerland; indeed, they
+were perceptible nearly as far as the great shock; the second,
+however, a little farther than the first, for it alone was noticed at
+such places as Vicenza, Forl&igrave;, and Florence. The shock at 6.29 was
+usually described as long and its vibrations as undulatory only; that
+at 8.51 as rather subsultory than undulatory and of very brief
+duration. The latter, however, was followed after an interval of a few
+seconds by another shock so weak that it generally passed unobserved.
+Both shocks were preceded by a rumbling sound.</p>
+
+<p><span class='pagenum'><a name="Page_168" id="Page_168">[168]</a></span>During the next two days, tremors and earth-sounds were frequent in
+the Riviera; once an hour, on an average, the greater part of the
+meizoseismal area was shaken by vibrations more or less slight. But,
+between one shock and another, at Diano Marina and Alassio, and even
+as far as Nice, it only required attention from a careful observer to
+perceive an almost continual throbbing of the ground.</p>
+
+<p>Only one of these shocks, that of February 24th, at 2.10
+<span class="fakesc">A.M.</span>, was strong enough to cause slight damage to buildings.
+It disturbed an area, not exceeded by any of the later shocks, the
+boundary of which, shown by the dotted line A in Fig. 33, extends to
+the north and east as far as Piacenza and Spezia, while to the west it
+includes Cannes. The centre of the curve so drawn lies on land, but,
+as the shock was not felt in Corsica, there is no evidence as to the
+southerly extension of the disturbed area; and it is probable, as
+Professor Mercalli suggests, that the shock originated in the eastern
+or Oneglia focus of the great earthquake.</p>
+
+<p>After February 25th, slight shocks were felt during the next
+fortnight, at the rate of three or four a day, until March 11th, when
+the last after-shock resulting in slight damage occurred at about 3.12
+<span class="fakesc">P.M.</span> The boundary of its disturbed area, represented in Fig.
+33 by the dotted line B, passes a little to the east of Savona, and
+then through Alessandria, Moncalieri, and Marseilles. The shock,
+however, was not observed in Corsica, so that the exact position of
+the epicentre is unknown; but Professor Mercalli believes it to
+coincide with the western or Nice epicentre of the principal
+earthquake. At the <span class='pagenum'><a name="Page_169" id="Page_169">[169]</a></span>moment of the shock, the sea was observed from
+Alassio to curl and to rise slightly, while the tide-gauge at Nice,
+which had traced a continuous curve earlier in the day, showed a
+characteristic notch about 3.7 <span class="fakesc">P.M.</span></p>
+
+<p>Of the remaining after-shocks, only two attained any notable degree of
+strength. One, on May 20th at about 8.15 <span class="fakesc">A.M.</span>, disturbed an
+area nearly concentric with that of the great earthquake, and with a
+boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on
+July 17th at 11.30 <span class="fakesc">P.M.</span>, occurred a shock felt over an area
+nearly as large as that disturbed on February 24th at 2.10
+<span class="fakesc">A.M.</span>, and situated in the same part of the country.</p>
+
+<p>Altogether, during the year following the Riviera earthquake,
+Professor Mercalli records 190 after-shocks, most of them slight or
+only just felt. With the exception of the first two (on February
+23rd), none was observed outside the isoseismal 4 of the principal
+earthquake (Fig. 33); and, of the rest, only the four whose dates are
+given above disturbed an area of more than one-eighth of that of the
+great shock. Some of them, like the shock of March 11th, were stronger
+in the western part of the meizoseismal area; but the majority
+affected most the eastern portion and seem to be closely associated
+with the Oneglia focus.</p>
+
+<p>From February 26th to April 20th, Professor Rumi made observations on
+the after-shocks by means of the Foucault pendulum erected at Genoa
+for demonstrating the rotation of the earth. In nearly every case, the
+oscillations took place along a north-east and south-west line, or in
+the same direction as the first great shock&mdash;a resemblance which
+supports <span class='pagenum'><a name="Page_170" id="Page_170">[170]</a></span>the inference that many of the after-shocks originated
+within the Oneglia focus.</p>
+
+<br />
+
+<h4>ORIGIN OF THE EARTHQUAKES.</h4>
+
+<p><i>Recent Movements in the Riviera.</i>&mdash;The earliest movements that
+resulted in the great range of the Maritime Alps and the Ligurian
+Apennines date from pre-Carboniferous times, when the central
+crystalline massifs in part emerged. At the end of the Liassic epoch,
+the secondary formations of the district were uplifted, and it was at
+this time that the range assumed its characteristic curved form. Later
+still, at the close of the Eocene period, an elevation of more than
+9000 feet took place, for upper Eocene beds are found at this height
+in the Maritime Alps.</p>
+
+<p>Since that time, other important movements have occurred. Pliocene
+deposits have been found in the Riviera at an altitude of 1,800 feet.
+Recent soundings in the Gulf of Genoa have also shown that all the
+valleys of the Riviera between Nice and Genoa are continued far below
+the level of the sea to depths of not less than 3000 feet. Thus, at
+the end of the Pliocene or beginning of the Quaternary period, there
+was an elevation of nearly 5000 feet, accompanied or followed by the
+erosion of the valleys which, later on, during the Quaternary period,
+were submerged about 3000 feet. Even in still more recent times,
+probably in the Pal&aelig;olithic age, minor movements continued. Traces of
+recent elevation, varying in amount from a few feet to sixty feet or
+more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi,
+and in Genoa; <span class='pagenum'><a name="Page_171" id="Page_171">[171]</a></span>while evidences of submergence are to be found near
+Monaco, at Beaulieu and at Diano Marina. It is important to notice
+that the great movements dating from the end of the Eocene period are
+almost confined to the Maritime Alps and the western portion of the
+Riviera. In the parts of Piedmont lying to the north of Cuneo and in
+the eastern Riviera, they produced hardly any sensible effect.</p>
+
+<p><i>Seismic History of the Riviera.</i>&mdash;The movements just referred to are
+those which, in course of time, have become sensible to the eye. They
+represent the sum of a long-continued series of displacements that may
+once have been on a large scale, but are now comparatively small. The
+earthquakes that occur in the Riviera show, however, that the final
+stage has not yet been reached. Their epicentres indicate the regions
+in which slips are still taking place, and the magnitude of these
+slips is roughly measured by the intensity of the resulting shocks.</p>
+
+<p>The map in Fig. 40 is one of a series drawn by Professor Mercalli to
+represent the distribution of seismic activity in Piedmont and the
+Riviera. It corresponds to the period from 1801 to 1895. The whole
+area is divided into a number of seismic districts, each of which is
+distinguished by a particular degree of activity. In estimating this
+quantity, Professor Mercalli takes intensity as well as frequency into
+account. Thus, the lowest degree, represented by the lightest tint of
+shading, corresponds to one or two strong earthquakes with a few
+moderate or slight shocks; the eighth and highest to four or five
+ruinous or disastrous earthquakes followed by trains of after-shocks.
+The map shows very clearly that, during the last century, the <span class='pagenum'><a name="Page_172" id="Page_172">[172]</a></span>seismic
+activity was greatest in the Maritime Alps and the western
+Riviera&mdash;that is, in the very districts in which the recent
+mountain-making movements have been most conspicuous.<a name="FNanchor_53_53" id="FNanchor_53_53"></a><a href="#Footnote_53_53" class="fnanchor">[53]</a></p>
+
+<div class="img"><a name="Fig40" id="Fig40"></a>
+<a href="images/fig40.jpg">
+<img border="0" src="images/fig40.jpg" width="60%" alt="Distribution of seismic activity in the Riviera." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 40.&mdash;Distribution of seismic activity in the Riviera. (<i>Mercalli.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>In all these districts, Professor Mercalli distinguishes several
+well-marked seismic centres, to each of which he traces the origin of
+two or more earthquakes. In the districts with which we are at present
+concerned, those of the Alpes Maritimes and the western Riviera, the
+most important centres are <span class='pagenum'><a name="Page_173" id="Page_173">[173]</a></span>situated near Oneglia (in the sea), near
+Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in
+the sea to the south of Nice. To the first of these centres belongs
+the disastrous earthquake of February 23rd, 1887, as well as its
+after-shocks on February 24th, May 20th, July 17th, and September 30th
+of the same year, also the ruinous earthquakes of 1612 and 1854, and
+several others of a lesser degree of intensity. All of these were
+longitudinal earthquakes, the axes of their meizoseismal areas being
+parallel to the neighbouring mountain-ranges. A few miles to the west
+of Oneglia lies the Taggia centre, with which were connected the
+disastrous earthquake of 1831, the violent earthquake of 1874, and
+other strong or very strong shocks. These were for the most part
+transversal earthquakes, their axes being perpendicular to those of
+the Oneglia centre.</p>
+
+<p>Some of the strongest earthquakes in this region originated in a
+centre lying to the north of Nice in the valleys of the Vesubia and
+Tinea. Among them may be mentioned the ruinous earthquakes of 1494,
+1556, 1564, and 1644, and probably also the disastrous earthquake of
+1227. A fourth centre, and one of considerable interest, is that which
+lies at sea, a short distance to the south of Nice, and nearly along
+the continuation of the valleys above-mentioned. This is the secondary
+centre of the earthquake of 1887, and probably also of that of
+December 29th, 1554. It is occasionally in action apart from the
+Oneglia centre, as on November 27th, 1771, June 19th, 1806, and
+December 21st, 1861; but such shocks, though rather strong, never
+reach a high degree of intensity.</p>
+
+<p><span class='pagenum'><a name="Page_174" id="Page_174">[174]</a></span><i>Origin of the Earthquakes of 1887.</i>&mdash;The most important feature in
+the principal earthquake of 1887 is its origination in two distinct
+foci, which are sometimes in action almost simultaneously, but more
+often separately. The earthquakes belonging to the two foci differ
+greatly in intensity and number, and the stronger part of the shock in
+1887 originated in the focus associated with the more disastrous and
+more frequent earthquakes.</p>
+
+<p>The existence of two foci would of course give rise to a meizoseismal
+area elongated in the direction of the line joining them. It is clear,
+however, that the Oneglia focus was also extended in the same
+direction; for, in the after-shock of February 24th, the isoseismals
+drawn by Professor Mercalli are parallel to this line; and this was
+also the case in the shock of March 11th. As both foci were under the
+sea, it is difficult to locate them with precision; but it seems very
+probable that they occupy portions of a submarine fault that runs
+parallel or nearly so to the Apennine axis between the meridians of
+Oneglia and Nice.</p>
+
+<p>A brief period of preparation is a characteristic of the Riviera
+earthquakes. In 1887, two at least of the preliminary shocks on
+February 23rd (those of about 2 and 5 <span class="fakesc">A.M.</span>) originated in the
+Oneglia focus. At 6.20 <span class="fakesc">A.M.</span> the first and weaker movement
+took place in the western focus; and, a few seconds after the
+resulting vibrations reached the eastern focus, the second and greater
+slip took place there. The occurrence of seismic sea-waves is probably
+evidence of the formation of a small, though sensible, fault-scarp in
+the same region. To relieve the additional stresses thus brought into
+action along the fault-surface, <span class='pagenum'><a name="Page_175" id="Page_175">[175]</a></span>numerous small slips took place in
+different parts, some as far to the west as the Nice focus, but the
+greater number probably within or close to the focus in the
+neighbourhood of Oneglia.</p>
+
+<br />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Bertelli, T.</span>&mdash;"Osservazioni fatte in occasione di una
+escursione sulle Riviera Ligure di ponente dopo i terremoti
+ivi seguiti nell' anno 1887." <i>Boll. Mens. dell' Oss. di
+Moncalieri</i>, vol. viii., 1888, Nos. 6, 7, 8.</p>
+
+<p class="hang">2. <span class="sc">Charlon, E.</span>&mdash;"Note sur le tremblement de terre du 23
+f&eacute;vrier 1887." <i>Bull. del Vulc. Ital.</i>, anno xiv., 1887, pp.
+18-23.</p>
+
+<p class="hang">3. <span class="sc">Denza, F.</span>&mdash;<i>Alcune notizie sul terremoto del 23 febbraio
+1887</i> (Turin).</p>
+
+<p class="hang">4. <span class="sc">Issel, A.</span>&mdash;"Il terremoto del 1887 in Liguria." <i>Boll.
+del R. Com. Geol. d'Italia</i>, anno 1887, supplemento, pp.
+1-207.</p>
+
+<p class="hang">5. <span class="sc">Mercalli, G.</span>&mdash;<i>I terremoti della Liguria e del
+Piemonte</i>. (Naples, 1897, 146 pp.)</p>
+
+<p class="hang">6. <span class="sc">Oddone, E.</span>&mdash;"I dati sismici della Liguria in rapporto
+alla frequenza ed alla periodicit&agrave;." <i>Boll. della Soc. Sismol.
+Ital.</i>, vol. ii., 1896, pp. 140-151.</p>
+
+<p class="hang">7. <span class="sc">Offret, A.</span>&mdash;"Sur le tremblement de terre du 23 f&eacute;vrier
+1887. Discussion des heures observ&eacute;s dans la zone
+&eacute;picentrale." Paris, <i>Acad. Sci., Compt. Rend.</i>, vol. civ.,
+1887, pp. 1150-1153.</p>
+
+<p class="hang">8. &mdash;&mdash;. "Tremblements de terre du 23 f&eacute;vrier 1887. Heures de
+l'arriv&eacute;e des secousses en dehors de l'&eacute;picentre." <i>Ibid.</i>,
+pp. 1238-1242.</p>
+
+<p class="hang">9. <span class="sc">Rossi, M.S. de.</span>&mdash;"Relazione sui terremoti del febbraio
+1887." <i>Bull. del Vulc. Ital.</i>, anno xiv., 1887, pp. 5-17.</p>
+
+<p class="hang">10. &mdash;&mdash;. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."
+<i>Ibid.</i>, pp. 60-62, 107-112, 115-128.</p>
+
+<p class="hang">11. <span class="sc">Taramelli, T.</span>, and <span class="sc">G. Mercalli.</span>&mdash;"Il terremoto
+ligure del 23 febbraio 1887." <i>Annali dell' Uff. Centr. di
+Meteor. e di Geodin.</i>, vol. viii., parte iv., 1888. (Roma, 298
+pp.)</p>
+
+<p class="hang"><span class='pagenum'><a name="Page_176" id="Page_176">[176]</a></span>12. <span class="sc">Uzielli, G.</span>&mdash;<i>Le commozioni telluriche e il terremoto
+del 23 febbraio 1887</i> (Turin).</p>
+
+<p class="hang">13. <i>Nature</i>, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi.,
+1887, pp. 4, 151-152.</p>
+
+<p class="hang">14. Paris, <i>Acad. Sci. Compt. Rend.</i>, vol. civ., 1887, pp. 556-557,
+606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766,
+822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245,
+1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol.
+cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274,
+660.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_47_47" id="Footnote_47_47"></a><a href="#FNanchor_47_47"><span class="label">[47]</span></a> The above times and all others in this chapter are given
+in Rome mean time, which is 50m. earlier than Greenwich mean time.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_48_48" id="Footnote_48_48"></a><a href="#FNanchor_48_48"><span class="label">[48]</span></a> Professor Uzielli has also published a map of the
+isoseismal lines for the Italian part of the disturbed area.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_49_49" id="Footnote_49_49"></a><a href="#FNanchor_49_49"><span class="label">[49]</span></a> It seems doubtful whether this movement was connected
+with the earthquake. M. Offret does not include Nice in his list of
+observatories at which magnetographs were disturbed.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_50_50" id="Footnote_50_50"></a><a href="#FNanchor_50_50"><span class="label">[50]</span></a> This is the time given by M. Offret. According to M.
+Mascart, it should be 6h. 25m. 40s.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_51_51" id="Footnote_51_51"></a><a href="#FNanchor_51_51"><span class="label">[51]</span></a> In order to test the truth of this explanation, M.
+Moureaux suspended a bar of copper at the Parc Saint-Maur observatory
+by two threads in the same way as the horizontal force-magnet. The
+direction of this bar was also registered photographically, and it
+remained unmoved during the Verny earthquake of July 12th, 1889, and
+the Dardanelles earthquake of October 25th, 1889, while one or more of
+the magnets were disturbed. The experiment, however, was ineffective;
+for, in order that the magnet may rest in a horizontal position, its
+centre of gravity must be at unequal distances from the two points of
+support.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_52_52" id="Footnote_52_52"></a><a href="#FNanchor_52_52"><span class="label">[52]</span></a> The hour-marks in Fig. 38 refer to Paris mean time, and
+those in Fig. 39 to Genoa mean time.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_53_53" id="Footnote_53_53"></a><a href="#FNanchor_53_53"><span class="label">[53]</span></a> In the seventeenth century, the maximum seismic activity
+was manifested in the neighbourhood of Nice, and in the eighteenth
+century in Piedmont.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_VII" id="CHAPTER_VII"></a><hr />
+<span class='pagenum'><a name="Page_177" id="Page_177">[177]</a></span><br />
+
+<h3>CHAPTER VII.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891.</h4>
+<br />
+
+<p>Although several years have elapsed since the occurrence of the
+greatest of Japanese earthquakes, the final report that will embody
+the labours of all its investigators is yet to be written. Several
+important contributions to it, however, have already been made.
+Professor Koto, in an admirable memoir, has traced the course of the
+great fault-scarp and discussed the origin of the earthquake;
+Professor Omori, with equal care and thoroughness, has investigated
+the unrivalled series of after-shocks; Mr. Conder studied the damaged
+buildings from an architect's point of view; Professor Tanakadate and
+Dr. Nagaoka devoted themselves to a re-determination of the magnetic
+elements of the central district,<a name="FNanchor_54_54" id="FNanchor_54_54"></a><a href="#Footnote_54_54" class="fnanchor">[54]</a> while, by the compilation of his
+great catalogue of Japanese earthquakes during the years 1885-92,
+Professor Milne has provided the materials for a further analysis of
+the minor shocks that preceded and followed the principal earthquake.</p>
+
+<p>The part of Japan over which the earthquake was <span class='pagenum'><a name="Page_178" id="Page_178">[178]</a></span>sensibly felt is
+shown in Fig. 41. The small black area in the centre is that in which
+the shock was most severe and the principal damage to life and
+property occurred. The other bands, more or less darkly shaded
+according to the greater or less intensity of the shock, will be
+referred to afterwards. Fig. 45 represents the meizoseismal area on a
+larger scale; and, as the greater part of it lies within the two
+provinces of Mino and Owari, the earthquake is generally known among
+the Japanese themselves as the Mino-Owari earthquake of 1891.</p>
+
+<div class="img"><a name="Fig41" id="Fig41"></a>
+<a href="images/fig41.png">
+<img border="0" src="images/fig41.png" width="55%" alt="Sketch-Map of Disturbed Area and Isoseismal Lines." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 41.&mdash;Sketch-Map of Disturbed Area and Isoseismal Lines. (<i>Masato.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<span class='pagenum'><a name="Page_179" id="Page_179">[179]</a></span><br />
+
+<h4>THE MEIZOSEISMAL AREA.</h4>
+
+<p>More than half of the meizoseismal area occupies a low flat plain of
+not less than 400 square miles in extent. On all sides but the south,
+the plain, which is a continuation of the depression forming the Sea
+of Is&eacute;, is surrounded by mountain ranges, those to the west, north,
+and north-east being built up mainly of Pal&aelig;ozoic rocks, and those on
+the east side of granite. A network of rivers and canals converts what
+might otherwise have been unproductive ground into one of the most
+fertile districts in Japan. A great garden, as it has been aptly
+termed, the whole plain is covered with rice-fields, and supports a
+population of about 787 to the square mile&mdash;a density which is
+exceeded in only six counties of England. As a rule, the soil is a
+loose, incoherent, fine sand, with but little clayey matter; and it
+is, no doubt, to its sandy nature that the disastrous effects of the
+earthquake were largely due. In the northern half of the district, the
+meizoseismal area is much narrower, and here it crosses a great
+mountain-range running from south-west to north-east and separating
+the river-systems of the Japan sea from those of the Pacific. To the
+north, the meizoseismal area terminates in another plain, in the
+centre of which lies the city of Fukui, where the destructiveness of
+the earthquake was only inferior to that experienced in the provinces
+of Mino and Owari. There is also a detached portion of the area lying
+to the east of Lake Biwa, but it is uncertain whether the exceptional
+intensity there was due to the nature of the ground or to the
+occurrence of a secondary or sympathetic earthquake in its immediate
+neighbourhood.</p>
+
+<div class="img"><a name="Fig42" id="Fig42"></a><span class='pagenum'><a name="Page_180" id="Page_180">[180]</a></span>
+<a href="images/fig42.png">
+<img border="0" src="images/fig42.png" width="50%" alt="General Plan of Geological Structure of Meizoseismal Area." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 42.&mdash;General Plan of Geological Structure of Meizoseismal Area. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The general plan of the geological structure of the central district
+is represented in Fig. 42. The thick line, partly continuous and
+partly broken, shows the course of the great fault, to the growth of
+which the earthquake chiefly owed its origin; while the thin
+continuous lines represent the changing direction of strike of the
+Pal&aelig;ozoic rocks which surround the Mino-Owari plain, and the
+arrowheads the direction of the dip. It will be seen that the
+direction of the strike forms an S-shaped curve, and it is clear that
+<span class='pagenum'><a name="Page_181" id="Page_181">[181]</a></span>the present torsion-structure of the district could not have been
+produced without the formation of many fractures at right angles and
+parallel to the lines of strike. Professor Koto points out that the
+regular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, and
+Itatori, indicated by broken lines in Fig. 42, have probably been
+excavated along a series of transverse fractures running from
+north-west to south-east; while fractures which are parallel to the
+line of strike may be responsible for the zigzag course of the
+valleys.</p>
+
+<br />
+
+<h4>DAMAGE CAUSED BY THE EARTHQUAKE.</h4>
+
+<p>The great earthquake occurred at 6.37 <span class="fakesc">A.M.</span>, practically
+without warning, and in a few seconds thousands of houses were
+levelled with the ground. Within the whole meizoseismal area there was
+hardly a building left undamaged. The road from Nagoya to Gifu, more
+than twenty miles in length, and formerly bordered by an almost
+continuous succession of villages, was converted into a narrow lane
+between two long drawn-out banks of <i>d&eacute;bris</i>. "In some streets," says
+Professor Milne, "it appeared as if the houses had been pushed down
+from the end, and they had fallen like a row of cards." Or, again, a
+mass of heaped-up rubbish might be passed, "where sticks and earth and
+tiles were so thoroughly mixed that traces of streets or indications
+of building had been entirely lost." At Gifu, Ogaki, Kasamatsu, and
+other towns, fires broke out after the earthquake. In Kasamatsu the
+destruction was absolutely complete; nothing was left but a heap of
+plaster, mud, tiles, and charred timbers. At Ogaki, not more than
+thirty <span class='pagenum'><a name="Page_182" id="Page_182">[182]</a></span>out of 8000 houses remained standing, and these were all much
+damaged. Within the whole district, according to the official returns,
+197,530 buildings were entirely destroyed, 78,296 half destroyed, and
+5,934 shattered and burnt; while 7,279 persons were killed, and 17,393
+were wounded.</p>
+
+<p>Next to buildings, the embankments which border the rivers and canals
+suffered the most serious damage, no less than 317 miles of such works
+having to be repaired. Railway-lines were twisted or bent in many
+places, the total length demolished being more than ten miles. In
+cuttings, twenty feet or more in depth, both rails and sleepers were
+unmoved; it was on the plains that the effects of the earthquake were
+most marked. The ground appeared as if piled up into bolster-like
+ridges between the sleepers, and in many places the sleepers had moved
+end-ways. When the line crossed a small depression in the general
+level of the plain, the whole of the track was bowed, as if the ground
+were permanently compressed at such places. "Effects of compression,"
+says Professor Milne, "were most marked on some of the embankments,
+which gradually raise the line to the level of the bridges. On some of
+these, the track was bent in and out until it resembled a serpent
+wriggling up a slope.... Close to the bridges the embankments had
+generally disappeared, and the rails and sleepers were hanging in the
+air in huge catenaries."</p>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>The land area disturbed by the earthquake and the different isoseismal
+lines are shown in Fig. 41. The "most severely shaken" district, that
+in which <span class='pagenum'><a name="Page_183" id="Page_183">[183]</a></span>the destruction of buildings and engineering works was
+nearly complete, contains an area of 4,286 square miles, or about
+two-thirds that of Yorkshire. This is indicated on the map by the
+black portion. Outside this lies the "very severely shaken" district,
+17,325 square miles in area, extending from Kobe on the west to
+Shizuoka on the east, in which ordinary buildings were destroyed,
+walls fractured, embankments and roads damaged, and bridges broken
+down. The third or "severely shaken" district contains 20,183 square
+miles; and in this some walls were cracked, pendulum clocks stopped,
+and furniture, crockery, etc., overthrown. Tokio and Yokohama lie just
+within this area. In the fourth region the shock was "weak," the
+motion being distinctly felt, but not causing people to run
+out-of-doors; and in the fifth it was "slight," or just sufficient to
+be felt. These two regions together include an area of 51,976 square
+miles.</p>
+
+<p>Thus, the land area disturbed amounts altogether to 93,770 square
+miles&mdash;<i>i.e.</i>, to a little more than the area of Great Britain.
+According to Professor Omori, the mean radius of propagation was about
+323 miles, and the total disturbed area must therefore have been about
+330,000 square miles, or nearly four times the area of Great Britain.
+Considering the extraordinary intensity of the shock in the central
+district, this can hardly be regarded as an over-estimate.</p>
+
+<p>The isoseismal lines shown in Fig. 41 are not to be regarded as drawn
+with great accuracy; for there is no marked separation between the
+tests corresponding to the different degrees of the scale of
+intensity. The seismographs at Gifu and Nagoya <span class='pagenum'><a name="Page_184" id="Page_184">[184]</a></span>were thrown down
+within the first few seconds, and failed to record the principal
+motion. But a great number of well-formed stone lanterns and
+tombstones were overturned, and, from the dimensions of these,
+Professor Omori calculated the maximum horizontal acceleration
+necessary for overturning them at fifty-nine places within the
+meizoseismal area.<a name="FNanchor_55_55" id="FNanchor_55_55"></a><a href="#Footnote_55_55" class="fnanchor">[55]</a> At five of these it exceeded 4000 millimetres
+per second per second, an acceleration equal to about five-twelfths of
+that due to gravity. Making use of these observations, Professor Omori
+has drawn two isoseismal lines within the central district, which are
+shown in Fig. 44. At every point of the curve marked 2, the maximum
+acceleration was 2000 millimetres per second per second, and of that
+marked 1, 800 millimetres per second per second. The dotted line
+within the curve marked 2 represents the boundary of the meizoseismal
+area, which, it will be observed, differs slightly from that given by
+Professor Koto (see Fig. 45). The difference, however, is apparently
+due to the standard of intensity adopted, Professor Koto's boundary
+agreeing rather closely with the curve marked 2 in Fig. 44.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>Little has yet been made known with regard to the nature of the shock,
+and the published records of the accompanying sound are so rare that
+it seems as a rule to have passed unheard. The seismographs <span class='pagenum'><a name="Page_185" id="Page_185">[185]</a></span>at Gifu
+and Nagoya registered the first half-dozen vibrations, and were then
+buried beneath the fallen buildings. In the following table, the data
+from these two stations are therefore incomplete:&mdash;</p>
+
+<br />
+
+<h4>PRINCIPAL MEASUREMENTS OBTAINED FROM SEISMOGRAPHIC RECORDS.</h4>
+
+<div class="centered">
+<table border="0" cellpadding="2" cellspacing="0" width="80%" summary="Principal Measurements Obtained From Seismographic Records." style="border: 1px solid black;">
+  <tr>
+  <td class="tdl tdball" width="40%">&nbsp;</td>
+  <td class="tdc tdball" width="15%">Gifu.</td>
+  <td class="tdc tdball" width="15%">Nagoya.</td>
+  <td class="tdc tdball" width="15%">Osaka.</td>
+  <td class="tdc tdball" width="15%">Tokio (Imp. Univ.).</td>
+  </tr>
+  <tr>
+  <td class="tdlp tdblrt">Maximum horizontal motion</td>
+  <td class="tdrp2 tdblrt">&gt; 18 mm.</td>
+  <td class="tdrp2 tdblrt">&gt; 26 mm.</td>
+  <td class="tdrp2 tdblrt">30 mm.</td>
+  <td class="tdrp2 tdblrt">&gt; 35 mm.</td>
+  </tr>
+  <tr>
+  <td class="tdlp tdblr">Period of ditto</td>
+  <td class="tdrp2 tdblr">2.0 secs.</td>
+  <td class="tdrp2 tdblr">1.3 sec.</td>
+  <td class="tdrp2 tdblr">1.0 sec.</td>
+  <td class="tdrp2 tdblr">2.0 secs.</td>
+  </tr>
+  <tr>
+  <td class="tdlp tdblr">Maximum vertical motion</td>
+  <td class="tdrp2 tdblr">&gt; 11.3 mm.</td>
+  <td class="tdrp2 tdblr">6.2 mm.</td>
+  <td class="tdrp2 tdblr">8 mm.</td>
+  <td class="tdrp2 tdblr">9.5 mm.</td>
+  </tr>
+  <tr>
+  <td class="tdlp tdblrb">Period of ditto</td>
+  <td class="tdrp2 tdblrb">0.9 sec.</td>
+  <td class="tdrp2 tdblrb">1.5 sec.</td>
+  <td class="tdrp2 tdblrb">1.0 sec.</td>
+  <td class="tdrp2 tdblrb">2.4 secs.</td>
+  </tr>
+</table>
+</div>
+
+<br />
+
+<p>If the period of the principal vibrations were known, the observations
+of Professor Omori on the overturning of bodies would enable us to
+determine the range of motion at different places. For instance, the
+maximum acceleration at Nagoya was found by these observations to be
+2,600 millimetres per second per second, and if we take the period of
+the greatest horizontal motion to be the same as that of the initial
+vibrations&mdash;namely, 1.3 second, the total range (or double amplitude)
+would be 223 millimetres, or 8.8 inches. With the same period, and the
+maximum acceleration observed (at Iwakura and Konaki) of more than
+4,300 millimetres per second per second, the total range would be
+greater than 14.5 inches.<a name="FNanchor_56_56" id="FNanchor_56_56"></a><a href="#Footnote_56_56" class="fnanchor">[56]</a></p>
+
+<p><span class='pagenum'><a name="Page_186" id="Page_186">[186]</a></span>In the meizoseismal area, many persons saw waves crossing the surface
+of the ground. At Akasaka, according to one witness, the waves came
+down the streets in lines, their height being perhaps one foot, and
+their length between ten and thirty feet. To the north of the same
+area, we are told that "the shoreline rose and fell, and with this
+rising and falling the waters receded and advanced." Even at Tokio,
+which is about 175 miles from the epicentre, the tilting of the ground
+was very noticeable. After watching his seismographs for about two
+minutes, Professor Milne next observed the water in an adjoining tank,
+80 feet long and 28 feet wide, with nearly vertical sides. "At the
+time it was holding about 17 feet of water, which was running across
+its breadth, rising first on one side and then on the other to a
+height of about two feet." Still clearer is the evidence of the
+seismographs in the same city. Instead of a number of irregular waves,
+all the records show a series of clean-cut curves. The heavy masses in
+the horizontal pendulums were tilted instead of remaining as steady
+points. They were not simply swinging, for the period of the
+undulations differed from that of the seismograph when set swinging,
+and also varied in successive undulations. It was ascertained
+afterwards, by measurement with a level, that to produce these
+deflections, the seismograph must have been tilted through an angle of
+about one-third of a degree.</p>
+
+<p><i>Direction of the Shock.</i>&mdash;Shortly after the earthquake, Professor
+Omori travelled over the meizoseismal area and made a large number of
+observations on the directions in which bodies were overturned, taking
+care to include only those in which the <span class='pagenum'><a name="Page_187" id="Page_187">[187]</a></span>direction of falling would not
+be influenced by the form of the base, such as the cylindrical stone
+lanterns so frequently found in Japanese gardens. At some places
+these bodies fell in various directions, at others with considerable
+uniformity in one direction. For instance, at Nagoya, out of 200 stone
+lanterns with cylindrical stems, 119 fell between west and south, and
+36 between east and north; the numbers falling within successive
+angles of 15&deg; being represented in Fig. 43. The mean direction of fall
+is W. 30&deg; S., coinciding with that in which the majority of the
+lanterns were overturned. Similar observations were made at forty-two
+other places within and near the meizoseismal area, and the resulting
+mean direction for each such place in the Mino-Owari district is shown
+by short lines in Fig. 44, the arrow indicating the direction towards
+which the majority of bodies at a given place were overturned. It will
+be seen from this map that the direction of the earthquake motion was
+generally at right angles, or nearly so, to that of <span class='pagenum'><a name="Page_188" id="Page_188">[188]</a></span>the neighbouring
+part of the meizoseismal zone, and that on both sides of it, the
+majority of overturned bodies at each place fell towards this zone.</p>
+
+<div class="img"><a name="Fig43" id="Fig43"></a>
+<a href="images/fig43.png">
+<img border="0" src="images/fig43.png" width="55%" alt="Plan of Directions of Fall of Overturned Bodies at Nagoya." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 43.&mdash;Plan of Directions of Fall of Overturned Bodies at Nagoya.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="img"><a name="Fig44" id="Fig44"></a>
+<a href="images/fig44.png">
+<img border="0" src="images/fig44.png" width="60%" alt="Map of Mean Directions of Shock and Isoseismal Lines in Central District." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 44.&mdash;Map of Mean Directions of Shock and Isoseismal Lines in Central District. (<i>Omori.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<br />
+
+<h4>VELOCITY OF THE EARTH-WAVES.</h4>
+
+<p>The times of the great earthquake and of sixteen minor shocks on
+October 28th and 29th and November 6th were determined at the Central
+Meteorological <span class='pagenum'><a name="Page_189" id="Page_189">[189]</a></span>Observatory at Tokio, and at either two or three of the
+observatories of Gifu, Nagoya, and Osaka, each of which is provided
+with a seismograph and chronometer. The after-shocks referred to
+originated near a point about 6 miles west of Gifu, and the difference
+between the distances of Tokio and Osaka from this point is 89&frac12;
+miles, of Tokio and Nagoya 147 miles, and of Tokio and Gifu 165 miles.
+The mean time-intervals between these three pairs of places were 67,
+111, and 128 seconds respectively; and these give for the mean
+velocity for each interval 2.1 kilometres (or 1.3 mile) per second.
+Thus there appears in these cases to be no sensible variation in the
+velocity with the distance from the origin.</p>
+
+<p>As might be expected, an earthquake of such severity was recorded by
+magnetometers at several distant observatories. Disturbances on the
+registers of Zikawei (China), Mauritius, Utrecht, and Greenwich have
+been attributed to the Japanese earthquake, but the times at which
+they commenced are too indefinite to allow of any determination of the
+surface-velocity of the earth-waves to great distances from the
+origin.</p>
+
+<br />
+
+<h4>THE GREAT FAULT-SCARP.</h4>
+
+<p>As in all disastrous earthquakes, the surface of the ground was
+scarred and rent by the shock. From the hillsides great landslips
+descended, filling the valleys with <i>d&eacute;bris</i>; and slopes which were
+formerly green with forest, after the earthquake looked as if they had
+been painted yellowish-white. Innumerable fissures cut up the plains,
+the general appearance of the ground, according to Professor Milne,
+being "as if gigantic ploughs, each cutting a trench from 3 to 12 feet
+deep, had been dragged up and down the river-banks." But by far the
+most remarkable feature of <span class='pagenum'><a name="Page_190" id="Page_190">[190]</a></span>the earthquake was a great rent or fault,
+which, unlike the fissures just referred to, pursued its course
+regardless of valley, plain, or mountain. Although at first sight
+quite insignificant in many places, and some time hardly visible to
+the untrained eye, Professor Koto has succeeded in tracing this fault
+along the<span class='pagenum'><a name="Page_191" id="Page_191">[191]</a></span> <span class='pagenum'><a name="Page_192" id="Page_192">[192]</a></span>surface for a distance of forty miles, and he gives good
+reasons for believing that its total length must be not less than
+seventy miles.</p>
+
+<div class="img"><a name="Fig45" id="Fig45"></a>
+<a href="images/fig45.png">
+<img border="0" src="images/fig45.png" width="50%" alt="Map of Meizoseismal Area." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 45.&mdash;Map of Meizoseismal Area. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="img"><a name="Fig46" id="Fig46"></a>
+<a href="images/fig46.jpg">
+<img border="0" src="images/fig46.jpg" width="50%" alt="Ploughshare Appearance of the Fault near Fujitani." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 46.&mdash;Ploughshare Appearance of the Fault near Fujitani. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="img"><a name="Fig47" id="Fig47"></a>
+<a href="images/fig47.jpg">
+<img border="0" src="images/fig47.jpg" width="60%" alt="The Fault-scarp at Midori." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 47.&mdash;The Fault-scarp at Midori. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The general character of the fault-scarp changes with the surface
+features. On flat ground, where the throw is small, it cuts up the
+soft earth into enormous clods, or makes a rounded ridge from one to
+two feet high, so that it resembles, more than anything else, <span class='pagenum'><a name="Page_193" id="Page_193">[193]</a></span>the
+pathway of a gigantic mole (Fig. 46). When the throw is
+considerable&mdash;and in one place it reaches from 18 to 20 feet&mdash;the
+fault-scarp forms a terrace, which from a distance has the appearance
+of a railway embankment (Fig. 47). Or, again, where the rent traverses
+a mountain ridge or a spur of hills, "it caused extensive landslips,
+one side of it descending considerably in level, carrying the forest
+with it, but with the trees complicatedly interlocked or prostrate on
+the ground."</p>
+
+<div class="imgr" style="width: 40%;"><a name="Fig48" id="Fig48"></a>
+<a href="images/fig48.png">
+<img border="0" src="images/fig48.png" width="85%" alt="Displacement of Field Divisions by the Fault near Nishi-Katabira." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 48.&mdash;Displacement of Field Divisions by the Fault near Nishi-Katabira. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>At its southern end, the fault was seen for the first time crossing a
+field near the village of Katabira. The field was broken into clods of
+earth, and swollen up to a height of 5&frac12; yards, while a great
+landslip had descended into it from an adjoining hill. A little
+farther to the north-west, the ground was sharply cut by the fault,
+the north-east side having slightly subsided and at the same time been
+shifted horizontally through a distance of 3&frac14; to 4 feet to the
+north-west Adjoining fields were formerly separated by straight mounds
+or ridges running north and south and east and west, and these mounds
+were cut through by the fault and displaced, as shown in Fig. 48. From
+this point the fault runs in a general north-westerly direction, the
+north-east side being always slightly lowered with respect to the
+other and shifted to the north-west. Near Seki it takes a more
+westerly direction, and continues so to a short distance east of
+<span class='pagenum'><a name="Page_194" id="Page_194">[194]</a></span>Takatomi, where the north side is lowered by five feet, and moved
+about 1&frac14; feet to the west. At the north end of Takatomi, a village
+in which every house was levelled with the ground, the fault is
+double, and the continuous lowering towards the north has converted a
+once level field into sloping ground. At this point, the small river
+Toba, flowing south, is partially blocked by the fault-scarp, and an
+area of about three-quarters of a square mile, on which two villages
+stand, was converted into a deep swamp (Fig. 49), so that, as the
+earthquake occurred at the time of the rice-harvest, the farmers were
+obliged to cut the grain from boats. After passing Takatomi, the fault
+again turns to the west-north-west, but, the throw being small, it
+resembles here the track of an enormous <span class='pagenum'><a name="Page_195" id="Page_195">[195]</a></span>mole. At Um&eacute;hara it crosses a
+garden between two persimmon trees, appearing on the hard face of the
+ground as a mere line; but the trees, which were before in an
+east-and-west line, now stand in one running north and south, without
+being in the least affected by the movement (Fig. 50). From here to
+Kimbara, where the fault enters the Neo valley, the north side is
+always depressed and shifted westwards by about 6&frac12; feet.</p>
+
+<div class="img"><a name="Fig49" id="Fig49"></a>
+<a href="images/fig49.png">
+<img border="0" src="images/fig49.png" width="60%" alt="Map of Swamp formed by stoppage of River Toba by Fault-scarp." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 49.&mdash;Map of Swamp formed by stoppage of River Toba by Fault-scarp. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="imgr" style="width: 30%;"><a name="Fig50" id="Fig50"></a>
+<a href="images/fig50.png">
+<img border="0" src="images/fig50.png" width="80%" alt="Shifting of Trees by fault at Um&eacute;hara." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 50.&mdash;Shifting of Trees by fault at Um&eacute;hara. (<i>Koto.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>It was in the Neo valley that the supreme efforts of the earthquake
+were manifested. Landslips were so numerous that the greater part of
+the mountain slopes had descended into the valley, the whole
+appearance of which had changed. "Unfamiliar obstacles," remarks
+Professor Koto, "made themselves apparent, and small hills covered
+with forest had come into sight which had not been seen before." But
+the ground was not only lowered and shifted by the fault; it was
+permanently compressed, plots originally 48 feet in length afterwards
+measuring only 30 feet. In fact, "it appears," in the words of
+Professor Milne, "as if the whole Neo valley had become narrower."</p>
+
+<p>A few miles after entering the Neo valley, the throw of the fault
+reaches its maximum at Midori. But instead of the relative depression
+of the east side, which prevails throughout the rest of the line, that
+side is here about 20 feet higher than the other. It is, however,
+shifted as usual towards the north, by about 13 feet; and this
+displacement is rendered <span class='pagenum'><a name="Page_196" id="Page_196">[196]</a></span>especially evident by the abrupt break in the
+line of a new road to Gifu (Fig. 47). That the east side has really
+risen is clear, for, a little higher up, the river has changed from a
+shallow rapid stream 30 yards wide into a small lake of more than
+twice the width, and so deep that a boatman's pole could not reach the
+bottom. At Itasho, about a mile north of Midori, both sides are nearly
+on the same level, the fault appearing like a mole's track; and seven
+miles farther, at Nagoshima, the east side is relatively depressed by
+more than a yard, and at the same time shifted about 6&frac12; feet to the
+north.</p>
+
+<div class="img"><a name="Fig51" id="Fig51"></a>
+<a href="images/fig51.png">
+<img border="0" src="images/fig51.png" width="45%" alt="Daily frequency of after-shocks at Gifu and Nagoya." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 51.&mdash;Daily frequency of after-shocks at Gifu and Nagoya.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>At Nogo, the main Neo valley turns off at right angles to the east,
+and the fault continues its course up a side valley, the east side,
+with respect to the other, being continually depressed and shifted
+towards the north. It was traced by Professor Koto through Fujitani
+(Fig. 46), where there were many unmistakable evidences of the
+violence of the shock, as far as the eastern shoulder of Haku-san; and
+here, after following the fault for 40 miles, the lateness of the
+season compelled him to return. There can be no doubt, however, that
+it runs as far as Minomata; and it is probable, from the <span class='pagenum'><a name="Page_197" id="Page_197">[197]</a></span>linear
+extension of the meizoseismal area, that it does not entirely die out
+before reaching the city of Fukui, 70 miles from its starting-point at
+Katabira.</p>
+
+<br />
+
+<h4>MINOR SHOCKS.</h4>
+
+<p>For some hours after the earthquake, shocks were so frequent in the
+meizoseismal area that the ground in places hardly ever ceased from
+trembling. Without instrumental aid, detailed record was of course
+impossible; but fortunately the buried seismographs at Gifu and Nagoya
+were uninjured, and in about seven hours both were once more in
+working order. To the energy by which this result was accomplished, we
+owe our most valuable registers of the after-shocks of a great
+earthquake.</p>
+
+<div class="img"><a name="Fig52" id="Fig52"></a>
+<a href="images/fig52.png">
+<img border="0" src="images/fig52.png" width="50%" alt="Monthly frequency of after-shocks at Gifu." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 52.&mdash;Monthly frequency of after-shocks at Gifu. (<i>Omori.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Until the end of 1893&mdash;that is, in little more than two years&mdash;the
+total number of shocks recorded at Gifu was 3,365, and at Nagoya
+1,298. None of these approached the principal earthquake in severity.
+Nevertheless, of the Gifu series, 10 were described as violent and 97
+strong; while of the remainder, 1,808 were weak, 1,041 feeble, and 409
+were sounds alone without any accompanying shock. The slight intensity
+of most of the shocks is also evident <span class='pagenum'><a name="Page_198" id="Page_198">[198]</a></span>from the inequality in the
+numbers recorded at Gifu and Nagoya, from which it appears that nearly
+two-thirds were imperceptible more than about 25 miles from the chief
+origin of the shocks. Only 70 of the after-shocks during the first two
+years were registered at Osaka, and not more than 30 at Tokio.</p>
+
+<p><i>Distribution of After-shocks in Time.</i>&mdash;The decline in frequency of
+the after-shocks was at first extremely rapid, the numbers recorded at
+Gifu during the six days after the earthquake being 303, 147, 116, 99,
+92, and 81, and at Nagoya 185, 93, 79, 56, 30, and 31; in fact, half
+of the shocks up to the end of 1893 occurred by November 23rd at Gifu,
+and by November 6th at Nagoya. The daily numbers at these two places
+are represented in Fig. 51, in which the crosses correspond to the
+numbers at Gifu, and the dots to those at Nagoya; and the curves drawn
+through or near the marks represent the average daily number of shocks
+from October 29th to November 20th. It will be seen that these curves
+are hyperbolic in form, the change from very rapid to very gradual
+decline in frequency taking place from five to ten days after the
+great earthquake. Fig. 52 illustrates the distribution in time of the
+after-shocks at Gifu to the end of 1893, the ordinates in these cases
+representing the number of shocks during successive months.<a name="FNanchor_57_57" id="FNanchor_57_57"></a><a href="#Footnote_57_57" class="fnanchor">[57]</a></p>
+
+<p><span class='pagenum'><a name="Page_199" id="Page_199">[199]</a></span>A similar rapid and then gradual decline in frequency characterises
+the strong and weak shocks recorded at Gifu. Of the ten violent
+shocks, only one occurred after the beginning of January 1892; and of
+the 97 strong shocks, only three after April 1892. But at the
+commencement of the series, feeble shocks (<i>i.e.</i>, shocks that could
+just be felt) and earth-sounds without any accompanying movement were
+comparatively rare, and did not become really prominent until two
+months had elapsed. Of the 308 after-shocks recorded in 1893, none
+could be described as strong, only 10 were weak, while 263 were feeble
+shocks and 35 merely earth-sounds.</p>
+
+<p>The last two diagrams show at a glance that the decline in frequency
+of after-shocks is very far from being uniform. Some of the
+fluctuations are due to the occurrence of exceptionally strong shocks,
+each of which is followed by its own minor train of after-shocks.<a name="FNanchor_58_58" id="FNanchor_58_58"></a><a href="#Footnote_58_58" class="fnanchor">[58]</a>
+Others seem to be periodic, and possibly owe their origin to external
+causes unconnected with the earthquake.<a name="FNanchor_59_59" id="FNanchor_59_59"></a><a href="#Footnote_59_59" class="fnanchor">[59]</a></p>
+
+<p><span class='pagenum'><a name="Page_200" id="Page_200">[200]</a></span><i>Method of representing the Distribution of After-shocks in
+Space.</i>&mdash;The maps in Figs. 54-57 show the distribution of the
+after-shocks in space during four successive intervals of two months
+each. They are founded on Professor Milne's great catalogue of
+Japanese earthquakes, which give, among other data, the time of
+occurrence and the position of the epicentre for every shock until the
+end of 1892. For the latter purpose, the whole country is divided by
+north-south and east-west lines into numbered rectangles, each
+one-sixth of a degree in length and breadth; and the position of an
+epicentre is denoted by the number of the rectangle in which it
+occurs. The area included within the maps is bounded by the parallels
+34&deg; 40' and 36&deg; 20' lat. N., and by the meridians 2&deg; 10' and 3&deg; 50'
+long. W. of Tokio, so that ten rectangles adjoin each side of the map.
+The number of epicentres lying within each rectangle having been
+counted, curves are then drawn through the centres of all rectangles
+containing the same number of epicentres, or through points which
+divide the line joining the centres of two rectangles in the proper
+proportion. Taking, for example, the curve marked 5, if the numbers in
+two consecutive rectangles are 3 and 7, the curve bisects the line
+joining their centres; if the numbers are 1 and 6, the line joining
+their centres is divided into five equal parts, and the curve passes
+through the first point of division reckoned from the centre of the
+rectangle in which six epicentres are found. Thus the meaning of the
+curve marked, say, <span class='pagenum'><a name="Page_201" id="Page_201">[201]</a></span>5 may be stated as follows:&mdash;If any point in the
+curve be imagined as the centre of a rectangle whose sides are
+directed north-south and east-west, and are respectively one-sixth of
+a degree of latitude and longitude in length; then the number of
+epicentres within this rectangle is at the rate of 5 for the time
+considered.</p>
+
+<p><i>Preparation for the Great Earthquake.</i>&mdash;At first sight, there appears
+to have been but little direct preparation for the great earthquake.
+Except for a rather strong shock on October 25th, at 9.14
+<span class="fakesc">P.M.</span>, it occurred without the warning of any preliminary
+tremors. But a closer examination of the evidence shows, as we should
+indeed expect, that there was a distinct increase in activity for many
+months beforehand. The region had become "seismically sensitive." Of
+the hundred rectangles included in the maps in Figs. 53-57, there are
+thirteen lying along the meizoseismal area of the earthquake of 1891,
+in which nearly all the after-shocks originated. During the five years
+1885-89, 53 out of 125 earthquakes (or 42 per cent.) had their
+epicentres lying within the thirteen rectangles; or, in other words,
+the average frequency in one of the rectangles of the meizoseismal
+area was five times as great as in one of those outside it. In 1890
+and 1891 (until October 27th), the percentage in the thirteen
+rectangles rose to 61, and the average frequency in one of them to ten
+times that of one of the exterior rectangles.</p>
+
+<p>The curves in Fig. 53 illustrate the distribution of epicentres during
+the latter interval. It will be seen that they follow roughly the
+course of the meizoseismal area southwards to the Sea of Is&eacute;, and that
+to the south-east they continue for several miles the short <span class='pagenum'><a name="Page_202" id="Page_202">[202]</a></span>branch of
+the meizoseismal area which surrounds the southern end of the
+fault-scarp.</p>
+
+<div class="img"><a name="Fig53" id="Fig53"></a>
+<a href="images/fig53.png">
+<img border="0" src="images/fig53.png" width="50%" alt="Distribution of preliminary Shocks in Space." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 53.&mdash;Distribution of preliminary Shocks in Space. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Thus, the preparation for the great earthquake is shown, first, by the
+increased frequency of earthquakes originating within its meizoseismal
+area; and, secondly, by the uniformity in the distribution of
+epicentres throughout the same region, the marked <span class='pagenum'><a name="Page_203" id="Page_203">[203]</a></span>concentration of
+effort which characterises the after-shocks being hardly perceptible
+during the years 1890-91.</p>
+
+<div class="img"><a name="Fig54" id="Fig54"></a>
+<a href="images/fig54.png">
+<img border="0" src="images/fig54.png" width="50%" alt="Distribution of After-shocks in Space (November-December 1891)." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 54.&mdash;Distribution of After-shocks in Space (November-December 1891). (<i>Davison</i>).<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Distribution of After-shocks in Space.</i>&mdash;We have seen that the
+after-shocks were subject to a fluctuating decline in frequency, rapid
+at first, and more gradual afterwards. It is evident, from Figs.
+54-57, that a <span class='pagenum'><a name="Page_204" id="Page_204">[204]</a></span>similar law governs the area within which the
+after-shocks originated. During the first two months, epicentres occur
+over nearly the whole of the meizoseismal area, but afterwards they
+are confined to a smaller district, which slowly, though not
+continually, decreases in size.</p>
+
+<div class="img"><a name="Fig55" id="Fig55"></a>
+<a href="images/fig55.png">
+<img border="0" src="images/fig55.png" width="50%" alt="Distribution of After-shocks in Space (January-February, 1892)." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 55.&mdash;Distribution of After-shocks in Space (January-February, 1892). (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<div class="img"><a name="Fig56" id="Fig56"></a>
+<a href="images/fig56.png">
+<img border="0" src="images/fig56.png" width="50%" alt="Distribution of After-shocks in Space (March-April)." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 56.&mdash;Distribution of After-shocks in Space (March-April). (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_205" id="Page_205">[205]</a></span>The most important feature in the distribution of the epicentres is
+the central region of extraordinary activity; but there are also
+districts of minor and more short-lived activity near the three
+extremities of the meizoseismal band. The seat of chief seismic action
+shifts slightly from one part to another of the epicentral region,
+especially about the end of 1891, as <span class='pagenum'><a name="Page_206" id="Page_206">[206]</a></span>will be seen by comparing the
+innermost curves of Figs. 54 and 55. Thus, with the decline in
+frequency of the after-shocks and the decrease in their sphere of
+action, there took place concurrently a gradual but oscillating
+withdrawal of that action to a more or less central region of the
+fault.</p>
+
+<div class="img"><a name="Fig57" id="Fig57"></a>
+<a href="images/fig57.png">
+<img border="0" src="images/fig57.png" width="50%" alt="Distribution of After-shocks in Space (May-June, 1892)." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 57.&mdash;Distribution of After-shocks in Space (May-June, 1892). (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_207" id="Page_207">[207]</a></span><i>Sound Phenomena of After-shocks.</i>&mdash;While comparatively few observers
+seem to have noticed any noise with the principal earthquake, many of
+the after-shocks were accompanied by sounds. Professor Omori describes
+them as belonging to two types. They were either rushing feeble noises
+like that of wind, or loud rumbling noises like those of thunder, the
+discharge of a gun, or the fall of a heavy body. In the Neo valley,
+sounds of the second type were most frequent and distinct, but they
+either occurred without any shock at all, or the attendant tremor was
+very feeble; while, on the other hand, severe sharp shocks were
+generally unaccompanied by distinctly audible sounds.</p>
+
+<p>It is remarkable, also, that sounds were less frequently heard with
+the early than with the later after-shocks. In November 1891, the
+percentage of audible shocks was 17, and from December to the
+following April always lay between 10 and 12. In May the percentage
+suddenly rose to 39, and until the end of 1892 was always greater than
+32, while in November 1892, it rose as high as 49. This, of course,
+agrees with Professor Omori's observation that sounds attended feeble
+shocks more often than strong ones.</p>
+
+<p>The distribution of the audible after-shocks in space is shown in Fig.
+58. These curves are drawn in the same way as those in Figs. 53-57,
+but they represent the percentages, not the actual numbers, of shocks
+accompanied by sound. It will be noticed that all three groups of
+curves lie along the meizoseismal area, or the continuation of the
+south-east branch; while the axis of the principal group of curves
+lies to the west of the central regions in which most after-shocks
+originated.</p>
+
+<div class="img"><a name="Fig58" id="Fig58"></a>
+<a href="images/fig58.png">
+<img border="0" src="images/fig58.png" width="60%" alt="Distribution of Audible After-shocks in Space (November 1891-December 1892)." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 58.&mdash;Distribution of Audible After-shocks in Space (November 1891-December 1892). (<i>Davison</i>.)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><span class='pagenum'><a name="Page_208" id="Page_208">[208]</a></span>The explanation of these peculiarities is no doubt connected with the
+comparative inability of the Japanese people to perceive the deep
+sounds which in Europe are always heard with earthquake shocks. The
+sounds are rarely heard by them more than a <span class='pagenum'><a name="Page_209" id="Page_209">[209]</a></span>few miles from the
+epicentre.<a name="FNanchor_60_60" id="FNanchor_60_60"></a><a href="#Footnote_60_60" class="fnanchor">[60]</a> We may therefore conclude that slight after-shocks
+originated nearer the surface than strong ones, that the mean depth
+of the foci decreased with the lapse of time, and that the axes of the
+systems of curves in Fig. 58 mark out approximately the lines of the
+growing faults. The separation of the two westerly groups of curves
+appears to show that the main branch of the meizoseismal area is
+connected with a fault roughly parallel to that traced by Professor
+Koto, but of which no scarp (if it existed) could be readily
+distinguished among the superficial fissures produced by the great
+shock.</p>
+
+<br />
+
+<h4>EFFECT OF THE EARTHQUAKE ON THE SEISMIC ACTIVITY OF THE ADJOINING DISTRICTS.</h4>
+
+<p>So great and sudden a displacement as occurred along the fault-scarp
+could hardly take place without affecting the stability of adjoining
+regions of the earth's crust, and we should naturally expect to find a
+distinct change in their seismic activity shortly after October 28th.
+In Fig. 59 two such regions are shown, bounded by the straight dotted
+lines. The district in which the principal earthquake and its
+after-shocks originated is enclosed within the undulating dotted
+lines. The continuous lines inside all three districts are the curves
+corresponding to 10 and 5 epicentres for the years 1885-92. Not far
+from the axes of the outer groups of curves there are <span class='pagenum'><a name="Page_210" id="Page_210">[210]</a></span>probably
+transverse faults, approximately parallel to the great fault-scarp and
+the main branch of the meizoseismal band, and distant from them about
+45 and 55 miles respectively.</p>
+
+<div class="imgl" style="width: 50%;"><a name="Fig59" id="Fig59"></a>
+<a href="images/fig59.png">
+<img border="0" src="images/fig59.png" width="85%" alt="Map of Adjoining Regions in which Seismic Activity was affected by the Great Earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 59.&mdash;Map of Adjoining Regions in which Seismic Activity was affected by the Great Earthquake. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>In the district represented in the north-east corner of Fig. 59, 29
+earthquakes originated between January 1st, 1885, and October 27th,
+1891, and 30 between October 28th, 1891, and December 31st, 1892, 7 of
+the latter number occurring in November 1891. In the south-west
+district, the corresponding figures before and after the earthquake
+are 20 and 36, 8 of the latter occurring in November 1891. Thus, in
+the north-east district, for every shock in the interval before the
+earthquake there were six in an equal time afterwards, and at the rate
+of 10 during November 1891; and in the south-west district, for every
+shock before the earthquake there were 10 afterwards, and at the rate
+of 16 during November 1891.</p>
+
+<p>Now, it is unlikely that the gradual increase of stress should be so
+nearly proportioned everywhere to the prevailing conditions of
+resistance as to give rise to a marked and practically simultaneous
+change in seismic activity over a large area; whereas the <span class='pagenum'><a name="Page_211" id="Page_211">[211]</a></span>paroxysmal
+occurrence of a strong earthquake might alter the surrounding
+conditions with comparative rapidity, and so induce a state of seismic
+excitement in the neighbourhood. It therefore seems very probable
+that the increased activity in the two districts here described was a
+direct consequence of the occurrence of the great earthquake.</p>
+
+<br style="clear: both;" />
+
+<h4>ORIGIN OF THE EARTHQUAKE.</h4>
+
+<p>The preponderance of preliminary earthquakes within the meizoseismal
+area and the outlining of the fault-system by the frequency curves of
+1890-91 (Fig. 53) point to the previous existence of the originating
+fault or faults, and to the earthquake being due, not to the formation
+of a new fracture, as has been suggested, but to the growth of an old
+fault.</p>
+
+<p>The last severe earthquake in the Mino-Owari plain occurred in 1859,
+so that for more than thirty years there had been but little relief to
+the gradually increasing stresses. Now, the distribution of stress
+must have been far from uniform throughout the fault-system, and also
+the resistance to displacement far from proportional to the stresses
+at different places. At certain points, therefore, the effective
+stress would be greater than elsewhere, and it would be at these
+points that fault-slips would first occur. Such slips tend to remove
+the inequalities in effective stress. Thus, the function of the slight
+shocks of 1890 and 1891 was, briefly, to equalise the effective stress
+over the whole fault-system, and so to clear the way for one or more
+great slips throughout its entire length.</p>
+
+<p>As to which side of the fault moved during the <span class='pagenum'><a name="Page_212" id="Page_212">[212]</a></span>great displacement, or
+whether both sides moved at once, we have no direct evidence but as
+regards the neighbourhood of Midori, and there the conditions were
+exceptional. Professor Koto thinks that it was probably the rock on
+the north-east side that was generally depressed and always shifted to
+the north-west But the disturbance in reality seems to have been more
+complicated. That this was the case, that displacement occurred along
+more than one fault, is probable from the branching of the
+meizoseismal area, the isolation of the audibility curves of the
+after-shocks (Fig. 58), and the sudden increase in seismic activity
+both to the north-east and south-west of the epicentre. The detached
+portion of the meizoseismal area near Lake Biwa may also point to a
+separate focus. The whole region, indeed, was evidently subjected to
+intense stresses, and the depression on the north-east side of the
+fault-scarp can hardly fail to have been accompanied by other
+movements, especially along a fault running near the western margin of
+the main branch of the meizoseismal area.</p>
+
+<p>The later stages of the movements are somewhat clearer. From a study
+of the after-shocks, we learn that the disturbed masses began at once
+to settle back towards the position of equilibrium. At first the slips
+were numerous and took place over the whole fault-system, but chiefly
+at a considerable depth, where no doubt the initial displacement was
+greatest. After a few months, stability was nearly restored along the
+extremities of the faults; slips were confined almost entirely to the
+central regions, while a much larger proportion of them took place
+within the superficial portions of the faults.</p>
+
+<p><span class='pagenum'><a name="Page_213" id="Page_213">[213]</a></span>The official records bring down the history to the end of 1893. Since
+that time more than one strong shock has been felt in the Mino-Owari
+plain; but the stage of recovery from the disturbances of 1891 is
+probably near its end, and we seem rather to be entering on a period
+in which the forces are once more silently gathering that sooner or
+later will result in another great catastrophe.</p>
+
+<br />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Conder, J.</span>&mdash;"An Architect's Notes on the Great
+Earthquake of October 1891." <i>Japan Seismol. Journ.</i>, vol.
+ii., 1893, pp. 1-91.</p>
+
+<p class="hang">2. <span class="sc">Davison, C.</span>&mdash;"On the Distribution in Space of the
+Accessory Shocks of the Great Japanese Earthquake of 1891."
+<i>Quart. Journ. Geol. Soc.</i>, vol. liii., 1897, pp. 1-15.</p>
+
+<p class="hang">3. &mdash;&mdash; "On the Effect of the Great Japanese Earthquake of 1891 on
+the Seismic Activity of the Adjoining Districts." <i>Geol.
+Mag.</i>, vol. iv., 1897, pp. 23-27.</p>
+
+<p class="hang">4. &mdash;&mdash; "On the Diurnal Periodicity of Earthquakes." <i>Phil. Mag.</i>,
+vol. xiii., 1896, pp. 463-476, especially pp. 466-468.</p>
+
+<p class="hang">5. &mdash;&mdash; "On Earthquake-Sounds." <i>Phil. Mag.</i>, vol. xlix., 1900, pp.
+31-70&mdash;especially pp. 49-53, 60-61.</p>
+
+<p class="hang">6. <span class="sc">Koto, B.</span>&mdash;"The Cause of the Great Earthquake in Central
+Japan, 1891." <i>Journ. Coll. Sci. Imp. Univ. Japan</i>, vol. v.,
+1893, pp. 295-353.</p>
+
+<p class="hang">7. <span class="sc">Masato, H.</span>&mdash;"Report on Earthquake Observations in
+Japan." <i>Cent. Meteor. Obs. of Japan</i> (Tokio, 1892), pp.
+16-18, 41, and map 30.</p>
+
+<p class="hang">8. <span class="sc">Milne, J.</span>&mdash;"A Note on the Great Earthquake of October
+28th, 1891." <i>Japan Seismol. Journ.</i>, vol. i., 1893, pp.
+127-151; <i>Brit. Assoc. Rep.</i>, 1892, pp. 114-128.</p>
+
+<p class="hang">9. &mdash;&mdash; "A Catalogue of 8,331 Earthquakes recorded in Japan between
+1885 and 1892." <i>Japan Seismol. Journ.</i>, vol. iv., 1895, pp.
+1-367&mdash;especially pp. 134-234, 303-353.</p>
+
+<p class="hang">10. &mdash;&mdash; "On Certain Disturbances in the Records of Magnetometers
+and the Occurrence of Earthquakes." <i>Brit. <span class='pagenum'><a name="Page_214" id="Page_214">[214]</a></span>Assoc. Rep.</i>, 1898,
+pp. 226-251&mdash;especially pp. 227, 232, 234, 241, and 245.</p>
+
+<p class="hang">11. <span class="sc">Milne, J.</span>, and <span class="sc">W.K. Burton.</span>&mdash;"The Great
+Earthquake in Japan." <i>Journ. Coll. Sci. Imp. Univ. Japan</i>,
+vol. v., 1893, pp. 295-352.</p>
+
+<p class="hang">12. <span class="sc">Omori, F.</span>&mdash;"On the After-shocks of Earthquakes."
+<i>Journ. Coll. Sci. Imp. Univ. Japan</i>, vol. vii., 1894, pp.
+111-200; abstract in <i>Japan Seismol. Journ.</i>, vol. iii., 1894,
+pp. 71-80.</p>
+
+<p class="hang">13. &mdash;&mdash; "A Note on the Great Mino-Owari Earthquake of October 28th,
+1891." <i>Pub. Earthquakes Inves. Com. in Foreign Languages</i>,
+No. 4, Tokio, 1900, pp. 13-24.</p>
+
+<p class="hang">14. &mdash;&mdash; "Sulla velocit&agrave; di propagazione e sulla lunghezza delle
+onde sismiche." <i>Ital. Soc. Sismol. Boll.</i>, vol. i., 1895, pp.
+52-60&mdash;especially pp. 52-57.</p>
+
+<p class="hang">15. &mdash;&mdash; "Sull' intensit&agrave; e sull' ampiezza del movimento nel gran
+terremoto giapponese del 28 ottobre 1891." <i>Ital. Soc. Sismol.
+Boll.</i>, vol. ii., 1896, pp. 189-200.</p>
+
+<p class="hang">16. &mdash;&mdash; "Note on the After-shocks of the Mino-Owari Earthquake of
+Oct. 28th, 1891." <i>Pub. Earthquakes Inves. Com. in Foreign
+Languages</i>, No. 7, Tokio, 1902, pp. 27-32.</p>
+
+<p class="hang">17. &mdash;&mdash; "Note on the relation between Earthquake Frequency and the
+Atmospheric Pressure." <i>Tokyo Phys.-Math. Soc. Reports</i>, vol.
+ii., 1904, No. 8.</p>
+
+<p class="hang">18. <span class="sc">Tanakadate, A.</span>, and <span class="sc">H. Nagaoka</span>. "The
+Disturbance of Isomagnetics attending the Mino-Owari
+Earthquake of 1891." <i>Journ. Coll. Sci. Imp. Univ. Japan</i>,
+vol. v., 1893, pp. 149-192.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_54_54" id="Footnote_54_54"></a><a href="#FNanchor_54_54"><span class="label">[54]</span></a> I have not referred to the results of this survey, for,
+though changes in all the magnetic elements (especially in horizontal
+intensity) have taken place between 1887 and 1891-92, these changes
+cannot be ascribed with confidence to the earthquake in the absence of
+a thorough knowledge of the secular variation.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_55_55" id="Footnote_55_55"></a><a href="#FNanchor_55_55"><span class="label">[55]</span></a> From the formula <i>a</i> = <i>xg</i> / <i>y</i>, where <i>a</i> is the
+maximum horizontal acceleration, <i>g</i> the acceleration due to gravity,
+<i>y</i> the height of the centre of gravity, and <i>x</i> its horizontal
+distance from the edge about which the body was overturned.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_56_56" id="Footnote_56_56"></a><a href="#FNanchor_56_56"><span class="label">[56]</span></a> These estimates are made, on the supposition of simple
+harmonic motion, from the formula 2<i>a</i> = &alpha;<i>t</i>&sup2; / (2&pi;&sup2;), where
+2<i>a</i> is the total range or double amplitude, <i>a</i> the maximum
+acceleration, and <i>t</i> the period of the vibration.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_57_57" id="Footnote_57_57"></a><a href="#FNanchor_57_57"><span class="label">[57]</span></a> Professor Omori finds that the mean daily number of
+earthquakes <i>y</i> during the month <i>x</i> (reckoned from November 1891) may
+be approximately represented by the equation&mdash;</p>
+
+<p class="pad"><i>y</i> = 16.9 / (<i>x</i> + 0.397);</p>
+
+<p class="noin">or, taking the semi-daily earthquake numbers during the five days
+between October 29th and November 2nd, 1891, by the equation&mdash;</p>
+
+<p class="pad"><i>y</i> = 440.7 / (<i>x</i> + 2.314),</p>
+
+<p class="noin">where <i>y</i> denotes the number of earthquakes observed during the twelve
+hours denoted by <i>x</i>, the time being measured from the first half of
+October 29th. It is interesting to notice that, taking account of the
+mean annual frequency of earthquakes in ordinary years, the number of
+shocks observed at Gifu during the two years 1898-99 should, according
+to the latter formula, be 163; the actual number recorded was 160.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_58_58" id="Footnote_58_58"></a><a href="#FNanchor_58_58"><span class="label">[58]</span></a> The last violent shock before the end of 1893 occurred
+on September 7th, 1892, and its effects on the frequency of
+after-shocks is shown by the daily numbers recorded at Gifu during the
+first fortnight in September. These are&mdash;2, 2, 2, 3, 5, 5, 28 (on
+September 7th), 8, 8, 5, 4, 3, 2, 4, 3.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_59_59" id="Footnote_59_59"></a><a href="#FNanchor_59_59"><span class="label">[59]</span></a> The periodicity of after-shocks is discussed in the
+papers numbered 4, 12, 16, and 17 at the end of this chapter. In
+these, the existence of diurnal and other periods is clearly
+established. Professor Omori also shows that the mean daily barometric
+pressure is subject to fluctuations with maxima occurring on an
+average every 5&frac12; days, and that earthquakes are least frequent on
+the days of the barometric maxima and minima, and more frequent in the
+days immediately preceding and following them.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_60_60" id="Footnote_60_60"></a><a href="#FNanchor_60_60"><span class="label">[60]</span></a> Of the Japanese earthquakes of 1885-92 originating
+beneath the land, twenty-six per cent. were accompanied by a recorded
+sound; but less than one per cent. of those originating beneath the
+sea and not more than ten miles from the coast.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_VIII" id="CHAPTER_VIII"></a><hr />
+<span class='pagenum'><a name="Page_215" id="Page_215">[215]</a></span><br />
+
+<h3>CHAPTER VIII.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896,<br />
+AND THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901.</h4>
+<br />
+
+<p>Among the earthquakes described in this volume, the Hereford and
+Inverness earthquakes hold but a minor place. The damage to buildings,
+though unusual for this country, was slight when compared with that
+caused by the preceding shocks; there was no loss of life, not a
+single person was injured by falling masonry. The interest of the
+earthquakes lies entirely in the detailed study rendered possible by
+numerous observations of the shock and sound,<a name="FNanchor_61_61" id="FNanchor_61_61"></a><a href="#Footnote_61_61" class="fnanchor">[61]</a> and in the bearing
+of this evidence on the general theory of the origin of earthquakes.</p>
+
+<br />
+
+<h4>THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896.</h4>
+
+<p>The principal earthquake of this series occurred at 5.32 <span class="fakesc">A.M.</span>
+on December 17th, and was preceded by at least nine minor shocks (the
+first of which was felt at about 11 or 11.30 <span class="fakesc">P.M.</span> on December
+16th), and followed by two others on the same day, and by a third and
+last on July 19th, 1897. The accounts <span class='pagenum'><a name="Page_216" id="Page_216">[216]</a></span>of these preliminary movements
+will be found on a later page, as their bearing will be more fully
+apparent after the discussion of the principal shock.</p>
+
+<div class="img"><a name="Fig60" id="Fig60"></a>
+<a href="images/fig60.png">
+<img border="0" src="images/fig60.png" width="70%" alt="Isoseismal and Isacoustic lines of Hereford earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 60.&mdash;Isoseismal and Isacoustic lines of Hereford earthquake. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>On the map in Fig. 60, the continuous curves represent isoseismal
+lines corresponding to the degrees 8, 7, 6, 5, and 4 of the
+Rossi-Forel scale. The isoseismal 8, which is the most accurately
+drawn of the series, is <span class='pagenum'><a name="Page_217" id="Page_217">[217]</a></span>an elongated oval, 40 miles long, 23 miles
+broad, and containing an area of 724 square miles. The longer axis is
+directed W. 44&deg; N. and E. 44&deg; S. Within this curve, there are 73
+places where buildings are known to have been damaged, 55 places being
+in Herefordshire, 17 in Gloucestershire, and one in Worcestershire.</p>
+
+<p>The most important damage occurred in the city of Hereford, which, in
+1901, contained 4,565 inhabited houses. Here, no fewer than 218
+chimneys had to be repaired or rebuilt. The Cathedral was slightly
+injured. The finial of a pinnacle of the Lady Chapel was thrown down,
+a fragment of a stone fell from one of the arches in the south
+transept, and the three pinnacles of the western front were fractured.
+Several churches suffered to a similar extent, while, at the Midland
+Railway Station, all the seven chimney-stacks were shattered. At
+Dinedor, Fownhope, Dormington, Withington, and a few other villages,
+the damage was also relatively greater than elsewhere, these places
+all lying within a small oval about 8&frac12; miles long, which surrounds,
+not the centre, but rather the north-west focus, of the isoseismal 8.</p>
+
+<p>The isoseismal 7, which includes places where the shock was strong
+enough to overthrow ornaments, vases, etc., is also very nearly an
+ellipse, whose axes are 80 and 56 miles in length, and whose area is
+3,580 square miles. Its longer axis, running from W. 42&deg; N. to E. 42&deg;
+S., is practically parallel to that of the inner curve. Next in
+succession comes the isoseismal 6, surrounding those places where the
+shock was strong enough to make chandeliers, pictures, etc., swing;
+but, as most of the observers seem to have slept in darkened rooms,
+the number of <span class='pagenum'><a name="Page_218" id="Page_218">[218]</a></span>determining points for this curve is less than usual,
+and its course is therefore laid down with a somewhat inferior degree
+of accuracy. The error, however, is probably small, and we may
+therefore regard the isoseismal 6 as another ellipse, 141 miles long,
+116 miles broad, and containing an area of 13,000 square miles. Its
+longer axis is again nearly parallel to those of the preceding
+isoseismals.</p>
+
+<p>The next two isoseismals are nearly circular in form. It will be
+noticed that large portions of them, and especially of the isoseismal
+4, traverse the sea. In these parts, the paths of the curves are to
+some extent conjectural. In drawing them, the chief guides are their
+trend before leaving the land and the known intensity along the
+neighbouring coastlines. The isoseismal 5 bounds the area within which
+the shock was perceptible as a sensible displacement and not merely a
+quiver. Its dimensions are 233 miles from north-west to south-east,
+and 229 miles from south-west to north-east, and its area 41,160
+square miles. The isoseismal 4, which includes places where the shock
+was strong enough to make doors, windows, etc., rattle, is 356 miles
+from north-west to south-east, and 357 miles from south-west to
+north-east, and 98,000 square miles in area; its centre coincides
+nearly with that of the small oval area in the neighbourhood of
+Hereford, where the damage to buildings was relatively greater than
+elsewhere.</p>
+
+<p>Outside the isoseismal 4, the earthquake was observed at several
+places. The shock was certainly felt at Middlesbrough, 12&frac12; miles
+from the curve, and probably at Killeshandra (in Ireland), 65 miles
+distant. Thus, if we consider the boundary of the <span class='pagenum'><a name="Page_219" id="Page_219">[219]</a></span>disturbed area to
+coincide with the isoseismal 4, its area would be 98,000 square
+miles, or 1-2/3 that of England and Wales; if it were a circle
+concentric with the isoseismal 4, and passing through Middlesbrough,
+its area would be 115,000 square miles, or nearly twice that of
+England and Wales; while, if it passed through Killeshandra, its area
+would be 185,000 square miles, or more than three times the area of
+England and Wales.<a name="FNanchor_62_62" id="FNanchor_62_62"></a><a href="#Footnote_62_62" class="fnanchor">[62]</a></p>
+
+<p><i>Position of the Originating Fault.</i>&mdash;The form, directions, and
+relative positions of the isoseismal lines furnish important evidence
+with regard to the originating fault. We conclude in the first place
+that its mean direction is parallel to the longer axes of the three
+innermost isoseismal lines&mdash;that is, north-west and south-east, or,
+more accurately, W. 43&deg; N. and E. 43&deg; S.<a name="FNanchor_63_63" id="FNanchor_63_63"></a><a href="#Footnote_63_63" class="fnanchor">[63]</a> In this case, the
+elongated forms of the isoseismal lines cannot be attributed to
+variations in the nature of the surface rocks. The district embraced
+contains about 13,000 square miles, and it is improbable that the axes
+of the three isoseismals should retain their parallelism over so large
+an area, if these variations had any considerable effect. Moreover, in
+the same district, an earthquake occurred in 1863, whose meizoseismal
+area was elongated from <span class='pagenum'><a name="Page_220" id="Page_220">[220]</a></span>north-east to south-west, or almost exactly
+perpendicular to the direction in 1896.</p>
+
+<p>Secondly, it will be noticed (Fig. 60) that the isoseismal lines are
+not equidistant from one another. On the north-east side, they are
+separated by distances of 20, 34, 55, and 51 miles; and on the
+south-west side by distances of 13&frac14;, 25, 60, and 77 miles. It
+follows from this that the fault-surface must hade or slope towards
+the north-east; for, near the epicentre, the intensity is greatest and
+dies out more slowly on the side towards which the fault hades.</p>
+
+<p>If we could ascertain any one place through which the fault passed,
+its position would thus be completely determined. Unfortunately, there
+is no decisive evidence on this point. There are, however, several
+places to the south-west of Hereford where the intensity of the shock
+was distinctly less than in the surrounding district, and it is
+possible that this was due to their neighbourhood to the fault-line
+(see p. 135). If so, the originating fault must have extended from a
+point about a mile and a half west of Hereford for a distance of about
+16 miles to the south-east; and a fault in this position would
+certainly satisfy all the details of the seismic evidence.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>Throughout the disturbed area, considerable variations were observed
+in the nature of the shock. These changes were due to the mere size of
+the focus, to its elongated form and, as will be seen, to its
+discontinuity, and also to the distance of the place of observation
+from the epicentre.</p>
+
+<p>At places near the epicentre, rapid changes in the <span class='pagenum'><a name="Page_221" id="Page_221">[221]</a></span>direction of the
+shock were observed owing to the large angle subtended by the focus;
+while, at considerable distances, this angle being small, the changes
+of direction were imperceptible. A further variation with the distance
+was an increase in the period of the vibrations. Close to the
+epicentre, the general impression was that of crossing the wake of a
+steamer in a very short rowing-boat, or of riding in a carriage
+without springs. At distances of a hundred miles or more, the movement
+is described as being of a pleasant, gentle, undulating character,
+like that felt during the rocking of a ship at anchor or in a carriage
+with well-appointed springs.</p>
+
+<p>The most remarkable feature of the shock, however, was its division
+into two distinct parts or series of vibrations, separated by an
+interval, lasting two or three seconds, of absolute rest and quiet.
+And this was no mere local phenomenon. With the exception of a narrow
+band that will be referred to presently, records of the double shock
+come from nearly all parts of the disturbed area, even from districts
+so remote as the Isle of Man and the east of Ireland. The two parts
+differed in intensity, in duration, and in the period of their
+constituent vibrations. For instance, at Oaklands (near Chard), a
+shivering motion was first felt, and then, after about three or four
+seconds, a distinct rocking from side to side. At Exeter, there was a
+sudden tremor lasting about two seconds, followed, after two or three
+seconds, by another and more severe shaking lasting four or five
+seconds. Again, at West Cross (near Swansea), an undulatory movement
+for about four seconds was followed soon after by a tremulous shock.
+At Liverpool, the durations of the first part, interval, and <span class='pagenum'><a name="Page_222" id="Page_222">[222]</a></span>second
+part were respectively estimated at about six, two, and four seconds.</p>
+
+<p>As a first result of the observations, then, it appears that in the
+south-east half of the disturbed area, the second part of the shock
+was the stronger, of greater duration and consisted of longer-period
+vibrations (as at <i>a</i>, Fig. 61); while, in the north-west half, the
+same features characterised the first part of the shock (<i>b</i>, Fig.
+61). A closer examination of the records shows, however, that the
+boundary between the two portions of the disturbed area was not a
+straight line, but slightly curved, the concavity facing the
+south-east. The broken line on the map (Fig. 60), which is hyperbolic
+in form, represents roughly the position of this curved boundary.<a name="FNanchor_64_64" id="FNanchor_64_64"></a><a href="#Footnote_64_64" class="fnanchor">[64]</a></p>
+
+<div class="img"><a name="Fig61" id="Fig61"></a>
+<a href="images/fig61.png">
+<img border="0" src="images/fig61.png" width="70%" alt="Nature of shock of Hereford earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 61.&mdash;Nature of shock of Hereford earthquake.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Along this hyperbolic boundary-line, or rather within a narrow band of
+which it is the central line, the shock lost its double character, and
+was manifested as a single series of vibrations gradually <span class='pagenum'><a name="Page_223" id="Page_223">[223]</a></span>increasing
+in intensity and then dying away. Close to the edges of this band,
+careful observers were able to distinguish two maxima of intensity
+connected by a continuous series of tremors (<i>c</i>, Fig. 61). Thus,
+within the band, the two series of vibrations, which elsewhere were
+isolated, must have been superposed on one another; while, near the
+edges of the band, the concluding tremors of the first series
+overlapped the initial tremors of the second.</p>
+
+<p><i>Origin of the Double Series of Vibrations.</i>&mdash;The Hereford earthquake
+thus belongs to the same class as the Neapolitan, Andalusian,
+Charleston, and Riviera earthquakes. As in these cases, the hypothesis
+of a single focus is inadmissible. The division of the disturbed area
+into two regions of opposite relative intensity, duration, etc., is
+sufficient proof that a single series of vibrations was not duplicated
+by reflection or refraction, or by separation into longitudinal and
+transverse waves. It is equally conclusive against a repetition of the
+impulse within the same focus. We must therefore infer that the focus
+consisted of two nearly or quite detached portions arranged along a
+north-west and south-east line, and that the impulse at the north-west
+focus was the stronger of the two. The only question that remains to
+be decided is whether the impulses at the two foci were simultaneous
+or not.</p>
+
+<p>Now, if the impulses occurred at the same instant, the waves from the
+two foci would travel with the same velocity, and would therefore
+coalesce along a straight band which would bisect at right angles the
+line joining the two epicentres. But we have already seen that this
+band is curved, and it thus follows that the two impulses were not
+simultaneous. Again, <span class='pagenum'><a name="Page_224" id="Page_224">[224]</a></span>since the concavity of the hyperbolic band faces
+the south-east, the waves from the north-west focus must have
+travelled farther than those from the south-east focus before the two
+met along the hyperbolic band; in other words, the impulse at the
+north-west focus must have occurred two or three seconds before the
+impulse at the other.</p>
+
+<p><i>Position and Dimensions of the Two Foci.</i>&mdash;There can be little doubt
+that the impulse at the north-west focus was responsible for the
+greater damage to buildings at Hereford, Dinedor, Fownhope, etc. The
+centre of its epicentral area must therefore lie about three miles
+south-east of Hereford. It is probable, also, that the corresponding
+centre of the other focus is similarly placed with respect to the
+south-east portion of the isoseismal 8&mdash;that is, about two or three
+miles north-east of Ross. These two points are eight or nine miles
+apart. Now, since, as we shall see, the mean surface-velocity of the
+earth-waves was about 3000 feet per second, and the mean duration of
+the quiet interval between the two series was 3&frac12; seconds, the
+nearest ends of the two foci must have been separated by a distance of
+not less than two miles. Moreover, since the series of vibrations from
+the north-west or Hereford focus lasted a few seconds longer than that
+from the south-east or Ross focus, the former must have been about two
+miles longer than the latter, and we may therefore estimate their
+lengths at about eight and six miles respectively. Including the
+undisturbed intermediate portion, this would give a total length of
+focus of about 16 miles, a result we have already inferred from the
+dimensions of the isoseismal 8.</p>
+
+<span class='pagenum'><a name="Page_225" id="Page_225">[225]</a></span><br />
+
+<h4>DIRECTION OF THE SHOCK.</h4>
+
+<p>Although no question was asked with regard to the direction of the
+shock, no fewer than 469 observers made notes on this point. As a
+general rule, their determinations are extremely rough, few referring
+to more than the eight principal points of the compass. Moreover, in
+any one place, the directions assigned to the shock are very varied.
+For instance, in the city and suburbs of Birmingham, eight observers
+give the direction along a north and south line, eight east and west,
+eleven north-west and south-east, and five north-east and south-west,
+while there are five other intermediate estimates. But, when these
+directions are plotted on a map of the district, it is seen at once
+that they are either nearly parallel or perpendicular to the roads in
+which the observers were living; that is, the apparent direction of
+the shock was at right angles to one of the principal walls of the
+house. This, of course, is a result to be anticipated, for, whatever
+be the direction of the earthquake-motion, a house tends to oscillate
+in a plane perpendicular to one or other of its walls.</p>
+
+<p>It is extraordinary to how great a distance the direction of the shock
+is perceptible. Records come from Brighton (137 miles from the
+epicentre), Maldon in Essex (144 miles), Harrogate (147 miles),
+Douglas in the Isle of Man (167 miles), Dublin (176 miles), and
+Baltinglass in Co. Wicklow (180 miles).</p>
+
+<p>Nevertheless, whatever the distance may be, the sense of direction
+must be most perceptible in those houses whose principal walls are at
+right angles <span class='pagenum'><a name="Page_226" id="Page_226">[226]</a></span>to the true direction of the earthquake-motion, and we
+should therefore expect to find the observations of direction most
+frequently made in such houses, or in others which approximate to this
+situation. Thus, the average of all the observations within a fairly
+small area should give a result not very far from the true direction
+of the shock; and, the smaller the area and the farther from the
+epicentre, the more reliable should be the result. Now, in Birmingham
+the mean direction of the shock is E. 39&deg; degrees N., which differs
+only by 2&deg; degrees from the line joining the city to the epicentre; in
+London it is E. 21&deg; degrees S., the difference being again 2&deg; degrees.
+In other cases, the observations from different counties are grouped
+together, and the mean direction is taken to correspond to the centre
+of the county. Yet, even then, there is often a close agreement
+between the mean direction of the shock and the direction of the
+county-centre from the epicentre; the difference being not more than
+two or three degrees in the counties of Buckingham, Devon, Stafford,
+Warwick, and York. In other cases, where the deviation exceeds this
+amount, either the number of observations is small or the county is
+near the epicentre and so subtends a large angle.</p>
+
+<p>Two results of some importance follow from this analysis: (1) that
+while, with a few isolated observations, the "method of directions" is
+almost sure to fail, with a large number of observations closely
+grouped, the position of the epicentre may be determined with a fair
+approach to accuracy; and (2) that, at any rate outside a radius of
+forty miles, the earth-waves travelled in approximately straight lines
+outwards from the epicentre.</p>
+
+<span class='pagenum'><a name="Page_227" id="Page_227">[227]</a></span><br />
+
+<h4>COSEISMAL LINES AND VELOCITY OF EARTH-WAVES.</h4>
+
+<p>Coseismal lines were defined by Mallet as long ago as 1849, but, owing
+to the difficulty of ascertaining the correct time, they have so far
+been of little service in the investigation of earthquakes. In the
+case of the Hereford earthquake, the distances traversed by the
+earth-waves are small; but, on the other hand, the time-records are
+numerous and frequently trustworthy to the nearest minute. Rejecting
+all estimates earlier than 5.32 <span class="fakesc">A.M.</span>, and later than 5.36, as
+well as a number at 5.35, there remain fairly good observations from
+381 places, and exceptionally accurate ones from 33 places. The latter
+were obtained from signalmen and other careful observers who were in
+possession of Greenwich time, or who compared their watches shortly
+afterwards with well-regulated watches.</p>
+
+<p>With evidence so abundant, a new method of drawing coseismal lines
+becomes possible. According to this method, each place of observation
+is indicated on the map by a mark corresponding to the particular
+minute recorded. If the records were quite correct, there would be a
+central area occupied by the marks corresponding to 5.32
+<span class="fakesc">A.M.</span>, surrounded by a series of zones in which the times were
+respectively 5.33, 5.34, and 5.35. The curves separating these zones
+would be coseismal lines corresponding to the times 5.32&frac12;,
+5.33&frac12;, and 5.34&frac12;.</p>
+
+<p>Owing, however, to the inevitable inaccuracy of all the time-records,
+these different zones intrude on one another, and the coseismal lines
+have therefore to be drawn about half-way through the <span class='pagenum'><a name="Page_228" id="Page_228">[228]</a></span>overlapping
+regions, special weight being attributed to the apparently more
+accurate observations.</p>
+
+<div class="img"><a name="Fig62" id="Fig62"></a>
+<a href="images/fig62.png">
+<img border="0" src="images/fig62.png" width="55%" alt="Coseismal lines of the Hereford earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 62.&mdash;Coseismal lines of the Hereford earthquake. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The coseismal lines obtained in this manner are represented by the
+continuous curves in Fig. 62. The isoseismals, which are added for the
+sake of comparison, are indicated by the dotted lines. It will be seen
+that the coseismal lines are elongated in the same direction as the
+isoseismals, but to a less extent, and this no doubt is due to the
+fact that the epoch selected by the majority of observers was <span class='pagenum'><a name="Page_229" id="Page_229">[229]</a></span>one not
+far from, and slightly preceding, that of the maximum intensity of
+the shock.</p>
+
+<p>Now, the average distance between the two inner coseismals is 32&frac34;
+miles, between the two outer ones (so far as drawn) 35-<span style="font-size: 90%;">1/6</span> miles, and
+between the first and third 67-<span style="font-size: 90%;">1/6</span> miles. The mean surface-velocity
+between the two inner coseismals is therefore 2,882 feet per second,
+and between the two outer ones 3,095 feet per second. There is thus an
+apparent increase in the velocity with the distance, but the accuracy
+of the coseismal lines is unequal to establishing this as a fact. The
+mean surface-velocity of 2,955 feet per second between the first and
+third coseismals is probably, however, the most accurate estimate of
+the surface-velocity yet made in a slight earthquake.</p>
+
+<br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p><i>Nature of the Sound.</i>&mdash;The sound which accompanied the shock was of
+the same character as that heard during all great earthquakes. It is
+often described in such terms as a deep booming noise, a dull heavy
+rumble, a grating roaring noise, or a deep groan or moan; more rarely
+as a rustling or a loud hissing rushing sound. As a rule, it began
+faintly, increased gradually in strength, and then as gradually died
+away; and this no doubt is the reason why it sometimes appeared as if
+an underground train or waggon were approaching quickly, rushing
+beneath the observer, and then receding in the opposite direction.
+Occasionally, the sound was very loud, being compared to the noise of
+many traction-engines heavily laden passing close at hand, <span class='pagenum'><a name="Page_230" id="Page_230">[230]</a></span>or to a
+heavy crash or peal of thunder. But its chief characteristic was its
+extraordinary depth, as if it were almost too low to be heard.
+According to one observer, it was a low rumbling sound, much lower
+than the lowest thunder; and another compared it to the pedal notes of
+a great organ, only of a deeper pitch than can be taken in by the
+human ear, a noise more <i>felt</i> than heard. It will be seen presently
+how the sound, from its very depth, was inaudible to many persons.</p>
+
+<p>A few observers described the sound in terms like those quoted above,
+but by far the larger number compared it to some more or less
+well-known type, and in many cases the resemblance was so close that
+the observer at first attributed it to the object of comparison. The
+descriptions, which present great varieties in detail, may be
+classified as follows: (1) One or several traction-engines passing,
+either alone or heavily laden, sometimes driven furiously past; a
+steam-roller passing over frozen ground or at a quicker pace than
+usual; heavy waggons driven over stone paving, on a hard or frosty
+road, in a covered way or narrow street, or over hollow ground or a
+bridge; express or heavy goods trains rushing through a tunnel or deep
+cutting, crossing a wooden bridge or iron viaduct, or a heavy train
+running on snow; the grating of a vessel over rocks, or the rolling of
+a lawn by an extremely heavy roller; (2) a loud clap or heavy peal of
+thunder, sometimes dull, muffled or subdued, but most often distant
+thunder; (3) a moaning, roaring, or rough, strong wind; the rising of
+the wind, a heavy wind pressing against the house; the howling of wind
+in a chimney, a chimney or <span class='pagenum'><a name="Page_231" id="Page_231">[231]</a></span>oil-factory on fire; (4) the tipping of a
+load of coal, stones, or bricks, a wall or roof falling, or the crash
+of a chimney through the roof; (5) the fall of a heavy weight or tree,
+the banging of a door, only more muffled, and the blow of a wave on
+the sea-shore; (6) the explosion of a boiler or cartridge of dynamite,
+a distant colliery explosion, distant heavy rock-blasting and the boom
+of a distant cannon; (7) sounds of a miscellaneous character, such as
+the trampling of many men or animals, an immense covey of partridges
+on the wing, the roar of a waterfall, the passage of a party of
+skaters, and the rending and settling together of huge masses of rock.</p>
+
+<p>The total number of comparisons made was 1,264. Of these, 45.4 per
+cent. refer to passing waggons, etc., 15.0 per cent. to thunder, 15.5
+to wind, 3.9 to loads of stones falling, 2.7 to the fall of a heavy
+body, 7.2 to explosions, and 10.3 per cent. to miscellaneous sounds.</p>
+
+<p>Generally, the sound adhered throughout to one of the types mentioned
+above, and, if it varied at all, varied only in intensity. At some
+places, however, the character of the sound was observed to change.
+For instance, one person described it as like the rumbling of a train
+going over a bridge, with a terrific crash, such as is heard in a
+thunderstorm at the instant when the shock was strongest, the rumbling
+dying away afterwards for some seconds.</p>
+
+<p><i>Inaudibility of the Sound to some Observers.</i>&mdash;The total number of
+observers who give a detailed account of the earthquake is 2,681, and,
+of these, 59 per cent. state that they heard the sound, 23 per <span class='pagenum'><a name="Page_232" id="Page_232">[232]</a></span>cent.
+give no information, while 18 per cent. distinctly say that they
+heard no sound; that is, roughly, out of every five observers, three
+heard the sound, one made no reference to it, and one failed to hear
+the sound.</p>
+
+<p>In a few cases, no doubt, this failure was due to the distance of the
+observer, but this is far from being a complete explanation; for, in
+Herefordshire, six out of 179, and in Gloucestershire 17 out of 227,
+observers heard no sound. Nor is the peculiarity a local one, for at
+Clifton two out of five observers who were awake did not hear the
+sound, at Birmingham four out of 23, and in London, eight out of 18.
+Even in the same house, it would happen that one observer would hear a
+sound as of a heavily-laden traction-engine passing, while to another
+it was quite inaudible.</p>
+
+<p>Again, a large number of observers who heard the sound expressly state
+that they were unconscious of any while the shock lasted. The noise at
+first resembled the approach of a steam-roller or traction-engine up
+the street, it became gradually louder, and then ceased more or less
+suddenly as the shock began; while, to others in the same places, the
+sound continued to grow in loudness until the strongest vibrations
+were felt.</p>
+
+<p>Even when observers in the same place agreed in hearing the sound, it
+presented itself to them under different aspects. Thus, at Hereford, a
+crash or bomb-like explosion was noticed by some, but not by all,
+observers; at Ledbury, the sound according to one began like a rushing
+wind and culminated in a loud explosive report, another heard a noise
+like distant thunder, which ended when the shock began, while a third
+heard no sound at all. At places more <span class='pagenum'><a name="Page_233" id="Page_233">[233]</a></span>distant from the epicentre, the
+same diversity, both in character and intensity, is manifested. Thus,
+at Birmingham, the accounts refer on the one hand to the distant
+approach of a train and the rising of the wind, on the other to the
+reports of large cannons and to a noise as if tons of <i>d&eacute;bris</i> had
+been hurled against the wall of the house; at Bangor, to muffled
+thunder, wind through trees, and a loud rumbling sound.</p>
+
+<p>The first explanation of these apparent anomalies which presents
+itself is inattention on the part of the observers; but it is one that
+will not bear examination, though it may apply in some cases. The
+sound is too loud, at any rate near the epicentre, to escape notice,
+and it is generally heard before the shock begins to be felt.
+Moreover, as described in the last chapter, three out of every four
+earthquakes in Japan are unaccompanied by recorded sound, and the
+Japanese as a race cannot be accused of such constant inattention. The
+defect, it can hardly be doubted, is inherent to the observer, and not
+dependent on the conditions in which he is placed.</p>
+
+<p>That the higher limit of audibility varies with different persons has
+long been known; and there can be no reason for doubting that there is
+a similar variability in the lower limit. Thus, to some observers, the
+sound remains inaudible throughout, however intently they may be
+listening. Again, it is found that, the deeper the sound, the greater
+must be the strength of the vibrations required to render them
+audible. As the vibrations which reach an observer increase in period,
+it may therefore happen that, sooner or later, the strength of some
+does not attain or exceed that limiting value, and, at that moment,
+<span class='pagenum'><a name="Page_234" id="Page_234">[234]</a></span>the sound will cease to be heard. Moreover, for vibrations of a given
+period, this limiting value varies for different persons. Thus, to one
+observer, the sound may become inaudible, while another may continue
+to hear it. Lastly, the vibrations which affect an observer at any
+moment are of various strength and period. One may hear all perhaps,
+while a second may be able to hear some and not others. Thus, to one
+observer, the sound may be like a rising wind, to another like a heavy
+traction-engine passing; one may hear the crashes which accompanied
+the strongest part of the shock, while a second may be deaf to the
+same vibrations; to one the sound may become continually louder and
+cease abruptly, to another it may increase to a maximum and then die
+away.</p>
+
+<p><i>Sound-Area.</i>&mdash;While the sound was a very prominent feature of the
+earthquake in and near the epicentral area, records at a great
+distance are naturally difficult to obtain, and, on this account, the
+number of stations for determining the boundary of the sound-area is
+too small to allow of it being accurately drawn. As a rule, however,
+it must lie between the isoseismals 5 and 4, but it is less nearly
+circular than either of these lines. Its length, from north-west to
+south-east, is 320 miles, its breadth 284 miles, and the area
+contained by it about 70,000 square miles, or roughly two-thirds that
+of the disturbed area.</p>
+
+<p><i>Isacoustic Lines.</i>&mdash;The dotted lines in Fig. 60 represent isacoustic
+lines&mdash;that is, lines which pass through all places where the
+percentage of observers who recorded their perception of the sound is
+the same. For instance, if we take any point in the line <span class='pagenum'><a name="Page_235" id="Page_235">[235]</a></span>marked 80 and
+describe a small circle with that point as centre, then 80 per cent.
+of the observers within that circle would hear the earthquake-sound.
+The isacoustic lines thus show how the audibility of the sound varies
+throughout the sound area. To draw the curves with a close approach to
+accuracy, the unit of area should be small and of constant dimensions;
+but, in the present case, owing to the comparative paucity of the
+observations, a smaller unit than the county would give unreliable
+results.<a name="FNanchor_65_65" id="FNanchor_65_65"></a><a href="#Footnote_65_65" class="fnanchor">[65]</a> At the centre of each county, the sound audibility may be
+regarded as proportional to the percentage of the total number of
+observers within the county who distinctly heard the sound. To draw
+the curve marked 50, the centre of every county in which the average
+percentage is less than 50 is joined to the centres of those adjoining
+counties in which it is above 50, and these lines are then divided in
+the proper ratio so as to give a point where the percentage would be
+exactly 50. A number of points at which the percentage is 50 is thus
+obtained, and the curve drawn through them is the required isacoustic
+line. The percentage of audibility varies from 87 in Herefordshire to
+23 in Essex and the east of Ireland, but the only isacoustic lines
+which can be completely drawn are those that correspond to the
+percentages between 80 and 50 inclusive.</p>
+
+<p>The peculiar form of the isacoustic lines will be evident at a glance.
+They bear little relation to the isoseismal lines. Their greatest
+extensions are not along the axes of those lines, but in two
+directions which are a little east of north-east and south of
+<span class='pagenum'><a name="Page_236" id="Page_236">[236]</a></span>south-west. They lie indeed along a hyberbolic line which, towards
+the south-west, agrees closely with the curvilinear axis of the
+hyperbolic band represented by the broken line in Fig. 60. Towards the
+north-east, the coincidence is not so close, but this is chiefly owing
+to the magnitude of the northern counties, which causes a deflection
+of the isacoustic lines towards the north.</p>
+
+<p>It will be remembered that the hyperbolic band is the area within
+which the vibrations from the two foci were superposed. Now, the sound
+accompanied each part of the shock, and ceased entirely during the
+interval between them. Also, the stronger series of vibrations was
+accompanied by the louder sound; but, while the difference in strength
+was considerable between the two parts of the shock, it was very
+slight between the two sounds. There is therefore no marked distortion
+of the isoseismal lines when crossing the hyperbolic band, while the
+isacoustic lines are completely diverted from their normal course.</p>
+
+<p>Thus, the study of the isacoustic lines strongly confirms the
+conclusions at which we have arrived above (p. 223)&mdash;namely, that
+there were two distinct foci arranged in a north-west and south-east
+line, and that the impulse at the former focus occurred a few seconds
+earlier than that at the latter.<a name="FNanchor_66_66" id="FNanchor_66_66"></a><a href="#Footnote_66_66" class="fnanchor">[66]</a></p>
+
+<p><span class='pagenum'><a name="Page_237" id="Page_237">[237]</a></span><i>Variations in the Nature of the Sound throughout the Sound-area.</i>&mdash;In
+one respect, the sound exhibited a marked uniformity all over the
+sound-area&mdash;namely, in its great depth; the word "heavy" being used in
+one out of every four accounts of the sound, whether close to the
+epicentre or near the boundary of the sound-area.</p>
+
+<p>The type of comparison employed varies in different parts of the
+sound-area. As we recede from the origin, the sound becomes on the
+average less like thunder or explosions and more like wind. The
+references to passing waggons, etc., are so numerous that it is
+possible to draw curves, in the same way as isacoustic lines, which
+represent equal percentages of comparison to this type out of the
+total number of comparisons. The curves are somewhat incomplete, but
+it is noteworthy that those corresponding to the higher percentages
+cling to the extremities of the hyperbolic band, probably because the
+uninterrupted duration of the sound is greater there than elsewhere.</p>
+
+<p>The effect of distance from the epicentre, however, is most noticeable
+in connection with changes in the character of the sound. It is only
+on the immediate neighbourhood of the origin that the explosive
+reports or crashes were heard in the midst of the rumbling sound. At a
+moderate distance, the sound before and after the shock became
+smoother, while the sound which accompanied the shock retained to a
+certain extent its rougher and more rumbling or grating character.
+Close to the boundary of the sound-area, the irregularities were still
+further smoothed away, and the only sound heard was like the low roll
+of distant thunder.</p>
+
+<p>The explanation of these changes depends on the <span class='pagenum'><a name="Page_238" id="Page_238">[238]</a></span>fact that, as we
+recede from the epicentre, the vibrations of every period tend to
+become inaudible. The limiting vibrations of the whole series will be
+the first to be lost, especially those of the longest period. Thus,
+near the epicentre, sound-vibrations of many different periods will be
+heard, and the sound will be more complex than it is elsewhere. The
+greater the distance, the narrower are the limits with regard to
+period between which the audible vibrations lie, until, near the
+boundary of the sound-area, the sound becomes an almost monotonous
+deep growl of nearly uniform intensity.</p>
+
+<p><i>Time-relations of the Sound and Shock.</i>&mdash;The principal epochs to be
+compared are the beginning, the epoch of maximum intensity, and the
+end. The beginning of the sound preceded that of the shock in 82 per
+cent. of the observations on this epoch, coincided with it in 12, and
+followed it in 6 per cent.; the epoch of maximum intensity preceded
+that of the shock in 21 per cent. of the records, coincided with it in
+73, and followed it in 6 per cent.; while the end of the sound
+preceded that of the shock in 22&frac12; per cent., coincided with it in
+27&frac12;, and followed it in 50 per cent. Thus, as a general rule, the
+beginning of the sound preceded that of the shock, the sound was
+loudest when the shock was strongest, and the end of the sound
+followed that of the shock. In other words, the duration of the sound
+was in most cases greater than that of the shock.</p>
+
+<br />
+
+<h4>MINOR EARTHQUAKES.</h4>
+
+<p>Of the twelve undoubted minor earthquakes, nine occurred before, and
+three after, the principal shock, the times of the first eleven lying
+between limits <span class='pagenum'><a name="Page_239" id="Page_239">[239]</a></span>about seven hours apart. With three exceptions, the
+records are insufficient to determine the positions of the epicentre
+with any approach to exactness.</p>
+
+<p>The first occurred at about 11 or 11.30 <span class="fakesc">P.M.</span> on December
+16th. The boundary of the disturbed area, which coincides nearly with
+that of the fifth shock (E, Fig. 63), is 97 miles long from north-west
+to south-east, 83 miles wide, and contains about 6,300 square miles.
+The focus was apparently situated between the two foci of the
+principal earthquake and partly coincided with them.</p>
+
+<div class="img"><a name="Fig63" id="Fig63"></a>
+<a href="images/fig63.png">
+<img border="0" src="images/fig63.png" width="55%" alt="Map of minor shocks of Hereford earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 63.&mdash;Map of minor shocks of Hereford earthquake. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Then came three slight shocks (at about 1 <span class="fakesc">A.M.</span> on December
+17th, 1.30 or 1.45 <span class="fakesc">A.M.</span>, and 2 <span class="fakesc">A.M.</span>), about <span class='pagenum'><a name="Page_240" id="Page_240">[240]</a></span>which
+little is known except that they probably originated somewhere near
+the Ross focus.</p>
+
+<p>The fifth shock (E, Fig. 63) occurred at about 3 <span class="fakesc">A.M.</span>, and
+disturbed an area 104 miles in length, 79 miles in width, and about
+6,400 square miles in area. Its boundary occupies approximately the
+position that would be taken by an isoseismal of intensity between 7
+and 6 of the principal earthquake. We may therefore infer that this
+shock and the principal earthquake were caused by slips along the same
+fault and in about the same region of the fault. Also, as there is no
+evidence of discontinuity in the vibrations of the minor shock, it is
+probable that the focus was continuous, and occupied the space between
+the two foci of the principal earthquake, as well as part or the whole
+of both these foci.</p>
+
+<p>The next four shocks occurred at about 3.30, 4, 5, and 5.20
+<span class="fakesc">A.M.</span>, and were more closely associated with the Ross than
+with the Hereford focus, and then followed the principal earthquake at
+5.32 <span class="fakesc">A.M.</span></p>
+
+<p>A few minutes later, at 5.40 or 5.45 <span class="fakesc">A.M.</span>, a very slight
+shock was felt, the focus of which was possibly situated in the
+central region between the two foci. The next, at about 6.15
+<span class="fakesc">A.M.</span> (K, Fig. 63), disturbed an area 41 miles long, 27 miles
+broad, and containing about 870 square miles. Its focus must have
+coincided approximately with the Ross focus of the principal
+earthquake, and this was also the case probably with the last shock of
+all, which occurred on July 19th, 1897, at 3.49 <span class="fakesc">A.M.</span></p>
+
+<br />
+
+<h4>ORIGIN OF THE EARTHQUAKES.</h4>
+
+<p>The greater part of the epicentral district is covered by a sheet of
+Old Red Sandstone (Fig. 64), but, just to the <span class='pagenum'><a name="Page_241" id="Page_241">[241]</a></span>north-east of the
+position laid down for the originating fault (indicated by the
+straight broken line), is the well-known Woolhope anticlinal, by which
+Silurian beds are brought to the surface. The anticlinal axis runs
+approximately north-west and south-east, and is thus roughly parallel
+to the earthquake-fault. Moreover, the thinning-out and occasional
+disappearance of some of the Silurian beds on the south-west side of
+the anticlinal (as compared with those on the north-east side) is
+suggestive of a north-west and south-east fault or rapid flexure at or
+<span class='pagenum'><a name="Page_242" id="Page_242">[242]</a></span>near the south-west junction of the Old Red Sandstone and the
+Silurian strata. If it be a fault, it must hade to the north-east, and
+would therefore satisfy two of the conditions determined by the
+seismic evidence. It would lie, however, about two miles too far to
+the north-east, being in fact to the north-east of the villages which
+suffered most from the earthquake.</p>
+
+<div class="img"><a name="Fig64" id="Fig64"></a>
+<a href="images/fig64.jpg">
+<img border="0" src="images/fig64.jpg" width="60%" alt="Geology of meizoseismal area of Hereford earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 64.&mdash;Geology of meizoseismal area of Hereford earthquake. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>But only a few miles to the south-east of the Woolhope anticlinal, and
+almost in the same line with it, there is a second anticlinal, that of
+May Hill. This is a triangular area, and is known to be bounded on all
+three sides by faults. The fault on the north-east side has an average
+north-west and south-east direction, and, if it were continued through
+the Old Red Sandstone towards the north-west, but bending at first a
+few degrees more to the west, it would pass through a point about
+1&frac12; miles west of Hereford. It is worthy of notice that both this
+fault and another nearly parallel to it, about half-a-mile farther
+north-east, stop, according to the Geological Survey map, at the
+points where they enter the Old Red Sandstone. The latter is an area
+which has never been investigated with thoroughness by modern
+stratigraphical methods, and in which it is difficult to trace faults.
+It therefore appears not improbable that the earthquakes were due to
+slips along a continuation of this fault.</p>
+
+<p>Whether this be the case or not, however, it is clear that the
+earthquake-fault must pass between the anticlinal areas of Woolhope
+and May Hill, the former being on the north-east, and the latter on
+the south-west, side of the fault. At the Hereford focus, the fault
+must hade to the north-east; and, at the Ross focus, it is probable,
+from the distribution of places <span class='pagenum'><a name="Page_243" id="Page_243">[243]</a></span>where damage occurred to buildings,
+that it hades to the south-west If this be the case, the fault must
+change in hade between the two foci.</p>
+
+<p>How long a time had elapsed since the last sign of growth in the
+earthquake-fault took place, it is impossible to say; but it must be
+many years in length. During this interval, the stresses tending to
+produce movement along the fault-service had been gradually
+increasing, until they were sufficient to overcome the resistance
+opposed to them. It is worthy of notice that the earliest perceptible
+movements were slight. Their function seems to have been to prepare
+the way for the great slips by equalising the difference between
+stress and resistance over a large area of the fault-surface. We
+cannot trace with accuracy the transference of the seat of movement
+from one part of the fault-surface to another. The first slip seems to
+have taken place chiefly in the region between the two foci of the
+principal earthquake; possibly it overlapped both of them partly. The
+next three slips were apparently in the neighbourhood of the Ross
+focus, and were followed by a fifth in the same area as the first.
+Then came a series of small movements that we cannot locate further
+than by saying that they were more closely connected with the Ross
+focus than the other.</p>
+
+<p>In consequence of the preliminary slips within and near the Ross
+focus, the effective stress in that portion of the fault was
+diminished; and this may be the reason why the first great slip took
+place at the Hereford focus. The immediate result of such a movement
+would naturally be an increase of stress in and beyond the terminal
+regions, and the next slip might have been expected in an area partly
+<span class='pagenum'><a name="Page_244" id="Page_244">[244]</a></span>overlapping the Hereford focus, and either to the north-west or
+south-east of it. Instead of this, for a distance of two miles in the
+latter direction, there was not the least perceptible movement during
+the principal earthquake, and the second great slip occurred in the
+region beyond occupied by the Ross focus. This second slip, moreover,
+occurred within two or three seconds after the other; that is, before
+the earth-waves had time to travel from the Hereford to the Ross
+focus. In other words, the slip at the Ross focus was not a
+consequence of the slip at the Hereford focus; but both were due to a
+single generative effort.</p>
+
+<p>Now, a section drawn parallel to the earthquake-fault and on the
+north-east side of it, would show an anticline near the Hereford focus
+and a corresponding syncline near the Ross focus, with an undisplaced
+portion in the intermediate region; while a parallel section on the
+other side of the fault would show a syncline near the Hereford focus,
+an anticline near the Ross focus, and again an undisplaced portion in
+the intermediate region. If further movements tending to accentuate
+such a structure were to occur (that is, if the anticlinals were to be
+made more anticlinal and the synclines more synclinal), there would
+therefore be two slips, one in each focus; while, along the
+fault-surface between, there would be practically no displacement. At
+any rate, the earlier stresses in that region may have been fully
+relieved by two slight preliminary slips (those causing the first and
+fifth minor earthquakes), and those resulting from the great
+displacements by the first after-slip which followed in about ten
+minutes.</p>
+
+<p>Half-an-hour later, another slip took place at the <span class='pagenum'><a name="Page_245" id="Page_245">[245]</a></span>Ross focus, and by
+this the equilibrium of the rock-masses was almost completely
+restored; for we have no certain evidence of any further movements
+until seven months have elapsed (July 19th, 1897), when there was a
+final slip in the same region of the fault.</p>
+
+<br />
+
+<h4>THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901.</h4>
+
+<p>Between the north-east end of Loch Ness and the Moray Firth at
+Inverness, there lies a tract of land not more than seven miles in
+length, which is notable as one of those most frequently shaken by
+earthquakes in the British Islands. In the intensity of its shocks it
+is inferior to the south-east of Essex and the centre of
+Herefordshire, and, in mere number, to the celebrated village of
+Comrie in Perthshire. But, in the interest of its seismic phenomena,
+in the light which they cast on the development of the earth's crust,
+the neighbourhood of Inverness has no equal in Great Britain, and not
+many superiors in any part of the world.</p>
+
+<p>For this importance from a seismological point of view, the district
+is indebted to the great fault which traverses Scotland along the line
+of the Caledonian Canal, and to the fact that this fault, although it
+dates from Old Red Sandstone times, has not yet finished growing. As
+results of its formation, we have the almost straight cliff along the
+south-east coast of Rossshire, and the long chain of lakes, beginning
+with Loch Dochfour and Loch Ness, and ending with Loch Oich, Loch
+Lochy, and Loch Linnhe. As evidences of its persistent though
+intermittent growth, we have the slight tremors and earth-sounds
+<span class='pagenum'><a name="Page_246" id="Page_246">[246]</a></span>occasionally observed at and near Fort William, and the much stronger
+shocks felt in the neighbourhood of Inverness.</p>
+
+<p>During the nineteenth century there were three strong earthquake
+shocks in this district. The first and most severe occurred on August
+13th, 1816, and was felt over the greater part of Scotland; the second
+on February 2nd, 1888; and the third and weakest on November 15th,
+1890. This last shock was followed by several slighter ones, the
+series ending with a rather smart shock on December 14th. Between this
+date and the summer of 1901 no earthquakes seem to have been felt at
+or anywhere near Inverness.</p>
+
+<br />
+
+<h4>PREPARATORY SHOCKS.</h4>
+
+<p>The date of the first shock of 1901 is not quite certain. One is said
+to have been felt at Aldourie (see Fig. 66) some time in June, and a
+second at Dochgarroch in July. These may have been succeeded by others
+too slight to attract much notice, but the first to be generally
+observed occurred on September 16th at 6.4 <span class="fakesc">P.M.</span> A weak
+tremor, accompanied by a faint sound, was perceived over a nearly
+circular area about 12 miles in diameter, and with its centre about
+1&frac12; miles south of Dochgarroch. On the next day, at 11
+<span class="fakesc">P.M.</span>, a quivering lasting two seconds was felt at Inverness,
+and a weak tremor, accompanied by sound, at Dochgarroch at 1.15
+<span class="fakesc">A.M.</span> on September 18th. Nine minutes later, at 1.24
+<span class="fakesc">A.M.</span>, occurred the principal earthquake, the shock of which
+would be called a strong one, even in Italy and Japan.</p>
+
+<span class='pagenum'><a name="Page_247" id="Page_247">[247]</a></span><br />
+
+<h4>EFFECTS OF THE SHOCK.</h4>
+
+<p>In Inverness, the damage to buildings, though seldom serious, was by
+no means inconsiderable. One brick building used as a smithy was
+destroyed, several chimneys or parts of them fell, and many
+chimney-cans were displaced or overthrown. At Dochgarroch and other
+places within the meizoseismal area, walls were cracked, chimneys
+thrown down, and lintels loosened.</p>
+
+<p>But, for this country, an unusual effect of the earthquake was a long
+crack made in the north bank of the Caledonian Canal near Dochgarroch
+Lochs. It occurred in the middle of the towing-path, and could be
+traced at intervals for a distance of 200 yards to the east of the
+Lochs, and 400 yards to the west, being often a mere thread, and in no
+place more than half-an-inch wide. Soon after its formation, however,
+the fissure was obliterated by heavy showers of rain.</p>
+
+<br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>The map (Fig. 65) shows the area over which the earthquake was
+perceptible. The isoseismal lines are drawn partly continuous and
+partly dotted&mdash;continuous where some confidence can be placed in their
+accuracy, and dotted where their course must be regarded as doubtful,
+owing to the rarity or absence of observations.</p>
+
+<p>The innermost isoseismal (shown on a larger scale in Fig. 66)
+corresponds to the intensity 8 of the Rossi-Forel scale, and includes
+the places where the shock was strong enough to cause slight
+structural damage to buildings. It is elliptical in form, 12 miles
+long, <span class='pagenum'><a name="Page_248" id="Page_248">[248]</a></span>7 miles broad, and 67 square mile in area, with its centre at a
+point about 1&frac12; mile east-north-east of Dochgarroch, and its longer
+axis running N. 33&deg; E. and S. 33&deg; W.</p>
+
+<div class="img"><a name="Fig65" id="Fig65"></a>
+<a href="images/fig65.png">
+<img border="0" src="images/fig65.png" width="55%" alt="Isoseismal lines of the Inverness earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 65.&mdash;Isoseismal lines of the Inverness earthquake. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The remaining isoseismals are less accurately drawn, owing to the
+scarcity of observations made in the west of Scotland. Except towards
+the west, however, the course laid down for the isoseismal 7 <span class='pagenum'><a name="Page_249" id="Page_249">[249]</a></span>may be
+trusted. Its length is 53&frac12; miles, width 35 miles, and area 1,500
+square miles. Its longer axis is almost exactly parallel to that of
+the preceding isoseismal, but the distance between the two curves is 9
+miles on the north-west, and 14 miles on the south-east, side. The
+isoseismal 6 is 105 miles long, 87 miles wide, and contains 7,300
+square miles; and the isoseismal 5, 157 miles long, 143 miles wide,
+and about 17,000 square miles in area.</p>
+
+<p>The isoseismal 4 may be regarded as the boundary of the disturbed area
+of the earthquake, for, so far as known, the shock was not noticed at
+any point outside it. Towards the north, it was felt at Wick,
+Castletown, and other intermediate places; towards the west at
+Tobermory in the island of Mull; and, towards the south, at Skelmorlie
+(in Ayrshire), Paisley, Belsyde (near Linlithgow), Gullane (near North
+Berwick), and Dunbar. Along the east coast of Scotland, between Wick
+and Dunbar, there are few places of any size where the shock was not
+felt. The disturbed area of the earthquake is thus 215 miles long from
+north-east to south-west, 198 miles wide, and contains about 33,000
+square miles.</p>
+
+<p><i>Position of the Originating Fault.</i>&mdash;The only isoseismals which are
+drawn accurately enough to determine the earthquake-fault are the two
+inner ones, those marked 8 and 7; but these are sufficient for the
+purpose. It is clear, from the direction of their longer axes, that
+the average direction of the fault must be N. 33 degrees E. and S. 33
+degrees W. Again, the isoseismals are farther apart towards the
+south-east than towards the north-west, implying that the fault hades
+to the south-east. Lastly, as the intensity of the shock is greater on
+the side towards which the <span class='pagenum'><a name="Page_250" id="Page_250">[250]</a></span>fault hades, it follows that the fault-line
+must lie a short distance (about a mile or so) on the north-west side
+of the centre of the isoseismal 8.</p>
+
+<p>Now, the great fault alluded to above occupies almost exactly the
+position indicated by the seismic evidence. Its mean direction from
+Tarbat Ness to Loch Linnhe is N. 35&deg; E. and S. 35&deg; W., it hades to the
+south-east, and the fault-line passes through a point about
+three-quarters of a mile to the north-west of the centre of the
+isoseismal 8 (Fig. 66). There can be little doubt, therefore, that the
+earthquake was caused by a slip of this fault; and the evidence of the
+after-shocks, as will be seen, offers additional support to this
+conclusion.</p>
+
+<p>The region in which the slip took place may be determined roughly from
+the position and form of the innermost isoseismal. Its centre must
+have been close to the point marked A in Fig. 66, which corresponds to
+a point about 1&frac12; mile east-north-east of Dochgarroch. In a
+horizontal direction, its length must have been at least five or six
+miles; otherwise, the isoseismal 8 would have been less elongated. It
+must therefore have reached from about half-a-mile north-east of Loch
+Ness to about half-a-mile south-west of Inverness. Its width, measured
+along the dip of the fault-surface is unknown; but the small distance
+between the centre of the isoseismal and the fault-line shows that the
+principal movement took place at a depth which was probably under,
+rather than over, one mile.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>We come now to the evidence afforded by the nature of the shock, in
+which there was but little variation throughout the disturbed area. At
+<span class='pagenum'><a name="Page_251" id="Page_251">[251]</a></span>Inverness, a gentle movement was first felt, followed by an
+extraordinary quivering, which increased in force for two or three
+seconds, and then decreased for two or three seconds; just as the
+quivering was about to cease, there was a distinct lurch or heave,
+after which the vibration was much more severe than before and lasted
+several seconds longer than the first part of the shock. Dalarossie
+lies about fourteen miles south-east of Inverness, and here the first
+indication was a loud sound, as of an express train, coming from the
+east, rushing close to, and then under, the house; this lasted for a
+few seconds, and towards the end of it the house vibrated. Then
+succeeded an interval of quietness for about a second, followed by a
+terrific burst or crash, not unlike the crash of a loud thunder peal,
+of about two seconds' duration, during which the house distinctly
+heaved up once and then sank back. After another brief interval of
+quietness, there was a low rumble, like the sound of a dying peal of
+thunder.</p>
+
+<p>It will be noticed, in this account, that the two parts of the shock
+were no longer consecutive. There was a short interval of rest between
+them, the intermediate vibrations observed at Inverness being too weak
+to be felt at Dalarossie. Still farther away, the extinction became
+more marked. At Aberdeen, for instance, the shock consisted of two
+parts, the first a tremble, followed, after an interval of a few
+seconds, by a swinging movement of longer duration than the tremble.</p>
+
+<p>In all parts of the disturbed area, the shock maintained the same
+character of division into two parts, the second of which was of
+greater duration and intensity than the first and consisted of
+vibrations <span class='pagenum'><a name="Page_252" id="Page_252">[252]</a></span>of longer period. A phenomenon of such wide occurrence was
+clearly not due to local influences. It must have been caused by two
+separate initial impulses, the stronger succeeding the other after an
+interval of a few seconds and taking place in nearly the same region
+of the fault.<a name="FNanchor_67_67" id="FNanchor_67_67"></a><a href="#Footnote_67_67" class="fnanchor">[67]</a></p>
+
+<br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p>Outside the isoseismal 5, there are but few records of the
+earthquake-sound; but it was heard faintly at Skelmorlie (in
+Ayrshire), Belsyde (near Linlithgow), and Gullane (near North
+Berwick). Towards the north, it was not observed beyond Wick and
+Wathen (in Caithness). The boundary of the sound-area cannot be laid
+down with any approach to accuracy, but it must have included a
+district containing about 27,000 square miles.</p>
+
+<p>Throughout the whole disturbed area, 84 per cent. of the observers
+heard the sound. The percentage varies in different counties, from 93
+in Inverness-shire to 77 in the counties of Perth and Aberdeen; but
+the records in the more distant regions are too few to allow of the
+construction of isacoustic lines.</p>
+
+<p>In its character, the sound resembled that usually heard with strong
+earthquakes, 39 per cent. of the observers having compared it to
+passing waggons, traction-engines, etc., 25 per cent. to thunder, 14
+to wind, 8 to loads of stones falling, 3 to the fall of heavy bodies,
+4 to explosions or the firing of heavy <span class='pagenum'><a name="Page_253" id="Page_253">[253]</a></span>guns, and 7 per cent. to
+miscellaneous sounds. The intensity of the sound gradually diminished
+outwards from the epicentre, and most rapidly near the isoseismal 7,
+which abounds approximately the area in which the sound was very loud
+from that in which it was distinctly fainter, and also includes nearly
+all the places at which loud explosive crashes were heard with the
+strongest vibrations.</p>
+
+<p>In the time-relations of the sound and shock, the Inverness earthquake
+resembles the Hereford earthquake of 1896. The beginning of the sound
+preceded that of the shock in 72 per cent. of the records, coincided
+with it in 20, and followed it in 8 per cent.; the epoch of maximum
+intensity of the sound preceded that of the shock in 20 per cent. of
+the records, coincided with it in 73, and followed it in 7 per cent.;
+while the end of the sound preceded that of the shock in 15 per cent.
+of the records, coincided with it in 34, and followed it in 52 per
+cent.</p>
+
+<p>Somewhat similar proportions hold over the greater part of the
+disturbed area, the percentages being nearly the same in the counties
+of Inverness, Ross, Nairn, Elgin, Banff, and the most distant
+counties. But in Aberdeenshire an exception occurs, the three epochs
+of sound and shock in most cases coinciding with one another. The
+majority of the observations in this county come from the southern
+part, and the line joining this district to the epicentre is nearly
+perpendicular to the line of the earthquake-fault. This result has an
+important bearing on the origin of the sound-vibrations. For, if the
+general precedence of the sound with respect to the shock were due to
+its superior velocity, the percentage of records in which the
+beginning of the sound preceded that of <span class='pagenum'><a name="Page_254" id="Page_254">[254]</a></span>the shock would vary only with
+the distance, and not with the direction from the origin. Indeed,
+with increasing distance from the origin, this percentage should
+continually approach 100; while that in which the end of the sound
+followed that of the shock should diminish to zero. There is, however,
+no trace of either tendency, the sound being heard after the shock at
+places close to the boundary of the sound-area. On the other hand, it
+the sound-vibrations were to start simultaneously, or nearly so, from
+all parts of the focus, but especially from its marginal regions,
+then, in the greater part of the disturbed area, the sound would be
+heard both before and after the shock; for the lateral margins of the
+focus would be the portions nearest to, and farther from, most
+observers; while, at places near the line through the epicentre at
+right angles to the earthquake-fault, the three principal epochs of
+the sound and shock should approximately coincide.</p>
+
+<p>The inference that the sound-vibrations heard before and after the
+shock come from the margins of the focus is also supported by the
+observations on the relative duration of the sound and shock. If we
+take only those records which are free from doubt, in 78 per cent. of
+the total number, the duration of the sound was greater than that of
+the shock; while, in Aberdeenshire, according to 93 per cent. of the
+observers, the durations of sound and shock were equal.</p>
+
+<p>We may imagine, then, that the slip within the seismic focus would be
+greatest in a central region, and that it would die outwards in all
+directions towards the edges. The friction arising from the slipping
+in the central region would produce chiefly <span class='pagenum'><a name="Page_255" id="Page_255">[255]</a></span>the comparatively large
+oscillations that formed the perceptible shock; the evanescent creep
+within the marginal regions would produce the small and rapid
+vibrations that were sensible only as sound.</p>
+
+<br />
+
+<h4>ORIGIN OF THE EARTHQUAKE.</h4>
+
+<p>While the seismic evidence enables us to determine the
+surface-position and the horizontal dimensions of the seismic focus,
+it unfortunately throws no light whatever on a point of some
+importance&mdash;namely, the direction of the movement which caused the
+earthquake. We cannot infer from it whether it was the rock on the
+south-east or north-west side of the fault that slipped or whether
+both sides slipped at once; nor, if that point had been settled, do we
+know if the movement of the displaced side was upward or downward. In
+the formation of the fault, however, it is clear that either the
+south-east side has been depressed or the north-west side elevated;
+and, as the bed of Loch Ness is below the level of the sea, that the
+former movement has predominated. If the displacements which gave rise
+to the earthquake were merely a continuation of the original series of
+movements&mdash;and this is, to say the least, a very probable view to
+take&mdash;then we may imagine that, for a distance of five or six miles,
+and at a depth of about a mile or less, there was a sudden sag
+downwards of the rock on the south-east side of the fault through a
+distance which perhaps in no part exceeded a fraction of an inch.</p>
+
+<p>Fig. 66 is an attempt to represent roughly the displacement which
+caused the principal earthquake. The diagram makes no pretence to
+accuracy, and <span class='pagenum'><a name="Page_256" id="Page_256">[256]</a></span>the scale in the vertical direction is enormously
+greater, perhaps a hundred thousand times greater, than that in the
+horizontal direction. The straight line is supposed to represent a
+straight line drawn before the earthquake on the surface of the rock
+adjoining the fault on the south-east side and at a depth of about a
+mile, and the curve the form of the same line after the earthquake.</p>
+
+<div class="img"><a name="Fig66" id="Fig66"></a>
+<a href="images/fig66.png">
+<img border="0" src="images/fig66.png" width="60%" alt="Diagram to illustrate supposed fault-displacement causing Inverness earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 66.&mdash;Diagram to illustrate supposed fault-displacement causing Inverness earthquake.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The effect of this great slip would obviously be to relieve the stress
+in the central region A, and to increase it suddenly in the parts
+denoted by the letters B and C. It is, therefore, in these parts
+especially that we should expect future slips to occur. Each slip
+would of course give rise to an after-shock, and would in like manner
+result in an increase of stress in its own terminal regions, though
+chiefly on the side remote from the centre A.</p>
+
+<br />
+
+<h4>THE AFTER-SHOCKS AND THEIR ORIGIN.</h4>
+
+<p>It is difficult to form any estimate of the total number of
+after-shocks. The list, compiled from the records of careful observers
+only, includes forty-six shocks and ten earth-sounds, the last of all
+occurring on November 21st. But the list is certainly incomplete. It
+contains, for instance, only one entry on <span class='pagenum'><a name="Page_257" id="Page_257">[257]</a></span>September 18th between 3.56
+and 9 <span class="fakesc">A.M.</span>; whereas, during the same interval, no fewer than
+eighteen slight shocks were felt by one observer at Dochgarroch, while
+another near Aldourie estimates the number of shocks up to October
+23rd at about seventy. The total number probably did not fall short of
+one hundred.</p>
+
+<div class="imgl" style="width: 40%;"><a name="Fig67" id="Fig67"></a>
+<a href="images/fig67.png">
+<img border="0" src="images/fig67.png" width="80%" alt="Map of epicentres of after-shocks of Inverness earthquakes." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 67.&mdash;Map of epicentres of after-shocks of Inverness earthquakes. (<i>Davison.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The majority were certainly very slight, and, at another time, would
+hardly have attracted any notice. There were, however, three of much
+greater importance than the rest. These occurred on September 18th at
+3.56 and 9 <span class="fakesc">A.M.</span>, and on September 30th at 3.39 <span class="fakesc">A.M.</span>
+The isoseismal lines of all three are elongated ovals, their longer
+axes are parallel to the fault, and their centres lie on the
+south-east side of the fault-line. The shocks were therefore evidently
+due to slips several miles in length along the fault. At present, we
+are concerned more with the position of their epicentres. These are
+indicated by the dots lettered B, C, D in Fig. 67; the dot marked A
+denoting the centre of the principal earthquake, and the continuous
+line the path of the fault.</p>
+
+<p>Thus, within two and a half hours, the great slip was followed by one
+with its centre at B, near the south-west margin of the principal
+focus. About five hours later, the scene of action was suddenly
+transferred to a region with its centre at C on the north-east margin.
+Both slips affected a portion of the fault-surface several miles in
+length, and must therefore have increased the area of displacement,
+slightly towards the north-east and considerably towards the
+south-west. Only small movements occurred during the next twelve days
+until 3.39 <span class="fakesc">A.M.</span> <span class='pagenum'><a name="Page_258" id="Page_258">[258]</a></span>on September 30th, when another long slip
+took place, with its centre at D, still farther to the south-west,
+and therefore again extending the area and amount of displacement in
+this direction.</p>
+
+<p>Turning now to the weaker after-shocks and earth-sounds, we find them
+affecting chiefly three regions of the fault. One of these is close to
+Dochgarroch, another near Inverness, and the third between Aldourie
+and Drumnadrochit; the effects of the slips in the last two districts
+being, as before, to extend the area of displacement a short distance
+(perhaps half a mile) to the north-east and not less than six miles to
+the south-west underneath Loch Ness.</p>
+
+<p>The unequal division of the after-shocks between the two sides of the
+principal centre (A, Fig. 67) is <span class='pagenum'><a name="Page_259" id="Page_259">[259]</a></span>very marked. The positions of the
+epicentres of forty-four shocks and earth-sounds can be determined
+with more or less accuracy, and, of these, only ten lie to the
+north-east of the principal centre, while thirty-four lie to the
+south-west, six or seven of the latter being beneath Loch Ness.</p>
+
+<p>One other point may be referred to before leaving these minor shocks.
+So far as regards the stronger shocks, there was a continual decrease
+in the depths of the seismic foci. This is shown by the progressive
+approach of their epicentres towards the fault-line; the distances in
+the three chief after-shocks being 1.7, 1.0, and 0.5 miles
+respectively; and in one of the latest shocks (that of October 13th at
+4.24 <span class="fakesc">P.M.</span>, E, Fig. 67) the distance is no more than one-tenth
+of a mile. The focus of this shock must, indeed, have been quite close
+to the surface near Dochgarroch. This constant diminution in the depth
+of the foci shows that the great slip was followed by a sudden
+increase of stress upwards as well as laterally, and explains why that
+slip did not leave any perceptible trace, either as fault-scarp or
+fissure, at the surface.</p>
+
+<br style="clear: both;" />
+
+<h4>SYMPATHETIC EARTHQUAKES.</h4>
+
+<p>It is remarkable that, of the 56 recorded after-shocks, at least six
+were felt or heard only at Dalarossie and other places in the valley
+of the Findhorn, a valley which lies about 13 or 14 miles to the
+south-east of the great fault. That they had no connection with that
+fault is certain, for two of them were so strong that, if they were so
+connected, they could not have escaped the notice of one or more of
+the watchful observers between Drumnadrochit and <span class='pagenum'><a name="Page_260" id="Page_260">[260]</a></span>Inverness. The
+probable explanation of these after-shocks is that they were due to
+slips of a fault running along the Findhorn valley;<a name="FNanchor_68_68" id="FNanchor_68_68"></a><a href="#Footnote_68_68" class="fnanchor">[68]</a> and that the
+great displacement near Inverness on September 18th led to a sudden
+increase of stress within the rocks for many miles around, which, at
+and near Dalarossie, was sufficient to precipitate the slips referred
+to.</p>
+
+<br />
+
+<h4>CONCLUSION.</h4>
+
+<p>At first sight, two earthquakes could hardly be more unlike than the
+Japanese earthquake of 1891 and the Inverness earthquake of 1901. In
+the rice-fields of central Japan, as we have seen, the roads for many
+leagues were edged with ruins, the fault-slip was prolonged up to the
+surface and visible as a scarp forty, if not seventy, miles in length,
+plots of ground were compressed and their boundaries altered, the
+hillsides were scored by landslips, places can now be seen from one
+another that formerly were hidden by a mountain ridge, and the total
+number of after-shocks within little more than two years amounted to
+above three thousand. On the other hand, when we examine the
+distribution of the after-shocks in space, we find that, though no
+part of the fault was exempt from slips, they favoured three regions
+in particular&mdash;one, the most important, a central region, yet not
+coincident with that in which the principal shock was most intense;
+and the other two surrounding the extremities of the fault. With <span class='pagenum'><a name="Page_261" id="Page_261">[261]</a></span>the
+lapse of time, the after-shocks on the whole became weaker and
+occurred less frequently, and the average depth of the foci gradually
+diminished. Moreover, in two districts distant forty-five and
+fifty-five miles from the fault, the frequency of the shocks during
+the month succeeding the earthquake was suddenly increased to ten and
+sixteen times the normal rate.</p>
+
+<p>It is interesting to notice so close a similarity in character,
+subsisting with so vast a difference in the scale of intensity. The
+identity of the powers at work in shaping the structure of both
+islands Is manifest. In Japan, we see the mountain-making forces
+acting with violence and producing effects that are only too apparent
+to the eye. In Scotland, whatever may have happened in former
+geological epochs, the changes in surface-structure are now taking
+place with almost infinite slowness, and hundreds or thousands of
+years must elapse before Loch Ness makes any visible progress in its
+march towards the sea.</p>
+
+<br />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Davison, C.</span>&mdash;<i>The Hereford Earthquake of December 17,
+1896.</i> (Birmingham, 1899.)</p>
+
+<p class="hang">2. &mdash;&mdash; "The Inverness Earthquake of Sept. 18, 1901, and its accessory
+shocks." <i>Quart. Journ. Geol. Soc.</i>, vol. lviii., 1902, pp.
+377-397.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_61_61" id="Footnote_61_61"></a><a href="#FNanchor_61_61"><span class="label">[61]</span></a> The study of the Hereford earthquake is based on 2,902
+records, coming from 1,943 places; that of the Inverness earthquake on
+710 records from 381 places.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_62_62" id="Footnote_62_62"></a><a href="#FNanchor_62_62"><span class="label">[62]</span></a> The disturbed area of the Hereford earthquake of 1896
+was probably greater than that of any other British earthquake of the
+nineteenth century; that of the Pembroke earthquake of 1892 being more
+than 56,000 square miles, of the Pembroke earthquake of 1893 about
+63,600 square miles, while that of the Essex earthquake of 1884 (a far
+stronger shock in the meizoseismal area) is estimated at about 50,000
+square miles.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_63_63" id="Footnote_63_63"></a><a href="#FNanchor_63_63"><span class="label">[63]</span></a> The approximate circularity of the two outer isoseismals
+is due to the fact that the vibrations propagated to such great
+distances are those which start from the comparatively small central
+region of the focus.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_64_64" id="Footnote_64_64"></a><a href="#FNanchor_64_64"><span class="label">[64]</span></a> The above statement summarises the evidence of the
+majority of the observers in each portion of the disturbed area. In
+this, as in other similar cases, discrepancies in the observations are
+unavoidable; but it is important to notice that they are least
+frequent in the observations evidently made with the greatest care.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_65_65" id="Footnote_65_65"></a><a href="#FNanchor_65_65"><span class="label">[65]</span></a> Except in the case of Yorkshire, where the three Ridings
+are regarded as separate counties.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_66_66" id="Footnote_66_66"></a><a href="#FNanchor_66_66"><span class="label">[66]</span></a> The Derby earthquake of March 24th, 1903, was also a
+twin earthquake. The centres of the two foci were situated near
+Ashbourne and Wirksworth, above eight or nine miles apart, along a
+line running N. 33&deg; E. and S. 33&deg; W. The two parts of the shock
+coalesced along a rectilineal band about five miles wide running
+centrally across the lower isoseismals in a direction at right angles
+to their longer axes. The isacoustic lines are also elongated in the
+direction of this band. In this case, the impulses at the two foci
+must have taken place at the same instant. (<i>Quart. Journ. Geo. Soc.</i>,
+vol. lx., 1904, pp. 215-232.)</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_67_67" id="Footnote_67_67"></a><a href="#FNanchor_67_67"><span class="label">[67]</span></a> If the foci of the two impulses had been detached, there
+would, with so small an interval between the two parts, have been a
+variation in the nature of the shock like that observed during the
+Hereford earthquake.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_68_68" id="Footnote_68_68"></a><a href="#FNanchor_68_68"><span class="label">[68]</span></a> This part of Inverness-shire has not yet been mapped by
+the Geological Survey, but a fault is known to exist in the Findhorn
+valley near Drysachan Lodge, which lies about eleven miles down the
+valley from Dalarossie.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_IX" id="CHAPTER_IX"></a><hr />
+<span class='pagenum'><a name="Page_262" id="Page_262">[262]</a></span><br />
+
+<h3>CHAPTER IX.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897.</h4>
+<br />
+
+<p>Very different from the shocks of Britain was the earthquake that
+overwhelmed so large a part of its great dependency on June 12th,
+1897&mdash;an earthquake which, if it is not without a rival, is certainly
+one of the most disastrous and most widely-felt of which we possess
+any record. That it was of the first magnitude was evident at once in
+Calcutta from the extensive injury to buildings, and its investigation
+was undertaken without delay by the members of the Geological Survey
+of India. The four officers who were at the headquarters in Calcutta
+were despatched to the area of greatest damage, letters and circulars
+were distributed as widely as possible, a large number of observers
+were induced to co-operate by keeping records of the after-shocks,
+and, later on, during the cold weather of 1897-98, Mr. R.D. Oldham,
+one of the superintendents of the Survey, made a tour through the
+epicentral district. To him, moreover, fell the much harder task of
+discussing the very numerous observations collected by himself and
+others; and the least that can be said of the valuable report prepared
+by him is that it is worthy of a great subject. Professor Omori also
+spent several months in studying the earthquake on behalf of the
+Japanese Government; but the account, <span class='pagenum'><a name="Page_263" id="Page_263">[263]</a></span>which is written in his own
+language, unfortunately remains a sealed book to western
+seismologists.</p>
+
+<div class="img"><a name="Fig68" id="Fig68"></a>
+<a href="images/fig68.png">
+<img border="0" src="images/fig68.png" width="90%" alt="Isoseismal Lines of Indian Earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 68.&mdash;Isoseismal Lines of Indian Earthquake. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<span class='pagenum'><a name="Page_264" id="Page_264">[264]</a></span><br />
+
+<h4>ISOSEISMAL LINES AND DISTURBED AREA.</h4>
+
+<p>In Fig. 68, which shows the area disturbed by the earthquake, Mr.
+Oldham has drawn two series of curves. In the absence of detailed
+records of the intensity&mdash;records that could not have been obtained
+from some parts of the disturbed area, and would have been difficult
+to procure in sufficient number from others&mdash;he has represented by the
+dotted curves a group of isoseismals in the form which he believes
+they would have assumed had the earth-waves been propagated in a
+homogeneous medium. The first includes all places, such as Shillong
+and Goalpara, where the destruction of brick and stone buildings was
+practically universal; the second, those, like Darjiling, in which
+damage to buildings was universal and often serious; the third,
+places, like Calcutta, where the earthquake was strong enough to
+injure all or nearly all brick buildings. Inside the fourth
+isoseismal, the shock was strong enough to disturb furniture and loose
+objects, but not to cause more than slight damage; within the fifth,
+it was generally noticed; and, beyond this, and as far as the sixth
+isoseismal, the earthquake was perceived only by a small number of
+sensitive persons at rest. The approximation of the curves towards the
+east and south-east, Mr. Oldham believes to be partly real, and not
+due to imperfect information.</p>
+
+<p>The continuous curves represent more closely the actual variation of
+intensity. The innermost curve A indicates the probable boundary of
+the epicentral tract, which is about 200 miles in length and more than
+6000 square miles in area. This will be referred to afterwards in
+greater detail. The next <span class='pagenum'><a name="Page_265" id="Page_265">[265]</a></span>curve B bounds the region within which
+serious damage to brick houses was common. Its irregular course is
+closely connected with the geological structure of the country, and is
+due to the fact, of which we have already met with several examples,
+that earthquakes are more destructive to houses built on alluvial
+ground than to those founded on rock. The area included within this
+curve is not less than 145,000 square miles; and, if we include the
+parts from which reports were not obtainable, it must amount to about
+160,000 square miles.</p>
+
+<p>The curve C represents the boundary of the disturbed area, so far as
+known, for about one-third of the area lies in regions from which no
+information was procurable, while another third is inhabited by
+ignorant and illiterate tribes. But, notwithstanding this, the shock
+is known to have been felt over an area of at least 1,200,000 square
+miles. If we include the detached region to the west, near Ahmedabad,
+the portion of the Bay of Bengal in which the shock would have been
+felt had the sea been replaced by land, and a large part of Thibet or
+Western China, from which no reports have come, but in which the shock
+was certainly sensible, this estimate, great as it is, must be raised
+to about 1,750,000 square miles.<a name="FNanchor_69_69" id="FNanchor_69_69"></a><a href="#Footnote_69_69" class="fnanchor">[69]</a></p>
+
+<p>Figures, such as those given above, convey but little <span class='pagenum'><a name="Page_266" id="Page_266">[266]</a></span>idea of the
+vastness of the area concerned. Transferring them to countries with
+which we are more familiar, we may say that the disturbed area was
+only a little less than half the size of Europe; the region in which
+serious damage occurred to masonry was more than twice as large as the
+whole of Great Britain; while, if the centre of the epicentral tract
+had been in Birmingham, nearly every brick and stone building between
+York and Exeter would have been levelled with the ground.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK.</h4>
+
+<p>Few and slight were the forerunners of the greatest of modern
+earthquakes. Early in June, faint tremors were felt by sensitive
+persons at Shillong. Others at the same place heard a rumbling sound
+for ten or fifteen seconds before the shock began, and at Silchar
+birds were seen to rise suddenly from trees before the movement became
+sensible to man. Except for these almost imperceptible warnings, the
+earthquake broke abruptly over the whole district.</p>
+
+<p>"At 5.15," writes one observer at Shillong, "a deep rumbling sound,
+like near thunder commenced, apparently coming from the south or
+south-west.... The rumbling preceded the shock by about two seconds
+... and the shock reached its maximum violence almost at once, in the
+course of the first two or three seconds. The ground began to rock
+violently, and in a few seconds it was impossible to stand upright,
+and I had to sit down suddenly on the road. The shock was of
+considerable duration, and maintained roughly the same amount of
+violence from the beginning to the end. It produced a very distinct
+<span class='pagenum'><a name="Page_267" id="Page_267">[267]</a></span>sensation of sea-sickness.... The feeling was as if the ground was
+being violently jerked backwards and forwards very rapidly, every
+third or fourth jerk being of greater scope than the intermediate
+ones. The surface of the ground vibrated visibly in every direction,
+as if it was made of soft jelly; and long cracks appeared at once
+along the road.... The road is bounded here and there by low banks of
+earth, about two feet high, and these were all shaken down quite flat.
+The school building, which was in sight, began to shake at the first
+shock, and large slabs of plaster fell from the walls at once. A few
+moments afterwards the whole building was lying flat, the walls
+collapsed, and the corrugated iron roof lying bent and broken on the
+ground. A pink cloud of plaster and dust was seen hanging over every
+house in Shillong at the end of the shock.... My impression at the end
+of the shock was that its duration was certainly under one minute, and
+that it had travelled from south to north.... The violence of the
+shock may be imagined when it is stated that the whole of the damage
+done was completed in the first ten or fifteen seconds of the shock."</p>
+
+<p>Other estimates of the duration are generally higher than that given
+above, ranging from three to five or even more minutes at Tura,
+Dhubri, Silchar, Calcutta, and other places. In some cases, it is
+possible that the immediately succeeding tremors were included as part
+of the great shock; but, in the central area, it is probable that the
+average duration of the shock did not differ much from three or four
+minutes.</p>
+
+<p>In this district, the movement was most complicated. Changes of
+direction were frequently noticed. At Silchar, for instance, the
+earthquake began with <span class='pagenum'><a name="Page_268" id="Page_268">[268]</a></span>an undulatory movement from north to south, like
+the swinging of a suspension bridge; it closed with a motion like
+that of a boat tossed in a choppy sea, or by the crossing of great
+waves which, whatever their dominant direction may have been,
+certainly did not travel from north to south. The vertical component
+of the motion must have been considerable; for, at Shillong, loose
+stones lying on the roads were tossed in the air "like peas on a
+drum," But this was even less pronounced than the horizontal movement,
+the range of which was at least eight or nine inches, and during which
+people felt as if they were being shaken like a rat by a terrier. The
+period of these vibrations was estimated at about a second.</p>
+
+<p>As they left the central region, the period of the waves lengthened,
+so that, at a distance, the shock no longer consisted of short jerks,
+but became a gentle rocking motion, causing in some people a sensation
+of nausea. At Calcutta, the undulations were regular and resembled the
+rolling of a mighty ship, the period being between one and two
+seconds. At Balasor, the motion was a long rolling one, such as would
+be felt on the deck of a ship in a fairly heavy sea; and, farther to
+the south as far as the limit of the disturbed area, the same
+undulatory movements were observed, gradually decreasing in intensity,
+and usually compared to the easy motion of a ship in a gentle sea.</p>
+
+<p><i>Visible Earth-Waves.</i>&mdash;A few examples have already been given of the
+observation of visible waves on the surface of the ground. They were
+seen at Charleston during the earthquake of 1886 (p. 110), and at
+Akasaka and other places in the meizoseismal area during the Japanese
+earthquake <span class='pagenum'><a name="Page_269" id="Page_269">[269]</a></span>of 1891 (p. 186). But they were more than usually
+prominent in the Indian earthquake; indeed, much of the difficulty
+experienced in standing during the shock seems to have been due to the
+passage of these surface-waves.</p>
+
+<p>At Shillong, according to an observer quoted above (p. 266), the
+surface of the ground vibrated visibly in every direction, as if it
+were made of soft jelly. Another describes it as presenting "the
+aspect of a storm-tossed sea, with this difference that the
+undulations were infinitely more rapid than any seen at sea." Near
+Maimansingh, earth-waves were watched approaching, exactly like
+rollers on the sea-coast, and, as they passed, the observers had a
+difficulty in standing. At Nalbari, the rice in the fields could be
+seen rising and falling at intervals during the transit of the waves.
+In the Assam valley, near Mangaldai, there were seen "waves coming
+from opposite directions and meeting in a great heap and then falling
+back; each time the waves seemed to fall back the ground opened
+slightly, and each time they met, water and sand were thrown up to a
+height of about 18 inches or so." Even as far as Midnapur, the ground
+was "distinctly billowy," and at Allahabad a series of waves was
+observed to cross the ground from south-south-west to
+north-north-east.</p>
+
+<p>It is obviously difficult to judge in any case of the magnitude of
+such waves. In the epicentral area, Mr. Oldham believes that, on an
+average, they were probably about thirty feet long and one foot in
+height, though some may have been both shorter and higher. These
+movements must have been comparatively slow, for their progress could
+be easily <span class='pagenum'><a name="Page_270" id="Page_270">[270]</a></span>followed by the eye; indeed, their rate, as one witness
+remarks, "though decidedly faster than a man could walk, was not so
+fast as he could run."</p>
+
+<br />
+
+<h4>ELEMENTS OF THE WAVE-MOTION.</h4>
+
+<p>In his study of the Neapolitan earthquake, Mallet showed how the
+amplitude and maximum velocity of the vibrations could be determined
+roughly from the displacement, projection, or overthrow of various
+bodies by the earthquake. Somewhat similar methods were employed by
+Mr. Oldham in the absence of seismographs from the epicentral area.
+His results are of course only approximate, but they lead nevertheless
+to a conclusion of great value and interest.</p>
+
+<div class="img"><a name="Fig69" id="Fig69"></a>
+<a href="images/fig69.png">
+<img border="0" src="images/fig69.png" width="50%" alt="Section of Tombs in the Cemetery at Cherrapunji." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 69.&mdash;Section of Tombs in the Cemetery at Cherrapunji. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p><i>Amplitude.</i>&mdash;The best measure of the amplitude was obtained at the
+cemetery at Cherrapunji, situated near the southern margin of the
+epicentral area. Here were two oblong masonry tombs (Fig. 69),
+standing close together with their longer axes pointing north and
+south. Their inner sides were partially destroyed. "On the outer
+sides, they are almost intact, but the tombs have been driven bodily
+down into the ground, and on either side to east and west, there is a
+depression with a vertical side <span class='pagenum'><a name="Page_271" id="Page_271">[271]</a></span>parallel to the outer surface of the
+tomb and a smooth flat bottom over which the base of the tomb has
+slid.... The edge of the western depression has the grass growing
+undisturbed up to the edge of it, and along the edge small fragments
+of lime and plaster show that this was originally in contact with the
+edge of the tomb, which has now moved away to a distance of 18 inches.
+On the east the edge of the depression is raised and the grass and
+earth forced upwards by the thrust of the tomb against it; the breadth
+of this depression is 10 inches."</p>
+
+<p>During the movement of the ground, the tombs, owing to their inertia,
+remained comparatively stationary, and the depressions were formed by
+the backward and forward movement of the ground against them. The
+movement on the east side was clearly arrested in some manner, and the
+range therefore cannot have been less than 10 inches. It may have been
+as much as 18 inches, and was probably, in Mr. Oldham's opinion, the
+mean of these two amounts&mdash;namely, 14 inches. This would give an
+amplitude of about 7 inches, a value which may be in excess of the
+average amount elsewhere in the district, as the cemetery is situated
+near the edge of a high sandstone scarp.</p>
+
+<p>At Tura, also within the epicentral area, a range of not less than 10
+inches was given by the sliding of a wooden house over the posts on
+which it rested. Six months after the shock, Mr. Oldham frequently
+noticed vacant spaces four or five inches across by the side of large
+boulders scattered over the Khasi hills, and he infers that
+"throughout the whole tract lying west of Shillong and Gauhati, as far
+as the hills extend, and probably over a large area of <span class='pagenum'><a name="Page_272" id="Page_272">[272]</a></span>the plains
+besides, the amplitude of the wave-motion was nowhere less than 3
+inches, while in many places it was over 6 inches."</p>
+
+<p><i>Maximum Velocity.</i>&mdash;The most trustworthy measure of the maximum
+velocity are those obtained from the projection of bodies. Mr. Oldham
+selects the following as most deserving of notice:&mdash;At Goalpara, an
+obelisk surmounting a tomb was broken off and thrown to one side,
+giving a maximum velocity of not less than 11 feet per second. At
+Gauhati, the coping of a small gate-pillar was shot off and fell at a
+distance of 4 feet 4 inches from the centre of the pillar; in this
+case the maximum velocity must have exceeded 8 feet per second. The
+highest velocity, of more than 16 feet per second, was measured at
+Rambrai, where a small group of monoliths were shot out of the ground,
+one of them to a distance of 6&frac12; feet. Lastly, at Silchar, a bullet
+was projected from the corner of a wooden post, acting as a rough form
+of seismometer, from which a maximum velocity of at least 1&frac12; feet
+per second was deduced.</p>
+
+<p><i>Maximum Acceleration.</i>&mdash;Estimates of the maximum horizontal
+acceleration were made from 28 overthrown pillars by means of
+Professor West's formula (p. 184, footnote). The measures obtained at
+the same place show some variation, but Mr. Oldham considers as fair
+average values those of 14 feet per second per second at Goalpara, 12
+at Gauhati, Shillong, and Sylhet, 10 at Cherrapunji, 9 at Dhubri, and
+4 feet per second per second at Silchar.</p>
+
+<p>Of the vertical component of the acceleration, not even the roughest
+numerical estimate can be formed. We know, however, that at Shillong,
+Gauhati, and <span class='pagenum'><a name="Page_273" id="Page_273">[273]</a></span>indeed throughout the epicentral area, stones were
+projected upwards, and this is evidence that the vertical component
+was greater than that of gravity&mdash;namely, 32 feet per second per
+second.</p>
+
+<p>Violent as the shock was at the places just mentioned, it must have
+been still greater in certain parts of the epicentral area. At Dilma,
+in the Garo hills, the shock seems to have been strong enough to
+disable men; and, in the neighbourhood of the faults that will be
+described in a later section, forest trees were snapped in two.
+Fortunately, as Mr. Oldham remarks, there were in these districts no
+towns or populous settlements to feel the full power of the earthquake
+to destroy.</p>
+
+<p><i>Anomalies in the above Measurements.</i>&mdash;If the movements of the ground
+followed the law of simple harmonic motion, any two of the four
+elements (period, amplitude, maximum velocity, and maximum
+acceleration) would suffice to determine the others (p. 4). Applying
+the usual formul&aelig; to the quantities obtained at Gauhati&mdash;namely, 8
+feet per second for the maximum velocity and 12 feet per second per
+second for the maximum acceleration, it follows that the amplitude
+would be 5 feet and the period 4 seconds&mdash;values, which are evidently
+inadmissible. Or, taking the maximum vertical component at 32 feet per
+second per second, the corresponding values would be 2 feet and 1&frac12;
+seconds, that of the amplitude being still too great. Again, at
+Rambrai, the maximum velocity was found to exceed 16 feet per second.
+The other elements are unknown, but, if the amplitude were one foot,
+Mr. Oldham shows that the maximum acceleration would be 256 feet per
+<span class='pagenum'><a name="Page_274" id="Page_274">[274]</a></span>second per second; or, taking the amplitude at the impossible amount
+of two feet, that the maximum acceleration would be 128 feet per
+second per second.</p>
+
+<p>It follows, therefore, that only part of the high velocities at
+Rambrai and elsewhere can be due to the elastic waves provoked by the
+initial disturbances. The remaining portion must be attributed to a
+bodily displacement of the earth's crust within the epicentral area&mdash;a
+displacement of which the fault-scarps and other distortions of that
+region furnish ample evidence.</p>
+
+<br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p>In the epicentral area, the sound that accompanied the earthquake was
+remarkable for its extraordinary loudness. At Shillong, the crash of
+houses falling within thirty yards was completely drowned by the roar
+of the earthquake.</p>
+
+<p>The sound was generally compared to distant thunder, the passage of a
+train or cart, etc.; but, whatever the type may be, it always implies
+a sound of deep pitch, close to the lower limit of audibility&mdash;a
+continuous rumbling or rattling noise, as a rule gradually becoming
+louder and then dying away. There was the usual conflict in the
+evidence of different observers due to the depth of the sound. In
+Calcutta, which lies well within the sound-area, some persons asserted
+that they heard a rumbling noise; others were positive that the only
+noise was that caused by falling buildings and furniture. Some, again,
+noticed that the shock was preceded by a loud roar; while others were
+certain that there was no <span class='pagenum'><a name="Page_275" id="Page_275">[275]</a></span>sound of any kind until the earthquake had
+become severe.</p>
+
+<p>As in the case of the disturbed area, it is impossible to define the
+boundary of the region over which the sound was heard. Like the shock,
+also, it seems to have been observed farther to the west than towards
+the east. Leaving out of account records that are possibly doubtful,
+the sound was heard for a distance of 330 miles to the west and
+south-west, and 290 miles to the east of the epicentral area&mdash;that is,
+allowing for the dimensions of that area, it must have been
+perceptible over a region measuring not less than 800 miles from east
+to west.</p>
+
+<br />
+
+<h4>VELOCITY OF THE EARTH-WAVES.</h4>
+
+<p>It is somewhat doubtful whether a more accurate estimate of the
+velocity is to be obtained from a violent earthquake or from one of
+moderate intensity. In the former case, the vast distances to which
+the shock is noticed lessen the effects of errors in the
+time-determinations, but this advantage is to a great extent
+compensated by the considerable duration of the shock and the
+consequent uncertainty whether all observers have timed the same phase
+of the movement. Also, in the Indian earthquake, there are further
+sources of error in the variety of standard times employed throughout
+the country and in the magnitude of the epicentral area.</p>
+
+<p>Of the numerous time-records collected by Mr. Oldham, the best are
+those which were obtained from a few self-recording instruments, from
+the more busy telegraph offices, from the larger railway stations, and
+in some cases from private individuals. <span class='pagenum'><a name="Page_276" id="Page_276">[276]</a></span>All records were in the first
+place subjected to a rigid process of selection; a large number were
+rejected on various grounds, and those only were retained which bore
+internal evidence of accuracy, due either to the conditions of the
+reporter's occupation or to the care taken by him to ensure exactness.
+To guard against any unconscious bias in making the selection, this
+process was carried out before the distances were calculated, and even
+before the position of the epicentral area was known.</p>
+
+<p>The boundary of this area is shown by the continuous line A in Fig.
+68. Its greatest length being about 200 miles from east to west, it is
+necessary in the first place to fix upon an equivalent centre within
+it, which may be regarded for this special purpose as the point of
+departure of the earth-waves. The more natural course perhaps would be
+to assume this point to coincide with the centre of the area. But, as
+the rate at which the initial movement spread over that area would
+probably differ little from the velocity of the earth-wave, and as all
+the time-stations lie towards the west, Mr. Oldham regards a point
+near the western boundary of the area (in lat. 25&deg; 45' N. and long.
+90&deg; 15' E.) as a sufficiently exact approximation to the position of
+the equivalent centre.</p>
+
+<p>The nearest place at which good time-observations were made is
+Calcutta, distant 255.5 miles from the assumed centre. One is
+indicated on the recording tide-gauge by a sudden rise of the water,
+while the others were obtained from the central telegraph office, the
+terminal railway stations, and from two careful readings by interested
+observers. They vary from 4h. 27m. 0s. to 4h. 28m. 37s. <span class="fakesc">P.M.</span>,
+all being liable to an error of half-a-minute. The arithmetic <span class='pagenum'><a name="Page_277" id="Page_277">[277]</a></span>mean for
+the beginning of the shock is 4h. 27m. 49s., and this is probably as
+accurate an estimate as the conditions allow.<a name="FNanchor_70_70" id="FNanchor_70_70"></a><a href="#Footnote_70_70" class="fnanchor">[70]</a></p>
+
+<p>Bombay lies outside the disturbed area, 1208.3 miles from the
+equivalent centre; and, for the time of arrival in that city, we have
+to depend on the records of the barograph and the three magnetographs.
+The horizontal force magnet was set in motion two and a half minutes
+before the others, no doubt by the advance tremors. The times given by
+the barograph and the vertical force-instrument differ by only one
+minute, and the best result seems to be that obtained by taking their
+mean&mdash;namely, 4h. 35m. 43s., which is probably accurate to within a
+minute.</p>
+
+<p>Assuming, then, that the time-interval between Calcutta and Bombay
+does not err by more than half-a-minute, it follows that the
+intervening velocity must lie between 2.8 and 3.2 kilometres per
+second, its probable value being 3 kilometres, or 2 miles, per second.</p>
+
+<p>The remaining records, which are of less value than those obtained in
+these cities, fall into two groups, the first consisting of a number
+of stations along a line running north and south between Calcutta and
+Darjiling or within a hundred miles on either side of the same, and
+the second a long series of stations crossing Northern India in a
+nearly westerly direction. The observations made at the Burmese
+stations were unfortunately affected by an error arising from the
+retardation of the Madras <span class='pagenum'><a name="Page_278" id="Page_278">[278]</a></span>time-signals through frequent repetition
+along the line.</p>
+
+<div class="img"><a name="Fig70" id="Fig70"></a>
+<a href="images/fig70.png">
+<img border="0" src="images/fig70.png" width="80%" alt="Time-curve of Indian earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 70.&mdash;Time-curve of Indian earthquake. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Individually, these records are not exact enough to be used in
+determining the velocity, but they may be employed collectively for
+the construction of the time-curve in Fig. 70. In this diagram,
+distances in hundreds of miles from the equivalent centre are
+represented along the horizontal line, and the time of occurrence in
+minutes past 4 <span class="fakesc">P.M.</span> along the perpendicular line. The small
+circles represent the observations at Calcutta and Bombay, the dots
+those at places lying nearly west of the origin, and the crosses those
+at places situated to the south or north-west. The continuous curve
+passes in an average manner through the series of points, and probably
+does not differ much from the true curve of the time of arrival of the
+shock at different places. The curve, it will be noticed, is at first
+concave, and afterwards convex, upwards; indicating that the times
+required to traverse successive equal distances at first increased,
+and then decreased. Thus, if the curve is an accurate representation
+of the facts, it would follow that the surface-velocity was subject to
+<span class='pagenum'><a name="Page_279" id="Page_279">[279]</a></span>a continual decrease outwards from the centre, until it was a minimum
+at a distance of about 280 miles, after which it increased.</p>
+
+<p>The deviation of the curve from a straight line is, however, so slight
+that we cannot feel much confidence in this conclusion. If we join the
+points corresponding to Calcutta and Bombay by a straight line (drawn
+dotted in Fig. 70), it does not in any part vary from the continuous
+line by a distance equivalent to more than half-a-minute. Indeed, if a
+very few discordant records are excluded, and if less weight is given
+to those times which are multiples of five minutes, the straight line
+represents the mean quite as fairly as the curved line does; and that
+this is the more probable interpretation will appear from the
+observations on the unfelt earthquake described in the next section.
+We may therefore conclude that the earth-waves travelled along the
+surface at an approximately uniform rate of 3 kilometres per second,
+or about 120 miles a minute&mdash;a result which Mr. Oldham considers may
+be accepted as accurate to within five per cent.</p>
+
+<p>If the two time-curves in Fig. 70 are continued to the right until
+they meet the time-scale, it will be seen that they intersect it near
+the point corresponding to 4.26 <span class="fakesc">P.M.</span>, implying that this
+would be approximately the time at which the shock was felt within the
+epicentral area. This agrees closely with the observed times of about
+4.25 at Parbatipur and Kuch Bihar, 4.26 at Siliguri, and 4.27 at
+Shillong and Goalpara; and it is probable that the error is not more
+than a quarter of a minute in defect or half-a-minute in excess. Thus,
+the time of arrival of the first sensible waves at the surface would
+lie between <span class='pagenum'><a name="Page_280" id="Page_280">[280]</a></span>4h. 25m. 45s., and 4h. 26m. 30s. <span class="fakesc">P.M.</span>, Madras
+time, or between 11h. 4m. 45s. and 11h. 5m. 30s. <span class="fakesc">A.M.</span>,
+Greenwich mean time.</p>
+
+<br />
+
+<h4>THE UNFELT EARTHQUAKE.</h4>
+
+<p>Of the crowd of vibrations that agitate the ground during an
+earthquake, part only combine to form the perceptible shock. Some are
+insensible owing to their small amplitude, others to the slowness of
+the motion. An interesting observation belonging to the latter class
+was made by an engineer near Midnapur, a place which lies just within
+the area of damage. At the time of the earthquake, he was taking
+levels on a railway bank, and was about to take a reading when he
+noticed the bubble of the level oscillating. In five or ten seconds
+the shaking began and appeared to last three or four minutes; but, for
+more than five minutes after it had apparently ceased, the level
+showed that the ground continued to rock.</p>
+
+<p>Again, in Burmah, at a place nineteen miles east of Tagaung and close
+to the border of the disturbed area, the water in a shallow tank,
+about 300 yards in length, was seen lapping up against the side in a
+manner that was at first attributed to elephants bathing. No shock was
+felt, but the shaking of the trees at the same time showed that the
+disturbance was due to the earthquake.</p>
+
+<p>Far beyond the limits of the disturbed area, however, the earthquake
+was recorded by many of the delicate instruments which have been
+employed during the last few years for the registration of distant
+shocks. Among the more important of these instruments are long
+vertical pendulums, horizontal <span class='pagenum'><a name="Page_281" id="Page_281">[281]</a></span>pendulums of various forms, and
+magnetographs. In the vertical, and some of the horizontal,
+pendulums, especially in those used in the Italian observatories, the
+masses carried are heavy, and the movements of the ground are
+magnified by lightly-balanced levers ending in points which trace
+their records on bands of smoked paper driven by clockwork. In the
+other horizontal pendulums and in the magnetographs, the method of
+registration is photographic. The paper required for the mechanical
+records being inexpensive, a high velocity (half-an-inch or more per
+minute) can be given to it, and the resulting diagrams are open and
+detailed. The Italian instruments also respond more readily than the
+others to the earlier and slighter tremors: while the apparatus in
+which photographic methods are used are sometimes so violently
+disturbed by the later undulations that the spot of light fails to
+leave any trace on the photographic paper. It is therefore from the
+Italian observatories that the more interesting records come. One of
+these, given by a horizontal pendulum at Rocca di Papa near Rome, is
+reproduced in Fig. 71; while the curve of the bifilar pendulum at
+Edinburgh (Fig. 72) is a good example of those obtained by the
+photographic method of registration.<a name="FNanchor_71_71" id="FNanchor_71_71"></a><a href="#Footnote_71_71" class="fnanchor">[71]</a></p>
+
+<p>All over Italy, from Ischia and Catania in the south to Pavia in the
+north, the different instruments employed began, one after the other,
+to write their <span class='pagenum'><a name="Page_282" id="Page_282">[282]</a></span>records of the movement as the unfelt earth-waves sped
+outwards from the centre. Italy passed, the tale was taken up by
+magnetographs at Potsdam and Wilhelmshaven, Pawlovsk (near St.
+Petersburg), Copenhagen, Utrecht, and Parc St. Maur (near Paris); by
+horizontal pendulums at Strassburg and Shide (in the Isle of Wight),
+and by a bifilar pendulum at Edinburgh. Shide is 4,891 miles from the
+centre of disturbance, but, as we shall see, the movement could be
+traced for a distance greater even than this.</p>
+
+<div class="img"><a name="Fig71" id="Fig71"></a>
+<a href="images/fig71.png">
+<img border="0" src="images/fig71.png" width="80%" alt="Seismographic Record of Indian Earthquake at Rocca di Papa." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 71.&mdash;Seismographic Record of Indian Earthquake at Rocca di Papa. (<i>Cancani.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>In the more complete records, and especially in those given by the
+Italian apparatus, Mr. Oldham distinguishes three phases of motion.
+The first consists of rapid and nearly horizontal movements of the
+ground. In Italy, it begins at about 11.17 <span class="fakesc">A.M.</span>&mdash;that is,
+about 12&frac12; minutes after the commencement of the shock at the
+epicentre (Fig. 71, <i>a</i>). Without any break in the movement, and after
+a further interval of about 8&frac12; minutes, the second phase begins;
+the vibrations are similar to the preceding, but <span class='pagenum'><a name="Page_283" id="Page_283">[283]</a></span>they are larger and
+more open, and are accompanied by an unmistakable tilting of the
+surface of the ground (Fig. 71, <i>b</i>). Lastly, after the lapse of about
+twenty minutes more, the second phase gives place, without
+interruption, to the third (Fig. 71, <i>c</i>),<a name="FNanchor_72_72" id="FNanchor_72_72"></a><a href="#Footnote_72_72" class="fnanchor">[72]</a> consisting of
+well-marked slow undulations, which have been aptly compared by
+Professor Milne to the movements caused by an ocean-swell. As they
+travelled across Europe, the surface of the ground was thrown into a
+series of flat waves, 34 miles in length, and 20 inches in maximum
+height, the complete period of each wave being 22 seconds. This phase
+is by far the longest of the three; in the more sensitive instruments,
+two or three hours elapsed before their traces ceased to show any sign
+of movement.</p>
+
+<div class="img"><a name="Fig72" id="Fig72"></a>
+<a href="images/fig72.png">
+<img border="0" src="images/fig72.png" width="50%" alt="Seismographic Record of Indian Earthquake at Edinburgh." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 72.&mdash;Seismographic Record of Indian Earthquake at Edinburgh. (<i>Heath.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Knowing the distances of the different observatories <span class='pagenum'><a name="Page_284" id="Page_284">[284]</a></span>from the
+epicentre, and the times taken by each phase to reach them, we can
+form some idea of the rates at which they travelled. If the early
+tremors moved in straight lines, their mean velocity for the first
+phase was 9.0, and for the second 5.3, kilometres per second; but, if
+they moved along curved paths through the body of the earth, their
+mean velocities must have exceeded these amounts. For the first
+undulations of the third phase, the velocity would be 2.9 kilometres
+per second if they travelled along straight lines, and 3.0 kilometres
+per second if they were confined to the surface of the earth.</p>
+
+<p>The existence of the second phase was noticed for the first time by
+Mr. Oldham in the records of the Indian earthquake, but he has since
+detected it in those of other shocks. He believes, in common with most
+seismologists, that the first phase corresponds to waves of elastic
+compression, or longitudinal waves, travelling through the body of the
+earth; and the second phase he attributes to waves of elastic
+distortion, or transversal waves, travelling in the same way, in which
+the particles move at right angles to the direction in which the wave
+travels, thus causing a slight tilting of the surface. It is probable
+that the waves of both phases move along curved, rather than straight,
+lines through the earth, that the curves are concave towards the
+surface, and that the velocity of the waves increases with the depth
+of their path below the surface.</p>
+
+<p>On the other hand, the surface-velocity of the first undulations of
+the third phase is practically constant for all distances from the
+epicentre, and, in the case of the Indian earthquake, it agrees almost
+exactly with that obtained for the velocity within the <span class='pagenum'><a name="Page_285" id="Page_285">[285]</a></span>disturbed area,
+and as far as Bombay. It is therefore difficult to resist the
+conclusion that the third phase consists of undulations which travel
+along the surface of the earth. Diverging in two dimensions only, they
+fade away much more slowly than the vibrations of the other two
+phases.</p>
+
+<p>We may thus imagine these surface-undulations speeding outwards from
+the epicentre in ever-widening circles until they have passed over a
+quarter-circumference of the earth, when they should begin to converge
+towards the antipodes. Here they should cross each other, and again
+spread out as circular waves, once more in their course passing the
+same observatories where they were first recorded, but in the opposite
+order. It has been reserved for the most violent earthquake of modern
+times to verify this interesting conclusion. Faint, but decided, are
+the traces of the second crossing. At Edinburgh, they occur at 2.6
+<span class="fakesc">P.M.</span>, at about the same time at Shide, at Leghorn 2.10,
+Catania 2.12&frac34;, while at Ischia there are several movements between
+2 and 3 <span class="fakesc">P.M.</span> At Rocca di Papa, near Rome, the time is
+slightly earlier, but the undulations, like those at the first
+crossing, have a complete period of about 20 seconds. The distances
+traversed by the waves are more than 20,000, instead of less than 5000
+miles; but the mean velocity with which they travelled is almost
+exactly the same as at first&mdash;namely, 2.95 kilometres per second.</p>
+
+<br />
+
+<h4>EARTH-FISSURES, SAND-VENTS, ETC.</h4>
+
+<p><i>Earth-Fissures.</i>&mdash;Among the superficial effects of the earthquake,
+none take a more important place <span class='pagenum'><a name="Page_286" id="Page_286">[286]</a></span>than the fissures formed in alluvial
+plains. Not only were they remarkably abundant, more so than in any
+other known earthquake, but they occurred over an unusually wide area.
+Wherever the necessary conditions prevailed, they were found to be
+numerous over a district bounded approximately by the isoseismal 1
+(Fig. 68), and measuring about 400 miles from east to west, and about
+300 miles from north to south; and they were present, though in
+smaller numbers, over an area nearly 600 miles long in an
+east-north-east and west-south-west direction. They were naturally
+more frequent near river-channels and reservoirs, on account of the
+absence of lateral support, and as a rule were parallel to the edge of
+the bank, a few hundred yards in length, and in width varying from
+some inches to four or five feet.</p>
+
+<p>Fissures in such positions are formed with every violent earthquake,
+and even with some of those more moderate shocks that visit the
+British Islands (see p. 247). But an interesting point established by
+the Indian earthquake is that they also occurred at a distance from
+any water-channel or excavation, often running parallel to, and along
+either side of, a road or embankment. In other situations, they showed
+a distinct tendency to range themselves parallel to one another; and,
+in these cases, it is possible that their formation was connected with
+the passage of the visible surface-waves. In an account already quoted
+(p. 247), it is stated that these waves came from opposite directions
+and that, as they separated after meeting, the ground opened slightly.</p>
+
+<div class="imgr" style="width: 45%;"><a name="Fig73" id="Fig73"></a>
+<a href="images/fig73.png">
+<img border="0" src="images/fig73.png" width="90%" alt="Displacement of alluvium at foot of a hill." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 73.&mdash;Displacement of alluvium at foot of a hill. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Among the Khasi and Garo hills (see Fig. 75), wherever the alluvium of
+the plains runs up to the foot of the hills, another form of fissure,
+represented <span class='pagenum'><a name="Page_287" id="Page_287">[287]</a></span>in Fig. 73, was constantly noticed. Close to the
+junction, there was a sudden drop, as at <i>a</i>, of from one to five
+feet, the vertical face having the appearance of a fault, but
+distinguished from one by following the windings of the hills. Then
+came a depressed band <i>b</i>, from ten to twenty feet wide, and outside
+this a low rounded ridge <i>c</i> raised above its former level, and
+merging beyond at <i>d</i> into the undisturbed plain. When Mr. Oldham
+visited the district in March 1898, the natives had flooded the
+rice-fields, and the features described were clearly depicted by the
+gathering of the water in the depression and the isolation of the
+ridge.</p>
+
+<p>The explanation of these peculiarities is evidently that given by Mr.
+Oldham. During the passage of repeated waves of compression, the
+thrust of the hill and plain against one another caused the heaping up
+of the alluvium in the ridge <i>c</i>; while the return movements resulted
+in the tearing of the alluvium away from the hillside, leaving the
+scarp <i>a</i> and the depression <i>b.</i></p>
+
+<p><i>Displacements of Alluvium.</i>&mdash;Many other remarkable evidences of
+compression were observed. Telegraph posts, originally set up in a
+straight line, were displaced, occasionally as much as ten or fifteen
+feet; sometimes without any apparent connection with neighbouring
+river-channels. In one part of the Assam-Bengal Railway, for nearly
+half a mile, the whole embankment, including borrow-pits and trees <span class='pagenum'><a name="Page_288" id="Page_288">[288]</a></span>on
+either side, was shifted laterally without any sign of wrenching from
+the adjoining ground, the maximum distance amounting to 6&frac34; feet. As
+the displacement took place parallel to the only river-course in the
+neighbourhood, Mr. Oldham attributes it to the sliding of the
+surface-layers over some yielding bed beneath. Again, throughout large
+areas of Northern Bengal, Lower Assam, and Maimansingh, rice-fields,
+which had been carefully levelled so that they might be uniformly
+flooded, were thrown into gentle undulations, the crests of which were
+occasionally two or three feet above the hollows. The piers of bridges
+were also moved parallel to, as well as towards, the streams, showing
+that the displacements extended to the depth of the foundations.</p>
+
+<p>The buckling of railway lines was often violent and took place over a
+large area. In the Charleston earthquake, every such bend was
+accompanied by a corresponding extension elsewhere (p. 113); but, in
+the Baluchistan earthquake of 1892, the neighbouring fish-joints were
+jammed up tight.<a name="FNanchor_73_73" id="FNanchor_73_73"></a><a href="#Footnote_73_73" class="fnanchor">[73]</a> In the one case, there was merely local
+compression; in the other, a permanent displacement of the earth's
+crust. The distortion of the Indian lines seems to belong to the
+former class. Repairs were of course generally made without delay; but
+all the information that could be obtained on this point showed that
+the compression causing the crumpling of the lines was accompanied by
+a compensating expansion, generally at a distance of about 300 yards.</p>
+
+<p><i>Sand-Vents.</i>&mdash;Shortly after the earthquake, large quantities of water
+and sand issued from fissures in the ground. At Dhubri, "innumerable
+jets of water, <span class='pagenum'><a name="Page_289" id="Page_289">[289]</a></span>like fountains playing, spouted up to heights varying
+from 18 inches to quite 3&frac12; or 4 feet. Wherever this had occurred,
+the land was afterwards seen to occupy a sandy circle with a
+depression in its centre. These circles ranged from 2 to 6 and 8 feet
+in diameter, and were to be seen all over the country. In some places,
+several were quite close together; in others they were at a distance
+of several yards." Near Maimansingh, they seem to have been almost as
+numerous, fifty-two, of four feet and less in diameter, being counted
+within an area 100 yards long and about 20 feet wide.</p>
+
+<p>The sand and water were ejected from the vents with some force. A few
+observers estimated the height of the spouts at about 12 feet, but
+this probably refers to stray splashes. It is clear, however, that the
+sand and water were forced not only up to the surface, but even in a
+continuous stream to heights of from two to ten feet above it. In many
+districts, trunks of trees or lumps of coal and fossil resin were
+washed up with the water, and even, in one or two cases, pebbles of
+hard rock weighing as much as half-a-pound.</p>
+
+<p>The origin of the sand-vents is to be sought in the presence of a
+water-bearing bed situated not far below the surface. In the central
+area, where there was a marked vertical component in the motion, this
+bed during the earthquake was compressed between those above and below
+it, and the resulting pressure was in places sufficient to force the
+water and sand, through the fissures formed by the earthquake, up to
+and beyond the surface. The gradual settling of the upper layer, cut
+up by the fissures, into the underlying quicksand, prolonged the
+process for some time after the shock was over; and, when the pressure
+was <span class='pagenum'><a name="Page_290" id="Page_290">[290]</a></span>at last relieved, some of the water was sucked back and so
+produced the crateriform hollows.</p>
+
+<p><i>Rise of River-Beds, etc.</i>&mdash;Over a large area, river-channels, tanks,
+wells, etc., were filled up, partly by the outpouring of the sand from
+vents, but chiefly, as shown by the forcing up of the central piers of
+bridges, by the elevation of the beds of the excavations. In the
+lowlands which lie between the Garo hills and the Brahmaputra, there
+were numerous channels from 15 to 20 feet in depth, the beds of which
+were pressed up until they became level with the banks, while a
+compensating subsidence took place close to the streams on either
+side. The general tendency of the earthquake was thus to obliterate
+the surface inequalities, so that, when the rivers rose later on, the
+district was extensively flooded.</p>
+
+<p>Besides these deferred floods, there occurred immediately after the
+earthquake a sudden rise in many rivers, amounting to from two to ten
+feet, followed by a gradual decline to the former state in two or
+three days. At Gauhati, for instance, the river-gauge showed that, at
+about three-quarters of an hour after the earthquake, the water stood
+7 feet 7 inches higher than on the morning of June 12th; at 7
+<span class="fakesc">A.M.</span> on June 13th it had fallen to 5 feet 8 inches, and at
+the same time on the two following days to 2 feet 7 inches and 6
+inches, showing that the water had returned nearly to its original
+level after the lapse of two and a half days.</p>
+
+<p>In most of the large rivers, the rise of water was due to the
+formation of partial dams formed by the local elevation of the
+river-beds described above. As the barriers were composed of loose
+sand, they were gradually scoured away and the material was spread
+over the bottom so as to leave the water at a level <span class='pagenum'><a name="Page_291" id="Page_291">[291]</a></span>slightly higher
+than that which it maintained before the earthquake.</p>
+
+<br />
+
+<h4>LANDSLIPS.</h4>
+
+<p>The distribution of landslips shows that their formation depends
+almost as much on local conditions as on the violence of the shock.
+The effect of the latter is manifested by their limitation to a
+certain central area. To the east of the North Cachar hills, few, if
+any, were to be seen; but, as far as Kohima, cracks or incipient
+landslips were formed on the hillsides. The Sylhet valley and a line
+to the west of Darjiling form the southern and western boundaries of
+the landslip area, which was therefore not less than 300 miles in
+length from east to west.</p>
+
+<p>Within this area, however, local conditions asserted their
+superiority. Among the more important may be mentioned the
+constitution of the hills and the presence of a thick superficial
+layer of subsoil or rock with an inner bounding surface of weak
+cohesion, the slope of the hillsides, and their height from base to
+crest. Thus, though the epicentral area was situated chiefly to the
+south of the Brahmaputra valley (Fig. 75), the east and west range of
+the landslips was more extensive in the Himalayas on the north side
+than in the Garo and Khasi hills on the south. In many places, the
+steep sides of the Himalayan valleys exist always in a critical
+condition of repose, and the effect of the Indian earthquake was such
+that all along the north side of the Brahmaputra valley, the range is
+scarred by landslips, even to the east of Tezpur.</p>
+
+<p>Again, along the southern edge of the Garo and Khasi hills, landslips
+were unusually prevalent. "Viewed from the deck of a steamer sailing
+up to <span class='pagenum'><a name="Page_292" id="Page_292">[292]</a></span>Sylhet," says Mr. Oldham, "the southern face of these hills
+presented a striking scene. The high sandstone hills facing the plains
+of western Sylhet, usually forest-clad from crest to foot, were
+stripped bare, and the white sandstone shone clear in the sun, in an
+apparently unbroken stretch of about 20 miles in length from east to
+west." At Cherrapunji, also, the deep valleys were so scored that,
+from a distance, there appeared to be more landslip than untouched
+hillside.</p>
+
+<p>But in no part, probably, were landslips more strikingly developed
+than in the small valley of the Mah&aacute;deo, which forms an amphitheatre
+about four miles long from east to west, and a mile and a half across,
+lying to the south of the B&aacute;lpakr&aacute;m and Pundengru hills. "Here,"
+remarks Mr. Oldham, "everything combined to favour the formation of
+landslips. The hills were composed of soft sandstone, they were
+steep-sided, high, and narrow from side to side, and consequently were
+doubtless thrown into actual oscillation as a whole; while the range
+of motion of the wave particle was not less than eight inches near the
+edge of the precipices. The result ... has been to produce an
+indescribable scene of desolation. Everywhere the hillsides facing the
+valley have been stripped bare from crest to base, and the seams of
+coal and partings of shale could be seen running in and out of the
+irregularities of the cliffs with a sharpness and distinctness which
+recalled the pictures of the ca&ntilde;ons of Colorado. At the bottom of the
+valley was a piled-up heap of <i>d&eacute;bris</i> and broken trees, while the old
+stream had been obliterated and the stream could be seen flowing over
+a sandy bed, which must have been raised many feet above the level of
+the old watercourse."</p>
+
+<p><span class='pagenum'><a name="Page_293" id="Page_293">[293]</a></span>In the sandstone districts of the area here considered, the landslips
+had some important secondary effects. Along the southern edge of the
+Garo and Khasi hills, great sand-fans spread over the fields, and the
+exposure of the hillsides formerly protected by forest left free scope
+for future denudation. Every stream of any size has in this way
+devastated many square miles of country. Among the hills themselves,
+more sand was brought down than the streams could carry away, and
+everywhere their beds were raised. "Ordinarily, the beds of these
+rivers, which are raging torrents when in flood, consist of a
+succession of deep pools separated by rocky rapids. After the rains of
+1897, it was found that the pools had been filled up, and the rapids
+obliterated by a great deposit of sand, over which the rivers flowed
+in a broad and shallow stream."</p>
+
+<p>A few valleys were for a short time barred across by landslips. In
+one, on the northern foot of the Garo hills, a landslip crossed the
+drainage channel and formed a shallow pond, which was not filled up by
+sand until the end of January 1898. Near Sinya, in the northern Khasi
+hills, an unusually large landslip formed a barrier, of which the
+remains are more than 200 feet above the level of the river-bed.
+Behind this, the water accumulated in a great lake until the beginning
+of September 1897, when the barrier burst and a flood of water rushed
+down the valley.</p>
+
+<br />
+
+<h4>ROTATION OF PILLARS, ETC.</h4>
+
+<div class="imgl" style="width: 20%;"><a name="Fig74" id="Fig74"></a>
+<a href="images/fig74.png">
+<img border="0" src="images/fig74.png" width="85%" alt="Twisting of monument at Chhatak." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 74.&mdash;Twisting of monument at Chhatak. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>A curious effect of earthquakes strong enough to damage buildings is
+that pillars, monuments, etc., may be fractured and the upper part
+rotated over the <span class='pagenum'><a name="Page_294" id="Page_294">[294]</a></span>lower without being overthrown. Even in Hereford and
+the surrounding villages, several pinnacles and chimney-stacks were
+twisted by the earthquake of 1896. The interest of the phenomenon,
+which has been known, since 1755,<a name="FNanchor_74_74" id="FNanchor_74_74"></a><a href="#Footnote_74_74" class="fnanchor">[74]</a> is mainly historical, for the
+endeavour to discover its cause was the origin of Mallet's views on
+the dynamics of earthquakes. Partly, also, it lies in the difficulty
+of finding a satisfactory explanation, or rather in deciding which of
+three or four possible explanations is the true one in any particular
+case.</p>
+
+<p>The Indian earthquake offered exceptional opportunities for studying
+the phenomenon in the large number of examples observed and the
+variety of objects rotated. None could be more striking than the
+twisted monument to George Inglis, represented in outline in Fig. 74.
+Chhatak, where this is situated, lies close to the southern boundary
+of the epicentral area. The monument is an obelisk, built of broad
+flat bricks or tiles on a base of 12 feet square, and originally more
+than 60 feet high. It was split by the earthquake into four portions.
+The <span class='pagenum'><a name="Page_295" id="Page_295">[295]</a></span>two upper, about six and nine feet long, were thrown down; while
+the third, 22 feet high, remains standing, but is twisted through an
+angle of 30&deg; with respect to the lowest part, which is unmoved. The
+upper of these two parts had evidently rocked on the lower, as the
+corners and edges were splintered, and below the fracture a slice of
+masonry about 15 inches thick, which was not bonded into the main
+mass, was split off by the pressure on its upper end. The plan of the
+parts still standing is shown in the lower part of Fig. 74.</p>
+
+<p>The possible explanations of the phenomenon are at least three in
+number. According to the first, which was given by Mallet in 1846, the
+adhesion of the twisted portion to its base is not uniform, and the
+resultant resistance to motion is not in the same vertical plane as
+the wave-movement.<a name="FNanchor_75_75" id="FNanchor_75_75"></a><a href="#Footnote_75_75" class="fnanchor">[75]</a> Some years later, Mallet offered another
+explanation. The body, he imagined, might be tilted on one edge by the
+earthquake, and, while still rocking, a second shock oblique to the
+first might twist it about that edge.<a name="FNanchor_76_76" id="FNanchor_76_76"></a><a href="#Footnote_76_76" class="fnanchor">[76]</a> In 1880, Professor T. Gray
+suggested that the column might be tilted on one corner and then
+twisted round it by later vibrations of the same shock.<a name="FNanchor_77_77" id="FNanchor_77_77"></a><a href="#Footnote_77_77" class="fnanchor">[77]</a></p>
+
+<p>None of these theories, Mr. Oldham argues, can give by itself a
+complete explanation of the phenomena observed in the central district
+of the Indian earthquake; and he therefore favours an extension of the
+second theory, which, though first proposed in 1882,<a name="FNanchor_78_78" id="FNanchor_78_78"></a><a href="#Footnote_78_78" class="fnanchor">[78]</a> was thought
+out independently and in greater <span class='pagenum'><a name="Page_296" id="Page_296">[296]</a></span>detail by himself. When the focus is
+of considerable dimensions, the shock at neighbouring places is
+constantly varying in direction, owing to the arrival of vibrations
+from different parts of the focus. Thus, instead of the two separate
+shocks required by Mallet's second explanation, we have a number of
+closely successive impulses frequently changing in direction and
+giving rise to what is known in the South of Europe as a vorticose
+shock. And, instead of a single twist of the pillars about one centre
+only, we have a series of small twists round a number of different
+centres, accompanied in consequence by a much smaller displacement of
+the centre of gravity than would have occurred had the same rotation
+been accomplished in one operation.</p>
+
+<p>The theory, it will be seen, accounts for the twisting of the pillar
+without overthrow, and for the splintering of the edges during the
+rocking of the column. It explains why in any district a number of
+similarly placed objects are generally twisted in the same direction.
+Moreover, a low column rocks to and fro more rapidly than a tall one
+similar in form and position, so that, at the instant when a later
+impulse comes from a different direction, two such columns might
+happen to be tilted on opposite edges, and would then be twisted in
+opposite directions. In certain cases, then, as occurred at several
+places during the Indian earthquake, an object may rotate in one
+direction, while others, similar in every respect but size, may be
+twisted in the opposite direction.</p>
+
+<br style="clear: both;" />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p><i>Frequency of After-Shocks.</i>&mdash;For some days after the great
+earthquake, the after-shocks by their very <span class='pagenum'><a name="Page_297" id="Page_297">[297]</a></span>frequency and by their wide
+distribution baffled close inquiry. During the first 24 hours,
+hundreds were felt at all points of the epicentral area; indeed, it is
+not too much to say that for several days the ground was never
+actually at rest. At the Bordwar tea-estate, which is traversed by one
+of the great fractures to be described in the next section, the
+surface of a glass of water on a table was for a whole week in a
+constant state of tremor; and at Tura a hanging lamp was kept
+continually swinging for the first three or four days.</p>
+
+<p>Most of these shocks were, of course, very slight; but, interspersed
+among them, were others of greater strength, and a few of considerable
+violence. One, on June 13th, about eight hours after the earthquake,
+was sensible beyond Allahabad&mdash;that is, for more than 520 miles from
+the epicentre; and another on the same day was felt in Calcutta,
+distant 255 miles. On June 14th, 22nd, and 29th, and again on August
+2nd and October 9th, shocks were noticed in that city; but, after the
+latter date, the disturbed area of no shock reached to so great a
+distance.</p>
+
+<p>To form any estimate of the total number of after-shocks is
+impossible, even for any one station. At first, lists were kept at
+isolated places, such as Shillong, Maimansingh, Dhubri, and a few
+others. Then, from July 15th, through Mr. Oldham's efforts, the
+records became more numerous until the end of the year, after which
+interest in the subject declined. Mr. Oldham's catalogue closes with
+the year 1898; but the register of a roughly-constructed seismograph,
+erected at Shillong in July 1897, continues to the present day.</p>
+
+<p>Imperfect as all non-instrumental registers must be, <span class='pagenum'><a name="Page_298" id="Page_298">[298]</a></span>they nevertheless
+furnish some idea of the frequency of the after-shocks. Thus, until
+the end of June, 679 shocks were recorded at Rangmahal (North
+Gauhati), 135 at Maimansingh, 89 at Kuch Bihar, and 83 at Kaunia
+(omitting those on June 12th). Again, from August 1st to 15th, 182
+were felt at Goalpara, 151 at Darangiri, 124 at Tura, 105 at Bijni, 94
+at Lakhipur, 94 at Krishnai, 48 at Dhubri, 28 at Rangpur, and 12 at
+Kuch Bihar; while at Borpeta, 113 shocks were reported during the
+first nine days of August. Turning to the registers of longer
+duration, we find that at Maophlang (near Shillong) 1,194 shocks were
+felt by one observer from September 12th, 1897, to October 7th, 1898;
+at the neighbouring station of Mairang, 1,065 from September 7th,
+1897, to December 31st, 1898; and at Tura, in the Garo hills, 1,145
+shocks from July 21st, 1897, to December 31st, 1898. The total number
+of earthquakes registered by the seismograph at Shillong from August
+1897 to the end of 1901 amounts to 1,274, and all of these were
+probably strong enough to arouse the observer from sleep. Outside the
+epicentral area, Mr. Oldham's list includes 88 shocks from June 12th
+to July 15th, about 950 from July 16th to December 31st (the period
+when the after-shocks were most carefully observed), and 296 shocks
+during the year 1898.</p>
+
+<p><i>Geographical Distribution of After-Shocks.</i>&mdash;When we endeavour to
+compare the lists of after-shocks at different places, we are at once
+met by two serious difficulties,&mdash;the imperfection of the records and
+the approximate character of the times of occurrence. Making every
+allowance, however, for these deficiencies, it is evident that very
+few of the <span class='pagenum'><a name="Page_299" id="Page_299">[299]</a></span>shocks felt at any one station were perceptible at its
+neighbours; in other words, that the shocks originated in a large
+number of foci scattered over a very wide area.</p>
+
+<p>For instance, two of the most carefully kept registers of after-shocks
+are those compiled at Maophlang (near Shillong), and at Mairang, only
+11 miles to the north-west. Now, between September 12th and September
+28th, 1897 (both dates inclusive), 92 shocks were felt at Maophlang
+and 83 at Mairang. Of the former, 37 were described as smart, 45
+slight, and 10 feeble; of the latter, 6 as smart, 9 slight, 65 feeble,
+and 3 very feeble. But, of the total number, only 20 were felt at both
+places at recorded times that were not more than fifteen minutes
+apart; 13 being described as smart&mdash;one at both places, one at Mairang
+alone, and the remaining 11 at Maophlang alone. When shocks occur so
+frequently, as in these cases, it is inevitable that, even if all were
+independent, some should coincide approximately in time of occurrence.
+It is therefore probable that only one in every eight shocks, and
+possibly only one in every twelve, was felt at both places.</p>
+
+<p>The actual numbers of shocks felt within stated periods at different
+places are perhaps hardly comparable, owing to the obvious
+imperfection of the records and the probably varying standards adopted
+by the reporters. But there can be little doubt that certain districts
+were more subject to after-shocks than others, especially such places
+as North Guahati, Shillong, and neighbouring villages, Tura,
+Darangiri, Goalpara, Bijni, Borpeta, Kaunia, and Rangpur. On the other
+hand, they seem to have been unusually scarce at Dhubri and in the
+district to the north-west, <span class='pagenum'><a name="Page_300" id="Page_300">[300]</a></span>and they became rare at Gauhati long
+before they ceased to be frequent at Borpeta. In the plain to the
+south of the Garo and Khasi hills, they were also uncommon, the
+combined records for Sylhet and Sonamganj for August 1-15 giving only
+20 shocks, and, neither to the east nor to the west of these places,
+is there any sign of greater frequency.</p>
+
+<p><i>Sound-Phenomena of After-Shocks.</i>&mdash;Many of the after-shocks were
+accompanied by sound, or else consisted of sound-vibrations only; and
+Mr. Oldham notices that such sounds were equally frequent both on the
+rocky ground of the hills and on alluvial plains nearly all the shocks
+that originated under the Borpeta plain being attended by distinctly
+audible rumblings.</p>
+
+<p>During his tour in the epicentral area in the winter of 1897-98, Mr.
+Oldham had many opportunities for observing these earth-sounds. They
+were, he says, close to the lower limit of audibility, less a note
+than a rumble, and very like distant thunder, though sometimes they
+consisted of a rapid succession of short sounds, such as is caused by
+a cart when driven rapidly over a rough pavement. "As a rule, they
+began as a low, almost inaudible rumble, gradually increasing in
+loudness, though to a very varying degree, and then gradually dying
+out after having lasted anything from 5 to 50 seconds. It cannot be
+said that there was any connection between the duration and the
+loudness of the sounds, some of the most prolonged never becoming
+loud, and some of those which lasted a shorter period being as loud as
+ordinary thunder at a distance of two or three miles."</p>
+
+<p>Mr. Oldham records an interesting fact in <span class='pagenum'><a name="Page_301" id="Page_301">[301]</a></span>connection with the
+distribution of the earth-sounds. At Naphak, in the Garo hills and
+about five miles south of Samin, 48 distinct rumbles were heard during
+23 hours on January 21-23, 1898, only seven of them being accompanied
+by a perceptible shock. At Samin, which was visited next, they were
+much less frequent, not more than 8 or 10 a day, and most of them
+attended by tremors. At Damra, a few miles to the north-east, they
+again became frequent; while, in the Chedrang valley, very few were
+heard, and only a small proportion of them were unaccompanied by
+sensible shocks. In the next section, it will be seen that the most
+conspicuous fault-scarps known in the epicentral area pass close by
+Samin and along the Chedrang valley. Thus, though the statement
+perhaps requires further confirmation, it would appear that
+earth-sounds were more common where the surface of the ground had been
+merely bent than where fractures extended right up to the surface.</p>
+
+<br />
+
+<h4>STRUCTURAL CHANGES IN THE EPICENTRAL AREA.</h4>
+
+<p>We come now to the important features which assign the Indian
+earthquake to a small class apart from nearly every other shock. Most
+earthquakes are due to movements that are entirely deep-seated. If
+strong enough, they may precipitate landslips or fissure the alluvial
+soil near river-channels. In the Neapolitan, Andalusian, and
+Charleston earthquakes, there were many such effects of the shock
+within the meizoseismal areas. In all three, however, the disturbances
+produced were superficial; no structural change, no fissuring that did
+not die out <span class='pagenum'><a name="Page_302" id="Page_302">[302]</a></span>rapidly downwards, was in any place perceptible. In the
+Riviera earthquake, the seismic sea-waves point to a small
+displacement of the ocean-bed; but it is only in the long fault-scarp
+of the central Japanese plain that we find a rival of the
+mountain-making movements that gave rise to the Indian earthquake.</p>
+
+<p>The boundary of the epicentral area, to the growth of which these
+distortions contributed, is represented by the curve marked A in Fig.
+68, and on a larger scale by the continuous line A in Fig. 75. A great
+part of the district is occupied by a group of hills known by various
+names locally, but which are conveniently included under the general
+term of the Assam range. To avoid the confusion of hill-shading, only
+the boundary of the range is indicated (by the broken line) in the map
+in Fig. 75. The Garo hills form the western part, and the Khasi and
+Jaintia hills the central and western parts, of the range as there
+depicted. They are formed chiefly of crystalline gneissic and granitic
+rocks and some metamorphic schists and quarzite, with cretaceous and
+tertiary rocks of varying thickness along its southern edge.</p>
+
+<p>Three stages have been distinguished in the history of the range.
+During the earliest, an old land-surface was worn down by rain and
+rivers till they were almost incapable of producing any further
+change. Traces of this surface are still visible in the plateau
+character of the mass. It was then elevated, not uniformly, but along
+a series of faults, so that it now consists of a succession of ranges,
+the face of each range being a fault-scarp, and its crest the edge of
+an adjoining plateau sloping away from the <span class='pagenum'><a name="Page_303" id="Page_303">[303]</a></span>summit. With this elevation
+began the third and last stage. The streams were able to work again,
+and deep gorges were cut out of the range, so that in parts its
+original character was nearly effaced. But the retention of that
+character in other districts is <span class='pagenum'><a name="Page_304" id="Page_304">[304]</a></span>of course evidence of the
+comparatively recent date of the final elevation.</p>
+
+<div class="img"><a name="Fig75" id="Fig75"></a>
+<a href="images/fig75.png">
+<img border="0" src="images/fig75.png" width="80%" alt="Epicentral Area of Indian Earthquake." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 75.&mdash;Epicentral Area of Indian Earthquake. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Owing to the great size of the epicentre and to the thickness of the
+forests which cover so much of its area, a comparatively small part of
+it could be traversed by Mr. Oldham during his tour in the winter of
+1897-98. The positions of the more important structural changes are
+indicated in Fig. 75. Of these, the fault-scarps are represented by
+continuous straight lines, the Bordwar fracture by the dotted straight
+line, pools and lakes not due to faulting by black ovals, reported
+changes in the aspects of the hills by circles, and the principal
+stations of the revised trigonometrical survey by crosses.</p>
+
+<p><i>Fault-Scarps.</i>&mdash;The most important fault-scarp is that called by Mr.
+Oldham the Chedrang fault, after the stream which coincides roughly
+with a great part of its course. The longer straight line in Fig. 75
+represents its position and general direction, and the sketch-map in
+Fig. 76 gives the plan of its southern half. From these, it will be
+seen that the fault follows on the whole a nearly straight path from
+south-south-east to north-north-west for not less than twelve miles,
+and that its throw, as indicated by the numbers to the right in Fig.
+76; is very variable, being zero in some places, and in one as much as
+35 feet or more. The upthrow is uniformly on the eastern side of the
+fault.</p>
+
+<p>At its southern end, as mapped in Fig. 76, there is no perceptible
+throw at the surface, but various marks of violence are manifested in
+the fissuring of the hillside and the snapping of small trees. About a
+quarter of a mile from this point, the fault crosses a tributary
+stream, where the throw amounts to two feet, and the <span class='pagenum'><a name="Page_305" id="Page_305">[305]</a></span>same distance
+farther on it meets the Chedrang river, the bed of which it crosses
+many times in its short course.</p>
+
+<div class="imgr" style="width: 25%;"><a name="Fig76" id="Fig76"></a>
+<a href="images/fig76.png">
+<img border="0" src="images/fig76.png" width="90%" alt="Plan of Chedrang fault." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 76.&mdash;Plan of Chedrang fault. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Mr. Oldham describes the fault in detail, as observed by him in
+February 1898. Here, it will be sufficient to refer to its more
+important features, and to its effects on the superficial drainage of
+the district. At the spot marked <i>a</i> (Fig. 76) the river, after
+running on the west or down-throw side of the fault for nearly half a
+mile, meets the scarp, and is ponded back by it for about a quarter of
+a mile upstream. For the next half-mile, the river keeps to the
+upthrow side of the fault, the scarp of which blocks the tributary
+streams from the west, forming a number of small pools. At the last of
+these, the total throw is not less than 25 feet. A little farther on,
+the fault crosses the Chedrang and causes the waterfall at <i>b</i>, the
+height of which, owing to the fall of dislodged fragments, does not
+exceed nine feet. The fault then runs along the old and now dry bed of
+the river, while the stream itself flows in a depression on the
+down-throw <span class='pagenum'><a name="Page_306" id="Page_306">[306]</a></span>side. About a quarter of a mile below the waterfall, the
+fault crosses the river, and soon after enters a large sheet of water
+at <i>c</i>, half a mile long, from 300 to 400 yards wide, and with a
+maximum depth of 18 feet. At first, the pool spreads on both sides of
+the fault, but the inequalities due to the scarp are evidenced by
+soundings. At the point where the fault leaves the pool, its throw is
+reduced to nothing, and it is just here that the water attains its
+greatest depth. To the north the throw increases rather rapidly, to 25
+feet in a quarter of a mile. But the peculiarity of this pool is that
+it is not, like the others mentioned above, dammed back by the
+fault-scarp. There is no barrier at its northern end, where the river
+escapes, except that formed by the gradually increasing throw of the
+fault. The pool is simply due to the reversal of the natural slope of
+the river-bed, caused by the formation of a roll or undulation in the
+ground on the upthrow side of the fault. Its recent origin is evident
+from the number of dead trees and bamboo clumps still standing in the
+water.</p>
+
+<p>For a mile after the fault leaves the pool, its throw varies
+considerably. It rises, as already mentioned, from zero to 25 feet. A
+little farther on, the fault runs up the side of a spur, the throw
+increasing to 31 feet; and, in this part, the violence of the shock
+was shown by the dislodgment of blocks of granite as much as 20 feet
+in diameter, and by the overthrow or destruction of many trees. After
+crossing the spur, the fault returns to the neighbourhood of the
+river, and crosses its bed four times, forming pools (<i>e</i>, <i>g</i>) or
+waterfalls (<i>d</i>, <i>f</i>) according as the scarp occurs on the downstream
+or upstream side. The throw of the fault then changes considerably
+within little more than half a <span class='pagenum'><a name="Page_307" id="Page_307">[307]</a></span>mile, from 18 feet to zero and again to
+20 feet, the undulation so formed producing a large pool (<i>h</i>)
+entirely on the upthrow side of the fault.</p>
+
+<p>At the point marked <i>i</i> on the map, the river once more crosses the
+fault; but the bottom of the valley is filled with alluvium, and,
+instead of a waterfall, a large sandy delta spreads down the stream.
+The scarp is, however, readily traced on the east side of the river, a
+throw of 32 feet being measured. After this, the alluvium becomes of
+considerable thickness, and the continuation of the fault is marked by
+a short slope, which tilts over the trees when it traverses
+forest-land. Leaving the valley of the Chedrang, the fault crosses an
+open plain, and is followed with some difficulty to the neighbourhood
+of Jhira, where, owing to the thick bed of alluvium, it forms a gentle
+roll or undulation of the surface, crossing the main channel of the
+Krishnai to the north-east of Jhira. On the west side of this barrier
+a large sheet of water, a mile and a half in length, three-quarters of
+a mile wide, and 12 feet in depth, gathered over the village of Jhira.
+"On the east side of the Jhira lake," says Mr. Oldham, "there is ample
+evidence of change of level, for part of the dry land was formerly ...
+perpetually under water, and at one place the remains of an old
+irrigation channel can be seen.... At the northern end of the lake the
+drainage now makes its escape in a broad and shallow sheet of water
+over what was once high land covered with <i>sal</i> forest."</p>
+
+<p>This is the last marked feature due to the Chedrang fault. Beyond the
+north of Jhira the throw rapidly diminishes, and perhaps dies out
+altogether before reaching the low hills lying to the north of that
+village.</p>
+
+<p><span class='pagenum'><a name="Page_308" id="Page_308">[308]</a></span>In several ways, this fault-scarp differs from that formed with the
+Japanese earthquake of 1891. Throughout its course the down-throw,
+wherever it is perceptible, is invariably to the west; in no place
+could any trace of horizontal shifting be detected; and the plane of
+the fault, when it traversed rock, is practically vertical.</p>
+
+<p>Whether the scarp was formed by the elevation of the rock to the east
+of the fault, or by the depression of that to the west, or by both
+such movements at once, there is no decisive evidence; but there are
+very good reasons for believing the first alternative to be the true
+one. The undulations in the ground which gave rise to the large pools
+at <i>c</i> and <i>h</i> (Fig. 76) occur on the east side of the fault. Also,
+between the outlet of the lake at Jhira and the point where the
+Krishnai rejoins its original channel, the gradient of the river
+approaches that of a mountain stream, although the new bed consists of
+alluvium, and not of rock. Now, the alluvial plain of this district is
+raised so slightly above the sea-level that no subsidence great enough
+to have caused the existing gradient could have occurred without the
+depressed area being flooded with water. Though some movements may
+have taken place on the west side of the fault, it seems clear, then,
+that elevation of the rock on the east side was the predominant, if
+not the sole, cause of the fault-scarp.</p>
+
+<p>As the Chedrang fault has been described somewhat fully, a brief
+reference to the rest will be sufficient The only other known scarp of
+any consequence lies about ten miles to the south of the Chedrang
+fault, and runs by the village of Samin, with an average course from
+E. 30&deg; S. to W. 30&deg; N. <span class='pagenum'><a name="Page_309" id="Page_309">[309]</a></span>Its total length does not exceed 2&frac12; miles.
+The down-throw is uniformly to the north, and the throw, which
+amounts to ten feet near its centre, gradually diminishes to zero at
+either end. Several pools are formed along the course of the
+fault-scarp by the blocking of small streams.</p>
+
+<p><i>The Bordwar Fracture.</i>&mdash;In the map of the epicentral area (Fig. 75),
+this remarkable fracture is represented by a dotted straight line. It
+is apparently an incipient fault. Though traceable for a distance of
+about seven miles, at no point is there any decisive evidence of
+either vertical or horizontal displacement; and, even if some doubtful
+indications of a change of level should be real, the throw must
+certainly be less than one foot. Yet, in the immediate neighbourhood
+of the fracture, the violence of the shock was extreme. "Trees have
+been overthrown or killed as they stood; a huge mass of rock,
+dislodged from near the crest of the hills, has rolled down the slope,
+scoring the side of the hill. On the opposite side an equally large
+block has been dislodged, and in its downward course cleared a
+straight track down the hill; and on the summit a gap has been cleared
+by the overthrow of trees along the line of fracture." Being only a
+few inches in width where it has rent the solid rock, the fracture was
+difficult to follow in many parts of its course. But, through
+forest-clad land, its track was marked by "a well-defined band of
+about half a mile broad, in which overturned trees are much more
+abundant than on either side, and towards the centre of this band the
+overturned trees are not only more numerous, but many of the smaller
+ones, up to six inches in diameter, have been snapped across by the
+violence of the shock."</p>
+
+<p><span class='pagenum'><a name="Page_310" id="Page_310">[310]</a></span><i>Lakes and Pools not due to Faulting.</i>&mdash;A few miles to the south of
+the Chedrang and Samin faults, and also of the Bordwar fracture,
+occurs a group of lakes or pools, represented on the map of the
+epicentral area (Fig. 75) by small black ovals. In the gradual
+increase in depth from either end, they resemble the two large sheets
+of water along the course of the Chedrang fault (<i>c</i> and <i>h</i>, Fig.
+76), but they differ from them in having no direct connection with any
+apparent fault.</p>
+
+<p>One of these pools lies in the valley of the Rongtham river, to the
+south of the Samin fault. It seemed, at first sight, to be nothing
+more than an ordinary pool, such as may be seen on any mountain
+stream. On the bottom, and close to the outlet, however, are coarse,
+partially rounded boulders, exactly resembling those farther down the
+river; and, as the old bed was followed up, these became coated with a
+slight deposit of sand and mud, pointing clearly to a change in the
+conditions under which they were formed. The water gradually deepened,
+until trees were met standing in the water, but killed by the recent
+submergence of their roots. The pool is nearly a quarter of a mile
+long, and its greatest depth (12 feet) occurs near the middle, just
+where the former stream, with an average depth of about a foot, was
+crossed by the track from Darangiri. Towards the upper end, the water
+shallows as gradually as it deepens at the other, and ends in a delta
+of boulders brought down by the stream above. As no fault could be
+discovered in the neighbourhood of the pool, it is evident that its
+formation was due to a bend of the river-bed, the maximum change of
+level, taking into account the river-slope, being not less than 24
+feet.</p>
+
+<p><span class='pagenum'><a name="Page_311" id="Page_311">[311]</a></span>Similar features characterise the other pools that were examined, some
+of which are smaller, and others larger, than that described above.
+One, higher up the valley of the Rongtham, has a length of about 1&frac12;
+mile and a maximum depth of 18 feet. Others of the same type, but of
+smaller size, were observed among the Khasi hills, about fifteen miles
+south of the Bordwar fissure; and it is probable that many others
+would have been found in the intermediate district, which Mr. Oldham
+was unable to visit.</p>
+
+<p><i>Changes in the Aspects of the Hills.</i>&mdash;There are, again, other facts
+of considerable interest which point to changes of level over a wide
+area; the places where they were noticed being indicated by small
+circles in Fig. 75. For instance, from Maophlang, near Shillong, a
+road leads to the neighbouring station of Mairang. Before the
+earthquake, only a short stretch of this road could be seen from the
+former place, as it rounded a spur about three miles away. Now, a much
+longer stretch is visible, and it can also be seen passing round the
+next, and previously hidden, spur. In this district the movements seem
+to have continued with the after-shocks; for, before the earthquake,
+the crest only of a ridge about a mile and a half to the west was
+visible; while, after it, a considerable portion could be seen, and
+much more some months later than immediately after the shock.</p>
+
+<p>Again, from a spot near the southern end of the Chedrang fault, it
+used to be only just possible to see the Brahmaputra over an
+intervening hill; whereas, now, the whole width of the river has come
+into view.</p>
+
+<p>Lastly, at Tura, which is 95 miles west of Maophlang, a battalion of
+military police were accustomed to signal by heliograph with another
+station, Rowmari, <span class='pagenum'><a name="Page_312" id="Page_312">[312]</a></span>15 miles farther to the west. This, formerly, could
+just be done by means of a ray which grazed a hill between the two
+places; it can now be done quite easily, and, in addition, a broad
+stretch of the plains east of the Brahmaputra is visible from the same
+spot.</p>
+
+<p><i>Revision of the Trigonometrical Survey.</i>&mdash;The movements described in
+the preceding pages are of course referred to points which may
+themselves have been displaced, and only a revision of the
+trigonometrical survey of the epicentral area and of part of the
+surrounding district could determine their absolute magnitude. During
+the cold weather of 1897-98, some of the triangles were re-measured by
+a member of the trigonometrical survey; but, as the time at his
+disposal was short, they were confined to the eastern part of the
+epicentral area, as the focus at that time was supposed to lie under
+the Khasi hills. The positions of some of these stations are indicated
+by crosses in Fig. 75; and in Fig. 77 the more important triangles are
+shown. In the revised work, all tower stations, consisting of brick
+towers built on alluvium, were omitted, as it could not be assumed
+that they had been undisturbed by displacements of the superficial
+beds.</p>
+
+<p>In re-calculating the lengths of the sides, the side
+Rangsanobo-Taramun Tila was adopted as the initial base, and the
+height of Rangsanobo as the initial height; a choice which later
+experience showed to be unfortunate, for Taramun Tila probably lies
+just outside, and Rangsanobo within, the epicentral area. Of the 16
+sides, whose old and new lengths were compared, only one was found to
+be apparently unchanged, two were shortened by an inch or two, while
+<span class='pagenum'><a name="Page_313" id="Page_313">[313]</a></span>the others were all lengthened by amounts varying from one to eight
+or nine feet, the numbers affixed to the sides in Fig. 77 denoting the
+calculated increases in feet. Assuming the new base-line to be
+unaltered by the earthquake movements, these changes imply the
+following displacements of the principal stations:&mdash;Thanjinath 6 feet,
+Mun 4, and Laidera 2, feet to the north; Mopen 5, Dinghei 9, Landau
+Modo 12, and Umter 11, feet to the north-west; and Mosingi 3, and
+<span class='pagenum'><a name="Page_314" id="Page_314">[314]</a></span>Mautherrican 5, feet to the west. At the same time, the height of
+most of the stations was found to be increased with reference to that
+of Rangsanobo: Mun by 2 feet, Thanjinath and Umter by 3, Mosingi by 4,
+Taramun Tila and Laidera by 6, Dinghei by 7, Landau Modo by 17, and
+Mautherrican by 24, feet; while the height of Mopen seems to have been
+diminished by 4 feet. Thus, at first sight, these calculations appear
+to indicate "a general elevation and extension of the hills, such as
+might follow on a bulging upwards of the surface due to the extension
+of a large mass of molten matter underground."</p>
+
+<div class="img"><a name="Fig77" id="Fig77"></a>
+<a href="images/fig77.png">
+<img border="0" src="images/fig77.png" width="55%" alt="Re-triangulation of Khasi hills." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 77.&mdash;Re-triangulation of Khasi hills. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Unfortunately, as Mr. Oldham shows, a very different, and more
+probable, interpretation may be given of these results; for all the
+calculated changes are rendered uncertain by the choice of the two
+stations which form the ends of the new base-line. One at least may
+have been displaced by the structural movements within the epicentral
+area; and, moreover, the line joining them runs nearly north and
+south. As compression in this direction is to be expected, it is
+probable that this line was shortened; and the assumption that its
+length was unchanged would therefore lead to an apparent expansion of
+all the other sides.</p>
+
+<p>The calculated changes seem to favour this explanation to a great
+extent. The sides joining Mopen, Rangsanobo, and Thanjinath run nearly
+east and west, and are apparently lengthened by 4.9 and 3.4 feet
+respectively; while, of the four sides joining these stations to
+Mosingi and Mun, lying next to the north, two are nearly or quite
+unchanged, and the others increased by 2.3 and 3.2 feet. Again, the
+estimated increase of the Mosingi-Mun line is 4.4 feet; while the <span class='pagenum'><a name="Page_315" id="Page_315">[315]</a></span>four
+sides joining these stations to the next northerly group are
+increased by small amounts&mdash;namely, 1.2, 2.6,-0.3, and 2.4 feet. Thus,
+the apparent expansion that should have occurred in these more or less
+northerly sides is lessened, or roughly compensated, probably by a
+compression of the whole region in a meridianal direction.</p>
+
+<p>For a similar reason, the slight general upheaval of the hills
+indicated by the repeated calculations, must be regarded as doubtful,
+for it depends on the assumed fixity of the station of Rangsanobo,
+whereas it is more probable that it was the height of Taramun Tila
+that remained unchanged. Reducing the calculated heights of all the
+other stations by six feet (the assumed rise of the latter), it
+follows that, on the whole, the height of the Khasi hills underwent
+but little change, except at Mautherrican and Landau Modo, and the
+secondary stations of Mairang and Kollong Rock, near Maonoi. The
+apparent elevations of 24, 17, 11, and 15 feet at these places exceed
+the probable error of the observations; and it is worthy of notice
+that all four stations lie close to the edge of fault-scarps, while
+Landau Modo is not far from two of the pools formed by distortion of
+the surface unaccompanied by faulting.</p>
+
+<p>If, then, the revised triangulation of the Khasi hills has failed to
+provide absolute measures of the displacements in the epicentral area,
+it has, nevertheless, proved that important movements, both horizontal
+and vertical, have taken place.</p>
+
+<p><i>Distribution of the Structural Changes.</i>&mdash;The boundary of the
+epicentral area, as drawn in Figs. 68 and 75, lays no claim to great
+accuracy; but its departure from the true line is probably in no place
+<span class='pagenum'><a name="Page_316" id="Page_316">[316]</a></span>considerable. It must evidently include all the districts where
+marked structural changes occurred, and must therefore extend east of
+Maophlang and west of Tura. Towards the north, these changes have been
+traced to the foot of the Garo hills, and there is some, though not
+very certain, evidence of alterations of level along the course of the
+Brahmaputra. The very large number of after-shocks recorded at Borpeta
+and Bijni also points to an extension of the epicentral area beyond
+these places. To the east, the course of the boundary becomes
+doubtful, but it must pass close to Gauhati and east of Shillong, and
+probably ends a short distance beyond Jaintiapur. The southern
+boundary must coincide nearly with the north edge of the alluvial
+plains of Sylhet, for there is no evidence of its intrusion into the
+plains. On the west side, the epicentral area includes the Garo hills
+and part of the alluvial plain to the west; and, from the large number
+of after-shocks felt at Rangpur and Kaunia, and the great violence of
+the shock at the former, we may infer that both places lie within the
+boundary-line. If, then, there is no great error in the mapping of
+this line, it follows that the epicentre was about 200 miles long from
+east to west, not less than 50, and possibly as much as 100, miles in
+maximum width, and contained an area of at least 6000 square miles.</p>
+
+<p>Near the boundary, the permanent displacements must have been
+comparatively small; but they were certainly marked in the northern
+part of the Assam hills for a distance of 100 miles from east to west.
+At the limits of the latter area, as Mr. Oldham remarks, "the evidence
+points to the changes being of the nature of long, low rolls, the
+change of slope being <span class='pagenum'><a name="Page_317" id="Page_317">[317]</a></span>insufficient to cause any appreciable change in
+the drainage channels. Then comes a zone in which the surface changes
+are more abrupt, the slopes of the stream beds have been altered so as
+to cause conspicuous changes in the nature of the streams, but any
+fracture or faulting which may have taken place has died out before
+the surface was reached. And north of this, close to the edge of the
+hills, the rocks have been fractured and faulted right up to the
+surface."</p>
+
+<br />
+
+<h4>ORIGIN OF THE EARTHQUAKE.</h4>
+
+<p>Almost every feature of the great earthquake points to an origin very
+different from that of the others described in this volume. The
+suddenness with which the shock began, its unusual duration, and the
+occurrence of many maxima of intensity, are inconsistent with a simple
+fault-displacement. Again, the excessive velocities of projection at
+Rambrai and elsewhere, the existence of isolated fault-scarps and
+fractures, the local changes of level, the compression indicated by
+the revised trigonometrical survey, the wide area over which these
+structural changes took place, and the numerous distinct centres of
+subsequent activity, all these phenomena demonstrate the intense and
+complex character of the initial disturbances, as well as the
+widespread bodily displacement of the earth's crust within the
+epicentral area. There may, it is conceivable, have been a number of
+foci, nearly or quite detached from one another, and giving rise to a
+group of nearly concurrent shocks. Or&mdash;and this is a far more probable
+supposition&mdash;there may have been one vast deep-seated centre, from
+which off-shoots ran up towards the surface, each partaking <span class='pagenum'><a name="Page_318" id="Page_318">[318]</a></span>to a
+greater or less degree in the movement within the parent focus.</p>
+
+<p>As Mr. Oldham points out, we have recently become acquainted with a
+structure exactly corresponding to that which is here inferred. The
+great thrust-planes, so typically developed in the Scottish Highlands,
+are only reversed faults which are nearly horizontal instead of being
+highly inclined; and they are accompanied by a number of ordinary
+reversed faults running upwards to the surface. In Fig. 78, the main
+features of a section drawn by the Geological Survey of Scotland are
+reproduced; T, T, representing thrust planes, and <i>t</i>, <i>t</i>, minor
+thrusts or faults. A great movement along one of the main
+thrust-planes would carry with it dependent slips along many of the
+secondary planes. Direct effects of the former might be invisible at
+the surface, except in the horizontal displacements that would be
+rendered manifest by a renewed trigonometrical survey; whereas the
+latter might or might not reach the surface, giving rise in the one
+case to fissures and fault-scarps, in the other to local changes of
+level, and in both to regions of instability resulting in numerous
+after-shocks.</p>
+
+<div class="img"><a name="Fig78" id="Fig78"></a>
+<a href="images/fig78.png">
+<img border="0" src="images/fig78.png" width="80%" alt="Diagram of Thrust-planes." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 78.&mdash;Diagram of Thrust-planes.<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The enormous dimensions of the parent focus will be obvious from the
+phenomena that have been <span class='pagenum'><a name="Page_319" id="Page_319">[319]</a></span>described above. Mr. Oldham has traced the
+probable form of the epicentre. It may in reality be neither so
+simple nor so symmetrical as is represented in Fig. 75, but there are
+good reasons for thinking that it does not differ sensibly either in
+size or form from that laid down. The part of the thrust-plane over
+which movement took place must therefore have been about 200 miles
+long, not less than 50 miles wide, and between 6000 and 7000 square
+miles in area. With regard to its depth, we have no decisive
+knowledge. It may have been about five miles or less; it can hardly
+have been much greater.</p>
+
+<p>It is a strain on the imagination to try and picture the displacement
+of so huge a mass. We may think, if we will, of a slice of rock three
+or four miles in thickness and large enough to reach from Dover to
+Exeter in one direction and from London to Brighton in the other; not
+slipping intermittently in different places, but giving way almost
+instantaneously throughout its whole extent; crushing all before it,
+both solid rock and earthy ground alike; and, whether by the sudden
+spring of the entire mass or by the jar of its hurtling fragments,
+shattering the strongest work of human hands as easily as the
+frailest. Such a thrust might well be sensible over half a continent,
+and give rise to undulations which, unseen and unfelt, might wend
+their way around the globe.</p>
+
+<br />
+
+<h4>REFERENCES.</h4>
+
+<div class="block"><p class="hang">1. <span class="sc">Agamennone, G.</span>&mdash;"Notizie sui terremoti osservati in
+Italia durante l'anno 1897 (Terremoto dell' India poco dopo
+il mezzogiorno del 12 giugno)." <i>Ital. Sismol. Soc. Boll.</i>,
+vol. iii., pte. ii., 1897, pp. 249-293.</p>
+
+<p class="hang"><span class='pagenum'><a name="Page_320" id="Page_320">[320]</a></span>2. &mdash;&mdash; "Il terremoto dell' India del 12 giugno 1897." <i>Ibid.</i>, vol.
+iv., 1898, pp. 33-40.</p>
+
+<p class="hang">3. &mdash;&mdash; "Eco in Europa del terremoto indiano del 12 giugno 1897."
+<i>Ibid.</i>, vol. iv., 1898, pp. 41-67. (See also the same volume,
+pp. 167-172.)</p>
+
+<p class="hang">4. <span class="sc">Baratta, M.</span>&mdash;"Il grande terremoto indiano del 12 giugno
+1897." <i>Ital. Soc. Geogr. Boll.</i>, vol. x., 1897, fasc. viii.</p>
+
+<p class="hang">5. <span class="sc">Cancani, A.</span>&mdash;"I pendoli orizzontali del R. Osservatorio
+geodinamico di Rocca di Papa, ed il terremoto indiano del 12
+giugno 1897." <i>Ital. Sismol. Soc. Boll.</i>, vol. iii., 1897, pp.
+235-240.</p>
+
+<p class="hang">6. <span class="sc">Heath, T.</span>&mdash;"Note on the Calcutta Earthquake (June 12th,
+1897) as recorded by the bifilar pendulum at the Edinburgh
+Royal Observatory." <i>Edinb. Roy. Soc. Proc.</i>, 1897, pp.
+481-488.</p>
+
+<p class="hang">7. <span class="sc">Oldham, R.D.</span>&mdash;"Report on the Great Earthquake of 12th
+June 1897." <i>Mems. Geol. Surv. of India</i>, vol. xxix., 1899,
+pp. i.-xxx., 1-379, with 44 plates and 3 maps.</p>
+
+<p class="hang">8. &mdash;&mdash; "List of After-shocks of the Great Earthquake of 12th June
+1897." <i>Ibid.</i>, vol. xxx., pt. i., 1900, pp. 1-102.</p>
+
+<p class="hang">9. &mdash;&mdash; "On Tidal Periodicity in the Earthquakes of Assam." <i>Journ.
+Asiat. Soc.</i>, vol. lxxi., 1902, pp. 139-153.</p></div>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_69_69" id="Footnote_69_69"></a><a href="#FNanchor_69_69"><span class="label">[69]</span></a> According to some reports, the earthquake was felt in
+Italy. At Livorno, the first movements were registered by seismographs
+at 11.17 <span class="fakesc">A.M.</span> (G.M.T.), and tremors were noticed by some
+persons at rest at about 11.15 <span class="fakesc">A.M.</span> At Spinea, a sensible
+undulatory shock from south-east to north-west, and lasting about four
+seconds, was felt at the moment when all the seismographs were set in
+motion by the Indian earthquake. In spite of the great distance, the
+perception of the earthquake in Italy is not impossible, but the
+records seem to me to refer to local tremors rather than to the very
+slow evanescent oscillations of a very distant earthquake.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_70_70" id="Footnote_70_70"></a><a href="#FNanchor_70_70"><span class="label">[70]</span></a> All the times in this section are referred to Madras
+mean time, which is 5h. 20m. 59.2s. in advance of Greenwich mean time.
+In the next section it will be found convenient to use the latter
+standard.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_71_71" id="Footnote_71_71"></a><a href="#FNanchor_71_71"><span class="label">[71]</span></a> It may be useful to give references to works in English
+in which the principal instruments for registering distant earthquakes
+are described. For Cancani's vertical pendulum, see <i>Brit. Assoc.
+Rep.</i>, 1896, pp. 46-47; Darwin's bifilar pendulum, <i>Brit. Assoc.
+Rep.</i>, 1893, pp. 291-303, and <i>Nature</i>, vol. 1., 1894, pp. 246-249;
+Milne's horizontal pendulum, <i>Seismology</i>, pp. 58-61;
+Rebeur-Paschwitz's horizontal pendulum, <i>Brit. Assoc. Rep.</i>, 1893, pp.
+303-308.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_72_72" id="Footnote_72_72"></a><a href="#FNanchor_72_72"><span class="label">[72]</span></a> The beginnings of the second and third phases are shown
+more clearly in the record of the vertical pendulum at Catania, a
+record, however, that will not bear the reduction necessary for these
+pages.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_73_73" id="Footnote_73_73"></a><a href="#FNanchor_73_73"><span class="label">[73]</span></a> <i>Geol. Mag.</i>, vol. x., 1893, pp. 356-360.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_74_74" id="Footnote_74_74"></a><a href="#FNanchor_74_74"><span class="label">[74]</span></a> <i>Irish Acad. Trans.</i>, vol. xxi, 1848, p. 52.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_75_75" id="Footnote_75_75"></a><a href="#FNanchor_75_75"><span class="label">[75]</span></a> <i>Irish Acad. Trans.</i>, vol. xxi., 1848, pp. 55-57.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_76_76" id="Footnote_76_76"></a><a href="#FNanchor_76_76"><span class="label">[76]</span></a> <i>Neapolitan Earthquake of 1857</i>, vol. i., 1862, pp.
+376-378.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_77_77" id="Footnote_77_77"></a><a href="#FNanchor_77_77"><span class="label">[77]</span></a> <i>Japan Seismol. Soc. Trans.</i>, vol. i., pt. II., 1880,
+pp. 33-35.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_78_78" id="Footnote_78_78"></a><a href="#FNanchor_78_78"><span class="label">[78]</span></a> <i>Geol. Mag.</i>, vol. ix., 1882, pp. 257-265.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="CHAPTER_X" id="CHAPTER_X"></a><hr />
+<span class='pagenum'><a name="Page_321" id="Page_321">[321]</a></span><br />
+
+<h3>CHAPTER X.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+
+<h4>CONCLUSION.</h4>
+<br />
+
+<p>In this concluding chapter, I propose to give a summary of the results
+at which we have arrived from the study of recent earthquakes, and
+this can, I think, be done best by describing what may be regarded as
+an average or typical earthquake, though it may be convenient
+occasionally to depart slightly from such a course. Few shocks have
+contributed more to our knowledge than the majority of those described
+in this volume; but, on certain points, we gain additional information
+from the investigation of other earthquakes, and these are referred to
+when necessary for the purpose in view.</p>
+
+<br />
+
+<h4>FORE-SHOCKS.</h4>
+
+<p>At the outset, we are met by a question of some interest and great
+practical importance&mdash;namely, whether there are any constant signs of
+the coming of great earthquakes by means of which their occurrence
+might be predicted and their disastrous effects mitigated.</p>
+
+<p>Excluding the Ischian earthquakes, which belong to a special class, it
+is evident that there is generally some slight preparation for a great
+earthquake. For a few hours or days beforehand, weak shocks and
+tremors are felt or rumbling noises heard within the <span class='pagenum'><a name="Page_322" id="Page_322">[322]</a></span>future
+meizoseismal area. But, unfortunately, it has not yet been found
+possible to distinguish these disturbances from others of apparently
+the same character which occur alone, so that for the present they
+fail to serve as warnings.</p>
+
+<p>In Japan, where the organisation of earthquake-studies is more
+complete than elsewhere, it is possible that a vague forecast might be
+made, if the distribution of the fore-shocks of the earthquake of 1891
+should prove to be a general feature of all great earthquakes. It was
+at first supposed that this earthquake occurred without preparation of
+any kind; but a closer analysis of the records shows that during the
+previous two years there was a very decided increase in the seismic
+activity of the district, and also that the distribution of the
+epicentres marked out the future fault-scarp, and at the same time
+exhibited a tendency to comparative uniformity over the whole
+fault-region.</p>
+
+<p>For the present, then, the only warning available is that given by the
+preliminary sound, which may precede the strongest vibrations by as
+much as five or ten or even more seconds. Though two or three seconds
+may elapse before its character is recognised, the fore-sound thus
+allows time for many persons to escape from their falling houses. Some
+races, however, are less capable of hearing the sound than others, and
+this may be one reason why Japanese earthquakes are so destructive of
+human life.</p>
+
+<br />
+
+<h4>DISTURBED AREA.</h4>
+
+<p>It is usual with some investigators to measure the intensity of an
+earthquake roughly by the extent of <span class='pagenum'><a name="Page_323" id="Page_323">[323]</a></span>its disturbed area. The depth of
+the seismic focus must of course have some influence on the size of
+this area, and this condition is only neglected because we have no
+precise knowledge of the depth in any case. Thus, Mr. Oldham regards
+the Indian earthquake of 1897 as rivalling the Lisbon earthquake of
+1755, which is generally considered to hold the first place, because
+its disturbed area was not certainly exceeded by that of the latter.</p>
+
+<p>That disturbed area is, however, an untrustworthy measure of intensity
+will be evident from the following table, in which the earthquakes
+described in this volume (omitting those of Ischia) are arranged as
+nearly as may be in order of intensity, beginning with the
+strongest:&mdash;</p>
+
+<div class="centered">
+<table border="0" cellpadding="1" cellspacing="0" width="40%" summary="Size of Disturbed area in order of intensity">
+  <tr>
+  <td class="tdl" width="70%">Earthquake.</td>
+  <td class="tdr" width="30%">Disturbed Area <br />in Sq. Miles.</td>
+  </tr>
+  <tr>
+  <td class="tdl">Indian</td>
+  <td class="tdr">1,750,000</td>
+  </tr>
+  <tr>
+  <td class="tdl">Japanese</td>
+  <td class="tdr">330,000</td>
+  </tr>
+  <tr>
+  <td class="tdl">Neapolitan</td>
+  <td class="tdr">39,200</td>
+  </tr>
+  <tr>
+  <td class="tdl">Charleston</td>
+  <td class="tdr">2,800,000</td>
+  </tr>
+  <tr>
+  <td class="tdl">Riviera</td>
+  <td class="tdr">219,000</td>
+  </tr>
+  <tr>
+  <td class="tdl">Andalusian</td>
+  <td class="tdr">174,000</td>
+  </tr>
+  <tr>
+  <td class="tdl">Hereford</td>
+  <td class="tdr">98,000</td>
+  </tr>
+  <tr>
+  <td class="tdl">Inverness</td>
+  <td class="tdr">33,000</td>
+  </tr>
+</table>
+</div>
+
+<br />
+
+<p>Here we see that the Charleston earthquake was perceptible over a
+greater area than the Indian earthquake, while the Neapolitan
+earthquake was inferior to that of Hereford in this respect. The
+explanation of course is that the boundaries of the disturbed areas
+are isoseismal lines corresponding to different degrees of intensity,
+the inhabitants of Great Britain and the United States being evidently
+more sensitive to weak tremors, or more observant, than <span class='pagenum'><a name="Page_324" id="Page_324">[324]</a></span>those of
+Italy, Spain, or Central Asia. The only disturbed areas that are
+bounded by isoseismals of the same intensity are the two last. Very
+roughly, then, we may say that the intensity of the Hereford
+earthquake was three times as great as that of the Inverness
+earthquake.</p>
+
+<br />
+
+<h4>POSITION OF THE EPICENTRE.</h4>
+
+<p>One of the first objects in the investigation of an earthquake is to
+determine the position and form of the epicentre. In a few rare cases,
+as in the Japanese and Indian earthquakes, when the fault-scarp is
+left protruding at the surface, only careful mapping is required to
+ascertain both data. But, in the great majority of earthquakes, the
+fault-slip dies out before reaching the surface and the position of
+the epicentre is then inferred by methods depending chiefly on the
+time of occurrence or on the direction or intensity of the shock.</p>
+
+<p>At first sight, methods that involve the time of occurrence at
+different places seem to be of considerable promise. No scientific
+instruments are so widely diffused as clocks and watches; but, on the
+other hand, few are so carelessly adjusted. It is the exception,
+rather than the rule, to find a time-record accurate to the nearest
+minute; and, as small errors in the time may be of consequence,
+methods depending on this element of the earthquake are seldom
+employed. If, however, the number of observations is large for the
+size of the disturbed area, the construction of coseismal lines may
+define approximately the position of the epicentre. In the Hereford
+earthquake of 1896, the centre of the innermost <span class='pagenum'><a name="Page_325" id="Page_325">[325]</a></span>coseismal line (Fig.
+62) is close to the region lying between the two epicentres.</p>
+
+<p>The method of locating the epicentre by means of the intersection of
+two or more lines of direction of the shock was first suggested by
+Michell in 1760,<a name="FNanchor_79_79" id="FNanchor_79_79"></a><a href="#Footnote_79_79" class="fnanchor">[79]</a> and has been employed by Mallet in investigating
+the Neapolitan earthquake, by Professors Taramelli and Mercalli in
+their studies of the Andalusian and Riviera earthquakes, as well as by
+other seismologists. The diversity of apparent directions at one and
+the same place caused its temporary neglect, until Professor Omori
+showed in 1894 that the mean of a large number of measurements gives a
+trustworthy result (p. 19). His interesting observations should
+reinstate the method to its former place among the more valuable
+instruments at the disposal of the seismologist.</p>
+
+<p>No observations, however, are at present so valuable for the purpose
+in view as those made on the intensity of the shock. For many years,
+it has been the custom to regard the epicentre as coincident with the
+area of greatest damage to buildings; and, when the area is small, the
+assumption cannot be much in error. It is of course merely a rough way
+of obtaining a result that is generally given more accurately by means
+of isoseismal lines; but there are exceptional cases, such as the
+Neapolitan and Ischian earthquakes, when the destruction wrought by
+the earthquake furnishes evidence of the greater value.</p>
+
+<p>A single isoseismal accurately drawn not only gives the position of
+the epicentre with some approach to exactness, but also by the
+direction of its longer axis determines that of the originating fault.
+When two <span class='pagenum'><a name="Page_326" id="Page_326">[326]</a></span>or three such lines can be traced, the relative position
+supplies in addition the hade of the fault (p. 219). The successful
+application of the method requires, it is true, a large number of
+observations, and these cannot as a rule be obtained except in
+districts that are somewhat thickly and uniformly populated, such as
+those surrounding the cities of Hereford and Inverness. In the
+Charleston earthquake, also, the position and form of the epicentres
+were deduced from the trend of isoseismal lines based on the damage to
+railway-lines and various structures within a sparsely inhabited
+meizoseismal area.</p>
+
+<p>In a few cases, of which the Indian earthquake may be regarded as
+typical, a fourth method has recently been found of service. The
+numerous after-shocks which follow a great earthquake originate for
+the most part within the seismic focus of the latter; and, as they
+usually disturb a very small area, it is not difficult to ascertain
+approximately the positions of their epicentres. Some, as in the
+Inverness after-shocks of 1901, result from slips in the very margin
+of the principal focus; but, as a rule, the seat of their activity
+tends to contract towards a central region of the focus. Bearing in
+mind, then, that some of the succeeding shocks originate at and beyond
+the confines of the focus, and that others may be sympathetic shocks
+precipitated by the sudden change of stress, it follows that the
+shifting epicentres of the true after-shocks map out, in part at any
+rate, the epicentral area of the principal earthquake.</p>
+
+<br />
+
+<h4>DEPTH OF THE SEISMIC FOCUS.</h4>
+
+<p>It is much to be regretted that we have no satisfactory method of
+determining so interesting an <span class='pagenum'><a name="Page_327" id="Page_327">[327]</a></span>element as the depth of the seismic
+focus. That it amounts to but a few miles at the most is certain from
+the limited areas within which slight shocks are felt or disastrous
+ones exhibit their maximum effects. Nor can we suppose that the rocks
+at very great depths are capable of offering the prolonged resistance
+and sudden collapse under stress that are necessary for the production
+of an earthquake.</p>
+
+<p>The problem is evidently beyond our present powers of solution, and
+its interest is therefore mainly historical. All the known methods are
+vitiated by our ignorance of the refractive powers of the rocks
+traversed by the earth-waves. But, even if this ignorance could be
+replaced by knowledge, most of the methods suggested are open to
+objection. Falb's method, depending on the time-interval between the
+initial epochs of the sound and shock, is of more than doubtful value.
+Dutton's, based on the rate of change of surface-intensity, is
+difficult to apply, and in any case gives only an inferior limit to
+the depth. Time-observations have been employed, especially in New
+Zealand; but the uncertainty in selecting throughout the same phase of
+the movement, and the large errors in the estimated depth resulting
+from small errors in the time-records, are at present most serious
+objections. There remains the method devised by Mallet, and, though he
+claimed for it an exaggerated accuracy, it still, in my opinion, holds
+the field against all its successors. When carefully applied, as it
+has been by Mallet himself, by Johnston-Lavis and Mercalli, we
+probably obtain at least some conception of the depth of the seismic
+focus.</p>
+
+<p>Professor Omori and Mr. K. Hirata have recently<a name="FNanchor_80_80" id="FNanchor_80_80"></a><a href="#Footnote_80_80" class="fnanchor">[80]</a> <span class='pagenum'><a name="Page_328" id="Page_328">[328]</a></span>lessened the chief
+difficulty in the application of Mallet's method. They have deduced
+the angle of emergence from the vertical and horizontal components of
+the motion as registered by seismographs, instead of from the
+inclination of fissures in damaged walls. In two recent earthquakes
+recorded at Miyako in Japan, they find the angle of emergence to be
+7.2&deg; and 9&deg; respectively, the corresponding depths of the foci being
+5.6 and 9.3 miles. These are probably the most accurate estimates that
+we possess, and it will be noted that they differ little from the mean
+values obtained for the Neapolitan, Andalusian, and Riviera
+earthquakes&mdash;namely, 6.6, 7.6, and 10.8 miles.</p>
+
+<br />
+
+<h4>NATURE OF THE SHOCK</h4>
+
+<p>In one respect, the earthquakes described above fail to represent the
+progress of modern seismology. They furnish no diagrams made by
+accurately constructed seismographs within their disturbed areas. The
+curve reproduced in Fig. 36, as already pointed out, is no exception
+to this statement. For another reason, the records that were obtained
+in Japan of the earthquake of 1891 are trustworthy for little more
+than the short-period initial vibrations; for, owing to the passage of
+the surface-waves, visible in and near the meizoseismal area, the
+Japanese seismographs registered the tilting of the ground rather than
+the elastic vibrations that traversed the earth's crust.</p>
+
+<p>Notwithstanding this defect, personal impressions of an
+earthquake-shock give a fairly accurate, if incomplete, idea of its
+nature. Nearly all observers placed under favourable conditions agree
+that an earthquake begins with a deep rumbling sound, <span class='pagenum'><a name="Page_329" id="Page_329">[329]</a></span>accompanied,
+after the first second or two, by a faint tremor which gradually, and
+sometimes rapidly, increases in strength until it merges into the
+shock proper, which consists of several or many vibrations of larger
+amplitude and longer period, and during which the attendant sound is
+generally at its loudest; the earthquake dying away, as it began, with
+tremors and a low rumbling sound.</p>
+
+<div class="img"><a name="Fig79" id="Fig79"></a>
+<a href="images/fig79.png">
+<img border="0" src="images/fig79.png" width="80%" alt="Seismographic Record of Tokio Earthquake of 1894." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 79.&mdash;Seismographic Record of Tokio Earthquake of 1894. (<i>Omori.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>The vibrations that produce the sensible shock are by no means all
+that are present during an earthquake. The Indian earthquake, for
+instance, seemed to last about three or four minutes at Midnapur; but
+the movements of the bubble of a level showed that the ground
+continued to oscillate for at least five minutes longer (p. 280). Many
+of these unfelt waves are rendered manifest by seismographs, although
+there are still others that elude registration either from the extreme
+shortness or the great length of their periods.</p>
+
+<p><span class='pagenum'><a name="Page_330" id="Page_330">[330]</a></span>In Fig. 79 is shown the principal part of a diagram obtained at Tokio
+during the Japanese earthquake of June 20th, 1894 (p. 18), the curve
+representing the N.E.-S.W. component of the horizontal motion during
+the first 25 seconds of the record. The instrument employed is one
+specially designed for registering strong earthquakes, and is
+unaffected by very minute tremors. Those which formed the commencement
+of this earthquake lasted for about 10 seconds, as shown by ordinary
+seismographs, and the vibrations had attained a range of a few
+millimetres before they affected the instrument in question. For the
+first 2&frac12; seconds, they occurred at the rate of four or five a
+second. The motion then suddenly became violent, and the ground was
+displaced 37 mm. in one direction, followed by a return movement of 73
+mm., and this again by one of 42 mm., the complete period of the
+oscillation being 1.8 seconds. The succeeding vibrations were of
+smaller amplitude and generally of shorter period for a minute and a
+half, then dying out during the last three minutes as almost
+imperceptible waves with a period of two or more seconds.<a name="FNanchor_81_81" id="FNanchor_81_81"></a><a href="#Footnote_81_81" class="fnanchor">[81]</a></p>
+
+<p>Though incomplete in some respects, this diagram illustrates clearly
+the division of the earthquake-motion into three stages&mdash;namely, the
+preliminary tremors, the principal portion or most active part of an
+earthquake, and the end-portion or gradually evanescent slow
+undulations. In all three stages, however, both tremors and slow
+undulations may be present; and, as the latter, owing to their long
+period, <span class='pagenum'><a name="Page_331" id="Page_331">[331]</a></span>are more or less insensible to human beings, the ripples of
+the final stage give the impression of a tremulous termination as
+described above. The duration of each stage varies considerably in
+different earthquakes. Thus, in a valuable study of 27 earthquakes
+recorded at Miyako, in Japan, during the years 1896-98, Messrs. Omori
+and Hirata show<a name="FNanchor_82_82" id="FNanchor_82_82"></a><a href="#Footnote_82_82" class="fnanchor">[82]</a> that the duration of the preliminary stage varies
+from 0 to 26 seconds, with an average of about 10 seconds; that of the
+principal portion from 0.7 to 26 seconds, also with an average of
+about 10 seconds; and that of the end portion from 28 and 105 seconds,
+with an average of about one minute. The total apparent duration,
+however, depends on the instrument employed; one of the earthquakes,
+that of April 23rd, 1898, disturbing the seismograph at Miyako for two
+minutes; while, at Tokio, a horizontal pendulum designed by Professor
+Omori oscillated for at least two hours. The periods of both ripples
+and slow undulations, again, vary from one earthquake to another; but
+it is worthy of notice that the average period of the undulations is
+almost constant in all three stages of the motion, being 1.1, 1.3, and
+1.3 seconds, respectively, for the east-west component of the
+horizontal motion, and 1.0 second throughout for the north-south
+component. For the ripples, the average period is .08 second in the
+preliminary stage, .10 second in the principal portion, and .08 second
+again in the end portion; those of the principal portion being
+slightly larger in amplitude, as well as longer in period, than the
+ripples of the first and third stages.</p>
+
+<span class='pagenum'><a name="Page_332" id="Page_332">[332]</a></span><br />
+
+<h4>SOUND-PHENOMENA.</h4>
+
+<p>Besides the ripples already mentioned, there are others of still
+smaller amplitude and shorter period that are sensible, but as a rule
+only just sensible, to us as sounds. All the known evidence points to
+the extraordinary lowness of the earthquake-sound. According to some
+observers, it seems as if close to their lower limit of audibility;
+while others, however intently they may listen, are unable to hear the
+slightest noise. In other words, the most rapid vibrations present in
+an earthquake do not recur at a rate of much more than about 30 to 50
+per second; or, if they do, they are not strong enough to impress the
+human ear.</p>
+
+<p>To most observers, the sound seems to increase and decrease in
+intensity with the shock, and so gradually and smoothly does this
+change take place that the sound is frequently mistaken for that of an
+underground train approaching the observer's house, passing beneath
+it, and receding in the opposite direction. Some persons, especially
+if situated within the meizoseismal area, hear also loud crashes in
+the midst of the rumbling sound and simultaneously with the strongest
+vibrations. At a moderate distance, say from 30 to 40 miles, the sound
+becomes more harsh and grating while the shock is felt; and, at a
+greater distance, even this change disappears, and nothing is heard
+but an almost monotonous sound like the low roll of distant thunder.
+The explanation of this is that the sound-vibrations are of different
+periods and varying amplitude, and the limiting vibrations tend to
+become inaudible with increasing distance, the lower on account of
+their <span class='pagenum'><a name="Page_333" id="Page_333">[333]</a></span>long period, the higher owing to their small amplitude.</p>
+
+<p>The magnitude of the sound-area depends, even more than that of the
+disturbed area, on the personal equation of the observers. The lower
+limit of audibility varies not only in different individuals, but also
+in different races. In Great Britain, it is doubtful whether an
+earthquake ever occurs unaccompanied by sound; and in the meizoseismal
+area the noise is heard by nearly all observers. With Italians, the
+average lower limit of audibility is higher than with the Anglo-Saxon
+race; slight shocks frequently occur without noticeable sound, but
+with strong ones, the larger number of observers is sure to include
+one or more capable of hearing the rumbling noise. The Japanese are,
+however, seldom affected by the most rapid earthquake-vibrations, and
+the strongest shocks may be unattended by any recorded sound. The
+result is manifest in the size of the sound-area in different
+countries. In the Hereford earthquake, the sound-area contained 70,000
+square miles; in the Neapolitan earthquake, about 3,300 square miles;
+while, in Japanese earthquakes, the sound is rarely heard more than a
+few miles from the epicentre.</p>
+
+<p>Another effect of this personal equation of the observers is that the
+sound-vibrations apparently outrace those of longer period. The
+Italians, for instance, generally hear the sound that precedes the
+shock, and more rarely the weaker sound that follows it. In Japan,
+only the earlier sound-vibrations, if any, seem to be audible. In
+Great Britain, on the contrary, the fore-sound is perceptible to four,
+and the after-sound to three, out of every five observers; <span class='pagenum'><a name="Page_334" id="Page_334">[334]</a></span>and these
+proportions are maintained roughly to considerable distances from the
+epicentre. It follows, therefore, that the sound-vibrations and those
+which constitute the shock must travel with nearly, if not quite, the
+same velocity; and that the greater duration of the sound is due
+either to the prolongation of the initial movement or to the
+overlapping of the principal focus by the sound-focus. Neither
+alternative can be regarded as improbable, but observations made on
+British earthquakes point to the latter explanation as the true one.</p>
+
+<p>It will be sufficient to refer to two phenomena in support of this
+statement. In the first place, the percentage of observers who hear
+the fore-sound varies with the direction from the epicentre. Thus,
+during the Inverness earthquake of 1901, the majority of observers in
+Aberdeenshire regarded the sound as beginning and ending with the
+shock; while, in counties lying more nearly along the course of the
+great fault, the sound was generally heard both before and after the
+shock (p. 253). In this case, then, the initial and concluding sound
+vibrations must have come chiefly from the margins of the seismic
+focus; and those from the margin nearest to an observer would be more
+sensible than those from the farther margin. Again, in slight
+earthquakes, such as the Cornwall earthquake of April 1, 1898,<a name="FNanchor_83_83" id="FNanchor_83_83"></a><a href="#Footnote_83_83" class="fnanchor">[83]</a> the
+curves of equal sound intensity, while their axes are parallel to
+those of the isoseismal lines, are displaced laterally with respect to
+these curves, owing to the arrival of the strongest sound-vibrations
+from the upper margin of an inclined seismic focus.</p>
+
+<p>When a fault-slip occurs, the displacement is <span class='pagenum'><a name="Page_335" id="Page_335">[335]</a></span>obviously greatest in
+the central region, and dies out gradually towards the margins of the
+focus. The phenomena described above show that the evanescent
+displacement within these margins generate sound-vibrations only; and
+that the greater slip within the central region produces also the more
+important vibrations that compose the shock. As the former are
+perceptible over a limited district, while the latter may be felt
+through half a continent, it is clear that the sound-area should bear
+no fixed relation in point of size to the disturbed area, but should
+be comparatively greater for a slight shock than for a strong one.</p>
+
+<br />
+
+<h4>VELOCITY OF THE EARTH-WAVES.</h4>
+
+<p>If we consider only the earthquakes here described, we see at once how
+great is the diversity in the estimated velocity of the earth-waves.
+On the one hand, we have a value as high as 5.2 kms. per sec. for the
+Charleston earthquake, and, at the other end of the scale, a value of
+0.9 km. per sec. for the Hereford earthquake. Between them, and
+equally trustworthy, lie the estimates of 3.0 km. per sec. for the
+Indian earthquake, and 2.1 kms. per sec. for the Japanese earthquake
+and its immediate successors.</p>
+
+<p>It is difficult to account entirely for such discordance. Errors of
+observation may be responsible for a small part of the differences.
+The initial strength of the disturbance appears to have some effect,
+and the nature of the rocks traversed must be a factor of consequence
+when the distances in question are not very great. In the Japanese and
+Hereford earthquakes, all three may have combined to produce the
+divergent results, the distance in these cases being only 275 and 142
+kms. respectively.</p>
+
+<p><span class='pagenum'><a name="Page_336" id="Page_336">[336]</a></span>In the Indian and Charleston earthquakes, the distances are much
+greater (1944 and 1487 kms.), and the variety of rocks traversed must
+tend to give a truer average. In the former, the result obtained (3.0
+kms. per sec.) agrees so closely with the velocity of the long-period
+undulations of distant earthquakes as to suggest that it was these
+waves that were timed at the stations west of Calcutta and disturbed
+the magnetographs at Bombay.<a name="FNanchor_84_84" id="FNanchor_84_84"></a><a href="#Footnote_84_84" class="fnanchor">[84]</a></p>
+
+<p>Omitting, then, the Indian estimate, we find that, for the Japanese
+and Charleston earthquakes, the velocity increases with the distance
+as measured along the surface. To a certain extent, such a result
+might have been expected, had we assumed the earthquake-waves to
+travel along the chords joining the focus to very distant places of
+observation.</p>
+
+<p>The wave-paths that penetrate the earth are straight lines, however,
+only when the conditions that determine the velocity are uniform
+throughout, and such uniformity we have no reason to expect. From what
+we know of the earth's interior, there can, indeed, be little doubt
+that the velocity of earthquake-waves increases with the depth below
+the surface, and that the wave-paths in consequence are curved lines
+with their convexity downwards. It would be out of place to state more
+than the principal result of the recent investigations by Dr. A.
+Schmidt<a name="FNanchor_85_85" id="FNanchor_85_85"></a><a href="#Footnote_85_85" class="fnanchor">[85]</a> and Prof. P. Rudzki<a name="FNanchor_86_86" id="FNanchor_86_86"></a><a href="#Footnote_86_86" class="fnanchor">[86]</a> <span class='pagenum'><a name="Page_337" id="Page_337">[337]</a></span>on this subject. These are based
+on the assumptions that the velocity increases with the depth below
+the surface, and that it is always the same at the same depth. From
+the focus of the earthquake, wave-paths diverge in all directions.
+Those which start horizontally curve upwards, and intersect the
+surface of the earth in a circle dividing the whole surface into two
+areas of very unequal size. Within the small area, the
+surface-velocity is infinite at the epicentre, and decreases outwards
+until it is least on the boundary-circle. In the larger region beyond,
+the surface-velocity increases with the distance from the epicentre,
+until, at the antipodes of that point, it is again infinite. But, as
+the depth of the focus is always slight compared with the radius of
+the earth, the small circular area surrounding the epicentre is
+practically negligible, and we may regard the surface-velocity of the
+waves that traverse the body of the earth as a quantity that
+continually increases with the distance from the epicentre.</p>
+
+<p>How fully this interesting theoretical result has been confirmed is
+well shown in Mr. Oldham's recent and very valuable investigation on
+the propagation of earthquake-motion to great distances.<a name="FNanchor_87_87" id="FNanchor_87_87"></a><a href="#Footnote_87_87" class="fnanchor">[87]</a> A study
+of the records of the Indian earthquake revealed the existence of
+three series of waves, the first two consisting in all probability of
+longitudinal and transversal waves travelling through the body of the
+earth, and the third of undulations spreading over its surface (pp.
+282-285). Extending his inquiries to ten other earthquakes originating
+in six different centres, Mr. Oldham distinguishes the same three
+phases in their movements; the third phase being the most <span class='pagenum'><a name="Page_338" id="Page_338">[338]</a></span>constantly
+recorded, the second less so, while the first phase is the most
+frequently absent. With the exception of a few very divergent records,
+the initial times of these phases and the maximum epoch of the third
+phase are plotted on the accompanying diagram (Fig. 80), in which
+distances from the epicentre in degrees of arc are represented along
+the horizontal line and the time-interval in minutes along the
+perpendicular line. The dots near the two lower curves refer to the
+records of the heavily weighted Italian instruments, and the crosses
+to those of the light horizontal pendulums, which respond somewhat
+<span class='pagenum'><a name="Page_339" id="Page_339">[339]</a></span>irregularly to the motion of the first two phases (p. 282). In the
+third phase, there is less divergence between the indications of the
+two classes of instruments, and dots are used in each case for the
+initial, and crosses for the maximum epoch.</p>
+
+<div class="img"><a name="Fig80" id="Fig80"></a>
+<a href="images/fig80.png">
+<img border="0" src="images/fig80.png" width="60%" alt="Time-curves of principal epochs of earthquake-waves of distant origin." /></a><br />
+<p class="cen" style="margin-top: .3em;"><span class="sc">Fig.</span> 80.&mdash;Time-curves of principal epochs of earthquake-waves of distant origin. (<i>Oldham.</i>)<span class="totoi"><a href="#toi">ToList</a></span></p>
+</div>
+
+<p>Of the smoothed curves drawn between these series of points, those
+marked A, B, and C represent the time-curves of the beginnings of the
+first, second, and third phases respectively, while D is the
+time-curve for the maximum of the third phase.</p>
+
+<p>The concavity of the two lower lines towards the horizontal base-line
+shows that the surface-velocity of the corresponding waves increases
+rapidly with the distance, far more so than would be possible with
+rectilinear motion. The rates at which these waves travel through the
+earth therefore increase with the depth, and the wave-paths must in
+consequence be curved lines convex towards the centre of the earth.</p>
+
+<p>If the time-curves A and B were continued backwards to the origin,
+their inclinations at that point to the horizontal line give the
+initial velocities of the corresponding waves, which prove to be about
+5 and 3 kms. per sec. respectively. Now, according to recent
+experiments made by Mr. H. Nagaoka on the elastic constants of
+rocks,<a name="FNanchor_88_88" id="FNanchor_88_88"></a><a href="#Footnote_88_88" class="fnanchor">[88]</a> the mean velocity of seven archaean rocks is 5.1 kms. per
+sec. for the longitudinal waves, and 2.8 kms. per sec. for the
+transversal waves&mdash;values which agree so closely with those obtained
+for the first two series of earthquake-waves as to leave little doubt
+with regard to their character.</p>
+
+<p>The other time-curves, C and D, corresponding to the initial and
+maximum epochs of the third phase, <span class='pagenum'><a name="Page_340" id="Page_340">[340]</a></span>are practically straight lines.
+Some of the records are slightly discordant for the average curve,
+especially for the initial epoch; but it is often difficult to define
+the commencement of this phase with precision. At any rate, the
+observations show no distinct sign of an increase in the
+surface-velocity of these waves with the distance from the origin. It
+may therefore be concluded that they travel along the surface with
+velocities which are practically constant for each individual
+earthquake, the largest waves at the rate of about 2.9 kms. per sec.,
+and the advance waves with a velocity of about 3.3 kms. per sec.,
+rising occasionally to over 4.0 kms. per sec.</p>
+
+<br />
+
+<h4>STRUCTURAL CHANGES IN THE EPICENTRAL AREA.</h4>
+
+<p>Changes of elevation have long been known as accompaniments of great
+earthquakes, though many of the earlier observations and measurements
+left much to be desired in accuracy and completeness. The Japanese
+earthquake of 1891, however, placed the reality of such movements
+beyond doubt, and revealed the existence of a fault-scarp, with a
+height in one place of 18 or 20 feet, and a length of at least 40, if
+not of 70, miles. In the Indian earthquake of 1897, the fault-scarps
+were shorter, though more pronounced in character, the largest known
+(the Chedrang fault) being about 12 miles long, and having a maximum
+throw at the surface of 35 feet. In some other recent earthquakes,
+also, remarkable fault-scarps have been developed. After the great
+shocks felt in Eastern Greece on April 20th and 27th, 1894, a fissure
+was traced for a distance of about 34 miles, running in an
+<span class='pagenum'><a name="Page_341" id="Page_341">[341]</a></span>east-south-east and west-north-west direction through the epicentral
+district, and varying in width from an inch or two to more than three
+yards. That it was a fault, and not an ordinary fissure, was evident
+from its great length, its uniform direction, and its independence of
+geological structure. The throw was generally small, in no place
+exceeding five feet.<a name="FNanchor_89_89" id="FNanchor_89_89"></a><a href="#Footnote_89_89" class="fnanchor">[89]</a> Again, in British Baluchistan, after the
+severe earthquake of December 20th, 1892, a fresh crack was observed
+in the ground running for several miles in a straight line parallel to
+the axis of the Khojak range. It coincided almost exactly with a line
+of springs, and was clearly produced by a fresh slip along an old line
+of fault, for before the earthquake it had the appearance of an old
+road, and the natives assert that the ground has always cracked along
+this line with every severe shock. In 1892, the change in relative
+height of the two sides of the fault was small, in one place where it
+was measured being only two inches.<a name="FNanchor_90_90" id="FNanchor_90_90"></a><a href="#Footnote_90_90" class="fnanchor">[90]</a></p>
+
+<p>But other changes, besides those in a vertical direction, occasionally
+take place; though, owing to their recent discovery, comparatively few
+examples are as yet known. While the throw of the Japanese fault
+varied greatly in amount, and once even in direction, there was also a
+constant shift towards the northwest of the ground on the north-east
+side of the fault, the displacement at one spot being as much as 13
+feet. In the fault-scarp formed in 1894 in Eastern Greece, a similar
+shift took place, though to what extent is unknown. There is,
+moreover, evidence of actual compression of the earth's crust at right
+angles <span class='pagenum'><a name="Page_342" id="Page_342">[342]</a></span>to the fault-line. The Neo valley, traversed by the Japanese
+fault, was apparently narrower after the earthquake than it was
+before, and plots of ground were reduced from 48 to 30 feet in
+length&mdash;<i>i.e.</i>, by nearly 40 per cent. In British Baluchistan, the
+formation of the fissure referred to above was accompanied both by
+compression perpendicular, and by shifting parallel, to the fault. The
+actual displacement in each direction is unknown, but the resultant
+was not less than 27 inches.</p>
+
+<p>There can be no doubt that a fault-scarp is formed in the first place
+with great rapidity. So abrupt, indeed, were the structural
+displacements in the epicentral area of the Indian earthquake, that
+they contributed very materially to the intensity of the shock, giving
+rise to the excessive velocities observed at Rambrai and elsewhere (p.
+273). The growth of the scarp does not, however, always cease with the
+first great earthquake, though it may take place in a contrary sense,
+as in the elevation connected with the Conception earthquake of 1835.
+The principal shock, according to Darwin, was followed during the few
+succeeding days "by some hundred minor ones (though of no
+inconsiderable violence), which seemed to come from the same quarter
+from which the first had proceeded; whilst, on the other hand, the
+level of the ground was certainly not raised by them; but, on the
+contrary, after an interval of some weeks, it stood rather lower than
+it did immediately after the great convulsion."<a name="FNanchor_91_91" id="FNanchor_91_91"></a><a href="#Footnote_91_91" class="fnanchor">[91]</a></p>
+
+<br />
+
+<h4>AFTER-SHOCKS.</h4>
+
+<p>A series of after-shocks, more or less long, is a <span class='pagenum'><a name="Page_343" id="Page_343">[343]</a></span>constant attendant
+on every great tectonic earthquake, and few are the earthquakes of
+any degree of strength that can be regarded as completely isolated.
+Even in those which visit this country, after-shocks are seldom
+absent. For instance, confining ourselves to the last few years, the
+Pembroke earthquake of 1892 was followed by 8 shocks, the Inverness
+earthquake of 1890 by at least 10, and possibly by 19 shocks, and that
+of the same district in 1901 by 15 well-defined after-shocks in
+addition to many others recorded by one observer. Of 300 Italian
+earthquakes strong enough to cause some damage to buildings, Dr.
+Cancani finds that every one was either preceded or followed, and
+chiefly followed, by its own train of minor shocks.</p>
+
+<p>For some hours, and even for days, after a great earthquake, the
+shocks are so numerous that it is often impossible to keep count of
+them. Many local centres spring into activity in different parts of
+the epicentral area; and, though only the strongest shocks can be
+identified elsewhere, it is clear that as a rule the shocks felt at
+any one station are quite distinct from those observed at another.</p>
+
+<p>The enormous number of after-shocks that follow some earthquakes can
+only be realised when they are subjected to continuous seismographic
+registration; and, even then, countless earth-sounds and the slightest
+tremors must escape detection. The shocks may, indeed, succeed one
+another so rapidly that one begins before another ends, and the result
+is an almost incessant tremulous motion rendered manifest by the
+quivering of water-surfaces or the swinging of chandeliers. Of the
+total number of after-shocks, we <span class='pagenum'><a name="Page_344" id="Page_344">[344]</a></span>may form some idea from recent
+records in Japan. After the Mino-Owari earthquake of 1891, 3,365
+shocks were recorded within little more than two years at Gifu, and
+1,298 at Nagoya, but neither of these figures includes the shocks felt
+within the first few hours. Of the Kumamoto earthquake of July 28th,
+1889, the after-shocks recorded at Kumamoto until the end of 1893
+amount to 922; and those of the Kagoshima earthquake of September 7th,
+1893, recorded at Chiran until the end of January 1894, to 480. During
+the first 30 days, the numbers recorded were 1,746 at Gifu, 340 at
+Kumamoto, and 278 at Chiran; showing, as Professor Omori remarks, that
+the after-shocks diminish in frequency with the size of the disturbed
+areas,<a name="FNanchor_92_92" id="FNanchor_92_92"></a><a href="#Footnote_92_92" class="fnanchor">[92]</a>&mdash;<i>i.e.</i>, roughly with the initial intensity of the shocks.</p>
+
+<p>Next to absolute number, the rapid decline in general frequency is the
+most marked characteristic of after-shocks. Professor Omori has shown
+that, excluding minor oscillations, it follows the law represented
+geographically by the curves in Fig. 51, and algebraically by the
+equation <span style="white-space: nowrap;"><i>y</i> = <i>k</i> / (<i>h</i> + <i>x</i>),</span> where <i>y</i> is the frequency at time
+<i>x</i> and <i>h</i> and <i>k</i> are constants for one and the same earthquake. By
+means of this formula, it is possible to estimate roughly the interval
+of time that must elapse before the seismic activity of the central
+district resumes its normal value. For the Mino-Owari earthquake, this
+proves to be about forty years, for the Kumamoto earthquake about
+seven or eight years, and for the Kagoshima earthquake about three or
+four years.</p>
+
+<p><span class='pagenum'><a name="Page_345" id="Page_345">[345]</a></span>In a recent memoir on Italian after-shocks,<a name="FNanchor_93_93" id="FNanchor_93_93"></a><a href="#Footnote_93_93" class="fnanchor">[93]</a> Dr. Cancani has urged
+that other factors besides initial intensity determine the duration of
+a seismic period, and prominently among these he places the depth of
+the seismic focus. When the depth is very small, the duration of the
+period is short, not much more than ten days; when the depth is
+moderate, the duration may extend to three months; and, when great, it
+may amount to several years.</p>
+
+<p>The principal law that governs the distribution of after-shocks in
+time may be regarded as well-established. It is otherwise with regard
+to their distribution in space. This has been examined only in the
+cases of the Japanese earthquake of 1891 and the Inverness earthquake
+of 1901. So far as we can judge from the evidence which they furnish,
+after-shocks appear to be most numerous within and near the central
+portion of the seismic focus; though the area of maximum activity is
+subject to continual oscillation. In this region, also, there is
+evidence of a gradual decrease in the depths of the after-shock foci;
+while, near the extremities of the epicentral area, there occur
+districts of slightly greater frequency than elsewhere. With the lapse
+of time, there seems therefore to be a constant extension, both
+upwards and longitudinally, of the area over which the principal
+fault-slip took place.</p>
+
+<br />
+
+<h4>ORIGIN OF EARTHQUAKES.</h4>
+
+<p>In the introductory chapter, a brief sketch is given of the different
+causes to which earthquakes are assigned. With those due to rock-falls
+in subterranean channels, we need have little to do. The <span class='pagenum'><a name="Page_346" id="Page_346">[346]</a></span>shocks are
+invariably slight, and the part they play in the shaping of the
+earth's crust is insignificant. Volcanic earthquakes possess a higher
+degree of interest. They represent, no doubt, incipient or
+unsuccessful attempts to produce an eruption. They may be the
+forerunners of a great catastrophe.</p>
+
+<p>Of far higher importance in the history of our globe is the third
+class of earthquakes, including all those connected with the manifold
+changes which the crust has undergone. In the slow annealing process,
+to which it has been subjected from the earliest times, the crust has
+been crumpled and fractured, elevated into the loftiest mountain
+ranges or depressed below the level of the sea. Every sudden yielding
+under stress is the cause of an earthquake. It is chiefly, perhaps
+almost entirely, in the formation of faults that this yielding is
+manifested. The initial fracturing may be the cause of one or many
+shocks, but infinitely the larger number must be referred to the slow
+growth of the fault, the intermittent slips, now in one part, now in
+another, which, after the lapse of ages, culminate in a great
+displacement. Of the length of time occupied in the formation of a
+single fault, we can make no estimate in years. The anticlinal fault
+of Charnwood Forest dates from a pre-carboniferous period. In 1893 it
+had not ceased to grow.<a name="FNanchor_94_94" id="FNanchor_94_94"></a><a href="#Footnote_94_94" class="fnanchor">[94]</a></p>
+
+<p>Still less can we conceive, however faintly, the number of elemental
+slips that constitute the history of a single fault. We may think, if
+we please, of the 143 tremors and earth-sounds noted at Comrie in
+Perthshire during the last three months of 1839, of the 306
+earthquakes felt in the Island of Zante during the year 1896, or the
+1,746 shocks recorded at Gifu during <span class='pagenum'><a name="Page_347" id="Page_347">[347]</a></span>thirty days in 1891; but we shall
+be as far as ever from realising the vast number of steps involved in
+the growth of a fault, let alone a mountain-chain.</p>
+
+<p>Yet, all over the land-surface of the globe, the crust is intersected
+by numberless faults, and hardly any portion is there in which some or
+many of these faults are not growing. One country, indeed, such as
+Great Britain, may have reached a condition of comparative stagnancy;
+the fault-slips are few and slight, and earthquakes in consequence are
+rare and generally inconspicuous. In another, like Eastern Japan and
+the adjoining ocean-bed, the movements are frequent, occasionally
+almost incessant, and few years pass without some great convulsion by
+which cities are wrecked and hundreds of human lives are lost. At such
+times, we magnify the r&ocirc;le of earthquakes, and are in some danger of
+forgetting that, in the formation of a mountain-chain or continent,
+they serve no higher purpose than the creaking of a wheel in the
+complex movements of a great machine.</p>
+
+<br />
+<hr style="width: 15%;" />
+<br />
+
+<h4>FOOTNOTES:</h4>
+
+<div class="footnote"><p class="noin"><a name="Footnote_79_79" id="Footnote_79_79"></a><a href="#FNanchor_79_79"><span class="label">[79]</span></a> <i>Phil. Trans.</i>, vol. li., pt. ii., 1761, pp. 625-626.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_80_80" id="Footnote_80_80"></a><a href="#FNanchor_80_80"><span class="label">[80]</span></a> <i>Journ. Sci. Coll. Imp. Univ.</i>, Tokyo, vol. xi., 1899,
+pp. 194-195.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_81_81" id="Footnote_81_81"></a><a href="#FNanchor_81_81"><span class="label">[81]</span></a> <i>Journ. Coll. Sci. Imp. Univ.</i>, Tokyo, vol. vii., pt.
+v., 1894, pp. 1-4; <i>Ital. Sismol. Soc. Boll.</i>, vol. ii., 1896, pp.
+180-188.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_82_82" id="Footnote_82_82"></a><a href="#FNanchor_82_82"><span class="label">[82]</span></a> <i>Journ. Coll. Sci. Imp. Univ.</i>, Tokyo, vol. xi., 1899,
+pp. 161-195.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_83_83" id="Footnote_83_83"></a><a href="#FNanchor_83_83"><span class="label">[83]</span></a> <i>Quart. Journ. Geol. Soc.</i>, vol. lvi., 1900, pp. 1-7.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_84_84" id="Footnote_84_84"></a><a href="#FNanchor_84_84"><span class="label">[84]</span></a> There is no reason why the surface-undulations of the
+Indian earthquake should not have produced a sensible shock even as
+far as Italy. Taking their amplitude in that country at 508 mm. and
+their period at 22 sec. (p. 283), the maximum acceleration would be
+about 40 mm. per sec., corresponding to the intensity 2 of the
+Rossi-Forel scale. (<i>Amer. Journ. Sci.</i>, vol. xxxv., 1888, p. 429.)</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_85_85" id="Footnote_85_85"></a><a href="#FNanchor_85_85"><span class="label">[85]</span></a> <i>Nature</i>, vol. lii., 1895, pp. 631-633.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_86_86" id="Footnote_86_86"></a><a href="#FNanchor_86_86"><span class="label">[86]</span></a> Gerland's <i>Beitr&auml;ge zur Geophysik</i>, vol. iii., pp.
+485-518.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_87_87" id="Footnote_87_87"></a><a href="#FNanchor_87_87"><span class="label">[87]</span></a> <i>Phil. Trans.</i>, 1900A, pp. 135-174.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_88_88" id="Footnote_88_88"></a><a href="#FNanchor_88_88"><span class="label">[88]</span></a> <i>Publ. of Earthq. Inves. Com. in For. Langs.</i> (Tokyo),
+No. 4, 1900, pp. 47-67.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_89_89" id="Footnote_89_89"></a><a href="#FNanchor_89_89"><span class="label">[89]</span></a> S.A. Papavasiliou, Paris, <i>Acad. Sci., Compt. Rend.</i>,
+vol. cxix., 1894, pp. 112-114, 380-381.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_90_90" id="Footnote_90_90"></a><a href="#FNanchor_90_90"><span class="label">[90]</span></a> <i>Geol. Mag.</i>, vol. x., 1893, pp. 356-360.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_91_91" id="Footnote_91_91"></a><a href="#FNanchor_91_91"><span class="label">[91]</span></a> <i>Geol. Soc. Trans.</i>, vol. v., 1840, pp. 618-619.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_92_92" id="Footnote_92_92"></a><a href="#FNanchor_92_92"><span class="label">[92]</span></a> The disturbed areas of these earthquakes contained,
+respectively, 221,000, 39,000, and 30,000 square miles.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_93_93" id="Footnote_93_93"></a><a href="#FNanchor_93_93"><span class="label">[93]</span></a> <i>Boll. Sismol. Soc. Ital.</i>, vol. viii., 1902, pp.
+17-48.</p></div>
+
+<div class="footnote"><p class="noin"><a name="Footnote_94_94" id="Footnote_94_94"></a><a href="#FNanchor_94_94"><span class="label">[94]</span></a> <i>Roy. Soc. Proc.</i>, vol. lvii., 1895, pp. 87-95.</p></div>
+
+<br />
+<br />
+<br />
+<br />
+<a name="INDEX" id="INDEX"></a><hr />
+<span class='pagenum'><a name="Page_349" id="Page_349">[349]</a></span><br />
+
+<h3>INDEX.<span class="totoc"><a href="#toc">ToC</a></span></h3>
+<br />
+
+<ul><li>Acceleration, maximum, of wave-motion in Japanese earthquake, <a href="#Page_184">184</a>, <a href="#Page_185">185</a>;
+  <ul class="nest">
+  <li>in Indian earthquake, <a href="#Page_272">272</a></li>
+  </ul>
+</li>
+
+<li>After-shocks, definition, <a href="#Page_4">4</a>;
+  <ul class="nest">
+  <li>frequency, <a href="#Page_198">198</a>, <a href="#Page_256">256</a>, <a href="#Page_296">296</a>, <a href="#Page_344">344</a>;</li>
+  <li>distribution in space, <a href="#Page_200">200</a>, <a href="#Page_203">203</a>, <a href="#Page_298">298</a>, <a href="#Page_326">326</a>, <a href="#Page_345">345</a>;</li>
+  <li>sound-phenomena, <a href="#Page_207">207</a>, <a href="#Page_300">300</a>;</li>
+  <li>connection with fault-scarps, <a href="#Page_300">300</a>;</li>
+  <li>outlining of epicentre by, <a href="#Page_326">326</a>;</li>
+  <li>origin of, <a href="#Page_257">257</a>;</li>
+  <li>of Neapolitan earthquake, <a href="#Page_40">40</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_56">56</a>, <a href="#Page_65">65</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_97">97</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_133">133</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_167">167</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_198">198</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_240">240</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_256">256</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_296">296</a>;</li>
+  <li>of British earthquakes, <a href="#Page_343">343</a>;</li>
+  <li>of Italian earthquakes, <a href="#Page_343">343</a>;</li>
+  <li>of Japanese earthquakes, <a href="#Page_344">344</a></li>
+  </ul>
+</li>
+
+<li>Agamennone, G., <a href="#Page_93">93</a>, <a href="#Page_94">94</a>, <a href="#Page_101">101</a>, <a href="#Page_319">319</a></li>
+
+<li>Alluvium, displacement of, by Indian earthquake, <a href="#Page_287">287</a></li>
+
+<li>Amplitude of wave-motion, definition, <a href="#Page_4">4</a>;
+  <ul class="nest">
+  <li>in Neapolitan earthquake, <a href="#Page_34">34</a>;</li>
+  <li>in Japanese earthquake, <a href="#Page_185">185</a>;</li>
+  <li>in Indian earthquake, <a href="#Page_270">270</a></li>
+  </ul>
+</li>
+
+<li>Andalusian earthquake, preparation for, <a href="#Page_75">75</a>;
+  <ul class="nest">
+  <li>investigation of, <a href="#Page_76">76</a>;</li>
+  <li>damage caused by, <a href="#Page_77">77</a>;</li>
+  <li>isoseismal lines and disturbed area, <a href="#Page_78">78</a>;</li>
+  <li>the unfelt earthquake, <a href="#Page_82">82</a>;</li>
+  <li>position of epicentre, <a href="#Page_84">84</a>;</li>
+  <li>depth of focus, <a href="#Page_85">85</a>;</li>
+  <li>nature of shock, <a href="#Page_87">87</a>;</li>
+  <li>sound-phenomena, <a href="#Page_91">91</a>;</li>
+  <li>velocity of earth-waves, <a href="#Page_92">92</a>;</li>
+  <li>connection between geological structure and intensity of shock, <a href="#Page_95">95</a>;</li>
+  <li>fissures, <a href="#Page_96">96</a>;</li>
+  <li>landslips, <a href="#Page_97">97</a>;</li>
+  <li>effect on underground water, <a href="#Page_97">97</a>;</li>
+  <li>after-shocks, <a href="#Page_97">97</a>;</li>
+  <li>origin of, <a href="#Page_99">99</a>;</li>
+  <li>bibliography, <a href="#Page_101">101</a></li>
+  </ul>
+</li>
+
+<li>Animals, effects of earthquakes on, <a href="#Page_143">143</a><br /><br /></li>
+
+
+<li>Baluchistan earthquake of 1892, <a href="#Page_288">288</a>, <a href="#Page_341">341</a></li>
+
+<li>Baldacci, L., <a href="#Page_70">70</a>, <a href="#Page_73">73</a></li>
+
+<li>Baratta, M., <a href="#Page_320">320</a></li>
+
+<li>Barrois, C., <a href="#Page_76">76</a></li>
+
+<li>Bergeron, C., <a href="#Page_76">76</a></li>
+
+<li>Bertelli, T., <a href="#Page_175">175</a></li>
+
+<li>Bertrand, M., <a href="#Page_76">76</a></li>
+
+<li>Birds, effects of earthquakes on, <a href="#Page_143">143</a></li>
+
+<li>Bordwar, crust-fracture at, <a href="#Page_309">309</a></li>
+
+<li>Br&eacute;on, R., <a href="#Page_76">76</a></li>
+
+<li>Burton, W.K., <a href="#Page_214">214</a><br /><br /></li>
+
+
+<li>Cancani, A., <a href="#Page_281">281</a>, <a href="#Page_282">282</a>, <a href="#Page_320">320</a>, <a href="#Page_343">343</a>, <a href="#Page_345">345</a></li>
+
+<li>Castro, M.F. de, <a href="#Page_76">76</a>, <a href="#Page_101">101</a></li>
+
+<li>Charleston earthquake, investigation of, <a href="#Page_102">102</a>;
+  <ul class="nest">
+  <li>damage caused by, <a href="#Page_103">103</a>;</li>
+  <li>isoseismal lines and disturbed area, <a href="#Page_104">104</a>;</li>
+  <li>preparation for, <a href="#Page_107">107</a>;</li>
+  <li>nature of shock, <a href="#Page_108">108</a>;</li>
+  <li>double epicentre, <a href="#Page_111">111</a>;</li>
+  <li>origin of double shock, <a href="#Page_120">120</a>;</li>
+  <li>depth of foci, <a href="#Page_122">122</a>;</li>
+  <li>velocity of earth-waves, <a href="#Page_126">126</a>;</li>
+  <li>fissures, <a href="#Page_130">130</a>;</li>
+  <li>sand-craters, <a href="#Page_130">130</a>;</li>
+  <li>effects on human beings, <a href="#Page_131">131</a>;<span class='pagenum'><a name="Page_350" id="Page_350">[350]</a></span></li>
+  <li>feeling of nausea, <a href="#Page_132">132</a>;</li>
+  <li>after-shocks, <a href="#Page_133">133</a>;</li>
+  <li>origin of, <a href="#Page_134">134</a>;</li>
+  <li>bibliography, <a href="#Page_137">137</a></li>
+  </ul>
+</li>
+
+<li>Charlon, E., <a href="#Page_175">175</a></li>
+
+<li>Chedrang, fault-scarp at, <a href="#Page_304">304</a></li>
+
+<li>Clocks, untrustworthiness of time-records of stopped, <a href="#Page_39">39</a>, <a href="#Page_94">94</a>, <a href="#Page_121">121</a>, <a href="#Page_127">127</a></li>
+
+<li>Conder, J., <a href="#Page_177">177</a>, <a href="#Page_213">213</a></li>
+
+<li>Coseismal lines, <a href="#Page_227">227</a>, <a href="#Page_324">324</a></li>
+
+<li>Covelli, N., <a href="#Page_67">67</a>, <a href="#Page_69">69</a><br /><br /></li>
+
+
+<li>Damage caused by Neapolitan earthquake, <a href="#Page_10">10</a>, <a href="#Page_24">24</a>;
+  <ul class="nest">
+  <li>by Ischian earthquakes, <a href="#Page_50">50</a>, <a href="#Page_56">56</a>;</li>
+  <li>by Andalusian earthquake, <a href="#Page_77">77</a>;</li>
+  <li>by Charleston earthquake, <a href="#Page_103">103</a>;</li>
+  <li>by Riviera earthquake, <a href="#Page_139">139</a>;</li>
+  <li>by Japanese earthquake, <a href="#Page_181">181</a>;</li>
+  <li>by Hereford earthquake, <a href="#Page_217">217</a>;</li>
+  <li>by Inverness earthquake, <a href="#Page_247">247</a></li>
+  </ul>
+</li>
+
+<li>Darwin, H., <a href="#Page_281">281</a></li>
+
+<li>Daubr&eacute;e, A., <a href="#Page_73">73</a></li>
+
+<li>Davison, C, <a  href="#Page_202">202-206</a>, <a href="#Page_208">208</a>, <a href="#Page_210">210</a>, <a href="#Page_213">213</a>, <a  href="#Page_215">215-261</a>, <a href="#Page_295">295</a></li>
+
+<li>Death-rate of Neapolitan earthquake, <a href="#Page_24">24</a>;
+  <ul class="nest">
+  <li>of Ischian earthquakes, <a href="#Page_50">50</a>, <a href="#Page_56">56</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_77">77</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_104">104</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_140">140</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_182">182</a></li>
+  </ul>
+</li>
+
+<li>Denza, F., <a href="#Page_155">155</a>, <a href="#Page_175">175</a></li>
+
+<li>Depth of seismic focus, methods of determining, <a href="#Page_25">25</a>, <a href="#Page_86">86</a>, <a href="#Page_122">122</a>, <a href="#Page_326">326</a>;
+  <ul class="nest">
+  <li>of Neapolitan earthquake, <a href="#Page_28">28</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_54">54</a>, <a href="#Page_61">61</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_86">86</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_122">122</a>, <a href="#Page_125">125</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_150">150</a>;</li>
+  <li>of Japanese earthquakes, <a href="#Page_328">328</a></li>
+  </ul>
+</li>
+
+<li>Derby earthquake of 1903, <a href="#Page_236">236</a></li>
+
+<li>Direction of shock, <a href="#Page_22">22</a>, <a href="#Page_33">33</a>, <a href="#Page_186">186</a>, <a href="#Page_225">225</a>, <a href="#Page_325">325</a></li>
+
+<li>Disturbed area, definition of, <a href="#Page_3">3</a>;
+  <ul class="nest">
+  <li>of Neapolitan earthquake, <a href="#Page_10">10</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_51">51</a>, <a href="#Page_58">58</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_80">80</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_107">107</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_145">145</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_183">183</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_219">219</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_249">249</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_265">265</a>;</li>
+  <li>connection between intensity of shock and, <a href="#Page_323">323</a></li>
+  </ul>
+</li>
+
+<li>Dolomieu, <a href="#Page_11">11</a></li>
+
+<li>Du Bois, F., <a href="#Page_73">73</a></li>
+
+<li>Dutton, C.E., <a  href="#Page_103">103-137</a></li>
+
+<li>Dutton's method of determining depth of seismic focus, <a href="#Page_122">122</a>, <a href="#Page_327">327</a><br /><br /></li>
+
+
+<li>Earthquake-motion, nature of, <a href="#Page_280">280</a>, <a href="#Page_282">282</a>, <a href="#Page_328">328</a>, <a href="#Page_330">330</a>, <a href="#Page_337">337</a>;
+  <ul class="nest">
+  <li>propagation of, to great distances, <a href="#Page_337">337</a></li>
+  </ul>
+</li>
+
+<li>Earth-sound, definition of, <a href="#Page_4">4</a></li>
+
+<li>Edinburgh, record of Indian earthquake at, <a href="#Page_281">281</a>, <a href="#Page_283">283</a>, <a href="#Page_285">285</a></li>
+
+<li>Ellis, W., <a href="#Page_83">83</a></li>
+
+<li>Emergence, angle of, <a href="#Page_13">13</a></li>
+
+<li>Epicentre, definition of, <a href="#Page_3">3</a>;
+  <ul class="nest">
+  <li>methods of determining position of, <a href="#Page_14">14</a>, <a href="#Page_52">52</a>, <a href="#Page_60">60</a>, <a href="#Page_324">324</a>;</li>
+  <li>of Neapolitan earthquake, <a href="#Page_22">22</a>, <a href="#Page_23">23</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_53">53</a>, <a href="#Page_60">60</a>, <a href="#Page_67">67</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_84">84</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_111">111</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_146">146</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_224">224</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_248">248</a>;</li>
+  <li>of Indian earthquakes, <a href="#Page_264">264</a>, <a href="#Page_276">276</a>, <a href="#Page_302">302</a></li>
+  </ul>
+</li>
+
+<li>Epomeo, <a href="#Page_45">45</a>, <a href="#Page_61">61</a>, <a href="#Page_71">71</a><br /><br /></li>
+
+
+<li>Falb's method of determining depth of seismic focus, <a href="#Page_86">86</a>, <a href="#Page_327">327</a></li>
+
+<li>Fallen pillars, evidence of, <a href="#Page_17">17</a>, <a href="#Page_19">19</a></li>
+
+<li>Fault, originating, of Hereford earthquake, <a href="#Page_219">219</a>;
+  <ul class="nest">
+  <li>of Inverness earthquake, <a href="#Page_249">249</a></li>
+  </ul>
+</li>
+
+<li>Fault-scarp of Japanese earthquake, <a href="#Page_189">189</a>;
+  <ul class="nest">
+  <li>general appearance, <a href="#Page_189">189</a>;</li>
+  <li>length, <a href="#Page_192">192</a>;</li>
+  <li>throw, <a href="#Page_193">193</a>;</li>
+  <li>horizontal shift, <a href="#Page_193">193</a>;</li>
+  <li>course, <a href="#Page_193">193</a>;</li>
+  <li>swamp formed by it, <a href="#Page_194">194</a></li>
+  </ul>
+</li>
+
+<li>Fault-scarps of Indian earthquakes, <a href="#Page_273">273</a>, <a href="#Page_304">304</a>;
+  <ul class="nest">
+  <li>Chedrang fault, <a href="#Page_304">304</a>;</li>
+  <li>Samin fault, <a href="#Page_308">308</a>;</li>
+  <li>of Greek earthquake of 1894, <a href="#Page_340">340</a>, <a href="#Page_341">341</a>;<span class='pagenum'><a name="Page_351" id="Page_351">[351]</a></span></li>
+  <li>of Baluchistan earthquake of 1893, <a href="#Page_341">341</a>, <a href="#Page_342">342</a>;</li>
+  <li>formation and growth of, <a href="#Page_342">342</a></li>
+  </ul>
+</li>
+
+<li>Fault-slips, tectonic earthquakes due to, <a href="#Page_5">5</a>, <a href="#Page_43">43</a>, <a href="#Page_100">100</a>, <a href="#Page_135">135</a>, <a href="#Page_174">174</a>, <a href="#Page_211">211</a>, <a href="#Page_219">219</a>, <a href="#Page_224">224</a>, <a href="#Page_241">241</a>, <a href="#Page_249">249</a>, <a href="#Page_255">255</a>, <a href="#Page_317">317</a>, <a href="#Page_346">346</a></li>
+
+<li>Fishes, destruction of, by Riviera earthquake, <a href="#Page_162">162</a></li>
+
+<li>Fissures, caused by Andalusian earthquake, <a href="#Page_96">96</a>;
+  <ul class="nest">
+  <li>by Charleston earthquake, <a href="#Page_130">130</a>;</li>
+  <li>by Inverness earthquake, <a href="#Page_247">247</a>;</li>
+  <li>by Indian earthquake, <a href="#Page_285">285</a></li>
+  </ul>
+</li>
+
+<li>Focus, seismic, definition of, <a href="#Page_3">3</a></li>
+
+<li>Focus, seismic, depth of, methods of determining, <a href="#Page_25">25</a>, <a href="#Page_86">86</a>, <a href="#Page_122">122</a>, <a href="#Page_326">326</a>;
+  <ul class="nest">
+  <li>of Neapolitan earthquake, <a href="#Page_28">28</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_54">54</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_86">86</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_122">122</a>, <a href="#Page_125">125</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_150">150</a>;</li>
+  <li>of Japanese earthquakes, <a href="#Page_328">328</a></li>
+  </ul>
+</li>
+
+<li>Focus, dimensions of seismic, of Hereford earthquake, <a href="#Page_224">224</a>;
+  <ul class="nest">
+  <li>of Inverness earthquake, <a href="#Page_250">250</a></li>
+  </ul>
+</li>
+
+<li>Fore-shocks, <a href="#Page_321">321</a>;
+  <ul class="nest">
+  <li>of Neapolitan earthquake, <a href="#Page_40">40</a>;</li>
+  <li>of Ischian earthquake, <a href="#Page_57">57</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_76">76</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_107">107</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_142">142</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_201">201</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_239">239</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_246">246</a></li>
+  </ul>
+</li>
+
+<li>Fouqu&eacute;, F., <a href="#Page_76">76</a>, <a href="#Page_84">84</a>, <a href="#Page_101">101</a></li>
+
+<li>Fracture, crust-, at Bordwar, <a href="#Page_309">309</a></li>
+
+<li>Fractures in buildings, evidence of, <a href="#Page_14">14</a>, <a href="#Page_15">15</a>, <a href="#Page_26">26</a></li>
+
+<li>Fuchs, C.W.C., <a href="#Page_102">102</a><br /><br /></li>
+
+
+<li>Galli, I., <a href="#Page_82">82</a></li>
+
+<li>Geological structure and intensity of shock, connection between, <a href="#Page_95">95</a>, <a href="#Page_106">106</a>, <a href="#Page_113">113</a>, <a href="#Page_115">115</a>, <a href="#Page_135">135</a>, <a href="#Page_164">164</a>, <a href="#Page_265">265</a></li>
+
+<li>Gifu, records of Japanese after-shocks at, <a href="#Page_183">183</a>, <a href="#Page_197">197</a></li>
+
+<li>Gray, T., <a href="#Page_295">295</a></li>
+
+<li>Great Glen fault and Inverness earthquakes, connection between, <a href="#Page_245">245</a></li>
+
+<li>Greek earthquake of 1894, fault-scarp of, <a href="#Page_340">340</a><br /><br /></li>
+
+
+<li>Hayden, E., <a href="#Page_103">103</a></li>
+
+<li>Heath, T., <a href="#Page_283">283</a>, <a href="#Page_320">320</a></li>
+
+<li>Hereford earthquake, investigation of, <a href="#Page_215">215</a>;
+  <ul class="nest">
+  <li>preparation for, <a href="#Page_215">215</a>, <a href="#Page_238">238</a>;</li>
+  <li>isoseismal lines and disturbed area of, <a href="#Page_216">216</a>;</li>
+  <li>damage caused by, <a href="#Page_217">217</a>, <a href="#Page_294">294</a>;</li>
+  <li>position of originating fault, <a href="#Page_219">219</a>;</li>
+  <li>nature of shock, <a href="#Page_220">220</a>;</li>
+  <li>origin of double series of vibrations, <a href="#Page_223">223</a>;</li>
+  <li>position and dimensions of the two foci, <a href="#Page_224">224</a>;</li>
+  <li>direction of the shock, <a href="#Page_225">225</a>;</li>
+  <li>coseismal lines and velocity of earth-waves, <a href="#Page_227">227</a>;</li>
+  <li>sound-phenomena, <a href="#Page_229">229</a>;</li>
+  <li>isacoustic lines and sound-area, <a href="#Page_234">234</a>;</li>
+  <li>fore-shocks, <a href="#Page_238">238</a>;</li>
+  <li>after-shocks, <a href="#Page_240">240</a>;</li>
+  <li>origin of earthquake, <a href="#Page_240">240</a>;</li>
+  <li>bibliography, <a href="#Page_261">261</a></li>
+  </ul>
+</li>
+
+<li>Hills, changes in aspect of, after Indian earthquake, <a href="#Page_311">311</a></li>
+
+<li>Hirata, K., <a href="#Page_327">327</a>, <a href="#Page_331">331</a></li>
+
+<li>Human beings, effects of Charleston earthquake on, <a href="#Page_131">131</a></li>
+
+<li>Hypocentre, <a href="#Page_3">3</a><br /><br /></li>
+
+
+<li>Iberian peninsula, earthquakes of, <a href="#Page_75">75</a></li>
+
+<li>Indian earthquake, investigation of, <a href="#Page_262">262</a>;
+  <ul class="nest">
+  <li>isoseismal lines and disturbed area, <a href="#Page_264">264</a>;</li>
+  <li>nature of shock, <a href="#Page_266">266</a>;</li>
+  <li>visible earth-waves, <a href="#Page_268">268</a>;</li>
+  <li>elements of wave-motion, <a href="#Page_270">270</a>;</li>
+  <li>sound-phenomena, <a href="#Page_274">274</a>;</li>
+  <li>velocity of earth-waves, <a href="#Page_275">275</a>;</li>
+  <li>the unfelt earthquake, <a href="#Page_280">280</a>;</li>
+  <li>earth-fissures, <a href="#Page_285">285</a>;</li>
+  <li>displacements of alluvium, <a href="#Page_287">287</a>;</li>
+  <li>sand-vents, <a href="#Page_288">288</a>;</li>
+  <li>rise of river-beds, etc., <a href="#Page_290">290</a>;</li>
+  <li>landslips, <a href="#Page_291">291</a>;</li>
+  <li>rotation of pillars, <a href="#Page_293">293</a>;</li>
+  <li>after-shocks, <a href="#Page_296">296</a>;</li>
+  <li>structural changes in epicentral area, <a href="#Page_301">301</a>, <a href="#Page_315">315</a>;</li>
+  <li>structure of epicentral district, <a href="#Page_302">302</a>;</li>
+  <li>fault-scarps, <a href="#Page_304">304</a>;</li>
+  <li>crust-fractures, <a href="#Page_309">309</a>;<span class='pagenum'><a name="Page_352" id="Page_352">[352]</a></span></li>
+  <li>lakes and pools not due to faulting, <a href="#Page_310">310</a>;</li>
+  <li>changes in aspects of hills, <a href="#Page_311">311</a>;</li>
+  <li>revision of trigonometrical survey, <a href="#Page_312">312</a>;</li>
+  <li>origin of earthquake, <a href="#Page_317">317</a>;</li>
+  <li>bibliography, <a href="#Page_319">319</a></li>
+  </ul>
+</li>
+
+<li>Inverness earthquake, preparation for, <a href="#Page_246">246</a>;
+  <ul class="nest">
+  <li>damage caused by, <a href="#Page_247">247</a>;</li>
+  <li>fissure in ground, <a href="#Page_247">247</a>;</li>
+  <li>isoseismal lines and disturbed area, <a href="#Page_247">247</a>;</li>
+  <li>position of originating fault, <a href="#Page_249">249</a>;</li>
+  <li>nature of shock, <a href="#Page_250">250</a>;</li>
+  <li>sound-phenomena, <a href="#Page_253">253</a>;</li>
+  <li>origin of earthquake, <a href="#Page_255">255</a>;</li>
+  <li>after-shocks and their origin, <a href="#Page_256">256</a>;</li>
+  <li>sympathetic earthquakes, <a href="#Page_259">259</a>;</li>
+  <li>comparison with Japanese earthquake, <a href="#Page_260">260</a>;</li>
+  <li>bibliography, <a href="#Page_261">261</a>.</li>
+  </ul>
+</li>
+
+<li>Investigation, Mallet's methods of, <a href="#Page_12">12</a>, <a href="#Page_21">21</a></li>
+
+<li>Isacoustic lines, <a href="#Page_234">234</a>;
+  <ul class="nest">
+  <li>of Hereford earthquake, <a href="#Page_235">235</a>;</li>
+  <li>of Derby earthquake, <a href="#Page_236">236</a></li>
+  </ul>
+</li>
+
+<li>Ischia, volcanic history of, <a href="#Page_45">45</a>, <a href="#Page_70">70</a>;
+  <ul class="nest">
+  <li>characteristics of eruptions, <a href="#Page_49">49</a>;</li>
+  <li>seismic history, <a href="#Page_49">49</a></li>
+  </ul>
+</li>
+
+<li>Ischian earthquake of 1881, investigation of, <a href="#Page_50">50</a>;
+  <ul class="nest">
+  <li>isoseismal lines and disturbed area, <a href="#Page_51">51</a>;</li>
+  <li>position of epicentre, <a href="#Page_52">52</a>;</li>
+  <li>depth of focus, <a href="#Page_54">54</a>;</li>
+  <li>nature of shock, <a href="#Page_55">55</a>;</li>
+  <li>after-shocks, <a href="#Page_56">56</a>;</li>
+  <li>origin of, <a href="#Page_70">70</a>;</li>
+  <li>bibliography, <a href="#Page_73">73</a></li>
+  </ul>
+</li>
+
+<li>Ischian earthquake of 1883, investigation of, <a href="#Page_56">56</a>;
+  <ul class="nest">
+  <li>preparation for, <a href="#Page_57">57</a>;</li>
+  <li>isoseismal lines and disturbed area, <a href="#Page_58">58</a>;</li>
+  <li>position of epicentre, <a href="#Page_60">60</a>;</li>
+  <li>depth of focus, <a href="#Page_61">61</a>;</li>
+  <li>nature of shock, <a href="#Page_64">64</a>;</li>
+  <li>landslips, <a href="#Page_64">64</a>;</li>
+  <li>after-shocks, <a href="#Page_65">65</a>;</li>
+  <li>origin of, <a href="#Page_70">70</a>;</li>
+  <li>bibliography, <a href="#Page_73">73</a></li>
+  </ul>
+</li>
+
+<li>Ischian earthquakes, characteristics of, <a href="#Page_66">66</a>; origin of, <a href="#Page_70">70</a></li>
+
+<li>Isoseismal lines, definition of, <a href="#Page_3">3</a>;
+  <ul class="nest">
+  <li>their use in determining position of epicentre, <a href="#Page_219">219</a>, <a href="#Page_249">249</a>, <a href="#Page_325">325</a>;</li>
+  <li>of Neapolitan earthquake, <a href="#Page_9">9</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_51">51</a>, <a href="#Page_58">58</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_78">78</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_104">104</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_143">143</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_178">178</a>, <a href="#Page_182">182</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_216">216</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_247">247</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_264">264</a></li>
+  </ul>
+</li>
+
+<li>Issel, A., <a href="#Page_139">139</a>, <a href="#Page_163">163</a>, <a href="#Page_164">164</a>, <a href="#Page_166">166</a>, <a href="#Page_175">175</a><br /><br /></li>
+
+
+<li>Japanese earthquake of 1887, <a href="#Page_18">18</a></li>
+
+<li>Japanese earthquake of 1891, investigation of, <a href="#Page_177">177</a>;
+  <ul class="nest">
+  <li>structure of meizoseismal area, <a href="#Page_179">179</a>;</li>
+  <li>damage caused by, <a href="#Page_181">181</a>;</li>
+  <li>isoseismal lines and disturbed area, <a href="#Page_182">182</a>;</li>
+  <li>nature of shock, <a href="#Page_184">184</a>;</li>
+  <li>the great fault-scarp, <a href="#Page_189">189</a>;</li>
+  <li>minor shocks, <a href="#Page_197">197</a>;</li>
+  <li>distribution of after-shocks in time, <a href="#Page_198">198</a>;</li>
+  <li>preparation for, <a href="#Page_201">201</a>;</li>
+  <li>distribution of after-shocks in space, <a href="#Page_203">203</a>;</li>
+  <li>sound-phenomena of after-shocks, <a href="#Page_207">207</a>;</li>
+  <li>sympathetic earthquakes, <a href="#Page_209">209</a>;</li>
+  <li>origin, of, <a href="#Page_211">211</a>;</li>
+  <li>bibliography, <a href="#Page_213">213</a></li>
+  </ul>
+</li>
+
+<li>Japanese earthquake of 1894, <a href="#Page_18">18</a>, <a href="#Page_329">329</a></li>
+
+<li>Johnston-Lavis, H.J., <a  href="#Page_50">50-72</a>, <a href="#Page_327">327</a><br /><br /></li>
+
+
+<li>Kilian, W., <a href="#Page_76">76</a></li>
+
+<li>Koto, B., <a href="#Page_177">177</a>, <a href="#Page_180">180</a>, <a href="#Page_181">181</a>, <a href="#Page_184">184</a>, <a  href="#Page_190">190-196</a>, <a href="#Page_209">209</a>, <a href="#Page_212">212</a>, <a href="#Page_213">213</a><br /><br /></li>
+
+
+<li>Lakes formed by bending of river-bed during Indian earthquake, <a href="#Page_310">310</a></li>
+
+<li>Lakes formed by fault-scarp of Japanese earthquake, <a href="#Page_194">194</a>;
+  <ul class="nest">
+  <li>of Indian earthquake, <a href="#Page_305">305</a></li>
+  </ul>
+</li>
+
+<li>Landslips caused by Ischian earthquake, <a href="#Page_64">64</a>;
+  <ul class="nest">
+  <li>by Andalusian earthquake, <a href="#Page_97">97</a>;</li>
+  <li>by Indian earthquake, <a href="#Page_291">291</a></li>
+  </ul>
+</li>
+
+<li>L&eacute;vy, M., <a href="#Page_76">76</a></li>
+
+<li>Lisbon earthquake of 1755, <a href="#Page_75">75</a>, <a href="#Page_82">82</a><br /><br /></li>
+
+
+<li>McGee, W.J., <a href="#Page_134">134</a></li>
+
+<li>Macpherson, J., <a href="#Page_101">101</a></li>
+
+<li>Magnetographs, earthquakes recorded by, <a href="#Page_82">82</a>, <a href="#Page_157">157</a>, <a href="#Page_160">160</a>, <a href="#Page_189">189</a>, <a href="#Page_277">277</a>, <a href="#Page_282">282</a></li>
+
+<li>Mallet, R., <a  href="#Page_7">7-44</a>, <a href="#Page_85">85</a>, <a href="#Page_102">102</a>, <a href="#Page_124">124</a>, <a href="#Page_150">150</a>, <a  href="#Page_294">294-296</a>, <a href="#Page_325">325</a><span class='pagenum'><a name="Page_353" id="Page_353">[353]</a></span></li>
+
+<li>Mallet's method of determining depth of focus, <a href="#Page_25">25</a>, <a href="#Page_327">327</a></li>
+
+<li>Masato, H., <a href="#Page_178">178</a>, <a href="#Page_213">213</a></li>
+
+<li>Mascart, E., <a href="#Page_159">159</a>, <a href="#Page_160">160</a></li>
+
+<li>May Hill anticlinal and Hereford earthquake, connection between, <a href="#Page_242">242</a></li>
+
+<li>Meizoseismal area, definition of, <a href="#Page_3">3</a>;
+  <ul class="nest">
+  <li>of Andalusian earthquake, <a href="#Page_99">99</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_179">179</a></li>
+  </ul>
+</li>
+
+<li>Mercalli, G., <a href="#Page_11">11</a>, <a href="#Page_57">57</a>, <a href="#Page_58">58</a>, <a href="#Page_60">60</a>, <a href="#Page_61">61</a>, <a href="#Page_63">63</a>, <a href="#Page_67">67</a>, <a  href="#Page_70">70-73</a>, <a href="#Page_76">76</a>, <a href="#Page_80">80</a>, <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_88">88</a>, <a href="#Page_90">90</a>, <a href="#Page_101">101</a>, <a  href="#Page_138">138-175</a>, <a href="#Page_325">325</a>, <a href="#Page_327">327</a></li>
+
+<li>Michell, J., <a href="#Page_325">325</a></li>
+
+<li>Milne, J., <a href="#Page_35">35</a>, <a href="#Page_177">177</a>, <a href="#Page_181">181</a>, <a href="#Page_182">182</a>, <a href="#Page_186">186</a>, <a href="#Page_189">189</a>, <a href="#Page_200">200</a>, <a href="#Page_213">213</a>, <a href="#Page_281">281</a>, <a href="#Page_283">283</a></li>
+
+<li>Minor shocks of Neapolitan earthquake, <a href="#Page_40">40</a>;
+  <ul class="nest">
+  <li>of Japanese earthquake, <a href="#Page_197">197</a></li>
+  </ul>
+</li>
+
+<li>Mountain ranges, effect of, on intensity of shock, <a href="#Page_95">95</a>, <a href="#Page_106">106</a></li>
+
+<li>Moureaux, T., <a href="#Page_161">161</a><br /><br /></li>
+
+
+<li>Nagaoka, H., <a href="#Page_177">177</a>, <a href="#Page_214">214</a>, <a href="#Page_339">339</a></li>
+
+<li>Nagoya, records of Japanese after-shocks at, <a href="#Page_183">183</a>, <a href="#Page_197">197</a></li>
+
+<li>Nature of shock, Neapolitan earthquake, <a href="#Page_30">30</a>;
+  <ul class="nest">
+  <li>Ischian earthquakes, <a href="#Page_55">55</a>, <a href="#Page_64">64</a>;</li>
+  <li>Andalusian earthquake, <a href="#Page_87">87</a>;</li>
+  <li>Charleston earthquake, <a href="#Page_108">108</a>;</li>
+  <li>Riviera earthquake, <a href="#Page_150">150</a>;</li>
+  <li>Japanese earthquake, <a href="#Page_184">184</a>;</li>
+  <li>Hereford earthquake, <a href="#Page_220">220</a>;</li>
+  <li>Inverness earthquake, <a href="#Page_250">250</a>;</li>
+  <li>Indian earthquake, <a href="#Page_266">266</a></li>
+  </ul>
+</li>
+
+<li>Nausea, feeling of, caused by Charleston earthquake, <a href="#Page_132">132</a></li>
+
+<li>Neapolitan earthquake, investigation of, <a href="#Page_7">7</a>, <a href="#Page_12">12</a>;
+  <ul class="nest">
+  <li>isoseismal lines and disturbed area, <a href="#Page_9">9</a>;</li>
+  <li>damage caused by, <a href="#Page_10">10</a>;</li>
+  <li>position of epicentre, <a href="#Page_14">14</a>;</li>
+  <li>depth of focus, <a href="#Page_25">25</a>;</li>
+  <li>nature of shock, <a href="#Page_30">30</a>;</li>
+  <li>sound-phenomena, <a href="#Page_37">37</a>;</li>
+  <li>velocity of earth-waves, <a href="#Page_39">39</a>;</li>
+  <li>minor shocks, <a href="#Page_40">40</a>;</li>
+  <li>origin, <a href="#Page_41">41</a>;</li>
+  <li>bibliography, <a href="#Page_44">44</a></li>
+  </ul>
+</li>
+
+<li>Ness, Loch, connection between Inverness earthquakes and formation of, <a href="#Page_255">255</a>, <a href="#Page_257">257</a>, <a href="#Page_261">261</a></li>
+
+<li>Nogu&eacute;s, A.F., <a href="#Page_101">101</a><br /><br /></li>
+
+
+<li>Oddone, E., <a href="#Page_175">175</a></li>
+
+<li>Offret, A., <a href="#Page_76">76</a>, <a href="#Page_158">158</a>, <a href="#Page_159">159</a>, <a href="#Page_175">175</a></li>
+
+<li>Oglialoro, A., <a href="#Page_73">73</a></li>
+
+<li>Oldham, R.D., <a  href="#Page_262">262-320</a>, <a href="#Page_337">337</a>, <a href="#Page_340">340</a></li>
+
+<li>Omori, F., <a href="#Page_19">19</a>, <a href="#Page_20">20</a>, <a href="#Page_177">177</a>, <a  href="#Page_183">183-186</a>, <a href="#Page_188">188</a>, <a  href="#Page_197">197-199</a>, <a href="#Page_207">207</a>, <a href="#Page_214">214</a>, <a href="#Page_262">262</a>, <a href="#Page_325">325</a>, <a href="#Page_327">327</a>, <a href="#Page_329">329</a>, <a href="#Page_331">331</a></li>
+
+<li>Origin of earthquakes, <a href="#Page_2">2</a>, <a href="#Page_5">5</a>, <a href="#Page_345">345</a>;
+  <ul class="nest">
+  <li>of Neapolitan earthquake, <a href="#Page_41">41</a>;</li>
+  <li>of Ischian earthquakes, <a href="#Page_70">70</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_101">101</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_134">134</a>;</li>
+  <li>of Riviera earthquakes, <a href="#Page_174">174</a>;</li>
+  <li>of Japanese earthquake, <a href="#Page_211">211</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_240">240</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_255">255</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_317">317</a></li>
+  </ul>
+</li>
+
+<li>Overturned bodies, maximum acceleration deduced from, <a href="#Page_184">184</a>, <a href="#Page_272">272</a><br /><br /></li>
+
+
+<li>Palmieri, L., <a href="#Page_57">57</a>, <a href="#Page_72">72</a>, <a href="#Page_73">73</a></li>
+
+<li>Periodicity of Japanese after-shocks, <a href="#Page_199">199</a></li>
+
+<li>Perrey, A., <a href="#Page_7">7</a></li>
+
+<li>Potenza, evidence of damaged church at, <a href="#Page_15">15</a>, <a href="#Page_26">26</a></li>
+
+<li>Prediction of earthquakes, possible, <a href="#Page_322">322</a></li>
+
+<li>Preparation for earthquakes, <a href="#Page_40">40</a>, <a href="#Page_57">57</a>, <a href="#Page_76">76</a>, <a href="#Page_107">107</a>, <a href="#Page_142">142</a>, <a href="#Page_201">201</a>, <a href="#Page_238">238</a>, <a href="#Page_246">246</a>, <a href="#Page_321">321</a><br /><br /></li>
+
+
+<li>Rails, flexure of, by Charleston earthquake, <a href="#Page_112">112</a>;
+  <ul class="nest">
+  <li>by Japanese earthquake, <a href="#Page_182">182</a>;</li>
+  <li>by Indian earthquake, <a href="#Page_288">288</a></li>
+  </ul>
+</li>
+
+<li>Railway-tunnels, observations of Riviera earthquake in, <a href="#Page_166">166</a></li>
+
+<li>Rebeur-Paschwitz, E. von, <a href="#Page_281">281</a></li>
+
+<li>River-beds, rise of, caused by Indian earthquake, <a href="#Page_290">290</a></li>
+
+<li>Riviera earthquake, investigation, <a href="#Page_138">138</a>;
+  <ul class="nest">
+  <li>damage caused by, <a href="#Page_139">139</a>;</li>
+  <li>preparation for, <a href="#Page_142">142</a>;</li>
+  <li>isoseismal lines and disturbed area, <a href="#Page_143">143</a>;</li>
+  <li>position of epicentre, <a href="#Page_146">146</a>;<span class='pagenum'><a name="Page_354" id="Page_354">[354]</a></span></li>
+  <li>depth of principal focus, <a href="#Page_149">149</a>;</li>
+  <li>nature of shock, <a href="#Page_150">150</a>;</li>
+  <li>sound-phenomena, <a href="#Page_156">156</a>;</li>
+  <li>the unfelt earthquake, <a href="#Page_157">157</a>;</li>
+  <li>effects of earthquake at sea, <a href="#Page_162">162</a>;</li>
+  <li>destruction of fishes, <a href="#Page_162">162</a>;</li>
+  <li>seismic sea-waves, <a href="#Page_163">163</a>;</li>
+  <li>connection between geological structure and intensity of shock, <a href="#Page_164">164</a>;</li>
+  <li>observations in railway-tunnels, <a href="#Page_166">166</a>;</li>
+  <li>after-shocks, <a href="#Page_167">167</a>;</li>
+  <li>recent movements in the Riviera, <a href="#Page_170">170</a>;</li>
+  <li>seismic history of the Riviera, <a href="#Page_171">171</a>;</li>
+  <li>origin of, <a href="#Page_171">171</a>;</li>
+  <li>bibliography, <a href="#Page_175">175</a></li>
+  </ul>
+</li>
+
+<li>Rocca di Papa, record of Indian earthquake at, <a href="#Page_281">281</a>, <a href="#Page_282">282</a>, <a href="#Page_285">285</a></li>
+
+<li>Rossi, M.S. de, <a href="#Page_57">57</a>, <a href="#Page_74">74</a>, <a href="#Page_82">82</a>, <a href="#Page_101">101</a>, <a href="#Page_175">175</a></li>
+
+<li>Rossi-Forel scale of seismic intensity, <a href="#Page_104">104</a>, <a href="#Page_216">216</a>, <a href="#Page_247">247</a></li>
+
+<li>Rotation of pillars, caused by Hereford earthquake, <a href="#Page_294">294</a>;
+  <ul class="nest">
+  <li>by Indian earthquake, <a href="#Page_293">293</a>;</li>
+  <li>explanation of, <a href="#Page_295">295</a></li>
+  </ul>
+</li>
+
+<li>Rudzki, P., <a href="#Page_336">336</a></li>
+
+<li>Rumi, Prof., <a href="#Page_169">169</a><br /><br /></li>
+
+
+<li>Samin, fault-scarp at, <a href="#Page_308">308</a></li>
+
+<li>Sand-craters caused by Charleston earthquake, <a href="#Page_130">130</a>;
+  <ul class="nest">
+  <li>by Indian earthquake, <a href="#Page_288">288</a></li>
+  </ul>
+</li>
+
+<li>Schmidt, A., <a href="#Page_336">336</a></li>
+
+<li>Seismic sea-waves of Riviera earthquake, <a href="#Page_142">142</a>, <a href="#Page_163">163</a></li>
+
+<li>Seismic vertical, <a href="#Page_12">12</a>, <a href="#Page_29">29</a>, <a href="#Page_62">62</a></li>
+
+<li>Seismographic records of Riviera earthquake, <a href="#Page_154">154</a>;
+  <ul class="nest">
+  <li>of Japanese earthquake of 1894, <a href="#Page_329">329</a></li>
+  </ul>
+</li>
+
+<li>Sekiya, S., <a href="#Page_18">18</a>, <a href="#Page_19">19</a></li>
+
+<li>Serpieri, A., <a href="#Page_74">74</a></li>
+
+<li>Shillong, nature of Indian earthquake at, <a href="#Page_266">266</a></li>
+
+<li>Sloan, E., <a href="#Page_103">103</a>, <a  href="#Page_117">117-119</a>, <a href="#Page_134">134</a>, <a href="#Page_135">135</a></li>
+
+<li>Sound-area, definition of, <a href="#Page_3">3</a>;
+  <ul class="nest">
+  <li>of Neapolitan earthquake, <a href="#Page_38">38</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_92">92</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_234">234</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_252">252</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_275">275</a></li>
+  </ul>
+</li>
+
+<li>Sound-phenomena, nature of sound, <a href="#Page_38">38</a>, <a href="#Page_229">229</a>, <a href="#Page_252">252</a>, <a href="#Page_332">332</a>;
+  <ul class="nest">
+  <li>inaudibility to some observers, <a href="#Page_231">231</a>, <a href="#Page_274">274</a>; its cause, <a href="#Page_233">233</a>;</li>
+  <li>isacoustic lines, <a  href="#Page_234">234-236</a>;</li>
+  <li>variations in nature of sound throughout sound-area, <a href="#Page_237">237</a>;</li>
+  <li>time-relation of sound and shock, <a href="#Page_238">238</a>, <a href="#Page_253">253</a>;</li>
+  <li>origin of earthquake-sounds, <a href="#Page_334">334</a>;</li>
+  <li>sound-phenomena of Neapolitan earthquake, <a href="#Page_37">37</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_91">91</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_133">133</a>;</li>
+  <li>of Riviera earthquake, <a href="#Page_156">156</a>;</li>
+  <li>of Japanese after-shocks, <a href="#Page_207">207</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_229">229</a>;</li>
+  <li>of Inverness earthquake, <a href="#Page_252">252</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_274">274</a></li>
+  </ul>
+</li>
+
+<li>Structural changes, distribution of, in Indian earthquake, <a href="#Page_315">315</a></li>
+
+<li>Subsultory shock, <a href="#Page_5">5</a></li>
+
+<li>Sympathetic earthquakes of Japanese earthquake, <a href="#Page_209">209</a>;
+  <ul class="nest">
+  <li>of Inverness earthquake, <a href="#Page_259">259</a><br /><br /></li>
+  </ul>
+</li>
+
+
+<li>Tanakadate, A., <a href="#Page_177">177</a>, <a href="#Page_214">214</a></li>
+
+<li>Taramelli, T., <a href="#Page_76">76</a>, <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_88">88</a>, <a href="#Page_90">90</a>, <a href="#Page_101">101</a>, <a href="#Page_138">138</a>, <a href="#Page_150">150</a>, <a href="#Page_165">165</a>, <a href="#Page_175">175</a>, <a href="#Page_325">325</a></li>
+
+<li>Tectonic earthquakes, <a href="#Page_5">5</a></li>
+
+<li>Thrust-plane, Indian earthquake due to movement along, <a href="#Page_318">318</a></li>
+
+<li>Time-curve of Indian earthquake, <a href="#Page_278">278</a>;
+  <ul class="nest">
+  <li>of principal epochs of earthquake-waves of distant origin, <a href="#Page_338">338</a></li>
+  </ul>
+</li>
+
+<li>Time-records, general inaccuracy of, <a href="#Page_324">324</a></li>
+
+<li>Time-relations of sound and shock in Hereford earthquake, <a href="#Page_238">238</a>;
+  <ul class="nest">
+  <li>in Inverness earthquake, <a href="#Page_253">253</a></li>
+  </ul>
+</li>
+
+<li>Trigonometrical survey, revised, of Khasi hills after Indian earthquake, <a href="#Page_312">312</a>;
+  <ul class="nest">
+  <li>interpretation of results, <a href="#Page_314">314</a></li>
+  </ul>
+</li>
+
+<li>Twin earthquakes, origin of, <a href="#Page_32">32</a>, <a href="#Page_89">89</a>, <a href="#Page_120">120</a>, <a href="#Page_153">153</a>, <a href="#Page_174">174</a>, <a href="#Page_223">223</a>;
+  <ul class="nest">
+  <li>Neapolitan earthquake, <a href="#Page_31">31</a>;</li>
+  <li>Andalusian earthquake, <a href="#Page_87">87</a>;</li>
+  <li>Charleston earthquake, <a href="#Page_108">108</a>;</li>
+  <li>Riviera earthquake, <a href="#Page_149">149</a>, <a href="#Page_150">150</a>;<span class='pagenum'><a name="Page_355" id="Page_355">[355]</a></span></li>
+  <li>Hereford earthquake, <a href="#Page_221">221</a><br /><br /></li>
+  </ul>
+</li>
+
+
+<li>Undulatory shock, <a href="#Page_5">5</a></li>
+
+<li>Unfelt earth-waves, Andalusian earthquake, <a href="#Page_82">82</a>;
+  <ul class="nest">
+  <li>Riviera earthquake, <a href="#Page_157">157</a>;</li>
+  <li>Indian earthquake, <a href="#Page_280">280</a></li>
+  </ul>
+</li>
+
+<li>Uzielli, G., <a href="#Page_143">143</a>, <a href="#Page_176">176</a><br /><br /></li>
+
+
+<li>Velocity, maximum, of wave-motion, in Neapolitan earthquake, <a href="#Page_35">35</a>;
+  <ul class="nest">
+  <li>in Indian earthquake, <a href="#Page_272">272</a></li>
+  </ul>
+</li>
+
+<li>Velocity of earth-waves, methods of determining, <a href="#Page_39">39</a>, <a href="#Page_93">93</a>, <a href="#Page_127">127</a>, <a href="#Page_229">229</a>;
+  <ul class="nest">
+  <li>variation with depth, <a href="#Page_336">336</a>;</li>
+  <li>form of wave-paths, <a href="#Page_336">336</a>;</li>
+  <li>velocity of different phases, <a href="#Page_339">339</a>;</li>
+  <li>of Neapolitan earthquake, <a href="#Page_39">39</a>;</li>
+  <li>of Andalusian earthquake, <a href="#Page_92">92</a>;</li>
+  <li>of Charleston earthquake, <a href="#Page_126">126</a>;</li>
+  <li>of Japanese earthquakes, <a href="#Page_188">188</a>;</li>
+  <li>of Hereford earthquake, <a href="#Page_229">229</a>;</li>
+  <li>of Indian earthquake, <a href="#Page_275">275</a>, <a href="#Page_279">279</a>, <a href="#Page_284">284</a></li>
+  </ul>
+</li>
+
+<li>Visible earth-waves in Charleston earthquake, <a href="#Page_110">110</a>;
+  <ul class="nest">
+  <li>in Japanese earthquake, <a href="#Page_186">186</a>;</li>
+  <li>in Indian earthquake, <a href="#Page_268">268</a></li>
+  </ul>
+</li>
+
+<li>Volcanic earthquakes, <a href="#Page_5">5</a>, <a href="#Page_70">70</a></li>
+
+<li>Vorticose shock, <a href="#Page_5">5</a><br /><br /></li>
+
+
+<li>Water, effect of Andalusian earthquake on underground, <a href="#Page_97">97</a></li>
+
+<li>Waterfalls caused by fault-scarps of Indian earthquake, <a href="#Page_305">305</a></li>
+
+<li>Wave-path, <a href="#Page_13">13</a></li>
+
+<li>West, C.D., <a href="#Page_272">272</a></li>
+
+<li>Woolhope anticlinal and Hereford earthquake, connection between, <a href="#Page_241">241</a></li>
+</ul>
+
+
+<br />
+<hr />
+<br />
+
+<div class="tr">
+<p class="cen"><a name="TN" id="TN"></a>Typographical errors corrected in text:</p>
+<br />
+Page &nbsp;&nbsp;54: &nbsp;Casamenello replaced with Casamenella<br />
+Page 117: &nbsp;'Captain Dutton' replaced with 'Major Dutton'<br />
+Page 119: &nbsp;'Capt. Dutton' replaced with 'Major Dutton'<br />
+Page 315: &nbsp;Rangsonobo replaced with Rangsanobo<br />
+Page 336: &nbsp;'per sec. per sec.' replaced with 'per sec.'<br />
+Page 337: &nbsp;negligeable replaced with negligible<br />
+</div>
+
+<br />
+<hr />
+<br />
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of Project Gutenberg's A Study of Recent Earthquakes, by Charles Davison
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diff --git a/LICENSE.txt b/LICENSE.txt
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+This eBook, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
+the "Copyright How-To" at https://www.gutenberg.org.
+
+No investigation has been made concerning possible copyrights in
+jurisdictions other than the United States. Anyone seeking to utilize
+this eBook outside of the United States should confirm copyright
+status under the laws that apply to them.
diff --git a/README.md b/README.md
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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #25062 (https://www.gutenberg.org/ebooks/25062)