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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Earthquakes and Other Earth Movements - -Author: John Milne - -Release Date: July 29, 2019 [EBook #60007] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK EARTHQUAKES, OTHER EARTH MOVEMENTS *** - - - - -Produced by deaurider, Robert Tonsing, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - THE INTERNATIONAL SCIENTIFIC SERIES - VOLUME LV - - - - - THE INTERNATIONAL SCIENTIFIC SERIES - - - - - EARTHQUAKES - AND - OTHER EARTH MOVEMENTS - - - BY - JOHN MILNE - -PROFESSOR OF MINING AND GEOLOGY IN THE IMPERIAL COLLEGE OF ENGINEERING, - TOKIO, JAPAN - - _WITH THIRTY-EIGHT FIGURES_ - - NEW YORK - D. APPLETON AND COMPANY - 1, 3, AND 5 BOND STREET - 1886 - - - - - PREFACE. - - • • • • • - - -In the following pages it has been my object to give a systematic -account of various Earth Movements. - -These comprise _Earthquakes_, or the sudden violent movements of the -ground; _Earth Tremors_, or minute movements which escape our attention -by the smallness of their amplitude; _Earth Pulsations_, or movements -which are overlooked on account of the length of their period; and -lastly, _Earth Oscillations_, or movements of long period and large -amplitude which attract so much attention from their geological -importance. - -It is difficult to separate these Earth Movements from each other, -because they are phenomena which only differ in degree, and which are -intimately associated in their occurrence and in their origin. - - • • • • • - -Because Earthquakes are phenomena which have attracted a universal -attention since the earliest times, and about them so many observations -have been made, they are treated of at considerable length. - -As very much of what might be said about the other Earth Movements is -common to what is said about Earthquakes, it has been possible to make -the description of these phenomena comparatively short. - -The scheme which has been adopted will be understood from the following -table:— - - I. EARTHQUAKES. - - 1. Introduction. - 2. Seismometry. - {(_a_) Theoretically. - 3. Earthquake Motion. {(_b_) As deduced from experiments. - {(_c_) As deduced from actual - Earthquakes. - 4. Earthquake Effects. {(_a_) On land. - {(_b_) In the ocean. - 5. Determination of Earthquake origins. - {(_a_) In space. - 6. Distribution of Earthquakes. {(_b_) In time (geological time, - { historical time, annual, - { seasonal, diurnal, &c.) - 7. Cause of Earthquakes. - 8. Earthquake prediction and warning. - - II. EARTH TREMORS. - - III. EARTH PULSATIONS. - - IV. EARTH OSCILLATIONS. - -In some instances the grouping of phenomena according to the above -scheme may be found inaccurate, as, for example, in the chapters -referring to the effects and causes of Earthquakes. - -This arises from the fact that the relationship between Earthquakes -and other Earth phenomena are not well understood. Thus the sudden -elevation of a coast line and an accompanying earthquake may be -related, either as effect and cause, or _vice versâ_, or they may both -be the effect of a third phenomenon. - -Much of what is said respecting Earthquake motion will show how little -accurate knowledge we have about these disturbances. Had I been writing -in England, and, therefore, been in a position to make references to -libraries and persons who are authorities on subjects connected with -Seismology, the following pages might have been made more complete, -and inaccuracies avoided. A large proportion of the material embodied -in the following pages is founded on experiments and observations -made during an eight years’ residence in Japan, where I have had the -opportunity of recording an earthquake every week. - -The writer to whom I am chiefly indebted is Mr. Robert Mallet. Not -being in a position to refer to original memoirs, I have drawn many -illustrations from the works of Professor Karl Fuchs and M. S. di -Rossi. These, and other writers to whom reference has been made, are -given in an appendix. - -For seeing these pages through the press, my thanks are due to -Mr. Thomas Gray, who, when residing in Japan, did so much for the -advancement of observational Seismology. - -For advice and assistance in devising experiments, I tender my thanks -to my colleagues, Professor T. Alexander, Mr. T. Fujioka, and to my -late colleague. Professor John Perry. - -For assistance in the actual observation of Earthquakes, I have to -thank my friends in various parts of Japan, especially Mr. J. Bissett -and Mr. T. Talbot, of Yokohama. For assistance in obtaining information -from Italian sources I have to thank Dr. F. Du Bois, from German -sources Professor C. Netto, and from Japanese sources Mr. B. H. -Chamberlain. For help in carrying out experiments, I am indebted to -the liberality of the British Association, the Geological Society of -London, the Meteorological and Telegraph departments of Japan, and to -the officers of my own institution, the Imperial College of Engineering. - -And, lastly, I offer my sincere thanks to those gentlemen who have -taken part in the establishment and working of the Seismological -Society of Japan, and to my publishers, whose liberality has enabled me -to place the labours of residents in the Far East before the European -public. - - JOHN MILNE. - - TOKIO, JAPAN; _June 30, 1883_ - - - - - CONTENTS. - - • • • • • - - CHAPTER I. - - INTRODUCTION. - - PAGE - Relationship of man to nature—The aspect of a country - is dependent on geological phenomena—Earthquakes an - important geological phenomenon—Relationship of seismology - to the sciences and arts—Earth movements other than - earthquakes—Seismological literature—(Writings of Perrey, - Mallet, Eastern writings, the Philosophical Transactions of - the Royal Society, the ‘Gentleman’s Magazine,’ the Bible, - Herodotus, Pliny, Hopkins, Von Hoff, Humboldt, Schmidt, - Seebach, Lasaulx, Fuchs, Palmieri, Bertelli, Seismological - Society of Japan)—Seismological terminology 1 - - - CHAPTER II. - - SEISMOMETRY. - - Nature of earthquake vibrations—Many instruments called - seismometers only seismoscopes—Eastern seismoscopes, columns, - projection seismometers—Vessels filled with liquid—Palmieri’s - mercury tubes—The ship seismoscope—The cacciatore—Pendulum - instruments of Kreil, Wagner, Ewing, and Gray—Bracket - seismographs—West’s parallel motion instrument—Gray’s conical - pendulums, rolling spheres, and cylinders—Verbeck’s ball and - plate seismograph—The principle of Perry and Ayrton—Vertical - motion instruments—Record receiver—Time-recording - apparatus—The Gray and Milne seismograph 12 - - - CHAPTER III. - - EARTHQUAKE MOTION DISCUSSED THEORETICALLY. - - Ideas of the ancients (the views of Travagini, Hooke, Woodward, - Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves - and vibrations—Possible causes of disturbance in the earth’s - crust—The time of vibration of an earth particle—Velocity - and acceleration of a particle—Propagation of a disturbance - as determined by experiments upon the elastic moduli of - rocks—The intensity of an earthquake—Area of greatest - overturning moment—Earthquake waves—Reflexion, refraction, - and interference of waves—Radiation of a disturbance 41 - - - CHAPTER IV. - - EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT. - - Experiments with falling weights—Experiments with - explosives—Results obtained from experiments—Relative - motion of two adjacent points—The effect of hills and - excavations upon the propagation of vibrations—The intensity - of artificial disturbances—Velocity with which earth - vibrations are propagated—Experiments of Mallet—Experiments - of Abbot—Experiments in Japan—Mallet’s results—Abbot’s - results—Results obtained in Japan 57 - - - CHAPTER V. - - EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION - ON EARTHQUAKES. - - Result of feelings—The direction of motion—Instruments as - indicators of direction—Duration of an earthquake—Period - of vibration—The amplitude of earth movements—Side of - greatest motion—Intensity of earthquakes—Velocity and - acceleration of an earth particle—Absolute intensity of an - earthquake—Radiation of an earthquake—Velocity of propagation 67 - - - CHAPTER VI. - - EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS. - - The destruction of buildings is not irregular—Cracks in - buildings—Buildings in Tokio—Relation of destruction - to earthquake motion—Measurement of relative motion of - parts of a building shaken by an earthquake—Prevention of - cracks—Direction of cracks—The pitch of roofs—Relative - position of openings in a wall—The last house in a row—The - swing of buildings—Principle of relative vibrational periods 96 - - - CHAPTER VII. - - EFFECTS PRODUCED UPON BUILDINGS (_continued_). - - Types of buildings used in earthquake countries—In Japan, - in Italy, in South America, in Caraccas—Typical houses - for earthquake countries—Destruction due to the nature of - underlying rocks—The swing of mountains—Want of support - on the face of hills—Earthquake shadows—Destruction due - to the interference of waves—Earthquake bridges—Examples - of earthquake effects—Protection of buildings—General - conclusions 122 - - - CHAPTER VIII. - - EFFECTS OF EARTHQUAKES ON LAND. - - 1. Cracks and fissures—Materials discharged from - fissures—Explanation of fissure phenomena. 2. Disturbances - in lakes, rivers, springs, wells, fumaroles, &c.—Explanation - of these latter phenomena. 3. Permanent displacement of - ground—On coast lines—Level tracts—Among - mountains—Explanation of these movements 146 - - - CHAPTER IX. - - DISTURBANCES IN THE OCEAN. - - Sea vibrations—Cause of vibratory blows—Sea waves: - preceding earthquakes; succeeding earthquakes—Magnitude - of waves—Waves as recorded in countries distant from the - origin—Records on tide gauges—Waves without earthquakes—Cause - of waves—Phenomena difficult of explanation—Velocity - of propagation—Depth of the ocean—Examples of - calculations—Comparison of velocities of earthquake waves - with velocities which ought to exist from the known depth of - the ocean 163 - - - CHAPTER X. - - DETERMINATION OF EARTHQUAKE ORIGINS. - - Approximate determination of an origin—Earthquake-hunting - in Japan—Determinations by direction of motion—Direction - indicated by destruction of buildings—Direction - determined by rotation—Cause of rotation—The use of time - observations—Errors in such observations—Origin determined - by the method of straight lines—The method of circles, the - method of hyperbolas, the method of co-ordinates—Haughton’s - method—Difference in time between sound, earth, and water - waves—Method of Seebach 187 - - - CHAPTER XI. - - THE DEPTH OF AN EARTHQUAKE CENTRUM. - - The depth of an earthquake centrum—Greatest possible depth of - an earthquake—Form of the focal cavity 213 - - - CHAPTER XII. - - DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME. - - General distribution of earthquakes—Occurrence along - lines—Examples of distribution—Italian earthquake of 1873—In - Tokio—Extension of earthquake boundaries—Seismic energy in - relation to geological time; to historical time—Relative - frequency of earthquakes—Synchronism of earthquakes—Secondary - earthquakes 226 - - - CHAPTER XIII. - - DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_) 234 - - - CHAPTER XIV. - - DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_). - - The occurrence of earthquakes in relation to the position of - the heavenly bodies—Earthquakes and the moon—Earthquakes - and the sun; and the seasons; the months—Planets and - meteors—Hours at which earthquakes are frequent—Earthquakes - and sun spots—Earthquakes and the aurora 250 - - - CHAPTER XV. - - BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN - TEMPERATURE AND EARTHQUAKES 266 - - - CHAPTER XVI. - - RELATION OF SEISMIC TO VOLCANIC PHENOMENA. - - Want of synchronism between earthquakes and volcanic - eruptions—Synchronism between earthquakes and volcanic - eruptions—Conclusion 270 - - - CHAPTER XVII. - - THE CAUSE OF EARTHQUAKES. - - Modern views respecting the cause of earthquakes—Earthquakes - due to faulting—To explosions of steam—To volcanic - evisceration—To chemical degradation—Attractive influence of - the heavenly bodies—The effect of oceanic tides—Variation in - atmospheric pressure—Fluctuation in temperature—Winds and - earthquakes—Rain and earthquakes—Conclusion 277 - - - CHAPTER XVIII. - - PREDICTION OF EARTHQUAKES. - - General nature of predictions—Prediction by the observation of - unusual phenomena (Alteration in the appearance and taste of - springs; underground noises; preliminary tremors; Earthquake - prophets—warnings furnished by animals, &c.)—Earthquake - warning 297 - - - CHAPTER XIX. - - EARTH TREMORS. - - Artificially produced tremors—Observations of Kater, Denman, - Airy, Palmer, Paul—Natural tremors—Observations of Zöllner, - M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With - seismoscopes, microphones, pendulums—Work in Italy—Bertelli, - Count Malvasia, M. S. di Rossi—Instruments employed in - Italy—Tromometers, microseismographs, microphones—Results - obtained in Italy—In Japan—Cause of microseismic motion 306 - - - CHAPTER XX. - - EARTH PULSATIONS. - - Definition of an earth pulsation—Indications of - pendulums—Indications of levels—Other phenomena indicating - the existence of earth pulsations—Disturbances in lakes and - oceans—Phenomena resultant on earth pulsations—Cause of earth - pulsations 326 - - - CHAPTER XXI. - - EARTH OSCILLATIONS. - - Evidences of oscillation—Examples of oscillation—Temple of - Jupiter Serapis—Observations of Darwin—Causes of oscillation 344 - - - APPENDIX 349 - - - INDEX 359 - - - - - EARTHQUAKES. - - • • • • • - - - - - CHAPTER I. - - INTRODUCTION. - - Relationship of man to nature—The aspect of a country is dependent - on geological phenomena—Earthquakes an important geological - phenomenon—Relationship of seismology to the sciences and - arts—Earth movements other than earthquakes—Seismological - literature—(Writings of Perrey, Mallet, Eastern writings, the - Philosophical Transactions of the Royal Society, the ‘Gentleman’s - Magazine,’ the Bible, Herodotus, Pliny, Hopkins, Von Hoff, - Humboldt, Schmidt, Seebach, Lasaulx, Fuchs, Palmieri, Bertelli, - Seismological Society of Japan)—Seismological terminology. - - -In bygone superstitious times lightning and thunder were regarded -as supernatural visitations. But as these phenomena became better -understood, and men learned how to avoid their destructive power, the -superstition was gradually dispelled. Thus it is with Earthquakes: -the more clearly they are understood, the more confident in the -universality of law will man become, and the more will his mental -condition be advanced. - -In his ‘History of Civilisation in England,’ Buckle has laid -considerable stress upon the manner in which earthquakes, volcanoes, -and other of the more terrible forms in which the workings of -nature reveal themselves to us, have exerted an influence upon the -imagination and understanding; and just as a sudden fright may affect -the nerves of a child for the remainder of its life, we have in the -annals of seismology records which indicate that earthquakes have not -been without a serious influence upon the mental condition of whole -communities. - -To a geologist there are perhaps no phenomena in nature more -interesting than earthquakes, the study of which is called Seismology. -Coming, as shocks often will, from a region of volcanoes, the study -of these disturbances may enable us to understand something about -the nature and working of a volcano. As an earthquake wave travels -along from strata to strata, if we study its reflections and changing -velocity in transit, we may often be led to the discovery of certain -rocky structures buried deep beneath our view, about which, without the -help of such waves, it would be hopeless ever to attain any knowledge. - -By studying the propagation of earthquake waves the physicist is -enabled to confirm his speculations respecting the transmission of -disturbances in elastic media. For the physicist earthquakes are -gigantic experiments which tell him the elastic moduli of rocks as -they exist in nature, and when properly interpreted may lead him to -the proper comprehension of many ill-understood phenomena. It is not -impossible that seismological investigation may teach us something -about the earth’s magnetism, and the connection between earthquakes and -the ‘earth currents’ which appear in our telegraph wires. These and -numerous other kindred problems fall within the domain of the physicist. - -It is of interest to the meteorologist to know the connections which -probably exist between earthquakes and the fluctuations of the -barometer, the changes of the thermometer, the quantity of rainfall, -and like phenomena to which he devotes his attention. - -Next we may turn to the more practical aims of seismology and ask -ourselves what are the effects of earthquakes upon buildings, and -how, in earthquake-shaken countries, the buildings are to be made to -withstand them. Here we are face to face with problems which demand -the attention of engineers and builders. To attain what we desire, -observation, common sense, and subtle reasoning must be brought to bear -upon this subject. - -In the investigation of the principle on which earthquake instruments -make their records, in the analysis of the results they give, in -problems connected with astronomy, with physics, and with construction, -seismology offers to the mathematician new fields for investigation. - -A study of the effects which earthquakes produce on the lower animals -will not fail to interest the student of natural history. - -A study like seismology, which leads us to a more complete knowledge -of earth-heat and its workings, is to be regarded as one of the -corner-stones of geology. The science of seismology invites the -co-operation of workers and thinkers in almost every department of -natural science. - -We have already referred to the influence exerted by earthquakes over -the human mind. How to predict earthquakes, and how to escape from -their dangers, are problems which, if they can be solved, are of -extreme interest to the world at large. - -In addition to the sudden and violent movements which we call -earthquakes, the seismologist has to investigate the smaller motions -which we call earth tremors. From observations which have been made -of late years, it would appear that the ground on which we dwell is -incessantly in a state of tremulous motion. - -A further subject of investigation which is before the seismologist is -the experimental verification of the existence of what may be called -‘earth-pulsations.’ These are motions which mathematical physicists -affirmed the existence of, but which, in consequence of the slowness of -their period, have hitherto escaped observation. - -The oscillations, or slow changes in the relative positions of land and -sea, might also be included; but this has already been taken up as a -separate branch of geology. - -These four classes of movements are no doubt interdependent, and -seismology in the widest sense might conveniently be employed to -include them all. In succeeding chapters we will endeavour to indicate -how far the first three of these branches have been prosecuted, and -to point out that which remains to be accomplished. It is difficult, -however, to form a just estimate of the amount of seismological work -which has been done, in consequence of the scattered and uncertain -nature of many of the records. Seismology, as a science, originated -late, chiefly owing to the facts that centres of civilisation are -seldom in the most disturbed regions, and that earthquake-shaken -countries are widely separated from each other. - -As every portion of the habitable globe appears to have been shaken -more or less by earthquakes, and as these phenomena are so terrible in -their nature, we can readily understand why seismological literature is -extensive. In the annals of almost every country which has a written -history, references are made to seismic disturbances. - -An idea of the attention which earthquakes have received may be -gathered from the fact that Professor Alexis Perrey, of Dijon, who -has published some sixty memoirs on this subject, gave, in 1856, a -catalogue of 1,837 works devoted to seismology.[1] In 1858 Mr. Robert -Mallet published in the Reports of the British Association a list of -several hundred works relating to earthquakes. Sixty-five of these -works are to be found in the British Museum. So far as literature is -concerned, earthquakes have received as much attention in the East as -in the West. In China there are many works treating on earthquakes, and -the attention which these phenomena received may be judged of from the -fact that in A.D. 136 the Government appointed a commission -to inquire into the subject. Even the isolated empire of Japan can -boast of at least sixty-five works on earthquakes, seven of which -are earthquake calendars, and twenty-three earthquake monographs.[2] -Besides those treating especially of earthquakes, there are innumerable -references to such disturbances in various histories, in the -transactions of learned societies, and in periodicals. To attempt to -give a complete catalogue of even the books which have been written -would be to enter on a work of compilation which would occupy many -years, and could never be satisfactorily finished. - -In the ‘Philosophical Transactions of the Royal Society,’ which were -issued in the eighteenth century, there are about one hundred and -eighty separate communications on earthquakes; and in the ‘Gentleman’s -Magazine’ for 1755 there are no less than fifty notes and articles on -the same subject. The great interest shown in earthquakes about the -years 1750–60 in England, was chiefly due to the terrible calamity -which overtook Lisbon in 1755, and to the fact that about this time -several shocks were experienced in various parts of the British -Islands. In 1750, which may be described as the earthquake year of -Britain, ‘a shock was felt in Surrey on March 14; on the 18th of the -same month the whole of the south-west of England was disturbed. On -April 2, Chester was shaken; on June 7, Norwich was disturbed; on -August 23, the inhabitants of Lancashire were alarmed; and on September -30 ludicrous and alarming scenes occurred in consequence of a shock -having been felt during the hours of Divine service, causing the -congregations to hurry into the open air.’[3] As might be expected, -these occurrences gave rise to many articles and notes directing -attention to the subject of earthquakes. - -Seismic literature has not, however, at all times been a measure of -seismic activity: thus, in Japan, the earthquake records for the -twelfth and sixteenth centuries scarcely mention any shocks. At first -sight it might be imagined that this was owing to an absence of -earthquakes; but it is sufficiently accounted for by the fact that -at that time the country was torn with civil war, and matters more -urgent than the recording of natural phenomena engaged the attention of -the inhabitants. Professor Rockwood, who has given so much attention -to seismic disturbances in America, tells us that during the recent -contest between Chili and Peru a similar intermission is observable. We -see, therefore, that an absence of records does not necessarily imply -an absence of the phenomena to be recorded. - -Perhaps the earliest existing records of earthquakes are those which -occur in the Bible. The first of these, which we are told occurred in -Palestine, was in the reign of Ahab (B.C. 918–897).[4] One of the most -terrible earthquakes mentioned in the Bible is that which took place -in the days of Uzziah, king of Judah (B.C. 811–759), which shook the -ground and rent the Temple. The awful character of this, and the deep -impression produced on men’s minds, may be learned from the fact that -the time of its occurrence was subsequently used as an epoch from which -to reckon dates. - -The writings of Herodotus, Pliny, Livy, &c., &c., show the interest -which earthquakes attracted in early ages. These writers chiefly -devoted themselves to references and descriptions of disastrous shocks, -and to theories respecting the cause of earthquakes. - -The greater portion of the Japanese notices of earthquakes is simply -a series of anecdotes of events which took place at the time of -these disasters. We also find references to superstitious beliefs, -curious occurrences, and the apparent connection between earthquake -disturbances and other natural phenomena. In these respects the -literature of the East closely resembles that of the West. The -earthquake calendars of the East, however, form a class of books which -can hardly be said to find their parallel in Europe;[5] while, on the -other hand, the latter possesses types of books and pamphlets which do -not appear to have a parallel elsewhere. These are the more or less -theological works—‘Moral Reflections on Earthquakes,’ ‘Sermons’ which -have been preached on earthquakes, ‘Prayers’ which have been appointed -to be read.[6] - -Speaking generally, it may be said that the writings of the ancients, -and those of the Middle Ages, down to the commencement of the -nineteenth century, tended to the propagation of superstition and to -theories based on speculations with few and imperfect facts for their -foundation. - -Among the efforts which have been made in modern times to raise -seismology to a higher level, is that of Professor Perrey, of Dijon, -who commenced in 1840 a series of extensive catalogues embracing -the earthquakes of the world. These catalogues enabled Perrey, and -subsequently Mallet in his reports to the British Association, to -discuss the periodicity of earthquakes, with reference to the seasons -and to other phenomena, in a more general manner than it had been -possible for previous workers to accomplish. The facts thus accumulated -also enabled Mallet to discuss earthquakes in general, and the various -phenomena which they present were sifted and classified for inspection. -Another great impetus which observational seismology received was Mr. -Mallet’s report upon the Neapolitan earthquake of 1857, in which new -methods of seismic investigation were put forth. These have formed -the working tools of many subsequent observers, and by them, as well -as by his experiments on artificially produced disturbances, Mallet -finally drew the study of earthquakes from the realms of speculation by -showing that they, like other natural phenomena, were capable of being -understood and investigated. - -In addition to Perrey and Mallet, the nineteenth century has produced -many writers who have taken a considerable share in the advancement of -seismology. There are the catalogues of Von Hoff, the observations of -Humboldt, the theoretical investigations of Hopkins, the monographs of -Schmidt, Seebach, Lasaulx, and others; the books of Fuchs, Credner, -Vogt, Volger; the records and observations of Palmieri, Bertelli, -Rossi, and other Italian observers. To these, which are only a few -out of a long list of names, may be added the publications of the -Commission appointed for the observation of earthquakes by the Natural -History Society of Switzerland, and the volumes which have been -published by the Seismological Society of Japan. - -Before concluding this chapter it will be well to define a few of the -more ordinary terms which are used in describing earthquake phenomena. -It may be observed that the English word _earthquake_, the German -_erdbeben_, the French _tremblement de terre_, the Spanish _terremoto_, -the Japanese _jishin_ &c., all mean, when literally translated, -_earth-shaking_, and are popularly understood to mean a sudden and more -or less violent disturbance. - -Seismology (σειμός an earthquake, λόγος a discourse) in its simplest -sense means the study of earthquakes. To be consistent with a Greek -basis for seismological terminology, some writers have thrown aside -the familiar expression ‘earthquake,’ and substituted the awkward word -‘seism.’ - -The source from which an earthquake originates is called the ‘origin,’ -‘focal cavity,’ or ‘centrum.’ - -The point or area on the surface of the ground above the origin is -called the ‘epicentrum.’ The line joining the centrum and epicentrum is -called the ‘seismic vertical.’ - -The radial lines along which an earthquake may be propagated from the -centrum are called ‘wave paths.’ - -The angle which a wave path, where it reaches the surface of the earth, -makes with that surface is called the ‘angle of emergence’ of the wave. -This angle is usually denoted by the letter _e_. - -As the result of a simple explosion at a point in a homogeneous medium, -we ought, theoretically, to obtain at points on the surface of the -medium equidistant from the epicentrum, equal mechanical effects. -These points will lie on circles called ‘isoseismic’ or ‘coseismic’ -circles. The area included between two such circles is an ‘isoseismic -area.’ In nature, however, isoseismic lines are seldom circles. -Elliptical or irregular curves are the common forms. - -The isoseismic area in which the greatest disturbance has taken place -is called the ‘meizoseismic area.’ Seebach calls the lines enclosing -this area ‘pleistoseists.’ - -These last-mentioned lines are wholly due to Mallet and Seebach. - -Many words are used to distinguish different kinds of earthquakes from -each other. All of these appear to be very indefinite and to depend -upon the observer’s feelings, which, in turn, depend upon his nervous -temperament and his situation. - -In South America small earthquakes, consisting of a series of rapidly -recurring vibratory movements not sufficiently powerful to create -damage, are spoken of as _trembelores_. - -The _terremotos_ of South America are earthquakes of a destructive -nature, in which distinct shocks are perceptible. It may be observed -that shocks which at one place would be described as _terremoto_ -would at another and more distant place probably be described as -_trembelores_. - -The _succussatore_ are the shocks where there is considerable vertical -motion. The terrible shock of Riobamba (February 4, 1797), which is -said to have thrown corpses from their graves to a height of 100 feet, -was an earthquake of this order. - -The _vorticosi_ are shocks which have a twisting or rotatory motion. - -Another method of describing earthquakes would be to refer to -instrumental records. When the vibrations of the ground have only -been along the line joining the observer and the epicentrum, the -disturbance might be called ‘euthutropic.’ A disturbance in which the -prominent movements are _transverse_ to the above direction might be -called ‘diagonic.’ If motions in both of these directions occur in the -records, the shock might be said to be ‘diastrophic.’ If there be much -vertical movement, the shock might be said to be ‘anaseismic.’ Some -disturbances could only be described by using two or three of these -terms. - - - - - CHAPTER II. - - SEISMOMETRY. - - Nature of earthquake vibrations—Many instruments called - seismometers only seismoscopes—Eastern seismoscopes, columns, - projection seismometers—Vessels filled with liquid—Palmieri’s - mercury tubes—The ship seismoscope—The cacciatore—Pendulum - instruments of Kreil, Wagner, Ewing, and Gray—Bracket - seismographs—West’s parallel motion instrument—Gray’s conical - pendulums, rolling spheres, and cylinders—Verbeck’s ball and - plate seismograph—The principle of Perry and Ayrton—Vertical - motion instruments—Record receivers—Time-recording apparatus—The - Gray and Milne seismograph. - - -Before we discuss the nature of earthquake motion, the determination -of which has been the aim of modern seismological investigation, the -reader will naturally look for an account of the various instruments -which have been employed for recording such disturbances. A description -of the earthquake machines which have been used even in Japan would -form a bulky volume. All that we can do, therefore, is to describe -briefly the more prominent features of a few of the more important of -these instruments. In order that the relative merits of these may be -better understood, we may state generally that modern research has -shown a typical earthquake to consist of a series of small tremors -succeeded by a shock, or series of shocks, separated by more or less -irregular vibrations of the ground. The vibrations are often both -irregular in period and in amplitude, and they have a duration of from -a few seconds to several minutes. We will illustrate the records of -actual earthquakes in a future chapter, but in the meantime the idea -that an earthquake consists of a single shock must be dismissed from -the imagination. - -To construct an instrument which at the time of an earthquake shall -move and leave a record of its motion, there is but little difficulty. -Contrivances of this order are called _seismoscopes_. If, however, -we wish to know the period, extent, and direction of each of the -vibrations which constitutes an earthquake, we have considerable -difficulty. Instruments which will in this way measure or write down -the earth’s motions are called _seismometers_ or _seismographs_. - -Many of the elaborate instruments supplemented with electro-magnetic -and clockwork arrangements are, when we examine them, nothing more than -elaborate seismoscopes which have been erroneously termed seismographs. - -The only approximations to true seismographs which have yet been -invented are without doubt those which during the past few years have -been used in Japan. It would be a somewhat arbitrary proceeding, -however, to classify the different instruments as seismoscopes, -seismometers, and seismographs, as the character of the record given -by certain instruments is sometimes only seismoscopic, whilst at other -times it is seismometric, depending on the nature of the disturbance. -Many instruments, for instance, would record with considerable accuracy -a single sudden movement, but would give no reliable information -regarding a continued shaking. - -_Eastern Seismoscopes._—The earliest seismoscope of which we find any -historical record is one which owes its origin to a Chinese called -Chôko. It was invented in the year A.D. 136. A description is given -in the Chinese history called ‘Gokanjo,’ and the translation of this -description runs as follows:— - -‘In the first year of Yōka, A.D. 136, a Chinese called Chôko invented -the seismometer shown in the accompanying drawing. This instrument -consists of a spherically formed copper vessel, the diameter of which -is eight feet. It is covered at its top, and in form resembles a -wine-bottle. Its outer part is ornamented by the figures of different -kinds of birds and animals, and old peculiar-looking letters. In the -inner part of this instrument a column is so suspended that it can -move in eight directions. Also, in the inside of the bottle, there is -an arrangement by which some record of an earthquake is made according -to the movement of the pillar. On the outside of the bottle there -are eight dragon heads, each of which holds a ball in its mouth. -Underneath these heads there are eight frogs so placed that they appear -to watch the dragon’s face, so that they are ready to receive the ball -if it should be dropped. All the arrangements which cause the pillar to -knock the ball out of the dragon’s mouth are well hidden in the bottle.’ - -[Illustration: FIG. 1.] - -‘When an earthquake occurs, and the bottle is shaken, the dragon -instantly drops the ball, and the frog which receives it vibrates -vigorously; any one watching this instrument can easily observe -earthquakes.’ - -With this arrangement, although one dragon may drop a ball, it is -not necessary for the other seven dragons to drop their balls unless -the movement has been in all directions; thus we can easily tell the -direction of an earthquake. - -‘Once upon a time a dragon dropped its ball without any earthquake -being observed, and the people therefore thought the instrument of -no use, but after two or three days a notice came saying that an -earthquake had taken place at Rōsei. Hearing of this, those who doubted -the use of this instrument began to believe in it again. After this -ingenious instrument had been invented by Chōko, the Chinese Government -wisely appointed a secretary to make observations on earthquakes.’ - -Not only is this instrument of interest on account of its antiquity, -but it is also of interest on account of the close resemblance it bears -to many of the instruments of modern times. - -Another earthquake instrument also of Eastern origin is the magnetic -seismoscope of Japan. - -On the night of the destructive earthquake of 1855, which devastated -a great portion of Tokio, the owner of a spectacle shop in Asakusa -observed that a magnet dropped some old iron nails and keys which had -been attached to it. From this occurrence the owner thought that the -magnet had, in consequence of its age, lost its powers. About two hours -afterwards, however, the great earthquake took place, after which the -magnet was observed to have regained its powers. This occurrence led -to the construction of the seismoscope, which is illustrated in a book -called the ‘Ansei-Kembun-Roku,’ or a description of the earthquake of -1855, and examples of the instrument are still to be seen in Tokio. -These instruments consist of a piece of magnetic iron ore, which holds -up a piece of iron like a nail. This nail is connected, by means of a -string, with a train of clockwork communicating with an alarm. If the -nail falls a catch is released and the clockwork set in motion, and -warning given by the ringing of a bell. It does not appear that this -instrument has ever acted with success. - -_Columns._—One of the commonest forms of seismoscope, and one which -has been very widely used, consists of a round column of wood, metal, -or other suitable material, placed, with its axis vertical, on a level -plane, and surrounded by some soft material such as loose sand to -prevent it rolling should it be overturned. The fall of such a column -indicates that a shaking or shock has taken place. Attempts have been -made by using a number of columns of different sizes to make these -indications seismometric, but they seldom give reliable information -either as to intensity or direction of shock. The indications as to -intensity are vitiated by the fact that a long-continued gentle shaking -may overturn a column which would stand a very considerable sudden -shock, while the directions in which a number of columns fall seldom -agree owing to the rotational motion imparted to them by the shaking. -Besides, the direction of motion of the earthquake seldom remains in -the same azimuth throughout the whole disturbance. - -An extremely delicate, and at the same time simple form of seismoscope -may be made by propping up strips of glass, pins, or other easily -overturned bodies against suitably placed supports. In this way bodies -may be arranged, which, although they can only fall in one direction, -nevertheless fall with far less motion than is necessary to overturn -any column which will stand without lateral support. - -_Projection Seismometers._—Closely related to the seismoscopes and -seismometers which depend on the overturning of bodies. Mallet has -described two sets of apparatus whose indications depend on the -distance to which a body is projected. In one of these, which consisted -of two similar parts arranged at right angles, two metal balls rest -one on each side of a stop at the lower part of two inclined \/ like -troughs. In this position each of the balls completes an electric -circuit. By a shock the balls are projected or rolled up the troughs, -and the height to which they rise is recorded by a corresponding -interval in the break of the circuits. The vertical component of the -motion is measured by the compression of a spring which carries the -table on which this arrangement rests. In the second apparatus two -balls are successively projected, one by the forward swing, and the -other by the backward swing of the shock. Attached to them are loose -wires forming terminals of the circuits. They are caught in a bed -of wet sand in a metal trough forming the other end of the circuit. -The throw of the balls as measured in the sand, and the difference -of time between their successive projections as indicated by special -contrivances connected with the closing of the circuits, enables -the observer to calculate the direction of the wave of shock, its -velocity, and other elements connected with the disturbance. It will be -observed that the design of this apparatus assumes the earthquake to -consist of a distinct isolated shock. - -Oldham, at the end of his account of the Cachar earthquake of 1869, -recommends the use of an instrument based on similar principles. In his -instrument four balls like bullets are placed in notches cut in the -corners of the upper end of a square stake driven into the ground. - -_Vessels filled with liquid._—Another form of simple seismoscope is -made by partially filling a vessel with liquid. The height to which the -liquid is washed up the side of the vessel is taken as an indication of -the intensity of the shock, and the line joining the points on which -maximum motion is indicated, is taken as the direction of the shock. If -earthquakes all lasted for the same length of time, and consisted of -vibrations of the same period, such instruments might be of service. -These instruments have, however, been in use from an early date. In -1742 we find that bowls of water were used to measure the earthquakes -which in that year alarmed the inhabitants of Leghorn. About the same -time the Rev. S. Chandler, writing about the shock at Lisbon, tells us -that earthquakes may be measured by means of a spherical bowl about -three or four feet in diameter, the inside of which, after being dusted -over with Barber’s puff, is filled very gently with water. Mallet, -Babbage, and De la Bêche have recommended the same sort of contrivance, -but, notwithstanding, it has justly been criticised as ‘ridiculous and -utterly impracticable.’[7] - -An important portion of Palmieri’s well-known instrument consists -of horizontal tubes turned up at the ends and partially filled with -mercury. To magnify the motion of the mercury, small floats of iron -rest on its surface. These are attached by means of threads to a pulley -provided with indices which move in front of a scale of degrees. We -thus read off the intensity of an earthquake as so many degrees, which -means so many millimetres of washing up and down of mercury in a tube. -The direction of movement is determined by the azimuth of the tube -which gives the maximum indication, several tubes being placed in -different azimuths. - -This form of instrument appears to have been suggested by Mallet, who -gives an account of the same in 1846. Inasmuch as the rise and fall -of the mercury in such tubes depend on its depth and on the period -of the earthquake together with its duration, we see that although -the results obtained from a given instrument may give us means of -making approximate comparisons as to the relative intensity of various -earthquakes, it is very far from yielding any absolute measurement. - -Another method which has been employed to magnify and register the -motions of liquid in a vessel has been to float upon its surface a -raft or ship from which a tall mast projected. By a slight motion of -the raft, the top of the mast vibrated through a considerable range. -This motion of the mast as to direction and extent was then recorded by -suitable contrivances attached to the top of the mast. - -A very simple form of liquid seismoscope consists of a circular trough -of wood with notches cut round its side. This is filled with mercury -to the level of the notches. At the time of an earthquake the maximum -quantity of mercury runs over the notches in the direction of greatest -motion. This instrument, which has long been used in Italy, is known as -a Cacciatore, being named after its inventor. It is a prominent feature -in the collection of apparatus forming the well-known seismograph of -Palmieri. - -_Pendulum instruments._—Mallet speaks of pendulum seismoscopes and -seismographs as ‘the oldest probably of seismometers long set up in -Italy and southern Europe.’ In 1841 we find these being used to record -the earthquake disturbances at Comrie in Scotland. - -These instruments may be divided into two classes: first, those which -at the time of the shock are intended to swing, and thus record the -direction of movement; and second, those which are supposed to remain -at rest and thus provide ‘steady points.’ - -To obtain an absolutely ‘steady point’ at the time of an earthquake, -has been one of the chief aims of all recent seismological -investigations. - -With a style or pointer projecting down from the steady point to -a surface which is being moved backward and forward by the earth, -such a surface has written upon it by its own motions a record of -the ground to which it is attached. Conversely, a point projecting -upwards from the moving earth might be caused to write a record on the -body providing the steady point, which in the class of instruments -now to be referred to is supposed to be the bob of a pendulum. It is -not difficult to get a pendulum which will swing at the time of a -moderately strong earthquake, but it is somewhat difficult to obtain -one which will not swing at such a time. During the past few years, -pendulums varying between forty feet in length and carrying bobs of -eighty pounds in weight, and one-eighth of an inch in length, and -carrying a gun-shot, have been experimented with under a great variety -of circumstances. Sometimes the supports of these pendulums have been -as rigid as it is possible to make a structure from brick and mortar, -and at other times they have intentionally been made loose and -flexible. The indices which wrote the motions of these pendulums have -been as various as the pendulums themselves. A small needle sliding -vertically through two small holes, and resting its lower end on a -surface of smoked glass, has on account of its small amount of friction -been perhaps one of the favourite forms of recording pointers. - -The free pendulums which have been employed, and which were intended -to swing, have been used for two purposes: first, to determine the -direction of motion from the direction of swing, and second, to see if -an approximation to the period of the earth’s motion could be obtained -by discovering the pendulum amongst a series of different lengths which -was set in most violent motion, this probably being the one which had -its natural period of swing the most nearly approximating to the period -of the earthquake oscillations. - -Inasmuch as all pendulums when swinging have a tendency to change the -plane of their oscillation, and also as we now know that the direction -of motion during an earthquake is not always constant, the results -usually obtained with these instruments respecting the direction of -the earth’s motion have been unsatisfactory. The results which were -obtained by series of pendulums of different lengths were, for various -reasons, also unsatisfactory. - -Of pendulums intended to provide a steady point, from which the -relative motion of a point on the earth’s surface could be recorded, -there has been a great variety. One of the oldest forms consisted of -a pendulum with a style projecting downwards from the bob so as to -touch a bed of sand. Sometimes a concave surface was placed beneath the -pendulum, on which the record was traced by means of a pencil. Probably -the best form was that in which a needle, capable of sliding freely up -and down, marked the relative horizontal motion of the earth and the -pendulum bob on a smoked glass plate. - -It generally happens that at the time of a moderately severe earthquake -the whole of these forms of apparatus are set in motion, due partly to -the motion of the point of support of the pendulum, and partly to the -friction of the writing point on the plate. - -Among these pendulums may be mentioned those of Cavallieri, Faura, -Palmieri, Rossi, and numerous others. It is possible that the -originators of some of these pendulums may have intended that -they should record by swinging. If this is so, then so far as the -determination of the actual nature of earthquake motion is concerned, -they belong to a lower grade of apparatus than that in which they are -here included. - -A great improvement in pendulum apparatus is due to Mr. Thomas Gray -of Glasgow, who suggested applying so much frictional resistance to -the free swing of a pendulum that for small displacements it became -‘dead beat.’ By carrying out this suggestion, pendulum instruments -were raised to the position of seismographs. The manner of applying -the friction will be understood from the following description of a -pendulum instrument which is also provided with an index which gives a -magnification of the motion of the earth. - -B B B B is a box 113 cm. high and 30 cm. by 18 cm. square. Inside this -box a lead ring R, 17 cm. in diameter and 3 cm. thick, is suspended as -a pendulum from the screw S. This screw passes through a small brass -plate P P, which can be moved horizontally over a hole in the top of -the box. These motions in the point of suspension allow the pendulum to -be adjusted. - -[Illustration: FIG. 2.] - -Projecting over the top of the pendulum there is a wooden arm W -carrying two sliding pointers H H, resting on a glass plate placed on -the top of the pendulum. These pointers are for the purpose of giving -the frictional resistance before referred to. If this friction plate -is smoked, the friction pointers will write upon it records of _large_ -earthquakes independently of the records given by the proper index, -which only gives satisfactory records in the case of shocks of ordinary -intensity. Crossing the inside of the pendulum R there is a brass bar -perforated with a small conical hole at M. A stiff wire passes through -M and forms the upper portion of the index I, the lower portion of -which is a thin piece of bamboo. Fixed upon the wire there is a small -brass ball which rests on the upper side of a second brass plate also -perforated with a conical hole, which plate is fixed on the bar O O -crossing the box. - -If at the time of an earthquake the upper part of the index I remains -steady at M, then by the motion at O, the lower end of the index which -carries a sliding needle at G, will magnify the motion of the earth in -the ratios M O: O G. In this instrument O G is about 17 cm. - -The needle G works upon a piece of smoked glass. In order to bring -the glass into contact with the needle without disturbance, the glass -is carried on a strip of wood K, hinged at the back of the box, and -propped up in front by a loose block of wood Y. When Y is removed -the glass drops down with K out of contact with the needle. The box -is carried on bars of wood C C, which are fixed to the ground by the -stakes A A. - -The great advantage of a pendulum seismograph working on a stationary -plate is, that the record shows at once whether the direction of motion -has been constant, or whether it has been variable. The maximum extent -of motion in various directions is also easily obtained. - -The disadvantage of the instrument is, that at the time of a large -earthquake, owing perhaps to a slight swing in the pendulum, the -records may be unduly magnified. - -On such occasions, however, fairly good records may be obtained from -the friction pointers, provided that the plates on which they work have -been previously smoked. It might perhaps be well to use two of these -instruments, one having a comparatively high frictional resistance, and -hence ‘dead beat’ for large displacements. - -Many attempts have been made to use a pendulum seismograph in -conjunction with a record-receiving surface, which at the time of the -earthquake should be kept in motion by clockwork. In this way it was -hoped to separate the various vibrations of the earthquake, and thus -avoid the greater or less confusion which occurs when the index of the -pendulum writes its backward and forward motion on a stationary plate. -Hitherto all attempts in this direction, in which a single multiplying -index was used, have been unsuccessful because of the moving plate -dragging the index in the direction of its motion for a short -distance, and then allowing it to fall back towards its normal position. - -In connection with this subject we may mention the pendulum -seismographs of Kreil, Wagener, Ewing, and Gray. - -In the bob of Kreil’s pendulum there was clockwork, which caused a disc -on the axis of the pendulum to continuously rotate. On this continually -revolving surface a style fixed to the earth traced an unbroken circle. -At the time of an earthquake, by the motion of the style, the circle -was to be broken and lines drawn. The number and length of these lines -were to indicate the length and intensity of the disturbance. - -Gray’s pendulum consisted of a flat heavy disc carrying on its upper -surface a smoked glass plate. This, which formed the bob of the -pendulum, was supported by a pianoforte steel wire. When set ready to -receive an earthquake, the wire was twisted and the bob held by a catch -so arranged that at the time of the earthquake the catch was released, -and the bob of the pendulum allowed to turn slowly by the untwisting of -the supporting wire. Resting on the surface of this rotating disc were -two multiplying indices arranged to write the earth’s motions as two -components. - -In the instruments of Wagener and Ewing, the clockwork and moving -surface do not form part of the pendulum, but rest independently on -a support rigidly attached to the earth. In Wagener’s instrument one -index only is used, while in Ewing’s two are used for writing the -record of the motion. - -A difficulty which is apparent in all pendulum machines is that when -the bob of such a pendulum is deflected it tends to fall back to its -normal position. To make a pendulum perfect it therefore requires -some compensating arrangement, so that the pendulum, for small -displacements, shall be in neutral equilibrium, and the errors due to -swinging shall be avoided. - -Several methods have been suggested for making the bob of an ordinary -pendulum astatic for small displacements. One method proposed by Gray -consists in fixing in the bob of a pendulum a circular trough of -liquid, the curvature of this trough having a proper form. Another -method which was suggested, was to attach a vertical spiral spring -to a point in the axis of the pendulum a little below the point of -suspension, and to a fixed point above it, so that when the pendulum is -deflected it would introduce a couple. - -Professor Ewing has suggested an arrangement so that the bob of the -pendulum shall be partly suspended by a stretched spiral spring, and at -the same time shall be partly held up from below by a vertically placed -strut, the weight carried by the strut being to the weight carried by -the spring in the ratio of their respective lengths. As to how these -arrangements will act when carried into practice yet remains to be seen. - -Another important class of instruments are _inverted pendulums_. These -are vertical springs made of metal or wood loaded at their upper end -with a heavy mass of metal. An arrangement of this sort, provided at -its upper end with a pencil to write on a concave surface, was employed -in 1841 to register the earthquakes at Comrie in Scotland. In Japan -they were largely employed in series, each member of a series having -a different period of vibration. The object of these arrangements -was to determine which of the pendulums, with a given earthquake, -recorded the greatest motion, it being assumed that the one which was -thrown into the most violent oscillation would be the one most nearly -approximating with the period of the earthquake. The result of these -experiments showed that it was usually those with a slow period of -vibration which were the most disturbed. - -_Bracket Seismographs._—A group of instruments of recent origin which -have done good work, are the bracket seismographs. These instruments -appear to have been independently invented by several investigators: -the germ from which they originated probably being the well-known -horizontal pendulum of Professor Zöllner. In Japan they were first -employed by Professor W. S. Chaplin. Subsequently they were used by -Professor Ewing and Mr. Gray. They consist essentially of a heavy -weight supported at the extremity of a horizontal bracket which is free -to turn on a vertical axis at its other end. When the frame carrying -this axis is moved in any direction excepting parallel to the length -of the gate-like bracket, the weight causes the bracket to turn round -a line known as the instantaneous axis of the bracket corresponding to -this motion of the fixed axis. Any point in this line may therefore be -taken as a steady point for motions at right angles to the length of -the supporting bracket. Two of these instruments placed at right angles -to each other have to be employed in conjunction, and the motion of -the ground is written down as two rectangular components. In Professor -Ewing’s form of the instrument, light prolongations of the brackets -form indices which give magnified representations of the motion, and -the weights are pivoted round a vertical axis through their centre. - -In the accompanying sketch B is a heavy weight pivoted at the end of -a small bracket C A K, which bracket is free to turn on a knife-edge, -K, above, and a pivot A, below, in the stand S. At the time of an -earthquake B remains steady, and the index P, forming a continuation -of the bracket, magnifies the motion of the stand, in the ratio of -A C : C N. - -[Illustration: FIG. 3.] - -In an instrument called a double-bracket seismograph, invented by Mr. -Gray, we have two brackets hinged to each other, and one of them to a -fixed frame. The planes of the two brackets are placed at right angles, -so as to give to a heavy mass supported at the end of the outer bracket -two degrees of horizontal freedom. - -In all bracket machines, especially those which carry a pivoted weight, -it is doubtful whether the weight provides a truly steady point -relatively to the plate on which the record is written for motion -parallel to the direction of the arm. - -[Illustration: FIG. 4.] - -_Parallel motion Instrument._—A machine which writes its record as -two components, and which promises great stability, is one suggested -by Professor C. D. West. Like the bracket machines it consists of -two similar parts placed at right angles to each other, and is as -follows: A bar of iron A is suspended from both sides on pivots at C C -by a system of light arms hinging with each other at the black dots, -between the upper and lower parts of the rigid frame B C. The arms are -of such a length that for small displacements parallel to the length -of the bar, C C practically move in a straight line, and the bar is -in neutral equilibrium. A light prolongation of the bar _d_ works the -upper end of the light index _e_, passing as a universal joint through -the rigid support F. A second index _e′ _ from the bar at right angles -also passes through F. The multiplying ends of these indices are -coupled together to write a resultant motion on a smoked glass plate S. - -_Conical Pendulums._—Another group of instruments which have also -yielded valuable records are the conical pendulum seismographs. The -idea of using the bob of a conical pendulum to give a steady point in -an earthquake machine was first suggested and carried into practice by -Mr. Gray. The seismograph as employed consists of a pair of conical -pendulums hung in planes at right angles to each other. The bob of each -of these pendulums is fixed a short distance from the end of a light -lever, which forms the writing index, the short end resting as a strut -against the side of a post fixed in the earth. The weight is carried by -a thin wire or thread, the upper end of which is attached to a point -vertically above the fixed end of the lever. - -_Rolling Spheres and Cylinders._—After the conical pendulum -seismographs, which claim several important advantages over the bracket -machines, we come to a group of instruments known as rolling sphere -seismographs. Here, again, we have a class of instruments for the -various forms of which we are indebted to the ingenuity of Mr. Gray. - -The general arrangement and principle of one of these instruments will -be readily understood from the accompanying figure. S is a segment -of a large sphere with a centre near C. Slightly below this centre a -heavy weight B, which may be a lead ring, is pivoted. At the time of -an earthquake C is steady, and the earth’s motions are magnified by -the pointer C A N in the proportion of C A : A N. The working of this -pointer or index is similar to that of the pointer in the pendulum. - -[Illustration: FIG. 5.] - -Closely connected with the rolling sphere seismographs, are Gray’s -rolling cylinder seismographs. - -These are two cylinders resting on a surface plate with their axes at -right angles to each other. Near to the highest point in each of these -cylinders, this point remaining nearly steady when the surface plate -is moved backwards and forwards, there is attached the end of a light -index. These indices are again pivoted a short distance from their ends -on axes connected with the surface plate. In order that the two indices -may be brought parallel, one is cranked at the second pivot. - -_Ball and Plate Seismograph._—Another form of seismograph, which is -closely related to the two forms of apparatus just described, is -Verbeck’s ball and plate seismograph. This consists of a surface plate -resting on three hard spheres, which in turn rests upon a second -surface plate. When the lower plate is moved, the upper one tends to -remain at rest, and thus may be used as a steady mass to move an index. - -_The Principle of Perry and Ayrton._—An instrument which is of interest -from the scientific principle it involves is a seismograph suggested -by Professors Perry and Ayrton, who propose to support a heavy ball on -three springs, which shall be sufficiently stiff to have an exceedingly -quick period of vibration. By means of pencils attached to the ball by -levers, the motions of the ball are to be recorded on a moving band of -paper. The result would be a record compounded of the small vibrations -of the springs superimposed on the larger, slower, wave-like motions -of the earthquake, and, knowing the former of these, the latter might -be separated by analysis. Although our present knowledge of earthquake -motion indicates that the analysis of such a record would often present -us with insuperable difficulties, this instrument is worthy of notice -on account of the novelty of the principle it involves, which, the -authors truly remark, has in seismometry been a ‘neglected’ one. - -_Instruments to record Vertical Motion._—The instruments which have -been devised to record vertical motion are almost as numerous as those -which have been devised to record horizontal motion. The earliest -form of instrument employed for this purpose was a spiral spring -stretched by weight, which, on account of its inertia, was supposed -at the time of a shock to remain steady. No satisfactory results have -ever been obtained from such instruments, chiefly on account of the -inconvenience in making a spring sufficiently long to allow of enough -elongation to give a long period of vibration. Similar remarks may -be applied to the horizontally placed elastic rods, one end of which -is fixed to a wall, whilst the opposite end is loaded with a weight. -Such contrivances, furnished with pencil on the weight to write a -record upon a vertical surface, were used in 1842 at Comrie, and we -see the same principle applied in a portion of Palmieri’s apparatus. -Contrivances like these neither give us the true amplitude of the -vertical motion, insomuch as they are readily set in a state of -oscillation; nor do they indicate the duration of a disturbance, for, -being once set in motion, they continue that motion in virtue of their -inertia long after the actual earthquake has ceased. They can only be -regarded as seismoscopes. - -[Illustration: FIG. 6.] - -The most satisfactory instrument which has yet been devised for -recording vertical motion is Gray’s horizontal lever spring seismograph. - -This instrument will be better understood from the accompanying sketch. -A vertical spring S is fixed at its upper end by means of a nut _n_, -which rests on the top of the frame F, and serves to raise or lower -the spring through a short distance as a last adjustment for the -position of the cross-arm A. The arm A rests at one end on two sharp -points, _p_, one resting in a conical hole and the other in a V-slot; -it is supported at B by the spring S, and is weighted at C with a lead -ring R. Over a pin at the point C a stirrup of thread is placed which -supports a small trough, _t_. The trough _t_ is pivoted at _a_, has -attached to it the index _i_ (which is hinged by means of a strip of -tough paper at _h_, and rests through a fine pin on the glass plate -_g_), and is partly filled with mercury. - -Another method of obtaining a steady point for vertical motion is that -of Dr. Wagener, who employs a buoy partly immersed in a vessel of -water. This was considerably improved upon by Mr. Gray, who suggested -the use of a buoy, which, with the exception of a long thin style, was -completely sunk. - -Among the other forms of apparatus used to record vertical motion may -be mentioned vessels provided with india-rubber or other flexible -bottoms, and partially filled with water or some other liquid. As the -vessel is moved up and down, the bottom tends to remain behind and -provides a more or less steady point. Pivoted to this is a light index, -which is again pivoted to a rigid frame in connection with the earth. -Instruments of this description have yielded good records. - -_Record Receivers._—A large number of earthquake machines having been -referred to, it now remains to consider the apparatus on which they -write their motions. The earlier forms of seismographs, as has already -been indicated, recorded their movements in a bed of sand; others -wrote their records by means of pencils on sheets of paper. Where we -have seismographs which magnify the motion of the earth, it will be -observed that methods like the above would involve great frictional -resistances, tending to cause motion in the assumed steady points of -the seismographs. One of the most perfect instruments would be obtained -by registering photographically the motions of the recording index by -the reflection of a ray of light. Such an instrument would, however, -be difficult to construct and difficult to manipulate. One of the best -practical forms of registering apparatus is one in which the record is -written on a surface of smoked glass. This can afterwards be covered -with a coat of photographer’s varnish, and subsequently photographed by -the ‘blue process’ so well known to engineers. - -To obtain a record of all the vibrations of an earthquake it is -necessary that the surface on which the seismograph writes should at -the time of an earthquake be in motion. Of record-receiving machines -there are three types. First, there are those which move continuously. -The common form of these is a circular glass plate like an old form of -chronograph, driven continuously by clockwork. On this the pointers of -the seismograph rest and trace over and over again the same circles. -At the time of an earthquake they move back and forth across the -circles, which are theoretically fine lines, and leave a record of -the earthquake. Instead of a circular plate, a drum covered with -smoked paper may be used, which, after the earthquake, possesses the -advantage, after unrolling, of presenting the record in a straight -line, instead of a record written round the periphery of a circle, as -is the case with the circular glass plates. Such records are easily -preserved, but they are more difficult to photograph. - -The second form of apparatus is one which is set in motion at the -time of a shock. This may be a contrivance like one of those just -described, or a straight smoked glass plate on a carriage. By means -of an electrical or a mechanical contrivance called a ‘starter,’ of -which many forms have been contrived, the earthquake is caused to -release a detent and thus set in motion the mechanism which moves the -record receiver. - -The great advantage of continuously-moving machines is that the -beginning and end of the shock can usually be got with certainty, while -all the uncertainty as to the action of the ‘starter’ is avoided. -Self-starting machines have, of course, the advantage of simplicity and -cheapness, while there is no danger of the record getting obliterated -by the subsequent motion of the plate under the index. - -_Time-recording Apparatus._—Of equal importance with the instruments -which record the motion of the ground, are those instruments which -record the time at which such motion took place. The great value of -time records, when determining the origin from which an earthquake -originates, will be shown farther on. The most important result which -is required in connection with time observations, is to determine -the interval of time taken by a disturbance in travelling from one -point to another. On account of the great velocity with which these -disturbances sometimes travel, it is necessary that these observations -should be made with considerable accuracy. The old methods of adapting -an apparatus to a clock which, when shaken, shall cause the clock to -stop, are of little value unless the stations at which the observations -are made are at considerable distances apart. This will be appreciated -when we remember that the disturbance may possibly travel at the rate -of a mile per second, that its duration at any station may often extend -over a minute, and that one set of apparatus at one station may stop, -perhaps, at the commencement of the disturbance, and the other near -the end. A satisfactory time-taking apparatus will therefore require, -not only the means for stopping a clock, but also a contrivance which, -at the same instant that the clock is stopped, shall make a mark on a -record which is being drawn by a seismograph. In this way we find out -at which portion of the shock the time was taken. - -[Illustration: FIG. 7.] - -Palmieri stops a clock in his seismograph by closing an electric -circuit. Mallet proposes to stop a clock by the falling of a column -which is attached by a string to the pendulum of the clock. So long as -the column is standing the string is loose and the pendulum is free -to move; but when the column falls, the string is tightened and the -pendulum is arrested. The difficulty which arises is to obtain a column -that will fall with a slight disturbance. The best form of contrivance -for causing a column to fall, and one which may also be used in drawing -out a catch to relieve the machinery of a record receiver, is shown in -the accompanying sketch. - -S is the segment of a sphere about 4·5 cm. radius, with a centre -slightly above C. L is a disc of lead about 7 cm. in diameter resting -upon the segment. Above this there is a light pointer, P, about 30 -cm. long. On the top of the pointer a small cylinder of iron, W, is -balanced, and connected by a string with the catch to be relieved. When -the table on which W P S rests is shaken, rotation takes place near -to C, the motion of the base S is magnified at the upper end of the -pointer, and the weight overturned. This catch may be used to relieve -a toothed bar axled at one end, and held up above a pin projecting -from the face of the pendulum bob. When this falls it catches the -projecting pin and holds the pendulum. - -Another way of relieving the toothed bar is to hold up the opposite -end to that at which it is axled by resting it on the extremity of a -horizontal wire fixed to the bob of a conical pendulum—for example, one -of the indices of a conical pendulum seismograph. The whole of this -apparatus, which may be constructed at the cost of a few pence, can be -made small enough to go inside an ordinary clock case. - -The difficulty which arises with all these clock-stopping arrangements -is that it is difficult for observers situated at distant stations -to re-start their clocks so that their difference in time shall -be accurately known. Even if each observer is provided with a -well-regulated chronometer, with which he can make comparisons, the -rating of these instruments is for all ordinary persons an extremely -troublesome operation. - -In order to avoid this difficulty the author has of late years used a -method of obtaining the time without stopping the clock. To do this a -clock with a central seconds hand is taken, and the hour and minute -hands are prolonged and bent out slightly at their extremities at right -angles to the face, the hour hand being slightly the longest. Each -hand is then tipped with a piece of soft material like cork, which is -smeared with a glycerine ink. A light flat ring, with divisions in it -corresponding to those on the face of the clock, is so arranged that -at the time of a shock it can be quickly advanced to touch the inked -pads on the hands of the clock and then withdrawn. This is accomplished -by suitable machinery, which is relieved either by an electro-magnet -or some other contrivance which will withdraw a catch. In this way an -impression in the form of three dots is received on the disc, and the -time known without either stopping or sensibly retarding the clock. - -For ordinary observers, if a time-taker is not used in conjunction -with a record receiver, as good results as those obtained by ordinary -clock-stopping apparatus are obtainable by glancing at an ordinary -watch. Subsequently the watch by which the observation was made should -be compared with some good time-keeper, and the local time at which the -shock took place is then approximately known. - -From what has now been said it will be seen that for a complete -seismograph we require three distinct sets of apparatus—an apparatus to -record horizontal motion, an apparatus to record vertical motion, and -an apparatus to record time. The horizontal and vertical motions must -be written on the same receiver, and if possible side by side, whilst -the instant at which the time record is made a mark must be made on -the edge of the diagram which is being drawn by the seismograph. Such -a seismograph has been constructed and is now erected in Japan. It is -illustrated in the accompanying diagram. - -_The Gray and Milne Seismograph._—In this apparatus two mutually -rectangular components of the horizontal motion of the earth are -recorded on a sheet of smoked paper wound round a drum, D, kept -continuously in motion by clockwork, W, by means of two conical -pendulum seismographs, C. The vertical motion is recorded on the same -sheet of paper by means of a compensated-spring seismograph, S L M B. - -The time of occurrence of an earthquake is determined by causing the -circuit of two electro-magnets to be closed by the shaking. One of -these magnets relieves a mechanism, forming part of a time-keeper, -which causes the dial of the timepiece to come suddenly forwards on -the hands and then move back to its original position. The hands are -provided with ink-pads, which mark their positions on the dial, thus -indicating the hour, minute, and second when the circuit was closed. -The second electro-magnet causes a pointer to make a mark on the paper -receiving the record of the motion. This mark indicates the part of the -earthquake at which the circuit was closed. - -[Illustration: FIG. 8.] - -The duration of the earthquake is estimated from the length of the -record on the smoked paper and the rate of motion of the drum. The -nature and period of the different movements are obtained from the -curves drawn on the paper. - -Mr. Gray has since greatly modified this apparatus, notably by the -introduction of a band of paper sufficiently long to take a record for -twenty-four hours without repetition. The record is written in ink by -means of fine siphons. In this way the instrument, which is extremely -sensitive to change of level, can be made to show not only earthquakes, -but the pulsations of long period which have recently occupied so much -attention. - - - - - CHAPTER III. - - EARTHQUAKE MOTION DISCUSSED THEORETICALLY. - - Ideas of the ancients (the views of Travagini, Hooke, Woodward, - Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves - and vibrations—Possible causes of disturbance in the Earth’s - crust—The time of vibration of an earth particle—Velocity and - acceleration of a particle—Propagation of a disturbance as - determined by experiments upon the elastic moduli of rocks—The - intensity of an earthquake—Area of greatest overturning - moment—Earthquake waves—Reflexion, refraction, and interference - of waves—Radiation of a disturbance. - - -_Ideas of Early Writers._—One of the first accounts of the varieties of -motion which may be experienced at the time of an earthquake is to be -found in the classification of earthquakes given by Aristotle.[8] It is -as follows:— - -1. Epiclintæ, or earthquakes which move the ground obliquely. - -2. Brastæ, with an upward vertical motion like boiling water. - -3. Chasmatiæ, which cause the ground to sink and form hollows. - -4. Rhectæ, which raise the ground and make fissures. - -5. Ostæ, which overthrow with one thrust. - -6. Palmatiæ, which shake from side to side with a sort of tremor. - -From the sixth group in this classification we see that this early -writer did not regard earthquakes as necessarily isolated events, but -that some of them consisted of a succession of backward and forward -vibratory motions. He also distinguishes between the total duration of -an earthquake and the length of, and intervals between, a series of -shocks. Aristotle had, in fact, some idea of what modern writers upon -ordinary earthquakes would term ‘modality.’ - -The earliest writer who had the idea that an earthquake was a -pulse-like motion propagated through solid ground appears to have been -Francisci Travagini, who, in 1679, wrote upon an earthquake which -in 1667 had overthrown Ragusa. The method in which the pulses were -propagated he illustrated by experiments. - -Hooke, who, in 1690, delivered discourses on earthquakes before the -Royal Society, divides these phenomena according to the geological -effects they have produced; thus there is a _genus_ producing -elevations, a _genus_ producing sinkings, a _genus_ producing -conversions and transportations, and a _genus_ which produces what, in -modern language, we should term metamorphic action. - -Woodward, in his ‘Natural History,’ written in 1695, speaks of -earthquakes as being agitations and concussions produced by water in -the interior of the earth coming in contact with internal fires. - -Stukeley observed that an earthquake was ‘a tremor of the earth,’ to -be explained as a vibration in a solid. The Rev. John Mitchell, writing -in 1760, says that the motion of the earth in earthquakes is partly -tremulous and partly propagated by waves. - -From these few examples, to which might be added many more, it will -be seen that an earthquake disturbance has usually been regarded as -a concussion, vibration, trembling, or undulatory movement. Further, -it can be seen in narratives of earthquakes that it had been often -observed that these tremblings and shakings continued over a certain -period of time. Although it had been noticed that large areas were -almost simultaneously affected by these disturbances, no definite idea -appears to have existed as to how earthquake motion was propagated. -Usually it was assumed that the disturbance spread through subterranean -channels. - -The first true conception of earthquake motion and the manner of its -propagation is due to Dr. Thomas Young, who suggested that earthquake -motion was vibratory, and it might be ‘propagated through the earth -nearly in the same manner as a noise is conveyed through the air.’ The -same idea was moulded into a more definite form by Gay Lussac. - -The first accurate definition of an earthquake is due to Mr. Robert -Mallet, who, after collecting and examining many facts connected with -earthquake phenomena, and reasoning on these, with the help of known -laws connected with the production and propagation of waves of various -descriptions, formulated his views as follows:— - -An earthquake is ‘_the transit of a wave or waves of elastic -compression in any direction from vertically upwards to horizontally, -in any azimuth, through the crust and surface of the earth, from any -centre of impulse or from more than one, and which may be attended -with sound and tidal waves, dependent upon the impulse and upon -circumstances of position as to sea and land_.’ - -In brief, so far as motion in the earth is concerned. Mallet defined -an earthquake as being a motion due to the transit of waves of elastic -compression. In many cases it is possible that this is strictly true, -but in succeeding pages it will be shown that earthquake motion may -also be due to the transit of waves of elastic distortion. - -To obtain a true idea of earthquake motion is a matter of cardinal -importance, as it forms the key-stone of many investigations. - -If we know the nature of the motion produced by an earthquake, we are -aided in tracking it to its origin, and in reasoning as to how it was -produced. If our knowledge of the nature of the motion of an earthquake -is incorrect, it will be impossible for us intelligently to construct -buildings to withstand the effects of these disturbances. We have thus -to consider, in this portion of seismology, a point of great scientific -importance, and shall deal with it at some length. - -_Nature of Elastic Waves and Vibrations._—When it is stated that an -earthquake consists of elastic waves of compression and distortion, the -student of physics has a clear idea of what is meant and a knowledge -of the mechanical laws which govern such disturbances. The ordinary -reader, however, and the majority of the inhabitants of earthquake -countries, who of all people have the greatest interest in this matter, -may not have so clear a conception, and it will, therefore, not be out -of place to give some general explanation on this point. - -The ordinary idea of a wave is that it is a disturbance similar to -that which we often see in water. Waves like these must not, however, -be confounded with elastic waves. A disturbance produced in water, -say, for instance, by dropping a stone into a pond, is propagated -outwards by the action of gravity. First, a ridge of water is raised -up by the stone passing beneath the surface. As this ridge falls -towards its normal position in virtue of its weight, it raises a second -ridge. This second ridge raises a third ridge, and so on. The water -moves vertically up and down, whilst the wave itself is propagated -horizontally. - -To understand what is meant by elastic waves, it is first necessary -to understand what is meant by the term elastic. In popular language -the term elastic is confined to substances like india-rubber, and but -seldom to rock-like materials, through which earthquake waves are -propagated. India-rubber is called elastic because after we remove a -compressive force it has a tendency to spring back to its original -shape. The elastic force of the india-rubber is in this case the force -which causes it to resist a change of form. Now, a piece of rock may, -up to a certain point, like the india-rubber, be compressed, and when -the compressing force is removed it will also tend to resume its -original form. However, as the rock offers more resistance to the -compressing force than the india-rubber offers, we say that it is the -more elastic. It may be here observed that a substance like granite -offers great resistance, not only to compression or a change of volume, -but also to a change of form or shape; whereas a substance like air, -which is also elastic, only offers resistance to compression, but not -to a change of shape. - -With these ideas before us we will now proceed to consider how, after -a body has been suddenly compressed or distorted, this disturbance -is propagated through the mass. For the elastic body let us take a -long spiral spring hung from the ceiling of a room and kept slightly -stretched by a weight. If we give this weight an upward tap from below, -say with a hammer, we shall observe a pulse-like wave which runs up the -spring until it reaches the ceiling of the room. Here it will, so to -speak, rebound, like a billiard ball from the end of a table, and run -towards the weight from which it started. Whilst this is going on we -may also observe that the weight is moving up and down. - -Here, then, we have two distinct things to observe—one being the -transmission of motion up to the ceiling, which we may liken to the -transmission of an earthquake wave between two distant localities on -the earth’s surface, and the other being the up and down motion of our -weight, which we may compare to the backward and forward swinging which -we experience at the time of an earthquake. - -These two motions—namely, the pulse-like wave produced by the -transmission of motion, and the backward and forward oscillation of the -weight or of any point on the spring—must be carefully distinguished -from each other. - -First, we will consider the backward and forward motion of the weight. -The distance through which the weight moves depends upon the force -of the blow. The number of up and down oscillations it makes, say -in a second, depends upon the stiffness of the spring. The weight, -supposing it to be always the same, will move more quickly at the end -of a stiff spring than at the end of a flaccid one; that is to say, its -velocity is quicker. As in any given spring the number of up and down -oscillations are always the same in a given interval of time, if these -oscillations are of great extent, the weight must move more quickly -with large than with small oscillations. - -At the time of an earthquake the manner in which we are moved backward -and forward is very similar to the manner in which the weight is moved. -If we stand on a hard rock-like granite, we are to a great extent -placed as if we were attached to a stiff quickly-vibrating spring. -If, however, we are on a soft rock, it is more like being on a loose -flaccid spring. - -All that has thus far been considered has been a backward and forward -kind of motion, where there is a rectilinear _compression_ and -_extension_ amongst the particles on which we stand. - -We might, however, imagine our rock, which for the moment we will -consider to be a square column, to be twisted, and thus have its -_shape_ altered. When the twisting force is taken off it seems evident -that the column would endeavour to untwist itself or regain its -original form. Now the force which a body offers against a change of -_volume_ may be very different from that which it offers against a -change of _form_. - -In disturbances which take place in the rocky crust of our earth, -it would seem possible that we may have vibrations set up which are -either compressions and extensions or twistings and distortions. These -may take place separately or simultaneously, or we may have resultant -motions due to their combination. - -The following are examples of possible causes which might give rise to -these different orders of disturbance:— - -1. Imagine a large area stretched by elevation until it reaches the -limit of its elasticity and cracks. After cracking, in consequence -of its elasticity, it will fly back over the whole area like a -broken spring, and each point in the area will oscillate round its -new position of equilibrium. In this case there will be no waves -of distortion excepting near the end of the crack, where waves are -transmitted in a direction parallel to the fissure. - -2. The ground is broken and slips either up, down, or sideways, as -we see to have taken place in the production of faults. Here we get -distortion in the direction of the movement, and waves are produced -by the elastic force of the rock, causing it to spring back from its -distorted form. In a case like this the production of a fissure running -north and south might give rise to north and south vibrations, which -would be propagated end on towards the north and south, but broadside -on towards the east and west. With disturbances of this kind, on -account of the want of homogeneousness in the materials in which -they are produced, we should expect to find waves of compression and -extension. - -3. A truly spherical cavity is suddenly formed by the explosion of -steam in the midst of an elastic medium. In this case all the waves -will be those of compression, each particle moving backward and forward -along a radius. - -Should the cavity, instead of being truly spherical, be irregular, it -is evident that, in addition to the normal vibration of compression, -transverse waves of distortion will be more or less pronounced, -depending upon the nature of the cavity. - -The combination of these two sets of vibrations may cause a point in -the earth to move in a circle, an ellipse, the form of a figure eight, -and in other curves similar to these, which are produced by apparatus -designed to show the combination of harmonic motion. From these -examples it will be seen that we have therefore to consider two kinds -of vibrations—one produced by compression or the alteration of volume, -and the other produced by an alteration in shape. - -Now the resistance which a body offers, either to a change in its -volume or in its shape, is called its elasticity, and the law which -governs the backward and forward motion of a particle under the -influence of this elasticity may be expressed as follows: - -If T be the time of vibration, or the time taken by a particle to make -one complete backward and forward swing, D the density of the material -of which this particle forms a part, and E the proper modulus of -elasticity of the material, then, - - _____ - T = 2π √ D/E - - _____ -From this formula, T = 2π √ D/E, we see that the time of -vibration of the earth during an earthquake, or the rate at which we -are shaken backwards and forwards, varies directly as the square root -of the density of the material on which we stand, and inversely as the -square root of a number proportional to its elasticity. - -_Velocity and Acceleration of an Earth Particle._—Another important -point, which the practical seismologist has often brought to his -notice, is the question of the velocity with which an earth particle - _____ -moves. According to the formula, T = 2π √ D/E, we should expect -that a particle would make each semi-vibration in an equal time, and -from a knowledge of the density and elastic moduli of a body this -time might be calculated. Although the time of a semi-oscillation may -be constant, we must bear in mind that, like the bob of a pendulum -during each of its swings, the particle starts from rest, increases in -velocity until it reaches the middle portion of its half swing, from -which it gradually decreases in speed until it reaches zero, when it -again commences a similar motion in the opposite direction. - -These pendulum-like vibrations are sometimes spoken of as simple -harmonic motions. If we know the distance through which an earthquake -moves in making a single swing, and the time taken in making this -swing, on the assumption that the motion is simple harmonic we can -easily calculate the maximum velocity with which the particle moves. - -Thus, if an earth particle takes one second to complete a -semi-oscillation, half of which, or the amplitude of the motion, equals -_a_, the maximum velocity equals π × _a_. - -Again, assuming the earth vibrations to be simple harmonic, the maximum -acceleration or rate of change in velocity will come about at the ends -of each semi-oscillation; and if V be the maximum velocity of -the particle, and _a_ the amplitude or half semi-oscillation, then the - V^2 -maximum acceleration equals ———. - _a_ - -Later on it will be shown, as the result of experiment, that certain of -the more important earth oscillations in an earthquake are not simple -harmonic motion. Nevertheless the above remarks will be of assistance -in showing how the velocity and other elements connected with the -motion of an earth particle, which are required by the practical -seismologist, may be calculated, irrespective of assumptions as to the -nature of the motion. - -_Propagation of a Disturbance._—We may next consider the manner in -which a disturbance, in which there are both vibrations of compression -and of distortion, is propagated. The first or normal set of vibrations -are propagated in a manner similar to that in which sound vibrations -are propagated. From a centre of disturbance these movements approach -an observer at a distant station, so to speak, end on. The other -vibrations have a direction of motion similar to that which we believe -to exist in a ray of light. These would approach the observer broadside -on. - -If the disturbance passed through a formation like a series of -perfectly laminated slates, each of these two sets of vibration might -be subdivided, and we should then obtain what Mallet has termed -ordinary and extraordinary normal and transverse vibrations. - -In consequence of the difference in the elastic forces on which the -propagation of these two kinds of vibration depends, the normal -vibrations are transmitted faster than the transversal ones—that is -to say, if an earthquake originated from a blow, the first thing that -would be felt at a point distant from the origin of the shock would be -a backward and forward motion in the direction to and from the origin, -and then, a short interval afterwards, a motion transversal, or at -right angles to this, would be experienced. - -From the mathematical theory of vibratory motions it is possible to -calculate the velocity with which a disturbance is propagated. As the -result of these investigations it has been shown that normal vibrations -travel more quickly than transverse vibrations. - -Deductions from experiments on small specimens are, however, -invalidated by the fact that the specimens used for experiments are, -of course, nearly homogeneous, whilst the earthquake passes through -a mass which is heterogeneous and more or less fissured. Mallet, by -experiments ‘on the compressibility of solid cubes of these rocks, -obtained the mean modulus of elasticity,’ with the result that ‘nearly -seven-eighths of the full velocity of wave-transit due to the material, -if solid and continuous, is lost by reason of the heterogeneity and -discontinuity of the rocky masses as they are found piled together in -nature.’ The full velocities of wave-transit, as calculated by Mallet -from a theorem given by Poisson, were— - - For slate and quartz transverse to lamination, 9,691 feet per second. - „ „ in line of lamination, 5,415 „ „ - -This more rapid transmission in a direction transverse to the -lamination, Mr. Mallet observes, may be more than counterbalanced by -the discontinuity of the mass transverse to the same direction. - -_The Intensity of an Earthquake._—The intensity of an earthquake -is best estimated by the intensity of the forces which are brought -to bear on bodies placed on the earth’s surface. These forces are -evidently proportional to the rate of change of velocity in the body, -and, as the destructive effect will be proportional to the maximum -forces, we may consistently indicate the intensity of an earthquake by -giving the maximum acceleration to which bodies were subject during -the disturbance. On the assumption that the motion of a point on -the earth’s surface is simple harmonic, the maximum acceleration is -directly as the maximum velocity and inversely as the amplitude of - _v_^2 -motion, or as ————— where _v_ indicates velocity and _a_ amplitude. - _a_ - -The next question of importance is to determine the manner in which -earthquake energy becomes dissipated—that is, to compare together -the intensity of an earthquake as recorded at two or more points at -different distances from the origin. First let us imagine the origin -of our earthquake to be surrounded by concentric shells, each of -which is the breadth of the vibration of a particle. Going outwards -from the centre, each successive shell will contain a greater number -of particles, this number increasing directly as the square of the -distance from the origin. Let the blow have its origin at the centre, -and give a vibratory movement to the particles in one of the shells -near the centre. - -This shell may be supposed to possess a certain amount of energy, -which will be measured by its mass and the square of the velocity of -its particles. In transferring this energy to the neighbouring shell -which surrounds it, because it has to set in motion a greater number -of particles than it contains itself, the energy in any one particle -of the second layer will be less than the energy in any one particle -in the first layer; the total energy in the second shell, however, -will be equal to the total energy in the first shell. Neglecting the -energy lost during the transfer, if the energy in a particle of the -first shell at any particular phase of the motion be K_{1}, -and the energy in a particle of the second shell K_{2}, these -quantities are to each other inversely as the masses of the shells—that -is, inversely as the squares of the mean radii of the shells. - - K_{2} _r__{1}^2 - In symbols, ————— = ————————— (1) - K_{1} _r__{2}^2 - -Assuming that energy is dissipated, - - K_{2} _r__{1}^2 _r__{1}^2 - ————— > ————————— = _f_ ————————— (2) - K_{1} _r__{2}^2 _r__{2}^2 - -where _f_ < 1 is the rate of dissipation of energy which is assumed to -be constant. - -_Area of greatest Overturning Moment._—Although the rate of dissipation -of the impulsive effects of an earthquake may follow a law like that -just enumerated, it must be remembered that if the depth of the origin -is comparable with the radius of the area which is shaken, the maximum -impulsive effect as exhibited by the actual destruction on the surface -may not be immediately above the origin where buildings have simply -been lifted vertically up and down, but at some distance from this -point, where the impulsive effort has been more oblique. - -At the _epicentrum_ we have the maximum of the true intensity as -measured by the acceleration of a particle, or the height to which -a body might be projected, but it will be at some distance from -this where we shall have the maximum intensity as exhibited by an -overturning effort. - -This will be rendered clear by the following diagram. - -In the accompanying diagram let O be the origin of a shock, and O C the -seismic vertical equal to _r_. Let the direct or normal shock emerge at -C, C_{1}, C_{2}, and at the angles θ_{1}, θ_{2}, &c. - -Assuming that the displacement of an earth particle at C equals C -B, and at C_{1} equals _c__{1} _b__{1}, and at C_{2} equals _c__{2} -_b__{2}, &c., and let these displacements C B, _c__{1} _b__{1}, _c__{2} -_b__{2}, &c., for the sake of argument, vary inversely as _r_, _r__{1}, -_r__{2}, &c. - -[Illustration: FIG. 9.] - -The question is to determine where the horizontal component C A of -these normal motions is a maximum. - -First observe that the triangle O C _c_ is similar to _a_, _b_, _c_. - - _h_ -Also _r_ = —————, and therefore the normal component _c__{1} - sin θ - - sin θ -_b__{1} at C_{1} is equal to C —————. - _h_ - -Also _c__{1} _a__{1} = _c__{1} _b__{1}, cos θ. - - sin θ cos θ _c_ sin 2θ - ∴ C_{1} _a_ = C ———————————— = ———— ∙ ———————, - _h_ _h_ 2 - -and sin 2θ is greatest when 2θ = 90° or θ = 45°. - -That is to say, the horizontal component reaches a maximum where the -angle of emergence equals 45°. - -This question has been discussed on the assumption that the amplitude -of an earth particle varies inversely as its distance from the origin -of the shock. Should we, however, assume that this amplitude varies -inversely as the square of the distance from the origin, we are led to -the result that the area of greatest disturbance is nearer to the point -where the angle of emergence is 55° 44′ 9″. Both of these methods are -referred to by Mallet, but the first is considered as probably the more -correct. - -_Earthquake Waves._—Hitherto we have chiefly considered earthquake -vibrations; now we will say a few words about earthquake waves. If -we strike a long iron rod at one end, we can imagine that, as in the -long spring, a pulse-like motion is transmitted. If the rod be struck -quickly, the pulses will rapidly succeed each other, and if struck -slowly the pulses will be at longer intervals. Each individual pulse, -however, will travel along the rod at the same rate, and hence the -distance between any two will remain constant; but that distance will -depend on the interval between the blows producing these pulses being -equal to the distance travelled by one pulse before the next blow is -struck. - -From this we see that an irregular disturbance will produce an -irregular succession of motions; some will be like long undulations in -a wide deep ocean, whilst others will be like ripples in a shallow bay. -Again, consider the bar to be struck one blow only, and then left to -itself. The bar will propagate a series of pulses along its length, due -to the out and in vibration of its end. These will succeed each other -at regular intervals, and will be mixed up with the pulses we have -previously considered. - -From this we see that in an earthquake, if it be produced by one blow, -the motion will be isochronous in its character; but if it be due to -a succession of blows at regular intervals, the motion will be the -resultant of a series of isochronous motions, and will be periodical. -If the impulses are irregular, you have a motion which is the resultant -of a number of isochronous motions due to each impulse, but these -compounded together in a different manner at each instant during the -earthquake, and giving as a result a motion which is in no sense -isochronous. This approaches more nearly to the actual motions we feel -as earthquakes. - -If we can imagine the ground shaken by an earthquake, made of a -transparent material which transmitted less light when compressed, -and we could look down upon a long extent of this at the time of an -earthquake, we should see a series of dark bands indicating strips of -country which were compressed. The distances between these bands might -be irregular. Keeping our attention on one particular band, this would -be seen to travel forward in a direction from the source. If we kept -our eye on one particular point, it would appear to open and shut, -becoming light and dark alternately. - -As to the existence of these elastic waves in actual earthquakes we -have no direct experimental evidence. The only kind of wave with which -we are familiar is a true surface undulation, which, although having -the appearance of a water-wave, may nevertheless represent a district -of compression. - - - - - CHAPTER IV. - - EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT. - - Experiments with falling weights—Experiments with - explosives—Results obtained from experiments—Relative motion - of two adjacent points—The effect of hills and excavations - upon the propagation of vibrations—The intensity of artificial - disturbances—Velocity with which earth vibrations are - propagated—Experiments of Mallet—Experiments of Abbot—Experiments - in Japan—Mallet’s results—Abbot’s results—Results obtained in - Japan. - - -_Experiments with Falling Weights._—A series of experiments, as the -nature of the disturbance produced in the surface of the earth when -a heavy weight is allowed to fall on it, was begun in November 1880 -by Mr. T. Gray and the author. These experiments were carried out at -the Akabane Engineering Works in Tokio. The weight used was a ball -of iron weighing about a ton, which in the different experiments was -allowed to fall from heights varying between ten and thirty-five feet. -The position of the place where the ball was allowed to fall was such -that in one direction the vibrations were transmitted up the side of -a steep hill, in another direction across a pond with perpendicular -sides, and in another direction across a level plain the material of -which consisted for the most part of hardened mud extending to a very -considerable depth. The vibrations produced by the fall of the ball -were transmitted through this hard mud with considerable intensity to a -distance of between 300 and 400 feet. - -The object of the experiment was to find the nature of the vibrations -produced in the crust of the earth by such a blow, the velocity of -transmission through this comparatively soft material, the effect of -hills and excavations in cutting off such disturbances, and the law -according to which the amplitude of the vibrations diminishes with the -distance from the source. - -A considerable variety of apparatus was used during these experiments, -but the most reliable results were obtained from the records of a -rolling sphere seismograph, which wrote the vibrations on a stationary -plate, and from the records of two bracket seismographs, similar to -Professor Ewing’s horizontal lever seismographs, which gave a record -of the vibrations as two rectangular components on a moving plate of -smoked glass. - -[Illustration: FIG. 10.] - -The general result as to the nature of the disturbance was that two -distinct sets of vibrations were set up by the blow. In one set the -direction of motion was along a line joining the point of observation -with the point from which the disturbance emanated; in the other set -the direction of motion was at right angles to that line. The nature -of the resultant motion will be gathered from fig. 10, which is taken -from the records drawn by the rolling sphere seismograph at a distance -of 50 feet, 100 feet, and 200 feet respectively from the point where -the ball struck the ground. The direct or normal vibrations reached -the instrument first, and were followed at an interval depending on -the distance of the instrument from the origin by the transverse -vibrations. From the records of these two sets of vibrations as -separated by the bracket seismographs, combined with the known rate of -motion of the glass plate, the velocity of transmission was found to -be, for normal vibrations 446–438 feet per second, and for transverse -vibrations 357–353 feet per second. - -The effect of the hill in cutting off the disturbance seemed to be -slight, but the direction of the vibrations which ascended the side -was mostly transverse. The pond, on the other hand, seemed completely -to cut off the disturbance, which, however, gradually crept round the -side, so that only a comparatively small triangular area was in shadow. - -The amplitude of the vibrations diminished directly as the distance -increased for some distance from the origin, but at greater distance -the rate of diminution seemed to be slower. The transverse vibration -seemed to die out less quickly than the normal vibrations.[9] - -These experiments were afterwards very considerably extended by the -author. In these later experiments charges of from one to two pounds of -dynamite were placed in bore-holes of various depths and exploded by -means of electricity. The results obtained confirmed the conclusions -already arrived at from the former experiments. The experiments on -velocity, however, seemed to indicate that the higher the initial -impulse the greater was the velocity. The velocity of propagation of -the transverse vibrations seemed to approach more and more to that of -the directed vibrations as the distance from the origin of disturbance -increased. Fig. 11 shows the nature of the record obtained from the -explosion of two pounds of dynamite at the bottom of a bore-hole eight -feet deep. These records show the interval of time which elapsed -between the arrival of the normal and the transverse vibrations at -points distant 100, 250, and 400 feet from the bore-hole. In the case -of the 100-feet station it will be observed that the motion towards the -origin is greater than that from the origin. It is also to be noticed -that the period of vibration becomes greater as the distance from the -origin increases. - -[Illustration: FIG. 11.—Records obtained at three stations -of the motion of the ground produced by the explosion of 2 lbs. of -dynamite.] - -_The Intensity of Artificial Disturbances._—The data which we have at -our disposal for determining the intensity of an earth particle which -has been caused to vibrate by the explosion of a charge of dynamite are -a series of records similar to that given on p. 60. These disturbances -are practically surface movements, and may be compared with the -movements of an earthquake which spreads over an area the radius of -which is great as compared with its depth. - -To find the mean acceleration of an earth particle, which quantity -has been taken to represent intensity, during any simple backward or -forward motion of the earth, it will be first necessary to determine -the amplitude of this motion and its maximum velocity, the mean - V^2 -acceleration being equal to ———. - 2A - -[Illustration: FIG. 12.] - -The second and third movements in a shock invariably exhibited the -greatest intensity, and to a distance of 400 feet from the origin, -where about three pounds of dynamite had been exploded in a bore-hole -about six feet deep, these intensities decreased directly as the -distance from the origin. The less intense movements also decreased -directly as the distance from the origin to a certain point, but after -that they decreased more slowly. A mean result of the more prominent -vibrations in four sets of experiments is shown in the curve, fig. 12, -where the horizontal measurements represent distance from the origin -in feet, and the vertical measurements mean acceleration in thousands -of millimetres per second. - -This curve approximates to an equi-angular hyperbola. The area between -the curve and its asymptotes is proportional to the whole energy of -the shock. The area of the diagram is proportional to the energy given -up to the ground by the explosion of three pounds of dynamite. If we -call the unit shock the effect produced by the explosion of one pound -of dynamite, the above artificial earthquake had an intensity equal to -three. - -The only other investigations which have been made in this interesting -branch of observational seismology are those by Mr. Robert Mallet,[10] -and those by General Henry L. Abbot.[11] - -_Mallet’s Results._—The velocity with which earth vibrations were -transmitted as deduced by Mr. Mallet were as follows:— - - Feet per second - In sand 824·915 - In contorted stratified rock, quartz, and slate - at Holyhead 1,088·669 - In discontinuous and much shattered granite 1,306·425 - In more solid granite 1,664·574 - -A striking result which was obtained by Mallet in his experiments at -Holyhead was that the transit velocity increases with an increase in -the intensity of the initial shock. Thus with a charge of 12,000 pounds -of powder the transit rate was 1,373 feet per second, whilst with 2,100 -pounds the transit rate was 1,099 feet per second. In these experiments -tremors were observed as preceding and following the main shock. - -_Abbot’s Results._—The important results obtained by General Abbot are -contained in the following table:— - - A. No. of Observation - B. Distance to Station in miles - C. Type of Seismometer - D. Velocity in feet per second - +--+----------------+--------------------------+--------+---+-------+ - | A| Date | Cause of Shock | B | C | D | - +--+----------------+--------------------------+--------+---+-------+ - | 1| Aug. 18, 1876 | 200 lbs. of dynamite | 5 ± | B | 5,280 | - | 2| Sept. 24, 1876 | Hallet’s Point Explosion | 5·134 | A | 3,873 | - | 3| „ | „ „ „ | 8·330 | B | 8,300 | - | 4| „ | „ „ „ | 9·333 | A | 4,521 | - | 5| „ | „ „ „ | 12·769 | B | 5,309 | - | 6| Oct. 10, 1876 | 70 lbs. dynamite | 1·360 | A | 1,240 | - | 7| Sept. 6, 1877 | 400 „ „ | 1·169 | A | 3,428 | - | 8| „ | „ „ „ | 1·169 | B | 8,814 | - | 9| Sept. 12, 1877 | 200 „ „ | 1·340 | A | 6,730 | - |10| „ | „ „ „ | 1·340 | B | 8,730 | - |11| „ | 70 „ „ | 1·340 | A | 5,559 | - |12| „ | „ „ „ | 1·340 | B | 8,415 | - +--+----------------+--------------------------+--------+---+-------+ - -A seismometer of type A means that the telescope used in observing the -tremor produced on the surface of a vessel of mercury by the passage of -the shock had a magnification of 6, whilst a telescope of the type B -had a magnification of 12. - -The mean velocity given by six observations with type A is 4,225 feet -per second, while that given by the same number with type B is 7,475 -feet per second. - -[Illustration: FIG. 13.] - -If we assume that the first tremor observed in the mercury is to -determine the true rate of transmission, General Abbot tells us that we -must reject all observations made with type A, inasmuch as they do not -reveal the velocity of the leading tremor. However, he also tells us -that a still higher power above 12 might have detected still earlier -tremors. - -When gunpowder was the explosive, the observers noted that the -disturbance observed in the mercury took a much longer time to reach a -maximum than it did when dynamite was employed. - -It was also observed that explosions fired beneath deep water gave -a higher velocity than similar explosions which took place beneath -shallow water. In the latter case much of the energy was probably -expended in throwing a jet of water into the air. - -Another point which was observed appears to have been that the rate -varied with the initial shock. Thus:— - - Feet per second - 400 lbs. of dynamite gave 8,814 - 200 „ „ 8,730 - 70 „ powder (deep) gave 8,415 - -Also it is probable that the rate of a wave diminished with its -advance. For, - - Feet per second - 200 lbs. of dynamite gave for 1 mile 8,730 - „ „ „ „ 5 miles 5,250 - 50,000 „ „ „ 8 „ 8,300 - „ „ „ „ 13½ „ 5,300 - -General Abbot’s general conclusions are:— - -1. A high magnifying power of telescope is essential in seismometric -observations. - -2. The more violent the initial shock the higher is the velocity of -transmission. - -3. This velocity diminishes as the general wave advances. - -4. The movements of the earth’s crust are complex, consisting of many -short waves first, increasing and then decreasing in amplitude; and -with a detonating explosive the interval between the first wave and -the maximum wave, at any station, is shorter than with a slow burning -explosive. - -_Results obtained in Japan._—From some experiments made by the author -in the grounds of the Meteorological Department in Tokio, the following -results were obtained:— - - A. Velocity in feet per second for the first 200 ft. (A to B) - B. Velocity in feet per second for the second 200 ft. (B to C) - C. Velocity in feet per second for 400 ft. (A to C) - D. Number of Cartridges of Dynamite (6 = 1 lb.) - - +--------------------+-----+-----+-----+------+ - | No. of Explosion | A | B | C | D | - +--------------------+-----+-----+-----+------+ - | { I. | 464 | 186 | 265 | 8·3 | - | Vertical { III. | -- | 211 | -- | 10·1 | - | vibrations { IV. | 352 | 234 | 281 | 7·1 | - | { V. | 343 | 232 | 277 | 5·0 | - | Normal { VI. | -- | -- | 407 | 10·0 | - | vibrations { VII. | -- | -- | 516 | 12·5 | - | Transverse } VIII. | -- | -- | 344 | 12·5 | - | vibrations } | | | | | - +--------------------+-----+-----+-----+------+ - -The general results to be deduced from the above appear to be:— - - 1. For vertical motion. - (_a_) For the first 200 feet. The velocity depends upon the initial - force—the greater the charge of dynamite the greater the - velocity. - (_b_) For the second 200 feet. The above law only appears in - experiments IV. and V., but it must be remembered that the - origins of I. and III. were farther removed from A than IV. - and V. - The speed of the wave during the second 200 feet is always less - than during the first 200 feet. - - 2. For normal vibrations. - Here the speed between A and C is all that was measured, but we - again see that the greater the initial force, or the nearer - we are to the origin of the disturbance, the greater is the - velocity. This velocity is greater than the velocity of the - vertical or transverse vibrations. - - 3. For transverse vibrations. - If we assume that the vertical vibrations are a component of the - transverse motions we see the same law as before—namely, that - the nearer we are to the origin of the disturbance the greater - is the speed with which that disturbance is propagated. - -It will be observed that the chief law here enunciated respecting the -decrease in speed of earth vibrations is the same as that pointed -out by General Abbot, from which it only differs by its being in all -cases proved without the introduction of personal errors, for the same -explosion, along the same line of ground and for different kinds of -vibrations. - - - - - CHAPTER V. - - EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION ON EARTHQUAKES. - - Result of feelings—The direction of motion—Instruments as indicators - of direction—Duration of an earthquake—Period of vibration—The - amplitude of earth movements—Side of greatest motion—Intensity - of earthquakes—Velocity and acceleration of an earth - particle—Absolute intensity of an earthquake—Radiation of an - earthquake—Velocity of propagation. - - -_Result of Feelings._—As the result of our experiences, and by -observations upon the movements produced in various bodies, we can -say that an ordinary earthquake consists of a number of backward and -forward motions of the ground following each other in quick succession. -Sometimes these commence and die out so gently that those who have -endeavoured to time the duration of an earthquake have found it -difficult to say when the shock commenced and when it ended. This was -a difficulty which Mr. James Bissett in Yokohama, and the author in -Tokio, had to contend against when, in 1878, they commenced to time -shocks between these two places. - -Sometimes these motions gradually increase to a maximum and then die -out as gradually as they commenced. - -Sometimes the maximum comes suddenly, and at other times during an -earthquake our feelings distinctly tell us that there are several -maxima. - -These have been the experiences of many observers, and have been -recorded by writers since the earliest times. Mallet devotes a -chapter to a consideration of the tremulous motion that precedes and -follows a shock, and he tells us that a single shock is an absolute -impossibility. In speaking of earthquakes, he says: ‘The almost -universal succession of phenomena recorded in earthquakes is, first a -trembling, then a severe shock, or several in quick succession, and -then a trembling gradually but rapidly becoming insensible.’ - -A quantitative and exact knowledge of the nature of earthquake -motion has only been attained of late years. The chief results which -investigators have aimed at have been the measurement of the amplitude, -the period, the direction, and the duration of the motions which -constitute an earthquake. Attention has also been given to the velocity -with which a disturbance is propagated. - -_The Direction of Motion._—One of the most ordinary observations -which are made about an earthquake is its direction. If we were to -ask the inhabitants of a town which had been shaken by an earthquake -the direction of the motion they experienced, it is not unlikely that -their replies would include all the points of the compass. Many, -in consequence of their alarm, have not been able to make accurate -observations. Others have been deceived by the motion of the building -in which they were situated. Some tell us that the motion had been -north and south, whilst others say that it was east and west. A certain -number have recognised several motions, and amongst the rest there will -be a few who have felt a wriggling or twisting. Leaving out exceptional -cases, the general result obtained from personal observation as to -the direction of an earthquake of moderate intensity is extremely -indefinite, and the only satisfactory information to be obtained is -that derived from instruments or from the effects of the earthquake -exhibited in shattered buildings and bodies which had been overturned -or projected. - -By the direction in which walls, columns, and other objects had been -overthrown or fractured, Mallet was enabled to determine the position -of the origin of the Neapolitan earthquake. Similar phenomena have many -times been taken advantage of by other investigators of earthquake -phenomena. Effects produced upon structures are, however, only to be -observed as the results of a destructive earthquake, at which time -cities may be regarded as collections of seismometers. (_See_ chapter -on Effects in Buildings.) - -To determine the direction of movement during a small earthquake, the -most satisfactory method appears to be an appeal to instruments. - -_Instruments as Indicators of Direction._—The relative values of -different kinds of instruments, such as columns, pendulums, and the -like, as indicators of direction have already been discussed. - -By the use of pendulum seismographs it has been shown that during an -earthquake the ground may move in one, two, or several directions (see -p. 21); and it is, generally speaking, only in those cases where we -experience a decided shock in the disturbance that we can determine -with any confidence the direction in which the motion has been -propagated. Such directions are usually indicated by the major axis of -certain more or less elliptical figures which have been drawn, which -in themselves appear to indicate the combination of two rectilinear -movements. - -Results similar to those indicated by the records of pendulum -seismographs have also been obtained upon moving plates with a double -bracket seismograph. Thus, in the earthquake which shook Tokio at 6 -A.M. on July 5, 1881, there were indications of the following -motions:— - -Near the commencement of the shock the motion was N. 112° E. One and a -half second after this, the direction of motion appears to have been -N. 50° E. In three-fourths of a second more it gradually changed to a -direction N. 145° E., and after a similar interval to N. 62° E. Half a -second after this it was N. 132° E., and four seconds later the motion -was again in the original direction—namely, N. 112° E. - -These particular directions of motion have been selected because they -were so definitely indicated. - -The commonest type of earthquake which is experienced in Japan, and -probably also in other earthquake-shaken districts, is the compound or -diastrophic form. - -That earthquakes often have motions compounded of two sets of -vibrations, has also been proved by the analysis of the records -obtained from two component seismographs. From an analysis of a record -of this description, Professor Ewing has shown that in the earthquake -felt in Tokio on March 11, 1881, there were approximate circular -(somewhat spiral) movements. - -This leads us to the consideration of the twisting and wriggling -motions which are said to be experienced by some observers. Motions -like these, which by the Italians and Mexicans are called _vorticosi_, -are usually supposed to be the cause of objects like chimneys and -gravestones being rotated. These phenomena, it will be seen from what -is said in the chapter upon the effects produced in buildings, can be -more easily explained upon the supposition of a simple rectilinear -movement. - -That at the time of an earthquake there may be motion in more than one -direction has been recognised since the time of Aristotle; and it is -possible that two sets of rectilinear motion, as, for instance, the -normal and transverse movements, may have led observers to imagine that -there has been a twisting motion taking place, and this especially when -the two sets of movements have quickly succeeded each other. - -Persons inside flexible buildings may possibly have experienced more -or less of a rotatory motion, although the shock was rectilinear; the -building assuming such a motion in consequence of its construction and -its position with regard to the direction of the shock. - -In the case of destructive earthquakes, especially at points situated -practically above the origin, the universal testimony, Mallet tells -us, is that a twisting, wriggling motion in different planes, attended -by an up-and-down movement of greater range, is experienced. To such -disturbances the word _sussultatore_ is sometimes applied. Mallet has -given many elliptical and other closed curves to illustrate the nature -of such motions. - -_Duration of an Earthquake._—When reading accounts of earthquakes -it is often difficult to determine the length of time a shaking was -continuous. In Japan, in A.D. 745, there was a shaking which is said -to have lasted sixty hours; and in A.D. 977 there were a series of -shakings lasting 300 days. Often we meet with records of disturbances -which have lasted from twenty to seventy days. - -At San Salvador, in 1879, more than 600 shocks were felt within ten -days; in 1850, at Honduras, there were 108 shocks in a week; in 1746, -at Lima, 200 shocks were felt in twenty-four hours; at the island of -St. Thomas, in 1868, 283 shocks were felt during about ten hours. - -Disturbances like these, which succeed each other with sufficient -rapidity to cause an almost continual trembling in the ground, may be -regarded as collectively forming one great seismic effort which may -last a minute, an hour, a day, a week, or even several years. Strictly -speaking, they are a series of separate earthquakes, the resultant -vibrations of which more or less overlap. Whenever a large earthquake -occurs it is generally succeeded by a large number of smaller shocks. - -The seismic disturbance as regards time is, as Mallet remarks, very -often ‘like an occasional cannonade during a continuous but irregular -rattle of musketry.’ In the New Zealand earthquake of 1848, shocks -continued for nearly five weeks, and during a large portion of the time -there were at least 1,000 shocks per day.[12] - -The earthquake of Lisbon, which in five minutes destroyed the whole -town, was followed by a series of disturbances lasting over several -months. After Basle had, on October 18, 1356, been laid in ruins, -it is stated shocks followed each other for a period of a year. The -Calabrian earthquake was continued with considerable strength for a -year, and it is said that the earth did not come completely to rest for -ten years. During this cannonade the heavy shocks announced, as they -do in most earthquake countries at the present day, a series of weaker -disturbances. In certain exceptional cases this order of events has -been inverted, and slight shocks have announced the coming of heavy -ones. Fuchs gives an example of this in the earthquake of Broussa, when -the first shock was on February 28, 1855. On March 9 and 23 there were -heavier shocks, but the heaviest did not arrive until March 28. - -Under certain conditions it is possible to have a sensible vibration -produced in the ground which is practically of unlimited duration; -thus, for instance, it has been noticed that the falling of water -at certain large waterfalls, by its continuous rhythmical impact on -the rocks, produces in them tremors which are to be observed at -great distances. Of this the author convinced himself at the Falls -of Niagara, where he observed the reflected and ever-moving image of -the sun in a pool of water. Under favourable circumstances almost -continual condensation of steam might take place in volcanic foci, each -condensation giving rise to a blow sufficiently powerful to produce -vibrations in the surrounding ground. Those who have stood near a large -geyser, like the one in Iceland, when it makes an ineffectual effort -to erupt, will recognise how powerful such a cause might be. Humboldt -has remarked shocks on Vesuvius and Pichincha which were periodic, -occurring twenty to thirty seconds before each ejection of vapour and -ashes. - -Earthquakes like these may be of vast extent, gradually spreading -further and further outwards. This spreading of earth vibrations may -be observed at a large factory containing heavy machinery or a steam -hammer. After the machinery comes to rest, it is probably some time -before the ground returns to rest. Examples of disturbances of this -nature are spoken of under the head of Earth Tremors. - -The record of the duration of an ordinary earthquake as observed at a -given point is dependent upon the sensibility of our instruments. - -Continuous motions perceptible to our senses without the aid of -instruments usually last from thirty seconds to about two or three -minutes. In Japan the shocks, as timed by watches, usually last from -twenty to forty seconds. Occasionally a continuous shaking is felt for -more than one and a half minutes, and cases have been recorded where -the motion has continued for as much as four minutes and thirty-three -seconds. - -Seismometers having a multiplication of 6 to 12 usually indicate that -motion continues longer than is perceptible to the senses. - -_Period of Vibration._—When an earthquake contains several prominent -vibrations which might be called the _shocks_ of the disturbance, our -feelings tell us that these have occurred at unequal intervals. - -About the time which is taken for the complete backward and forward -oscillation of the ground which constitutes the shock a little has -already been said. This was deduced from the records of disturbances as -drawn by seismographs. From the same sources we can readily obtain the -period of all the prominent vibrations in a disturbance. - -In any given earthquake there are irregularities in period, and -different earthquakes differ from each other. About the early attempts -to determine the period of earth vibrations something has been said in -the chapter on Earthquake Instruments. - -In the earthquake of March 11 (referred to on p. 70) we find that both -components commenced with a series of small vibrations, about five or -six to the second; next came the shock, consisting of two complete -vibrations executed in two seconds. In this it is to be observed that -the motion eastwards was performed much more quickly than the motion -westwards. Next, by reference to the east and west component, it is -seen that there are a number of large vibrations, about one per second, -on which a number of smaller motions are superposed. As the motion -proceeds, these become less and less definitely pronounced and more -irregular in their intervals, until finally the motion dies away. - -This earthquake, as recorded at the author’s house in Tokio, lasted -about one and a half minute. - -The same earthquake, as recorded by Professor Ewing at a station -situated about one and a half mile distant, but on flat ground, appears -to have lasted four and a half minutes. The largest wave had a period -of 0·7 second. - -In the earthquake of March 8, 1881, there were on an average 1·4 -vibrations per second. These vibrations were executed in a direction -transverse to the line joining the observing station and the locality -from which the disturbance must have originated as determined by time -observations. It can, therefore, be assumed that these vibrations, -having so slow a period, were transverse motions, this slowness or -sluggishness being due to the fact that the modulus for distortion -is less than the modulus which governs the propagation of normal -vibrations. - -_The Amplitude of Earth Movements._—In making estimates of the -distances through which we are moved backward and forward at the time -of an earthquake, if we judge by our feelings, we may often be misled. -If a person is out of doors and walking, an earthquake may take place -sufficiently strong to cause chimneys to fall and unroof houses, which, -so far as the actual shaking of the ground is concerned, will be passed -by unnoticed. On the other hand, to persons indoors, especially on an -upper story, it is impossible even for a tremor to pass by without -creating considerable alarm by the angular movement that has been taken -up by the building. - -Many observers have endeavoured to make actual measurements of the -maximum extent through which the earth moves at the time of an -earthquake. Among the reports of the British Association for 1841 is -the report of a committee which had been appointed ‘for obtaining -instruments and registers to record shocks of earthquakes in Scotland -and Ireland’. We read that in one earthquake which had been measured -the displacement of the ground had been half an inch, and in another -it had been less than half an inch. The instruments used to make these -observations depended upon the inertia of pendulums which at the time -of the disturbance were supposed to remain at rest. Observations -similar to these have been made in Japan. One long series were made by -Mr. E. Knipping for Dr. Gr. Wagener. They extended from November 1878 -to April 1880, and were as follows:— - - Number of Maximum horizontal - Earthquakes motion of the ground - 10 ·0 to 0·15 mm. - 7 ·15 „ 0·5 „ - 8 ·5 „ 2·5 „ - 2 2·5 „ more „ - -With his apparatus for vertical motion Dr. Wagener also made -observations on the absolute vertical motion. This seldom reached ·02 -mm. The greatest value was that observed for the destructive shock of -Feb. 22, 1880, which was ·56 mm. - -By means of a number of instruments distributed at various localities -round Tokio, the chief of which were pendulums with friction pointers -to render them ‘_dead beat_,’ and with magnifying apparatus to show -the actual motion of the ground, the author arrived at results similar -to those obtained by Dr. Wagener—namely, that the earth’s maximum -horizontal motion at the time of a small earthquake was usually only -the fraction of a millimetre, and it seldom exceeded three or four -millimetres. When we get a motion of five or six millimetres, we -usually find that brick and stone chimneys have been shattered. - -The results obtained for vertical motion were also very small. In Tokio -it is seldom that vertical motion can be detected, and when it is -recorded it is seldom more than a millimetre. - -These results, which were put forward some years ago, have since -received confirmation by the use of a variety of instruments in the -hands of different observers. - -Mallet, in his account of the Neapolitan earthquake of 1857, -approximated to the amplitude of an earth particle by observing the -width, at the level of the centre of gravity, of fissures formed -through and remaining in great masses of very inelastic masonry. - -Taking stations situated on or very nearly on the same line passing -through the seismic vertical (_epicentrum_), Mallet observed the -amplitude increased as some function of the distance, as will be seen -from the following table:— - - +-------------------------+------+-------+-------+---------+-------+ - | Station |Polla |La Sala|Certosa|Tramutola|Sarconi| - +-------------------------+------+-------+-------+---------+-------+ - | Distance from Seismic } | | | | | | - | Vertical in } | 3·45 | 11·60 | 16·50 | 20·60 | 26·7 | - | geographical miles } | | | | | | - | Amplitude in inches | 2·5 | 3·5 | 4·0 | 4·5 | 4·75 | - +-------------------------+------+-------+-------+---------+-------+ - -The possibility of a law such as this having an existence for places at -a distance from the seismic vertical comparable with the vertical depth -of the centrum will be shown farther on. - -With regard to the maximum displacement of an earth particle. Mallet -was of opinion that there was evidence to show that it had in some -cases been over one foot. M. Abella, in an earthquake which occurred in -the Philippines in 1881, made a rough observation of the motion of the -earth to a distance of about _two metres_. This, as might be expected, -was beyond the elastic limits of the material, and caused fissures to -be formed, which were seen to open and shut. - -_Intensity of Earthquakes._—In speaking of the strength of an -earthquake, we usually employ terms like ‘weak,’ ‘strong,’ ‘violent,’ -&c. Although these expressions, accompanied by illustration of the -effects which an earth quake has produced, convey a general idea of -the strength of a shock as felt at some particular locality, our ideas -nevertheless wanting in definiteness; and if we endeavour to compare -one shock with another, as a whole, our want of exactness is augmented. -We have seen that Palmieri’s seismograph indicates intensity by a -certain number of degrees, which, to a certain extent, is a measure of -the violence of the motion as indicated at a particular locality. The -degrees, as before stated, refer to the height to which in consequence -of the shaking, a certain quantity of mercury was washed in a tube, -which is a function of the depth of mercury in the tube, and also of -the duration of the disturbance. - -From this it seems possible that a very slow motion of small amplitude, -continuing over a sufficient period of time, might, if it agreed with -the period of the mercury, indicate an earthquake of many degrees of -intensity, whilst residents in the neighbourhood might not have noticed -the disturbance; and, on the other hand, a short but intense shock -creating considerable destruction might have been recorded as of only a -few degrees of intensity. - -Although objections like these might be raised to such a method of -recording intensity, in practice it would appear that such results are -not pronounced, and the indications of the instrument usually give us -approximate indications of relative intensity. - -In writing about the Neapolitan earthquake of 1857, Mallet says that -‘area alone affords no test of seismic energy.’ - -The area over which a shock is felt will depend not only upon the -initial force of the disturbance, but also upon the focal depth of -a shock, the form and position of that focus, the duration of the -disturbance, and the nature and arrangement of the materials which are -shaken. - -From observations in Japan, it is clearly shown that massive mountain -ranges exert a considerable influence upon the extension of seismal -disturbances. On one side of a large range of mountains large cities -might be laid in ruins, whilst on the other side the disturbance -creating this destruction might not be noticed. - -_Velocity and Acceleration of an Earth Particle._—We now pass on to -methods of determining the intensity of an earthquake which are less -arbitrary than those which have just been discussed. These methods have -already been discussed when speaking of artificial disturbances, where -it was shown that the intensity of an earthquake as measured by its -destructive effects greatly depended upon the suddenness with which the -backward and forward motions of the ground were commenced or ended. - -Amongst the earlier investigators of seismic phenomena who observed -that there existed a connection between the distance to which bodies -had been projected during an earthquake and the suddenness or initial -velocity with which the ground had been moved beneath them, was -Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks -from his chimneys had, by the New England earthquake of 1755, been -thrown thirty feet. From this and the known height of the chimney, he -calculates that the bricks had been projected with an initial velocity -of twenty-one feet per second.[13] - -The calculations made by Mallet respecting the maximum velocity of -an earth particle at the time of the Neapolitan earthquake in 1857 -depended upon the overthrow, projection, and fracture of bodies. - -The principles which guided him in making the calculations will be -understood from the following illustration. - -[Illustration: Fig. 14.] - -If a column, A B C D, receive a shock or be suddenly moved in the -direction of the arrow, the centre of gravity, G, of this column will -revolve round the edge, and tend to describe the path G O. If it passes -O, the column will fall. The work done in such a case as this is equal -to lifting the column through the height _o_ _h_. - -If G A = _a_, the angle G A _h_ = φ, and the weight of the body = W, -then the above work equals - - W_a_ (1 - cos φ). - -This must equal the work acquired—that is to say, the kinetic energy of -rotation of the body, or - - W _w_^2 K^2 - W_a_ (1 - cos φ) = ———————————. - 2 _g_ - -Where _w_ is the angular velocity of the body at starting, K the radius -of gyration round A, and _g_ the velocity acquired by a falling body in -one second. Whence - - _w_^2 K^2 = 2 _ga_ (1 - cos φ), - -but _w_, the angular velocity, is equal to the statical couple applied, -divided by the moment of inertia, or, - - V_a_ cos φ - _w_ = ——————————, - K^2 - -squaring and substituting - - K^2 1 - cos φ - V^2 = 2_g_ × ——— × —————————, - _a_ cos^2 φ - - K^2 -and since the length of the corresponding pendulum is _l_ = ———, - _a_ - - 1 - cos φ - V^2 = 2_gl_ × —————————. - cos^2 φ - -To apply this to any given case we must find the value of _l_ or of - -K^2 -———. -_a_ - -Mallet finds these values for the cube, solid and hollow rectangular -parallelopipeds, solid and hollow cylinders, &c. In these formulæ we -have a direct connection between the dimensions and form of a body and -the velocity with which the ground must move beneath it to cause its -overthrow. - -[Illustration: FIG. 15.] - -Not only is the case discussed for horizontal forces, but also for -forces acting obliquely. Similar reasonings are applied to the -productions of fractures in walls, but as there is uncertainty in our -knowledge of the co-efficient of force necessary to produce fracture -_through joints across_ beds of masonry, the deductions ought not to -be applied as the measures of velocity. Where the fractures occur at -the base or in horizontal planes, or in those of the continuous beds of -the masonry, or through homogeneous bodies, the uncertainty is not so -great, and for cases like these Mallet gives several illustrations. The -distance to which bodies had been projected, as, for example, ornaments -from the tops of pedestals, coping-stones from the edges of roofs, were -also used as means of determining the angle at which the shock had -emerged, or, if this be known, for determining the velocity. - -Thus by a shock in the direction O C, a ball, A, on the top of a -pedestal would describe a trajectory to the point C. Let the angle -which O C makes with the horizon be _e_, the vertical height through -which the ball has fallen be _b_, and the horizontal distance of -projection be _a_; then - - _a_^2 - _b_ = _a_ tan _e_ + ————————————, - 4H cos^2 _e_ - -H being the height due to the velocity of projection. Whence - - ___________________ - 2H ± √4H(H + _b_) - _a_^2 - Tan _e_ = ——————————————————————————. - _a_ - - _a_^2 _g_ - V^2 = ———————————————————————————————. - 2 cos^2 _e_ (_b_ - _a_ tan _e_) - -For the back motion or subnormal wave in the direction C O, - - ___________________ - 2H ± √4H(H + _b_) - _a_^2 - Tan _e_ = ——————————————————————————. - _a_ - - _a_^2 _g_ - V^2 = ———————————————————————————————. - 2 cos^2 _e_ (_b_ + _a_ tan _e_) - -A serious error which may enter into calculations of this description -when practically applied has been pointed out when speaking of columns -as seismometers. It was then shown that such bodies before being -overthrown may often be caused to rock, and therefore that their final -overthrow may not have any direct connection with the impulse of the -succeeding shock. - -Another point to which attention must be drawn respecting the above -calculations is that if there was no friction or adherence between the -projected body and its pedestal, in consequence of its inertia it -would be left behind by the forward motion of the shock, and simply -drop at the foot of its support. In the case of frictional adherence it -would be carried forward by the velocity acquired before this adherence -was broken, and thrown in a direction _opposite_ to that given in the -figure—that is to say, in the direction of the shock.[14] - -_The Absolute Intensity of the Force exerted by an Earthquake._—No -doubt it has occurred to many who have experienced an earthquake -that the power which gave birth to such a disturbance must have been -enormously great. The estimates which we shall make of the absolute -amount of energy represented by an earthquake cannot, on account of the -nature of the factors with which we deal, be regarded as accurate. They -may, however, be of assistance in forming estimates of quantities about -which we have at present no conception. One method of obtaining the -result we seek is that which was employed by Mallet in his calculations -respecting the Neapolitan earthquake. Although disbelieving in the -general increment of temperature as we descend in the earth at an -average rate of 1° F. for every fifty or sixty feet of descent, for -want of better means. Mallet assumes this law to be true, and, knowing -from a variety of observations the depth of various parts of the cavity -from which the disturbance sprang, he calculates the temperatures of -this cavity in various parts as due to its depth beneath the surface. -Next, it is assumed that steam was suddenly admitted into this cavity, -which might exert the greatest possible pressure due to the maximum -temperature. This was calculated as being about 684 atmospheres. - -Next, he determined the column of limestone necessary to balance such a -pressure, which is about 8,550 feet in height. As the least thickness -of strata above this cavity was 16,700 feet, the pressure of 684 -atmospheres was not sufficient to blow away its cover, but if suddenly -admitted or generated in the cavity it might have produced the wave of -impulse by the sudden compression of the walls of the cavity. - -The pressure of 684 atmospheres is equivalent to about 4·58 tons on -the square inch, and, as the total area of the walls of the cavity is -calculated at twenty-seven square miles, the total accumulated pressure -would be more than 640,528 millions of tons. Mallet, however, shows -that it is probable that the temperature of the focal cavity was much -greater than that due to the hypogeal increment, and that therefore the -pressure may have been greater. - -The capability of producing the earthquake impulse, however, depends -on the _suddenness_ with which the steam is flashed off. According -to the experiments of Boutigny and others, Mallet tells us that the -most sudden production of steam would take place at a temperature of -500°-550° C., which is but a few degrees below that calculated for the -mean focal depth. - -Assuming the above calculated pressure to be true, and knowing the -co-efficient of compression of the materials on which it acted, the -volume of the wave at a given moment near the instant of starting—that -is, at the focus—can be calculated, and from this the wave amplitude on -reaching the surface may be deduced. - -Proceeding backwards, if we have observed the wave amplitude, -calculated the depth of the focus, and know the co-efficient of -expansion, then the total compression may be calculated and the -temperature due to the pressure producing this may be arrived at. In -this way earthquakes may be used as a means of calculating subterranean -temperature at depths that can never be attained experimentally. - -A method of proceeding which is probably more definite than that -adopted by Mallet would be the application of the method indicated when -speaking of the intensity of artificial disturbances. - -If for a given earthquake the origin of which is known we have -determined by seismographs the mean acceleration of an earth particle -at two or more stations at different distances from that origin, we are -enabled to construct a curve of intensity the area between which and -its asymptotes was shown to be a measure of the total intensity of the -shock. Comparing this area with that of a unit disturbance produced, -say, by the explosion of a pound of dynamite, one may approximately -calculate in terms of this unit the initial intensity of the earthquake. - -_Radiation of an Earthquake._—The tremors preceding the more violent -movements of an earthquake may be due, as Mallet has suggested,[15] -to the free surface waves reaching a distant point before the direct -vibrations. - -The fact that earth vibrations produced by striking a blow on or near -the surface of the ground are wholly obliterated in reaching a cutting -or valley, there being no underground waves of distortion to crop up -on the opposite side of the valley, indicates that the disturbance is -one that travels on the surface; the same fact is illustrated when we -endeavour to transmit vibrations through the side of a hill into a -tunnel. - -In the tunnel, although the distance may be small, no sensible effects -are produced, whilst the same disturbance may be recorded at a long -distance from its origin on the surface of the ground outside the -tunnel. - -Lastly, we may refer to the experiences of miners underground. - -Occasionally it has happened that miners when deep underground, as in -the Marienberg in the Saxon Erzgebirge, have felt shocks which have not -been noticed on the surface. These observations are rare, and it is -possible that they may be explained by the caving in of subterranean -excavations. - -The usual experience is, that if a shock is felt underground it is also -felt on the surface, as for example in the lead mines in Derbyshire at -the time of the Lisbon disturbance (1755). - -The most frequent observation, however, is that a shock may be felt on -the surface while it is not remarked by the miners beneath the surface, -as at Fahlun and Presburg in November, 1823. - -At the Comstock Lode in Colorado about twelve years ago many -earthquakes were felt. On one particular day twenty-four were counted. -Superintendent Charles Foreman told the author when he visited Virginia -City in 1882, that special observations were made to determine whether -these shocks were felt as severely deep down in the mines as on the -surface, where they were on the verge of being destructive. The -universal testimony of many observers was that in most cases they were -not felt at all underground, and when a shock was felt it was extremely -feeble. At Takashima Colliery, in Japan, it is seldom that shocks are -felt underground. - -The explanation of these latter observations appears to be either -that, in consequence of a smaller amplitude of motion in the solid -rocks beneath the surface as compared with the extent of motion on -the surface, the disturbances are passed by unnoticed, or else the -disturbance is, at a distance from its origin, practically confined to -the surface. - -_Velocity of Propagation of an Earthquake._—Although many have written -upon earthquakes and have endeavoured to give to us the velocity with -which they were propagated, the subject is one about which we have as -yet but little exact information. - -The importance of this branch of investigation is undoubtedly great. -By knowing the velocity with which an earthquake has travelled in -various directions we are assisted in determining the locality of -its origin; we may possibly make important deductions respecting the -nature of the medium through which it has passed; perhaps also we -may learn something regarding the intensity of the disturbance which -created the earthquake. In the Report of the British Association for -1851 Mallet gives the table on next page, in which are placed together -the approximate rates of transit of shocks of several earthquakes -which he discusses. Some of these, it will be observed, are records of -disturbances which must have passed through or across the bed of the -ocean. - -In Mallet’s British Association Report for 1858, he gives data compiled -by Mr. David Milne[16] respecting the Lisbon earthquakes of 1755 and -1761, from which data the tables of velocities (p. 89) have been -calculated, omitting those which Mr. Mallet has marked as uncertain. - -The distances are marked in degrees of seventy English miles each, and -the time is reduced to Lisbon time. - - +---------------------+-------+--------------------------+----------+ - | |Approx.| | | - | |rate in| Formation constituting | | - | Occasion and Place |feet | Range on surface as far |Authority | - | | per | as known or conjectured | | - | |second | | | - +---------------------+-------+--------------------------+----------+ - |Rev. John Mitchell’s |1,760 |Sea bottom, probably on | Mitchell | - | guesses from the | | slates, secondary and | | - | Lisbon earthquakes | | crystalline rocks | | - |Von Humboldt’s |1,760 |From observations in | Humboldt | - | guesses from South | to | various South American | | - | America |2,464 | rocks in great part | | - | | | volcanic | | - | | | | | - | _Lisbon Earthquake | | | | - | of 1761._ | | | | - |Lisbon to Corunna |1,994 |Transition, carboniferous | ‘Annual | - | | | and granitoid |Register’ | - |Lisbon to Cork |5,228 |Transition, carboniferous | „ | - | | | crystalline slates and | | - | | | granitoid, probably, | | - | | | under sea bottom | | - |Lisbon to Santa Cruz |3,261 |The same with many | „ | - | | | alterations | | - | | | | | - | _Antilles._ | | | | - |Pointe à Pitre to |6,586 |Probably volcanic rocks |Stier and | - | Cayenne (doubtful) | | under sea bottom | Perrey’s | - | | | | memoran- | - | | | |dum, Dijon| - | | | | | - | _India._ | | | | - |Cutch to Calcutta, |1,173 |Alluvial, secondary, | ‘Royal | - | 1819 | | granitoid and later | Asiatic | - | | | igneous rocks | Journal’ | - |India, Nepauls, and | | | | - | basin of the Ganges,| | | | - | 1834:-- | | | | - |Rungpur to Arrah |2,314 }|Deep alluvia, with | ‘Royal | - |Monghyr to Gorackpur |3,520 }| occasional transition, | Asiatic | - |Rungpur to Monghyr | 990 }| carboniferous, granitoid,| Journal’ | - |Rungpur to Calcutta |1,210 }| and later igneous rocks | | - |Ships ‘Rambler’ and |1,056 |Sea bottom resting on |‘Nautical | - | ‘Millwood,’ at sea, | | unknown rock |Magazine’ | - | 1851; between lat. | | | | - | 16° 30′ N.L., 54° | | | | - | 30′ W., and lat. 23°| | | | - | 30′ N.L., 58° 0′ W. | | | | - +---------------------+-------+--------------------------+----------+ - - THE LISBON EARTHQUAKE ON NOVEMBER 1, 1755. - - +----------------------------+---------+--------+--------+ - | | Moment |Distance|Velocity| - | Localities |observed | from |in feet | - | |of shock |presumed| per | - | | | origin | second | - +----------------------------+---------+--------+--------+ - | | h. m. | ° ′ | | - | Presumed focus of shock, | } 9 23 | -- | -- | - | lat. 30°, long. 11° W. | } | | | - | A ship at sea in lat. 38°, | } 9 24 | 0 30 | 3,091 | - | long. 10° 47′ W. | } | | | - | Colares | 9 30 | 1 30 | 1,325 | - | Lisbon | 9 32 | 1 30 | 1,030 | - | Oporto | 9 38 | 2 30 | 1,030 | - | Ayamont | 9 50 | 4 0 | 916 | - | Cadiz | 9 48 | 5 0 | 1,236 | - | Tangier and Tetuan | 9 46 | 5 30 | 1,478 | - | Madrid | 9 43 | 6 0 | 1,855 | - | Funchal | 10 1 | 8 30 | 1,382 | - | Portsmouth | 10 3 | 12 30 | 1,431 | - | Havre | 10 23 | 13 0 | 1,339 | - | Reading | 10 27 | 13 30 | 1,304 | - | Yarmouth | 10 42 | 15 0 | 1,174 | - | Amsterdam | 10 6 | 17 0 | 2,444 | - | Loch Ness | 10 42 | 18 0 | 1,409 | - +----------------------------+---------+--------+--------+ - - - THE LISBON EARTHQUAKE OF MARCH 31, 1761.[17] - - +----------------------------+---------+--------+--------+ - | | Moment |Distance|Velocity| - | Localities |observed | from |in feet | - | |of shock |presumed| per | - | | | origin | second | - +----------------------------+---------+--------+--------+ - | | h. m. | ° ′ | | - | Presumed focus, lat. 43°, | } 11 51 | -- | -- | - | long. 11° W. | } | | | - | Ship at sea in lat. 43°, | } | | | - | many leagues from coast | } 11 52 | 0 30 | 3,091 | - | of Portugal | } | | | - | Ship in lat. 44° 8′ and | } 11 54 | 1 45 | 3,607 | - | about 80 leagues from | } | | | - | coast | } | | | - | Corunna | 11 51 | 2 30 | 2,576 | - | Ship lat. 44° 8′ and 80 | } | | | - | leagues NNW. of Cape | } 11 58 | 3 30 | 3,091 | - | Finisterre | } | | | - | Lisbon | noon | 4 30 | 3,091 | - | Madeira | 12 6 | 10 0 | 4,122 | - | Cork | 12 11 | 9 30 | 2,937 | - +----------------------------+---------+--------+--------+ - -These tables, owing to the nature of the materials which Mallet had at -his disposal, are but rude approximations to the truth. Two interesting -facts are, however, observable: the first being that the velocities -for the earthquake of 1761 are much higher than those obtained for the -earthquake of 1755; and, secondly, that in both cases the velocities as -determined from the observations of ships at sea closely approximate -to each other, in all cases being nearly the same as that with which a -sound wave would travel through water. - -The great differences in transit velocity obtained for different -earthquakes is a point worthy of attention. - -Seebach’s velocity is a _true_ transit velocity, and its determination -is dependent on the assumption that the shock radiated from the -_centrum_ and not from the _epicentrum_, Seebach’s method is explained -when speaking about the determination of origins. - -Some interesting observations on the velocity with which the earthquake -of October 7, 1874, was propagated, are given by M. S. di Rossi.[18] - -One assumption is that the disturbance radiated from an origin -to surrounding points of observation, whilst another is that the -disturbance followed natural fractures, the direction of which is -derived from the crest of certain mountain ranges. These velocities are -as follows, Maradi being at or near the origin of the disturbance:— - - +-----------------------------+-----------------------------------+ - | Velocity in feet per second | Velocity in feet per second by | - | with direct radiation | propagation along mountain chains | - +-----------------------------+-----------------------------------+ - | Modigliana 820 | By the Valley of Marenzo 1,080 | - | Bologna 656 | „ „ Saveno 1,080 | - | Forli 874 | „ „ Montone 1,080 | - | Modena 518 | „ „ Panaro {1,080 | - | | { 984 | - | Firenze 273 | „ „ Sieve 540 | - | Compiobbi 328 | „ „ „ 540 | - +-----------------------------+-----------------------------------+ - -Another set of interesting results are those of P. Serpieri on the -earthquake of March 12, 1873. The curious manner in which this shock -radiated is described in the chapter on the Geographical Distribution -of Earthquakes (see p. 231). Two large areas appear to have been -almost simultaneously struck, so that, there being no time for elastic -yielding, the velocities calculated between places situated on either -of the areas are exceedingly great.[19] - - From Ragusa to Venice the velocity was 2,734 feet per second - „ Spoleto „ „ 4,101 „ „ - „ Perugia to Orvieto „ 601 „ „ - „ „ „ Ancona „ 1,640 „ „ - „ „ „ Rome „ { 1,640 „ „ - {or, 2,186 - -The following are examples of approximate earthquake velocities which -have been determined in Japan. - -_The Tokio Earthquake of October 25, 1881._—From records respecting -this earthquake it appears to have been felt over the whole of Yezo -and the northern and eastern coast of Nipon, a little farther south -than Tokio. It was severest at Nemuro and Hakodate, and at the former -place a little damage was done. From these facts, together with the -indications of instruments recording direction of movement and a -general inspection of the time records, it seems that the disturbance -must have originated beneath the sea on the east coast of Yezo at a -very long distance to the north-east of Tokio, from which place it -passed in a practically direct line on to Yokohama. - -As the disturbance was felt at Yokohama twenty-one seconds later than -at Tokio, and the distances between these two places is about sixteen -geographical miles, for this portion of its course the disturbance -must have travelled at a rate of at least 4,300 _feet per second_. If -we assume that the shock, after having reached Hakodate, travelled on -at the same rate as it did between Tokio and Yokohama in order to reach -Saporo, where the shaking was felt eighteen seconds after Hakodate, -it must have had about thirteen geographical miles to travel after -Hakodate was shaken before Saporo felt its effect. - -Drawing from Hakodate a tangent to the eastern side of a circle of -thirteen miles radius described round Saporo, the origin of the -disturbance must be on the line bisecting this tangent at right angles. -As it also lies on a line drawn through Tokio and Yokohama, it lies in -a position about 41 N. lat. and 144° 15′ E. long., which is a position -somewhat nearer to Nemuro than Hakodate, as we should anticipate. If -this be taken as approximately indicating the origin, then the shock, -after reaching Hakodate from the Hakodate _homoseist_, travelled about -218 miles to reach Tokio in 128 seconds, which gives a _velocity of -10,219 feet per second_. - -The method here followed is equivalent to that of the hyperbola and one -direction (see p. 204). The hyperbola is described on the assumption -that the velocity deduced from the time taken to travel between Tokio -and Yokohama is correct, and also that the earth waves travelled with -approximately the same velocity in the vicinity of Saporo as near -Tokio. The probability, however, is that they travelled more quickly. -If this be so, then the origin is thrown somewhat to the south-east and -the velocity between the Hakodate homoseist and Tokio reduced. Thus, -if the velocity in the Saporo district be double that observed in the -Tokio district, the origin is shifted about twenty-eight miles to the -south-west, and the last-mentioned velocity is reduced to about 9,000 -feet per second. - -If we work by the method of circles, and assume the velocity to have -been constant in all directions, then this velocity must have been -about 6,000 feet per second. If we assume that the indications of -direction obtained from seismographs and other sources give to us by -this intersection a proper origin, the velocity in some directions may -have been as much as 17,000 feet per second. - -An origin thus determined, or even if determined by the method of -circles, is in discord with the fact that places like Nemuro, in the -north-east of Yezo, were nearer to the origin than any of the other -places which have been mentioned. - -The conclusion which we are therefore led to with regard to this shock, -assuming of course that the time observations are tolerably correct, -is that the velocity of propagation was variable, being greater when -measured between points near to the origin than between points at a -distance. The velocities estimated vary between 4,000 and 9,000 feet -per second. - -In the case of the earthquake which has just been discussed, we have -an example of a disturbance which must have passed between Tokio and -Yokohama in what was almost a straight line from the origin. As this -direction ought to give the maximum time of transit if all earthquakes -are propagated with the same velocity, the following table is given of -the interval between the time of observation of several shocks at these -two stations:— - - FROM YOKOHAMA TO TOKIO. FROM TOKIO TO YOKOHAMA. - - 1880 December 20th 36 seconds | 1882 October 25th 21 seconds - 1881 January 7th 14–31 „ | 1883 February 6th 23 „ - „ March 8th 60 „ | „ March 1 53 „ - „ „ 17th. 66 „ | „ „ „ 63 „ - „ November 15th 31 „ | „ 8th 27 „ - 1882 February 16th 22 „ | „ „ 11th 26 „ - -As these are observations which have been made with the assistance of a -telegraphic signal daily employed to correct and rate the clocks from -which the observations were obtained, they may be regarded as being -tolerably, correct. - -The disturbance of February 6, the two shocks of March 1, appear, like -that of October 25, to have passed in almost a direct line from an -origin in the N.N.E, through Tokio on to Yokohama. Their velocities of -propagation as calculated from the above intervals are approximately -3,900, 1,900, and 1,400 feet per second. The shock of February 16 -appears to have had its origin near to a point in Yedo Bay about eight -miles east of Yokohama. Assuming this to be the case, the shock between -the Yokohama homoseist and Tokio travelled at the rate of 2,454 feet -per second, but between the Tokio homoseist and Chiba at the rate -of 750 feet per second; that is to say, the velocity of propagation -rapidly decreased as the disturbance spread outwards. - -At Yokohama it was recorded at 5.31.54, at Tokio at 5.32.16, and at -Chiba at 5.33.48. These times are given in Tokio mean time. - -The shock of March 11, which was recorded at Tokio at 7.51.22 P.M. -and at Yokohama at 7.51.33 P.M., appears, from the indications of -instruments which were exceptionally definite in their records, to have -originated in the N.E. corner of Yedo Bay, about nineteen miles S.S.W. -from Chiba. This shock was rather severe, fracturing several chimneys. -From the Tokio homoseist it appears to have travelled on to Yokohama at -the rate of about 2,200 feet per second. Assuming these observations -to be _approximately_ accurate, if we take them with the records of -previous observers they lead us to the following conclusions:— - -1. Different earthquakes, although they may travel across the same -country, have very variable velocities, varying between several -hundreds and several thousands of feet per second. - -2. The same earthquake travels more quickly across districts near to -its origin than it does across districts which are far removed. - -3. The greater the intensity of the shock the greater is the velocity. - - - - - CHAPTER VI. - - EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS. - - The destruction of buildings is not irregular—Cracks in - buildings—Buildings in Tokio—Relation of destruction to - earthquake motion—Measurement of relative motion of parts of a - building shaken by an earthquake—Prevention of cracks—Direction - of cracks—The pitch of roofs—Relative position of openings in a - wall—The last house in a row—The swing of buildings—Principle of - relative vibrational periods. - - -The subject of this chapter is, from a practical point of view, one -of the most important with which a seismologist has to deal. We -cannot prevent the occurrence of earthquakes, and unless we avoid -earthquake-shaken regions, we have not the means of escaping from -them. What we can do, however, is in some degree to protect ourselves. -By studying the effects produced by earthquakes upon buildings of -different construction and variously situated, we are taught how to -avoid or at least to mitigate calamities which, in certain regions of -the world, are continually repeated. The subject is an extensive one, -and what is here said about it must be regarded only as a contribution -to the work of future writers who may give it the attention it -deservedly requires. - -_The Destruction produced by Earthquakes is not irregular._—If we -were suddenly placed amongst the ruins of a large city which had -been shattered by an earthquake, it is doubtful whether we should at -once recognise any law as to the relative position of the masses of -_débris_ and the general destruction with which we are surrounded. -The results of observation have, however, shown us that, amongst the -apparently chaotic ruin produced by earthquakes, there is in many cases -more or less law governing the position of bodies which have fallen, -the direction and position of cracks in walls, and the various other -phenomena which result from such destructive disturbances. - -Mallet, at the commencement of his first volume, describing the -Neapolitan earthquake of 1857, discusses the general effect produced by -various shocks upon differently constructed buildings. First he shows -that, if we have a rectangular building, the walls at right angles to -the shock will be more likely to be overthrown than those which are -parallel to it. Experience teaches a similar lesson. Thus Darwin, when -speaking of the earthquake at Concepcion in 1835,[20] tells us that -the town was built in the usual Spanish fashion, with all the streets -running at right angles to each other. One set ranged S.W. by W. and -N.E. by E. and the other N.W. by N. and S.E. by S. The walls in the -former direction certainly stood better than those in the latter. The -undulations came from the S.W. - -In Caraccas it is said that every house has its _laga securo_, or safe -side, where the inhabitants place their fragile property. This _laga -securo_ is the north side, and it was chosen because about two out of -every three destructive shocks traversed the city from west to east, -so that the walls in these sides of a building have been stricken -broadside on.[21] - -_Cracks in Buildings._—Results like the above come from destructive -earthquakes rather than from movements such as those we have to deal -with ordinarily. When a building is subjected to a slight movement -it is assumed that the walls at right angles to the direction of the -shock move backwards and forwards as a whole, and there is little or -no tendency for them to be fractured at their weaker parts, these -weaker parts being those over the various openings. The walls, however, -which are parallel to the direction of the movement are, so to speak, -extended and contracted along their length, and in consequence they may -be expected to give way over the various openings. This tendency for -extension and contraction of a wall along its length may be supposed, -for instance, to be due to the different portions of a wall, owing to -differences in dimensions and elasticity, having different periods of -natural vibration, or possibly for two portions of a long line of wall -to be simultaneously affected by portions of waves in different phases. - -As an illustration of the giving way of a building in the manner here -suggested we may take the case of a large brick structure which was -recently being erected in Tokio. This building, at the time of the -earthquake, was only some fourteen or fifteen feet above the surface -of the ground. The length of the building stretched from N.W. to S.E., -and it was intersected by many walls at right angles to this direction. -Through all the walls of this building there were many arched openings. -In the central part of the transverse walls, which walls were fully -five feet in thickness, the arches which joined them together were 4 -feet 4 inches in thickness. The arches therefore formed a comparatively -lightly constructed link between heavy masses of brickwork. - -On March 3, 1879, at 4.43 P.M., an earthquake was felt throughout -Tokio, the strength of which, as judged by our feelings, was above that -of an average shock. As registered by one of Palmieri’s instruments, it -had a direction S.S.W. to N.N.E. and an intensity of 11°. On the same -day there were several smaller shocks having the same direction, and -these were succeeded by others on the 9th of the month. - -Immediately after these shakings it was discovered that almost every -arch in the internal walls of the building here referred to had been -cracked across the crown in a direction about N. 40° W. All the -other arches of the building, of which there were a great number in -walls at right angles to the direction of the shock, were found not -to have sustained any injury. To this statement, however, there was -one exception, which was subsequently proved to have been due to a -settlement taking place. - -After examining these cracks the only cause to which they could be -attributed was the series of shakings which they had just experienced. -It seemed as if the heavy walls right and left of the arches had been -in vibration without synchronism in their periods, and as a consequence -the arches which connected them had been torn asunder. - -Although the time at which the cracks were formed and the peculiar -positions in which they were only to be found pointed distinctly to -their origin, to be certain that they were not due to settlement of the -foundations, horizontal lines were ruled upon the brickwork and from -time to time subsequently observed. - -The points to which the various cracks extended were also marked and -observed. Beneath the walls as foundations there were beds of concrete -about three feet thick and about ten feet in width. These had been -under the pressure of the partially built walls for two years before -the arches had been put in. As these foundations were unusually strong, -being intended to carry so very much greater weight than that to which -they had been subjected, if any settlement had been detected it would -have been a matter of surprise. - -Some weeks after the formation of these cracks it was observed that -they gradually closed. This was probably due to the gradual falling -inwards of the two broken portions of the arch, their position when -open being one of instability. - -[Illustration: FIG. 16.—Cracks in a corner house, Belluno, June 29, -1873 (Bittner).] - -Had this building been more complete at the time of the shock, and -the heavy walls been tied together at higher points, although the -archways would have been points of weakness, it is quite possible -that fracture would not have taken place. This illustration shows us -that when a building is shaken in a definite direction there will be -some rule as to the positions in which fractures occur. As another -example, we may take the observations of Alexander Bittner upon the -buildings of Belluno after the shock of June 29, 1873 (see Beiträge -zur Kenntniss des Erdbebens von Belluno am 29 Juni, 1873, p. 40. Von -Alexander Bittner. Aus dem LXIX. Bande der Sitzungsb. der K. Akad. der -Wissensch., II. Abth., April-Heft. Jahrg. 1874). - -Speaking generally, he remarks that ‘Houses similarly situated have -suffered in corresponding walls and corners in a similar manner. In -Belluno there is a certain kind of damage which is repeated everywhere, -making a peculiar system of splits in the S.W. and N.E. corners of the -houses.’ This is well shown in the accompanying sketch, which evidently -illustrates the effect of a shock oblique to the direction of two walls -at right angles to each other. - -[Illustration: FIG. 17.—Brick buildings in Tokio, showing -fractures.] - -_Buildings in Tokio._—For the purpose of finding out what has been the -effect produced by earthquakes upon the buildings of Tokio, and at -the same time for ascertaining whether blocks of buildings ranging in -different directions suffered to the same extent, the author examined, -in company with Mr. Josiah Conder, a large number of foreign-built -houses in the district of the Ginza. The chief reason for choosing -this was because it was the only district where a large number of -_similar_ buildings could be found. By examining houses or buildings of -different constructions, the effects produced upon them by earthquakes -are very often likely to show so many differences that it becomes -almost an impossibility to determine what the general effect has -been—unsymmetrical construction involving unsymmetrical ruin. - -A number of similarly constructed buildings in one locality may be -regarded as a number of seismographs, the effect upon any one of them -being judged of by the average of the general effect which has been -produced upon the whole. The general form of two of these houses -which were examined is shown in fig. 17. In this figure the general -character of the fractures which have been produced can also be seen. -The houses are built of brick, and are in many cases faced with a thin -coat of white plaster. Projecting from the level of the upper floor -there is a balcony fronted by a low balustrade. This is supported by -small beams which at their outer extremity are carried on a row of -cylindrical columns. This forms a covered way in front of each row of -houses. The roofs are covered with thick tiles. It will be observed -that the arches of the upper windows spring _sharply_ from their -abutments, and at their crown they carry a heavy key-stone. The lower -openings, which have a span of 9 feet, have evidently been constructed -in imitation of the open front of an ordinary Japanese house. These -archways curve out _gently_ from their abutments. The outside walls -have a thickness of 13½ inches. - -The results obtained from a careful examination of 174 houses in -streets running N.E. and 156 houses in streets running N.W., all of -these houses being similar, were as follows:— - -1. In the upper windows nearly all the cracks ran from the springing, -which formed an angle with the abutment. - -2. In the lower arches, which _curved_ into the abutments, not a single -crack was observed at the springway. The cracks in these arches were -near the crown, where beams projected to carry the balcony. In many -instances the cracks proceeded from such beams, even if there were -no arch beneath. That cracks should occur in peculiar positions, as -is here indicated, is shown in the illustrations which accompany the -accounts of many earthquakes. - -3. The houses which were most cracked were in the streets running -parallel to the direction in which the greater number and most powerful -set of shocks cross the city. - -The results showed that, in order to avoid the effects of small shocks, -all walls containing principal openings should be placed as nearly -as possible at right angles to the direction in which the shocks of -the districts usually travel. The blank walls, or those containing -unimportant openings, would then be parallel to the direction of the -shocks—that is, presuming our building to be made up of two sets of -walls at right angles to each other. - -Another point of importance would be to build archways _curving_ into -the supporting buttresses; the archways over doors and windows which we -find in earthquake countries do not appear to be in any way different -from those which are built in countries free from earthquakes. In -the one country these structures have simply to withstand vertical -pressures applied statically; in the other, they have to withstand more -or less horizontal stresses, applied suddenly. - -_Relation of Destruction to Earthquake Motion._—The relations which -exist between the overturning and projection of bodies and the motion -of the ground have already been discussed. It may be interesting -to call attention to the fact that in the formulæ showing three -relationships, it was the _shape_ rather than the _weight_ of a body -which determined whether it should be overturned or projected by a -motion at its base. - -As an interesting proof that light bodies may be overturned as easily -as heavy ones. Mallet refers to the overturning of several large -haystacks as one of the results of the Neapolitan earthquake. - -If masses of material are displaced or fractured, then Mallet remarks - _____ -that the maximum velocity will exceed √2_gh_, where _h_ is the -amplitude of the wave. Should the maximum velocity be less than this -quantity, the masses which are acted upon will be simply raised and -lowered, and there will be no relative displacements even if the -emergence of the wave be nearly or quite vertical. - -When we get a vertical wave acting upon an irregular mass of masonry, -the heavier portions of the masonry, by their inertia, tend to descend -relatively to the remaining portions, and in this way vertical fissures -will be produced. For this reason it would not be advisable to use -heavy materials above archways, heavy roofs, or heavy floors. The -vertical fissures, Mallet remarks, would have their widest opening at -the base. - -In considering cases of fracture produced by earthquake motion, it -must be remembered that these are due to stresses applied _suddenly_, -and that if the same amount of stress had been _slowly_ applied to a -building, fractures might not have occurred. - -If a disturbance is horizontal, and has a direction parallel to the -length of a wall, the wall is carried forward at its foundations. This -motion is opposed by the inertia of the upper portion of the wall and -the various loads it carries. The wall being elastic, distortion takes -place, and cracks, which are widest at the top, will be formed. In a -uniform wall the two most prominent fissures ought to be near the ends. - -If the horizontal backward and forward movement has a direction -oblique to the plane of the wall, the wall will be either overthrown, -fractured, or have a triangular fragment thrown off towards the origin -from the end last reached. - -Should the wave emerge steeply, diagonal fissures at right angles to -the direction of transit will be formed, or else triangular pieces will -be projected. - -The accompanying figures are reduced from Mallet’s ‘Account of the -Neapolitan Earthquake of 1857.’ - -[Illustration: FIG. 18.—Cathedral Church, Potenza (Mallet).] - -Taking _a_ _b_ as the general direction of the fractures in fig. 18, -then _c_ _d_ will represent the direction in which the shock emerged, -which is at an angle of 23°·20′ to the horizon. It might be argued that -the direction of these fractures was due to the direction in which -surface undulations had travelled, or to the relative strengths and -proportions of different portions of the building. The directions of -cracks in a building are undoubtedly due to a complexity of causes, but -for buildings situated in the region of shock the impulsive effect of -the shock is probably the most important function to be considered. -The method of applying the directions of emergence, deduced from -observations on fractures, to determine the origin of a disturbance -will be referred to in Chapter X. - -[Illustration: FIG. 19.—The Cathedral, Paterno (Mallet). -Neapolitan Earthquake of 1857.] - -Mallet observed that, although two ends of a building might be nearly -the same, the fissures and joints do not occur at equal distances from -the ends, nor are they equally opened. - -The end where the joints are the most opened is that which was first -acted upon, and this phenomenon may be sufficiently well pronounced -to indicate the direction in which we must look to find the origin -of a disturbance. Amongst possible explanations for this disposition -of fractures in a wall. Mallet suggests that they may be due to real -differences in the two semiphases of the wave of shock, the second -semiphase being described with a somewhat slower velocity than the -first. This, it will be observed, is contrary to the indications of -seismographs. - -Fig. 19, of the cathedral at Paterno, shows the effect of a subnormal -shock striking a wall obliquely and projecting one of its corners. - - - MEASUREMENTS OF THE RELATIVE MOTION OF PARTS OF A - BUILDING AT THE TIME OF AN EARTHQUAKE. - -In 1880 a series of observations was made in Tokio to determine whether -at the time of an earthquake the various parts of the arched openings -which we see in many buildings synchronised in their vibrations, or, -for want of synchronism, were caused to approach and recede from each -other. The arches experimented on were heavy brick arches forming the -two corridors of the Imperial College of Engineering. The direction of -one set of these corridors is N. 40° E. and that of the other N. 50° W. - -The thickness of the walls in which these arches are placed is 1 ft. -11 in. They are built of Japanese bricks bound together with ordinary -lime. The span of the arches is 8 ft. 3 in., and the height of the arch -from the springing-line to the crown 4 ft. 1 in. The height of the -abutments is 7 ft. 1½ in. The voussoirs of the arch are formed of a -light grey soft volcanic rock, and on their faces show a depth of 12 -inches. The width of the intermediate columns between the arches is 4 -ft. 6⅞ in. - -To determine whether at the time of an earthquake there was any -variation in the dimensions of these arches, a light stiff deal -rod, about 2 in. by ½ in. in cross section, was placed across the -springing-line of the arch. One end of this was firmly fixed to the -top of one abutment by means of a spike; on the other end, which was -to indicate any horizontal movement if the abutments approached each -other, there was fixed a pointer made out of a piece of steel wire. -This rested on a piece of smoked glass fixed to the ledge on which -the loose end of the rod was resting. If the abutments approached or -receded from each other a line would be drawn measuring the extent of -the motion. As a further indication of motion, a second smoked glass -plate was fixed on the transverse rod, which plate was marked on by a -pointer attached to a vertical rod hanging down from the crown of the -arch. - -As a general result of these experiments it may be said that the -portions of the building which were examined usually either did not -move at all, or else they practically synchronised in their movements. -When they did move, the extent of motion was small, and the small -differences in movement which were observed were in every probability -far within the elastic limits of the structure. - -_Observations on Cracks._—To determine whether the walls of a building -which have once been cracked, when subjected to a series of shocks, -similar to those which they experienced before being cracked, still -continued to give way, the extremities of a considerable number of -cracks in the N.E. end of the museum buildings of the Engineering -College were marked with pencil. Although since the time of marking -there had been many severe shocks, these cracks did not visibly extend. -These marks were made on the outside wall of the building. On the -inside, one of these same cracks showed itself as a fissure about ¼ -inch in width. Across this crack a horizontal steel wire pointer was -placed. One end of this wire was fixed in the wall; the other end, -which was pointed, rested on the surface of a smoked glass plate placed -on the other side of the crack. After small earthquakes there was no -indication of motion having taken place, but after a shock on February -21, as indicated by a line upon the smoked glass plate, it was seen -that the sides of the crack had approached and receded from each other -through a distance of about 1/16 inch. - -By similar contrivances placed on cracks in a neighbouring building -exactly similar results were obtained, namely, that during small -earthquakes the two sides of the crack had retained their relative -positions, but at the time of a large shock this position had been -changed. - -In this building it was also observed that the cracks in many instances -increased their length. - -By attaching levers to the end of the pointers to multiply any motion -that might take place, no doubt the indications would be more frequent -and more definite. It would also be easier to note the relative -distances of motion in two directions, namely, how far the cracks -had closed and how far they had opened. As to whether motion would -occur or not, much would no doubt depend upon the direction of the -earthquake. - -_Prevention of Fractures._—One conclusion which may perhaps be drawn -from these observations is, that a cracked building at the time of an -earthquake shows a certain amount of flexibility. Whether a building -which had been designed with cracks or joints between those parts -which were likely to have different periods of vibration would be more -stable, so far as earthquake shakings are concerned, than a similar -building put up in an ordinary manner, is a matter to be decided by -experiment. Certainly some of the cracks which have been examined -indicate that if they had not existed, the strain upon the portion of -the building where they occur would have been extremely great. - -_Direction of Cracks._—In looking at the cracks produced by small -earthquakes it is interesting to note the manner of their extension. -The basements of the buildings which have been most carefully examined -are, for a height of two or three feet, built of large rectangular -blocks of a greyish-coloured volcanic rock. In these parts the cracks -pass in and out between the joints of the stone, indicating that -the stones have evidently been stronger than the mortar which bound -them together, and as a consequence the latter had to give way. -Above this basement when the cracks enter the brickwork, they no -longer exclusively confine themselves to the joints, but run in an -irregular line through all they meet with, sometimes across the bricks -and sometimes through the mortar joints. In places where they have -traversed the brickwork, we can say that the mortar has been stronger -than the bricks. This traversing of the bricks rather than the joints -is, I think, the general rule for the direction of the cracks in the -brickwork of Tokio buildings. - -_The Pitch of Roofs._—From observation of the effects produced by -earthquakes, it appears to us that the houses which lost the greater -number of tiles appear to be those with the steepest pitch, and those -where the tiles were simply laid upon the roof and not in any manner -fastened down. It would seem that destruction of this sort might to a -great extent be obviated by giving the roofs a less inclination and -fixing the tiles with nails. It was also noticed that the greatest -disturbance amongst the tiles was upon the ridges of the roofs. -Destruction of this sort might be overcome by giving especial attention -to these portions during the construction of the roof. - -[Illustration: FIG. 20. FIG. 21.] - -_Relative Position of Openings in Walls._—From what has been said about -the fractures in the buildings of Tokio it will have been seen that, -with but few exceptions, they have all taken place above openings -like doorways and windows. If architecture demands that openings like -arches should be placed one above another in heavy walls of this kind, -as in fig. 17, there will be lines of weakness running through the -openings parallel to the dotted lines. As arches are only intended -to resist vertical thrusts, special construction must be adopted to -make them strong enough to resist horizontal pulls. For instance, a -flat arch would offer more resistance to horizontal pulls than an -arch put together with ordinary voussoirs, there being in the former -case more friction to prevent the component parts sliding over each -other. Or again, above each arch an iron girder or wooden lintel might -be inserted in the brick or stone arch. It was suggested to me by my -colleague, Mr. Perry, that the best form calculated to give a wall -uniform strength, would be to build it so that the openings of each -tier would occupy alternate positions, that is to say, along lines -parallel to the struts and ties of a girder. In this way we should have -our materials so arranged that they would offer the same resistance to -horizontal as to vertical movements. Such a wall is shown in fig. 20: -the dotted lines running through the openings, and all similar lines -parallel to the former, representing lines of weakness. If we compare -this with fig. 21, we shall see that in the case of a horizontal -movement _a_ _b_ or of a vertical movement _c_ _d_, we should rather -expect to find fractures in a house built like fig. 21 than in one -built like fig. 20. If, however, these two buildings were shaken by a -shock which had an angle of emergence of about 45° in the direction _e_ -_f_, the effects might be reversed. Usually, however, and always in a -town like Tokio which is visited by shocks originating at a distance, -the movements are practically horizontal ones, and, therefore, -buildings erected on the principles illustrated by fig. 20 should -be much superior, so far as resisting earthquakes is concerned, to -buildings constructed in the ordinary manner, as in fig. 21. Fractures -following a vertical line of weakness are shown in the accompanying -drawing, fig. 22, of the Church of St. Augustin, at Manilla, shattered -by the earthquakes of 1880. - -_The last House in a Row._—When an earthquake shock enters a line -of buildings, and proceeds in a direction coincident with that of -the buildings, we should expect that the last of these houses, being -unsupported on one side, would be in the position of the last person in -Tyndall’s row of boys. From this it would seem that the end house in a -row would show the greatest tendency to fly away from its neighbours. -If the last house stood upon the edge of a deep canal or a cliff, -there would be a layer of ground, equal in thickness to the depth of -the canal or to the height of the cliff, as the case may be, which -would also be in a position to be thrown forward. The effect which is -sometimes produced upon an end building is shown in fig. 23, which is -taken from the photograph of a house shattered in 1868 at San Francisco. - -[Illustration: FIG. 22.—Church of St. Augustin, Manilla. -Earthquakes of July 18–20, 1880.] - -_The Swing of Buildings._—The distance through which buildings are -moved at the time of an earthquake depends partly on their construction -and partly on the extent, nature, and duration of the movement -communicated to them at their foundations. By violent shocks buildings -may be completely overthrown. In the case of small earthquakes, the -upper portion of a house may frequently move through a much greater -distance than the ground at its foundation. For instance, during the -Yokohama earthquake of February, 1880, when the maximum amplitude of -the earth’s motion was probably under ¾ of an inch, from the slow swing -of long Japanese pictures, from three to six feet in length, which -oscillated backwards and forwards on the wall, it is very probable -that the extent through which the upper portion of houses moved was -very considerable. In some instances these pictures seem to have swung -as much as two feet, and from the manner in which they swung they -evidently synchronised with the natural swing of the house. - -[Illustration: FIG. 23.—Webber House, San Francisco. Oct. 21, 1868.] - -From this it would seem that such a house must have rocked from side to -side one foot out of its normal perpendicular position. That the motion -was great is testified by nearly all who tried to stand at the time of -the shock, it having been impossible to walk steadily across the floor -of a room in an upper story. The houses here referred to are either -those which are purely Japanese, or else those which are framed of wood -and built on European models, a class of building which is very common -in Tokio and Yokohama. - -Perry and Ayrton calculated the period of a complete natural vibration -of different structures. For a square house whose outer and inner -sections were respectively 30 and 26 feet, and whose height was 30 -feet, the period calculated would be about ·06 second. - -At the time of the above earthquake many houses seem to have moved like -inverted pendulums. On the morning after the shock my neighbour, who -was living upstairs in a tall wooden house with a tile roof, told me -that he endeavoured to count the vibrations, and was of the impression -that to make a complete swing it took about 2 seconds. - -Assuming now that the distance through which the top of a wooden house -moved was about 1 foot, and the number of vibrations which it made per -second was about ·5, then the greatest velocity of a point on the top -of such a house must have been about 6 feet per second. - -Mallet, who made observations upon the vibrations of various -structures, tells us that Salisbury spire moves to and fro in a gale -more than 3 inches. A well-constructed brick and mortar wall, 40 feet -high and 1 foot 6 inches thick, was observed to vibrate in a gale 2 -feet transversely before it fell. - -An octagonal chimney with a heavy granite capping, 160 feet high, was -observed instrumentally to vibrate at the top nearly 5 inches.[22] - -At the time of a severe earthquake it does not seem impossible but that -a building may be swung completely over. The accompanying illustration, -fig. 24, taken from a photograph,[23] apparently indicates a movement -of description. - -[Illustration: FIG. 24.—Stud Mill at Haywards, California. Oct. 21, -1868.] - -_Principle of relative Vibrational Period._—If a lath or thin pole -loaded at one end with a weight fixed to the ground, so as to stand -vertically, be shaken by an earthquake it will be caused to rock to -and fro like an inverted pendulum. The period of its swing will be -chiefly dependent on its dimensions, its elasticity, and its load. In -a building we have to consider the vibration of a number of parts, -the periods of which, if they were independent of each other, would be -different. On account of this difference in period, whilst one portion -of a building is endeavouring to move towards the right, another -is pulling towards the left, and, in consequence, either the bonds -which join them or else they themselves are strained or broken. This -was strikingly illustrated by many of the chimneys in the houses at -Yokohama, which by the earthquake of February 20, 1880, were shorn off -just above the roof. The chimneys were shafts of brick, and probably -had a slower period of vibration than the roof through which they -passed, this latter vibrating with the main portion of the house, which -was framed of wood. - -A particularly instructive example of this kind which came under my -notice is roughly sketched in fig. 25. - -[Illustration: FIG. 25.] - -This is a chimney standing alone, which, for the sake of support, was -strapped by an iron band to an adjoining building. It would seem that -at the time of the shock, the building moving one way and the chimney -another, the swing of the heavy building gave the chimney a sharp jerk -and cut it off. The upper portion, being then loose upon the lower -part, rotated under the influence of the oscillations in manner similar -to that in which gravestones are rotated. - -Mallet made observations similar to these in Italy. He tells us that a -buttress may often not have time to transmit its stability to a wall. -The wall and the buttress have different periods of vibration, and -therefore they exert impulsive actions on each other. Effects like -these were strikingly observable in many of the rural Italian churches -where the belfry tower is built into one of the quoins of the main -rectangular building. - -Not only have we to consider the relative vibrations of the various -parts of a building amongst themselves, but we have to consider the -relation of the natural vibrations of any one of them or the vibration -of the building as a whole, with regard to the earth, the vibrations of -which it must be remembered are not strictly periodic. - -Some of the more important results dependent upon the principle of -‘relative vibrational periods’ may be understood from the following -experiments:— - -[Illustration: FIG. 26.] - -In fig. 26 A, B, and C are three flat springs made out of strips of -bamboo, and loaded at the top with pieces of lead. At the bottom they -are fixed into a piece of board D E, and the whole rests on a table F -G. The legs of this table being slightly loose, by placing the fingers -on the top of it, a quick short backward and forward movement can be -produced. The weights on A and B are the same, but they are larger than -the weight on C. Consequently the periods of A and B are the same, but -different to the period of C. The dimensions of these springs are as -follows: height, 18 inches; A and B each carry weights equal to 320 -grammes, and they make one vibration per second; C has a weight of 199 -grammes, and makes 0·75 vibrations per second. - -_First Experiment._—It will be found that by giving the table a gentle -backward and forward movement, the extent of which movement may be so -small that it will be difficult to detect it with the eye, either A -and B may be made to oscillate violently whilst C remains still; or -_vice versâ_, C may be caused to oscillate whilst A and B remain still. -In the one case the period of shaking will have been synchronous with -the natural period of A and B, whilst in the latter it will have been -synchronous with that of C. This would seem to show us that if the -natural period of vibration of a house, or of parts of it, at any time -agree with the period of the shock, it may be readily thrown into a -state of oscillation which will be dangerous for its safety. - -_Second Experiment._—Bind A and B together with a strip of paper -pasted between them. (The paper used was three-eighths of an inch -broad and would carry a weight of nearly three pounds.) If the table -be now shaken as before, A and B will always have similar movements, -and tend to remain at the same distance apart, and as a consequence -the strip of paper will not be broken. From this experiment it would -seem that so long as the different portions of a building have almost -the same periods of vibration, there will be little or no strain upon -the tie-rods or whatever contrivance may be used in connecting the -different parts. - -_Third Experiment._—Join A and C, or B and C with a strip of paper in a -manner similar to the last experiment. If the table be now shaken with -a period approximating either to that of A and B, or with that of C, -the paper will be suddenly snapped. - -This indicates that if we have different portions of a building of -such heights and thicknesses that their natural periods of vibration -are different, the strain upon the portions which connect such parts -is enormous, and it would seem, as a consequence, that either the -vibrators themselves, or else their connections, must, of a necessity, -give way. This was very forcibly illustrated in the Yokohama earthquake -of February 1880 by the knocking over of chimneys. The particular case -of the chimneys is, however, better illustrated by the next experiment. - -_Fourth Experiment._—Take a little block of wood three-quarters of an -inch square and about one inch high, and place it on the top of A, B, -or C. It will be found that, although the spring on which it stands -is caused to swing backwards and forwards through a distance of three -inches, the little block will retain its position. - -This little block we may regard as the upper part of a chimney standing -on a vibrating stack, and we see that, so long as this upper portion is -light, it has no tendency to fall. - -_Fifth Experiment._—Repeat the fourth experiment, having first placed a -small leaden cap on the top of the block representing the chimney. (The -cap used only weighed a few grammes.) When vibration commences it will -be found that the block quickly falls. This would seem to indicate that -chimneys with heavy tops are more likely to fall than light ones. - -_Sixth Experiment._—Bind A and B together with a strip of paper and -stand the little block upon the top of either. It will be found that -the block will stand as in the fourth experiment. - -_Seventh Experiment._—Bind A and C, or B and C together, and place -the block upon the top of either of them. When vibration commences, -although the paper may not be broken, the little block will quickly -fall. - -_Eighth Experiment._—Take two pencils or pieces of glass tube and -place them under the board D E. If the table F G be now shaken in the -direction D E, it will be found that the springs will not vibrate. - -In a similar manner if a house or portion of a house were carried on -balls or rollers, as has already been suggested, it would seem that the -house might be saved from much vibration. - -_Ninth Experiment._—Set any of the springs in violent vibration by -gently shaking D E instead of the table, and then suddenly cease the -actuating motion. It will be observed that at the moment of cessation -the board and the springs will have a sudden and very decided motion -of translation in the same direction as that in which the springs were -last moving, and although the springs were at the time swinging through -a considerable arc, all motion will suddenly cease. - -This shows, that if a house is in a state of vibration the strain at -the foundations must be very great. - -It would not be difficult to devise other experiments to illustrate -other phenomena connected with the principle of relative vibrational -periods, but these may perhaps be sufficient to show to those who have -not considered this matter its great importance in the construction -of buildings. Perhaps the greater portion of what is here said may by -many be regarded as self-evident truisms hardly worth the trouble of -demonstration. Their importance, however, seems to be so great that I -hope that their discussion has not been altogether out of place. - -I may remark that in the rebuilding of chimneys in Yokohama the -principles here enunciated were taken advantage of by allowing the -chimneys to pass freely through the roofs without coming in contact -with any of the main timbers. - -In putting up buildings to resist the effects of an earthquake, besides -the idea of making everything strong because the earthquake is strong, -there are several principles which, like the one just enunciated, might -advantageously be followed which as yet appear to have received but -little attention. - - - - - CHAPTER VII. - - EFFECTS PRODUCED UPON BUILDINGS (continued). - - Types of buildings used in earthquake countries—In Japan, in Italy, - in South America, in Caraccas—Typical houses for earthquake - countries—Destruction due to the nature of underlying rocks—The - swing of mountains—Want of support on the face of hills—Earthquake - shadows—Destruction due to the interference of waves—Earthquake - bridges—Examples of earthquake effects—Protection of - buildings—General conclusions. - - -_Types of buildings used in earthquake countries._—In Japan there -are excellent opportunities of studying various types of buildings. -The Japanese types, of course, form the majority of the buildings. -The ordinary Japanese house consists of a light framework of 4 or -5 inch scantling, built together without struts or ties, all the -timbers crossing each other at right angles. The spaces are filled -in with wattle-work of bamboo, and this is plastered over with mud. -This construction stands on the top of a row of boulders or of square -stones, driven into the surface soil to a distance varying from a few -inches to a foot. The whole arrangement is so light that it is not an -uncommon thing to see a large house rolled along from one position to -another on wooden rollers. In buildings such as these after a series of -small earthquake shocks, we could hardly expect to find more fractures -than in a wicker basket. - -The larger buildings, such as temples and pagodas, are also built of -timber. These are built up of such a multitude of pieces and framed -together in such an intricate manner that they also are capable of -yielding in all directions. The European buildings are, of course, -made of brick and stone with mortar joints. Some of these, as the -buildings of the Ginza in Tokio, are not designed for great strength. -On the other hand, others have thick and massive walls and are equal in -strength to those we find in Europe. - -The third type of buildings are those which are built in blocks; and -these blocks being bound together with iron rods traversing the walls -in various directions are especially designed to withstand earthquakes. -A system somewhat similar to this has been patented in America, and -examples of these so-called earthquake-proof buildings are to be found -in San Francisco. - -Speaking of Japanese buildings, Mr. R. H. Brunton, who has devoted -especial attention to them says that,[24] ‘to imagine that slight -buildings, such as are seen here (i.e. in Japan), are the best -calculated to withstand an earthquake shock is an error of the most -palpable kind.’ After describing the construction of a Japanese house -in pretty much the same terms as we have used, he says ‘that with its -unnecessarily heavy roof and weak framework it is a structure of all -others the worst adapted to withstand a heavy shock.’ He tells us, -further, that these views are sustained by the truest principles of -mechanics. In order to render buildings to some extent proof against -earthquakes, some of the heavy roofs in Tokio have been so constructed -that they are capable of sliding on the walls. Mr. Brunton mentions -a design for a house, the upper part of which is to rest on balls, -which roll on inverted cups fixed on the lower part of the building, -which is to be firmly embedded in the earth. A similar design was, -at the suggestion of Mallet, used to support the tables carrying the -apparatus of some of the lighthouses erected in Japan by Mr. Brunton. -The very existence of these designs seems to indicate that the ordinary -European house, however solidly and strongly it may be built, is not -sufficient to meet the conditions imposed upon it. What is required, -is something that will give way—an approximation to the timber frame -of a Japanese house, so strongly condemned by Mr. Brunton and others. -The crucial test of the value of the Japanese structure, as compared -with the modern buildings of brick and stone, is undoubtedly to be -found by an appeal to the buildings themselves. So far as my own -experience has gone, I must say that I have never seen any signs in -the Japanese timber buildings which could be attributed to the effects -of earthquakes, and His Excellency Yamao Yozo, Vice Minister of Public -Works, who has made the study of the buildings of Japan a speciality, -told me that none of the temples and palaces, although many of them -are several centuries old, and although they have been shaken by small -earthquakes and also by many severe ones, show any signs of having -suffered. The greatest damage wrought by large earthquakes appears to -have resulted from the influx of large waves or from fires. In every -case where an earthquake has been accompanied by great destruction, -by consulting the books describing the same, it can be seen, from the -illustrations in these books portraying conflagrations, that this -destruction was chiefly due to fire. When we remember that nearly -all Japanese houses are constructed of materials that are readily -inflammable, it is not hard to imagine how destruction of this kind -has come about. To a Japanese, living as he does in a house which has -been compared to a tinder-box, fire is one of his greatest enemies, -and in a city like Tokio it is not at all uncommon to see during the -winter months many fires which sweep away from 100 to 500 houses. In -one winter I was a spectator of three fires, each of which was said to -have destroyed upwards of 10,000 houses. - -Although it would appear that the smaller earthquakes of Japan produce -no visible effect upon the native buildings, it is nevertheless -probable that small effects may have been produced, the observation -of which is rendered difficult by the nature of the structure. If we -look at buildings of foreign construction, by which are meant buildings -of brick and stone, the picture before us is quite different, and -everywhere the effects of earthquakes are palpable even to the most -casual observer. Of these effects numerous examples have already been -given. Not only are these buildings damaged by the cracking of walls -and the overturning of chimneys, but they also appear to be affected -internally. For instance, in the timbers of the roof of the museum -attached to the Imperial College of Engineering in Tokio, there are a -number of diagonal faces acting as struts or ties intended to prevent -more or less horizontal movements taking place. Those which are rigidly -joined together with bolts and angle irons have apparently suffered -from their rigidity, being twisted and bent into various forms. The -buildings in Tokio, which are strongly put together, being especially -designed to withstand earthquakes, appear to have suffered but little. -I know only one example which at the time of the severe shock of 1880 -had several of its chimneys damaged. - -The ordinary houses in Italy, though built of stone and mortar, are -but poorly put together, and, as Mallet has remarked, are in no way -adapted to withstand the frightful shakings to which they are subjected -from time to time. - -In the large towns, like Naples, Rome, and Florence, where happily -earthquakes are of rare occurrence, although the building may be -better than that found in the country, the height of the houses and -the narrowness of the streets are sufficient to create a shudder, when -we think of the possibility of the occurrence of a moderately severe -earthquake. - -In South America, although many buildings are built with brick -and stone, the ordinary houses, and even the larger edifices, are -specially built to withstand earthquakes. In Mr. James Douglas’s -account of a ‘Journey Along the West Coast of South America,’ we read -the following[25]: ‘The characteristic building material of Guayaquil -is bamboo, which grows to many inches in thickness, and which, when -cut partially through longitudinally at distances of an inch or so, -and once quite through, can be opened out into fine elastic boards of -serviceable width. Houses, and even churches, of a certain primitive -beauty are built of such reeds, so bound together with cords that few -nails enter into the construction, and which, therefore, yield so -readily to the contortions of the earth during an earthquake as to be -comparatively safe.’ - -Here we have a house, which, so far as earthquakes are concerned, is -an exaggerated example of the principles which are followed in the -construction of an ordinary Japanese dwelling. - -Another plan adopted in South America can be gathered from the same -author’s writings upon Lima, about which he says, ‘To build high houses -would be to erect structures for the first earthquake to make sport of, -and, therefore, in order to obtain space, safety, and comfort, the -houses of the wealthy surround court after court, filled with flowers, -and cooled with fountains, connected one with another with wide -passages which give a vista from garden to garden.’ - -History would indicate that houses of this type have been arrived at as -the results of experience, for it is said that when the inhabitants of -South America first saw the Spaniards building tall houses, they told -them they were building their own sepulchres.[26] - -In Jamaica, we find that even as early as 1692 experience had taught -the Spaniards to construct low houses, which withstood shakings better -than the tall ones.[27] - -In Caraccas, which has been called the city of earthquakes, it is -said that the earthquakes cause an average yearly damage amounting to -the equivalent of a _per capita_ tax of four dollars. To reduce this -impost to a minimum much attention is paid to construction. ‘Projecting -basement corners (giving the house a slightly pyramidal appearance) -have been found better than absolutely perpendicular walls; mortised -corner-stones and roof beams have saved many lives when the central -walls have split from top to bottom; vaults and key-stone arches, no -matter how massive, are more perilous than common wooden lintels, and -there are not many isolated buildings in the city. In many streets -broad iron girders, riveted to the wall, about a foot above the house -door, run from house to house along the front of an entire square. -Turret-like brick chimneys, with iron top ornaments, would expose the -architect to the vengeance of an excited mob; the roofs are flat, -or flat terraced; the chimney flues terminate near the eaves in a -perforated lid.’[28] - -_Typical houses for earthquake countries._—From what has now been said -about the different buildings found in earthquake countries, it will be -seen that if we wish to put up a building able to withstand a severe -shaking, we have before us structures of two types. One of these types -may be compared with a steel box, which, even were it rolled down a -high mountain, would suffer but little damage; and the other, with a -wicker basket, which would equally withstand so severe a test. Both -of these types may be, to some extent, protected by placing them upon -a loose foundation, so that but little momentum enters them at their -base. One suggestion is to place a building upon iron balls. Another -method would be to place them upon two sets of rollers, one set resting -upon the other set at right angles. The Japanese, we have seen, place -their houses on round stones. The solid type of building is expensive, -and can only be approached partially, whilst the latter is cheap, and -can be approached closely. In the case of a solid building it would be -a more difficult matter to support it upon a movable foundation than in -the case of a light framework. Such a building is usually firmly fixed -on the ground, and consequently at the time of an earthquake, as has -already been shown by experiment, must be subjected to stresses which -are very great. In consequence also of the greater weight of the solid -structure, more momentum will enter it at its base than in the case of -the light structure. Also, we must remember that the rigidity favours -the transmission of momentum, and with rigid walls we are likely to -have ornaments, coping-stones, and the comparatively freer portions -forming the upper part of a building displaced; whilst, with flexible -walls absorbing momentum in the friction of their various parts, such -disturbances would not be so likely. Mr. T. Ronaldson, referring to -this, says, that in 1868, at San Francisco, the ornamental stone work -in stone and cement buildings was thrown from its position, whilst -similar ornaments in neighbouring brick buildings stood. - -To reduce the top weight of a building, hollow bricks might be -employed. To render a building more homogeneous and elastic, the -thickness of bricks might be reduced. Inasmuch as the elasticity -of brick and timber are so different, the two ought to be employed -separately. For internal decorations plaster mouldings might be -replaced by _papier mâché_ and _carton-pierre_, the elastic yielding of -which is comparatively great.[29] Houses, whether of brick and stone, -or of timber, ought to be broad and low, and the streets three or four -times as wide as the houses. The flatter the roofs the better. - -One of the safest houses for an earthquake country would probably be a -one-storied strongly framed timber house, with a light flattish roof -made of shingles or sheet-iron, the whole resting on a quantity of -small cast-iron balls carried on flat plates bedded in the foundations. -The chimneys might be made of sheet-iron carried through holes free of -the roof. The ornamentation ought to be of light materials. - -At the time of severe earthquakes many persons seek refuge from their -houses by leaving them. In this case accidents frequently happen from -the falling of bricks and tiles. Others rush to the doorways and -stand beneath the lintels. Persons with whom the author has conversed -have suggested that strongly constructed tables and bedsteads in -their rooms would give protection. To see persons darting beneath -tables and bedsteads would undoubtedly give rise to humiliating and -ludicrous exhibitions. This latter idea is not without a value, and -most certainly, if applied in houses of the type described, would be -valuable. - -The great danger of fire may partially be obviated by: the use of -‘earthquake lamps,’ which are so constructed that before they overturn -they are extinguished. It is said that in South America some of the -inhabitants are ready at any moment to seek refuge in the streets, -and they have coats prepared, stocked with provisions and; other -necessaries, which, if occasion demands, will enable them to spend the -night in the open air. These coats, called ‘earthquake coats,’ might -also, with properly constructed houses, be rendered unnecessary. - -_Destruction due to the nature of the underlying rocks._—That the -nature of the ground on which a building stands is intimately related -with the severity of the blow it receives is a fact which has often -been demonstrated. - -One cause of destruction is due to placing a building on foundations -which are capable of receiving the full effects of a shock, and -transmitting it to the buildings standing on them. - -For instance, the reason why a soft bed might possibly make a good -foundation, is, as has been pointed out by Messrs. Perry and Ayrton, -because the time of transmission of momentum is increased; in fact, the -soft bed is very like a piece of wood interposed between a nail and -the blows of a hammer—it lengthens the duration of impact. For this -reason we are told that a quaking bog will make a good foundation. When -a shock enters loose materials its waves will be more crowded, and it -is possible that a line of buildings may rest on more than one wave -during a shock. There are many examples on record of the stability of -buildings which rested on beds of particular material at the time of -destructive earthquakes. As the observations which have been made -by various writers on this subject appear to point in a contrary -direction, I give the following examples:— - -In the great Jamaica earthquake of 1692, the portions of Port -Royal which remained standing were situated on a compact limestone -foundation; whilst those on sand and gravel were destroyed (‘Geological -Observer,’ p. 426). Again, on p. 148 of the same work, we read, -‘According to the observations made at Lisbon, in 1737, by Mr. Sharpe, -the destroying effects of this earthquake were confined to the tertiary -strata, and were most violent on the blue clay, on which the lower part -of the city is constructed. Not a building on the secondary limestone -or on the basalt was injured.’ - -In the great earthquakes of Messina, those portions of the town -situated on alluvium, near the sea, were destroyed, whilst the high -parts of the town, on granite, did not suffer so much. Similar -observations were made in Calabria, when districts consisting of -gravel, sand, and clay became, by the shaking, almost unrecognisable, -whilst the surrounding hills of slate and granite were but little -altered. At San Francisco, in 1868, the chief destruction was in the -alluvium and made ground. - -At Talcahuano, in 1835, the only houses which escaped were the -buildings standing on rocky ground; all those resting on sandy soil -were destroyed. - -From the results of observations like these, it would seem the harder -rocks form better foundations than the softer ones. The explanation of -this, in many cases, appears to lie in the fact that the soft strata -were in a state of unstable equilibrium, and by shaking, they were -caused to settle. Observations like the following, however, point out -another reason why soft strata may sometimes afford a bad foundation. - -‘Humboldt observed that the Cordilleras, composed of gneiss and -mica-slate, and the country immediately at their foot, were more shaken -than the plains.’[30] - -‘Some writers have asserted that the wave-like movements (of the -Calabrian earthquake in 1783) which were propagated through recent -strata from west to east, became very violent when they reached the -point of junction with the granite, as if a reaction was produced when -the undulatory movement of the soft strata was suddenly arrested by the -more solid rocks.’ - -Dolomieu when speaking of this earthquake says, the usual effect ‘was -to disconnect from the sides of the Apennines all those masses (of sand -and clay) which either had not sufficient bases for their bulk, or -which were supported only by lateral adherence.’ - -These intensified actions taking place at and near to lines of -junction between dissimilar strata is probably due to the phenomena of -reflection and refraction. - -When referring to the question as to whether buildings situated on -loose materials suffered more or less than those on solid rocks, -Mallet, in his description of the Neapolitan earthquake of 1857, -remarks: ‘We have in this earthquake, towns such as Saponara and -Viggiano, situated upon solid limestone, totally prostrated; and we -have others such as Montemarro, to a great extent based upon loose -clays, totally levelled. We have examples of almost complete immunity -in places on plains of deep clay as that of Viscolione, and in places -on solid limestone, like Castelluccio, or perched on mountain tops like -Petina.’[31] - -After reading the above, we see that the probable reason why, in -several cases, beds of soft materials have not made good foundations, -consists in the fact that they have either been of small extent or else -have been observed only in the neighbourhood of lines which divided -them from other formations, which lines are always those of great -disturbances. - -At the end of his description of the Neapolitan earthquake of 1857, -Mallet says that more buildings were destroyed on the rock than on the -loose clay. This, however, he remarks, is hardly a fact from which we -can draw any valuable deductions, because it so happened that more -buildings were constructed on the hills than on the loose ground.[1] - -Professor D. S. Martin, writing on the earthquake of New England in -1874, remarks that in Long Island the shock was felt where there was -gneiss between the drift. Around portions to the east the observations -were few and far between. He also remarks that generally the shocks -were felt more strongly and frequently on rocky than on soft ground.[32] - -From these examples, it would appear that the hard ground, which -usually means the hills, forms a better foundation than the softer -ground, which is usually to be found in the valleys and plains. Other -examples, however, point to a different conclusion. For instance, a -civil engineer, writing about the New Zealand earthquake of 1855, when -all the brick buildings in Wellington were overthrown, says that ‘it -was most violent on the sides of the hills at those places, and least -so in the centre of the alluvial plains.’[33] - -In this example it must be noticed that the soft alluvium here referred -to was of large extent, and not loose material resting on the flanks of -rocks, from which it was likely to be shaken down, as in most of the -previous examples. - -The results of my own observations on this subject point as much in one -direction as in the other. In Tokio, from instrumental observations -upon the slopes and tops of hills, the disturbance appears to be -very much less than it is in the plains. Thus, at my house, situated -on the slope of a hill about 100 feet in height, for the earthquake -of March 11, 1882, I obtained a maximum amplitude of motion of from -three to four millimètres only, whilst Professor Ewing, with a similar -instrument, situated on the level ground at about a mile distant, -found a motion of fully seven millimètres. This calculation has been -confirmed by observations on other earthquakes. Thus, for instance, in -the destructive earthquake of 1855, when a large portion of Tokio was -devastated, it was a fact, remarked by many, that the disturbance was -most severe on the low ground and in the valleys, whilst on the hills -the shock had been comparatively weak. As another illustration, I may -mention that within three-quarters of a mile from my house in Tokio -there is a prince’s residence which has so great a reputation for the -severity of the shakings it receives that its marketable value has been -considerably depreciated, and it is now untenanted. - -In Hakodadi, which is a town situated very similarly to Gibraltar, -partly built on the slope of a high rocky mountain and partly on a -level plain, from which the mountain rises, the rule is similar to -that for Tokio, namely, that the low, flat ground is shaken more -severely than the high ground. At Yokohama, sixteen miles south-west -from Tokio, the rule is reversed, as was very clearly demonstrated -by the earthquake of February 1880, when almost every house upon the -high ground lost its chimney, whilst on the low ground there was -scarcely any damage done; the only places on the low ground which -suffered were those near to the base of the hills. The evidence as to -the relative value of hard ground as compared with soft ground, for -the foundation of a building, is very conflicting. Sometimes the hard -ground has proved the better foundation and sometimes the softer, and -the superiority of one over the other depends, no doubt, upon a variety -of local circumstances. - -These latter observations open up the inquiry as to the extent to which -the intensity of an earthquake may be modified by the topography of the -disturbed area. - -_The swing of mountains._—If an earthquake wave is passing through -ground the surface of which is level, so long as this ground is -homogeneous, as the wave travels further and further we should expect -its energy to become less and less, until, finally, it would insensibly -die out. If, however, we have standing upon this plain a mountain, -judging from Mallet’s remarks, this mountain would be set in a state -of vibration much in the same way as a house is set in vibration, -and it would tend to oscillate backward and forward with a period of -vibration dependent upon the nature of its materials, size, and form. -The upper portion of this mountain would, in consequence, swing through -a greater arc than the lower portion, and buildings situated on the top -of it would swing to and fro through a greater arc than those which -were situated near its foot. This explanation why buildings situated -on the top of a mountain should suffer more than those situated on -a plain, is one which was offered by Mallet when writing of the -Neapolitan earthquake. He tells us that towns on hills are ‘rocked as -on the top of masts,’ and if we accept this explanation it would, in -fact, be one reason why the houses situated on the Bluff at Yokohama -suffered more than those situated in the settlement. This explanation -is given on account of the great authority it claims as a consequence -of its source. It is not clear how the statement can be supported, -as different portions of the mountain receive momentum in opposite -directions at the same time. - -_Want of support on the faces of hills._—When a wave of elastic -compression is propagated through a medium, we see that the energy of -motion is being continually transmitted from particle to particle of -that medium. A particle, in moving forwards, meets with an elastic -resistance of the particles towards which it moves, but, overcoming -these resistances, it causes these latter particles to move, and in -turn to transmit the energy to others further on. So long as the medium -in which this transfer of energy is continuous, each particle has a -limit to its extent of motion, dependent on the nature of the medium. -When, however, the medium, which we will suppose to be the earth, is -not continuous, but suddenly terminates with a cliff or scarp, the -particles adjacent to this cliff or scarp, having no resistance offered -to their forward motion, are shot forward, and, consequently, the -ground here is subjected to more extensive vibrations than at those -places where it was continuous. This may be illustrated by a row of -marbles lying in a horizontal groove; a single marble rolled against -one end of this row will give a concussion which will run through the -chain, like the bumping of an engine against a row of railway cars, and -as a result, the marble at the opposite end of the row, being without -support, will fly off. Tyndall illustrates the same thing with his -well known row of boys, each one standing with his arms stretched out -and his hands resting upon the shoulders of the boy before him. A push -being given to the boy at the back, the effect is to transmit a push -to the first boy, who, being unsupported, flies forward. - -In the case of some earthquakes, most disastrous results have occurred -which seem only to admit of an explanation such as this. A remarkable -instance of this kind occurred when the great earthquake of 1857 ‘swept -along the Alps from Geneva to the east-north-east, and its crest -reached the edge of the deep glen between Zermatt and Visp. Then the -upper part of the wave-movement, a thousand or two thousand feet in -depth from the surface, came to an end; the forward pulsation acted -like the breaker of the sea, and heavy falls of rock encumbered the -western side of the valley.’ - -_Earthquake shadows._—If a mountain stands upon a plain through which -an elastic wave is passing, which is almost horizontal, the mountain -is, so to speak, in the _shadow_ of such a wave. If we only consider -the normal motion of this wave, we see that the only motion which the -mountain can obtain will be a wave of elastic distortion produced -by a shearing force along the plain of the base. Should, however, -the wave approach the mountain from below, and emerge into it at a -certain angle, only the portion of the mountain on the side from -which the wave advanced could remain in shadow, whilst the portion -on the opposite side would be thrown into a state of compression and -extension. Portions in shadow, however, would be subject to waves -of elastic distortion. In a manner similar to this we may imagine -that certain portions of the bluff, so far as the advancing wave was -concerned, were in shadow, and thus saved from the immediate influence -of the direct shock. A hypothetical case of such a shadow is shown in -the accompanying section, illustrating the contour of the ground at -Yokohama. The situation which might be in the shadow of one shock, -however, it is quite possible might not be in that of another. We -must also remember that a place in shadow for a direct shock might be -affected by reflected waves, and also by the transverse vibrations of -the direct shock. These effects are over and above the effects produced -by the waves of elastic distortion just referred to. It might be asked -whether whole countries, like England, which are but seldom shaken, are -in shadow. - -[Illustration: FIG. 27.—Hypothetical section at Yokohama.] - -_Destruction due to the interference of waves._—Referring to the -section of the ground at Yokohama (Fig. 27), it will be seen that -both the settlement and the bluff stand upon beds of gravel capping -horizontal beds of grey tuff. The gravel of that portion of the -settlement on the seaboard originally formed the line of a shingle -beach. That portion of the settlement back from the sea stands upon -ground which was originally marshy. In the central portions of the -settlement this bed of gravel is very thick, perhaps 100 feet or so, -but as you near the edge of the bluff it probably becomes thinner, -until it finally dies out upon the flanks of the scarps. - -On the top of the bluff, the beds of gravel will, in every probability, -be generally thinner than they are upon the lower level. The beds of -tuff, which is a soft grey-coloured clay-like rock, produced by the -solidification of volcanic mud, appear, when walking on the seaboard, -to be horizontally stratified. If there is a dip inland, it is in all -probability very slight. Here and there the beds slightly faulted. -Taken as a whole we may consider these beds as being tolerably -homogeneous, and an earthquake in passing through them would meet -with but little reflection or refraction. At the junction of these -beds with the overlying gravels, both reflection and refraction would -comparatively be very great. - -On entering the gravel, as the wave would be passing into a less -elastic medium, the direction of the wave would be bent towards the -perpendicular to the line of junction, and the angle of emergence at -the surface would consequently be augmented. At the surface certain -reflection would also take place, but the chief reflections would be -those at the junction of the tuff and the alluvium. - -Under the settlement it is probable that all the reflections which -took place would be single. Thus wave fronts like A_{1} advancing in -a direction parallel to the line _a__{1}; would be reflected in a -direction _a__{2} and give rise to a series of reflected waves A_{2}. -These are shown by thicker lines. Similarly all the neighbouring waves -to the right and left of A_{1} would give rise to a series of reflected -waves. If the lines drawn representing wave fronts are districts of -compression, then, where two of the lines cross each other, there would -be double energy in producing compression. Similarly, districts of -rarefaction might accord, and, again, compression of one wave might -meet with the rarefaction of another and a neutralisation of effect -take place. A diagram illustrating concurrence and interference of this -description is given in Le Conte’s ‘Elements of Geology,’ p. 115. The -interference which has been spoken of, however, is not the greatest -which would occur. The greatest would probably be beneath the bluff and -the scarps which run down to join the level ground below. This would be -the case because it is a probability that there might not only be cases -of interference of single reflected waves, but also of waves which -had been not only twice but perhaps thrice reflected. For example, a -wave like B_{1} (which is parallel to A_{1} of the first supposition), -advancing in a direction parallel to _b__{1} might be reflected along -the line _b__{2} giving rise to waves like B_{2}, which in turn might -be reflected along _b__{3} giving rise to waves like B_{3}. The number -of districts where there would be concurrence and interference would, -in consequence of the number of times waves might be reflected, be -augmented. Here the violence of the shock would, at certain points, be -considerably increased, but as a general result energy must be lost, -so that even if some of the reflected waves found their way into the -portion we have regarded as being in shadow, their intensity would not -be so great as if they had entered it directly. - -The shaking down of loose materials from the sides of hills may be -partially explained on the assumption of an increased disturbance due -to interference. - -_Earthquake bridges._—In certain parts of South America there appear -to exist tracts of ground which are practically exempt from earthquake -shocks, whilst the whole country around is sometimes violently shaken. -It would seem as if the shock passes beneath such a district as water -passes beneath a bridge, and for this reason these districts have been -christened ‘bridges.’ - -This phenomenon appears to depend upon the nature of the underlying -soil. When an elastic wave passes from one bed of rock to another of a -different character, a certain portion of the wave is reflected, while -the remainder of it is transmitted and refracted, and ‘bridges’ we -may conceive of as occurring where the phenomenon of total reflection -occurs. - -In the instances given of soft materials having proved good -foundations, it was assumed that they had chiefly acted as absorbers -of momentum. They have also acted as reflecting surfaces, and where no -effects have been felt by those residing on them, this may have been -the result of total reflection, and the soft beds thus have played the -part of bridges. - -Fuchs gives an example taken from the records of the Syrian earthquake -of 1837, where not only neighbouring villages suffered differently, but -even neighbouring houses. In one case a house was entirely destroyed, -whilst in the next house nothing was felt. - -In Japan, at a place called Choshi, about 55 miles east of the capital, -earthquakes are but seldom felt, although the surrounding districts may -be severely shaken. - -From descriptions of this place it would appear that there is a large -basaltic boss rising in the midst of alluvial strata. The immunity from -earthquakes in this district has probably given rise to the myth of -the Kanam rock, which is a stone supposed to rest upon the head of a -monstrous catfish (Namadzu), which by its writhings causes the shakings -so often felt in this part of the world.[34] - -Prof. D. S. Martin, writing on the earthquake of New England in 1874, -says that it was felt at four points; it was felt in the heart of -Brooklyn all within a circle of half a mile across; ‘and this fact -would suggest that a ridge of rock perhaps approaches the surface at -that point, though none is known to appear.’[35] - -The subject of special districts, which are more or less protected from -severe shakings, will be again referred to, and it will be seen that -after a seismic survey has been made even of a country like Japan, -where there are on the average at least two earthquakes per day, it -is possible to choose a place to build in as free from earthquakes as -Great Britain. - -_General examples of earthquake effects._—The following examples of -earthquake effects are drawn from Mallet’s account of the Neapolitan -earthquake of 1857. - -At a town called Polla there was great destruction. Judging from -the fissures in the parts that remained standing it seemed that the -emergence of the shock had been more vertical in the upper part of the -town than in the lower, proving that whatever had been the angle below, -the hill had itself vibrated, which, being horizontal, had modified the -angle of the fissures. - -Diano suffered but little, partly because it was well built, and partly -on account of its situation, which was such that before the shock -reached it the disturbance had to pass from beds of clay into nearly -vertically placed beds of limestone. Also a great portion of the shock -was cut off by the Vallone del Raccio to the north and north-west of -the town. Here the effects of the partial extinction of the wave on the -‘free outlaying stratum’ were visible in the masses of projected rock. - -Castellucio did not suffer because its well buttressed knoll was end -on to the direction of shock, and on account of a barrier of vertical -breccia beds protecting it upon the east. - -Pertosa stands on a mound. The destruction was least in the southern -part of the town. From the relation of the beds of breccia on which the -town stands, and the direction of the wave path, it is evident that the -southern part of the town received the force of the shock through a -greater thickness of the breccia beds than the other parts did. - -Petina, standing on a level limestone spur jutting out from a mountain -slope, suffered nothing, whilst Anletta five miles to the south-west, -and Pertosa six miles distant, were in great part prostrated. (1) The -terrace did not vibrate, and (2) between Petina and Anletta there is -almost 6,000 feet of piled up limestone, so that any shock emergent at -a steep angle had to pass up transversely through these beds. - -_Protection of buildings._—In addition to giving proper construction -to our buildings, choosing proper foundations and positions for them, -something might possibly be done to ward off the destructive effects of -an earthquake. We read that the Temple of Diana at Ephesus was built on -the edge of a marsh, in order to ward off the effect of earthquakes. -Pliny tells us that the Capitol of Rome was saved by the Catacombs, -and Elisée Reclus[36] says that the Romans and Hellenes found out that -caverns, wells, and quarries retarded the disturbance of the earth, -and protected edifices in their neighbourhood. The tower of Capua was -saved by its numerous wells. Vivenzis asserts that in building the -Capitol the Romans sunk wells to weaken the effects of terrestrial -oscillations. Humboldt relates the same of the inhabitants of San -Domingo. - -Quito is said to receive protection from the numerous cañons in the -neighbourhood, whilst Lactacunga, fifteen miles distant, has often been -destroyed. - -Similarly, it is extremely probable that many portions of Tokio have -from time to time been protected more or less from the severe shocks of -earthquakes by the numerous moats and deep canals which intersect it. - -Although we are not prepared to say how far artificial openings of this -description are effectual in warding off the shocks of earthquakes, -from theoretical considerations, and from the fact that their use has -been discovered by persons who, in all probability, were without the -means of making theoretical deductions, the suggestions which they -offer are worthy of attention. - -_General conclusions._—The following are a few of the more important -results which may be drawn from the preceding chapter:— - -1. In choosing a site for a house find out by the experience of others -or experimental investigation the localities which are least disturbed. -In some cases this will be upon the hills, in others in the valleys and -on the plains. - -2. A wide open plain is less likely to be disturbed than a position on -a hill. - -3. Avoid loose materials resting on harder strata. - -4. If the shakings are definite in direction, place the blank walls -parallel to such directions, and the walls with many openings in them -at right angles to such directions. - -5. Avoid the edges of scarps or bluffs, both above and below. - -6. So arrange the openings in a wall, that for horizontal stresses the -wall shall be of equal strength for all sections at right angles. - -7. Place lintels over flat arches of brick or stone. - -8. To withstand destructive shocks either rigidly follow one or other -of the two systems of constructing an earthquake-proof building. The -light building on loose foundations is the cheaper and probably the -better. - -9. Let all portions of a building have their natural periods of -vibration nearly equal. - -10. If it is a necessity that one portion of a building should have a -very different period of vibration to the remainder, as for instance a -brick chimney in a wooden house, it would seem advisable either to let -these two portions be sufficiently free to have an independent motion, -or else they must be bound together with great strength. - -11. Avoid heavy topped roofs and chimneys. If the foundations were free -the roof might be heavy. - -12. In brick or stone work use good cement. - -13. Let archways curve into their abutments. - -14. Let roofs have a low pitch, and the tiles, especially those upon -the ridges, be well secured. - - - - - CHAPTER VIII. - - EFFECTS OF EARTHQUAKES ON LAND, - - 1. Cracks and fissures—Materials discharged from - fissures—Explanation of fissure phenomena. 2. Disturbances in - lakes, rivers, springs, wells, fumaroles, &c.—Explanation of - these latter phenomena. 3. Permanent displacement of ground—On - coast lines—Level tracts—Among mountains—Explanation of these - movements. - - -_Cracks and fissures formed in the ground._—Almost all large -earthquakes have produced cracks in the ground. The cracks which were -found in the ground at Yokohama (February 22, 1880) were about two or -three inches wide, and from twenty to forty yards in length. They could -be best seen as lines along a road running near the upper edge of some -cliffs which overlook the sea at that place. The reason that cracks -should have occurred in such a position rather than in others was -probably owing to the greater motion at such a place, due to the face -of the cliff being unsupported, and there being no resistance opposed -to its forward motion. It often happens that earthquake cracks are many -feet in width. At the Calabrian earthquake of 1783, one or two of the -crevasses which were formed were more than 100 feet in width and 200 -feet in depth. Their lengths varied from half a mile to a mile.[37] -Besides these large cracks, many smaller ones of one or two feet in -breadth and of great length were formed. In the large fissures many -houses were engulfed. Subsequent excavations showed that by the closing -of the fissures these had been jammed together to form one compact -mass. These cracks are usually more or less parallel, and at the same -time parallel to some topographical feature, like a range of mountains. -For example, the cracks which were formed by the Mississippi earthquake -of 1812 ran from north-east to south-west parallel to the Alleghanies. -By succeeding shocks these crevasses are sometimes closed and sometimes -opened still wider. Their permanency will also depend upon the nature -of the materials in which they are made. - -During an earthquake large cracks may suddenly open and shut. - -During the convulsions of 1692 which destroyed Port Royal, it is said -that many of the fissures which were formed, opened and shut. In some -of these, people were entirely swallowed up and buried. In others they -were trapped by the middle, and even by the neck, where if not killed -instantaneously they perished slowly. Subsequently their projecting -parts formed food for dogs.[38] - -The earthquake which, July 18, 1880, shook the Philippines caused many -fissures to be found, which in some places were so numerous that the -ground was broken up into steps. Near to the village of San Antonio the -soil was so disturbed that the surface of a field of sugar-canes was -so altered that in some cases the top of one row of full grown plants -was on a level with the roots of the next. Into one such fissure a boat -disappeared, and into another, a child. - -Subsequently the child was excavated, and its body, which was found a -short distance below the surface, was completely crushed.[39] - -At the time of the Riobamba earthquake, not only were men engulfed, -but animals, like mules, also sank into the fissures which were formed. - -The fissures which were formed at the time of the Owen’s Valley -earthquake in 1872 extended for miles nearly parallel to the -neighbouring Sierras. In some places the ground between the fissures -sank twenty or thirty feet, and at one place about three miles east of -Independence, a portion of the road was carried eighteen feet to the -south by a fissure.[40] - -Speaking generally, it may be said that all large earthquakes are -accompanied by the formation of fissures. The Japanese have a saying -that at the time of a large earthquake persons must run to a bamboo -grove. - -The object of this is to escape the danger of being engulfed in -fissures, the ground beneath a bamboo grove being so netted together -with fine roots that it is almost impossible for it to be rent open. - -_Materials discharged from fissures._—Together with the opening of -cracks in the earth it often has happened that water, mud, vapours, -gases, and other materials, have been ejected. - -At the time of the Mississippi earthquake water, mixed with sand and -mud, was thrown out with such violence that it spurted above the tops -of the highest trees. In Italy such phenomena have often been repeated. - -From the fissures which were formed in 1692 at the time of the -earthquakes in Sicily, water issued which in some instances was -salt.[41] - -By the Cachar earthquake (January 10, 1869) numerous fissures were -formed parallel to the banks of a river, from this water and mud were -ejected. Dr. Oldham, who describes this earthquake, says that the -first shot of dry mud or sand was mistaken for smoke or steam. The -water was foul, and hotter than surface water at the time, but only -slightly so; and the sulphurous smell was nothing more than you would -perceive in stirring up the mud at the bottom of any stagnant pool -which had lain undisturbed for some time.[42] - -In 1755, when Tauris was destroyed, boiling water issued from the -cracks which were formed. Similar phenomena were witnessed at a place -eight miles from La Banca in Mexico, in the year 1820. Part of this hot -water was pure and part was muddy. - -Sometimes the water which has been ejected has been so muddy that the -mud has been collected to form small hills. This was the case at the -time of the Riobamba earthquake. The mud in this case consisted partly -of coal, fragments of augite, and shells of infusoria. - -At the time of the Jamaica earthquake men who had fallen into crevices -were in some cases thrown out again by issuing water. - -Sometimes, as has already been mentioned, vapour, gases, and even -flames issue from fissures. Vapour of sulphur appears to be exceedingly -common. Kluge says that many fish were killed in consequence of the -sulphurous vapours which rose in the sea near to the coast of New -Zealand in 1855. - -On December 14, 1797, an insupportable smell of sulphur was observed to -have accompanied the earthquake which at that time shook Cumana, which -was greatest when the disturbance was greatest. - -Sulphurous fumes which were combustible were belched out of the -earth at the time of the Jamaica earthquake in 1692. The smell which -accompanied this was so powerful that it caused a general sickness -which swept away about 3,000 persons.[43] - -From the fissures formed at Concepcion in 1835, water, which was black -and fœtid, issued.[44] - -The earthquakes of New England in 1727 were accompanied by the -formation of fissures, from which sand and water boiled out in -sufficient quantity to form a quagmire. In some places ash and sulphur -are said to have been ejected. - -At one house the stink of sulphur accompanying the earthquake was so -great that the family could not bear to remain in doors.[45] - -Emanations of gas sometimes appear to have burst out from submarine -sources. - -Thus the earthquake at Lima, in March, 1865, was accompanied with great -agitation of the water and an odour of sulphuretted and carburetted -hydrogen. This former gas was developed to such an extent that the -white paint of the U.S. ship ‘Lancaster’ was blackened.[46] With the -smell, flames have sometimes been observed, as, for instance, at the -time of the Lisbon earthquake. - -At the time of the earthquakes of 1811 and 1813, in the Mississippi -valley, steam and smoke issued from some of the fissures which were -formed. - -Instances are recorded where stones have been shot up from fissures -unaccompanied by water, as, for instance, at the earthquake of Pasto -(January, 1834). It is imagined that the propelling power must have -been the sudden expansion of escaping gases. - -It has been suggested that flames seen above fissures might perhaps -be due to the burning of materials like sulphur. Mr. D. Forbes, who -examined the effects of the earthquakes of Mendoza, which were felt for -a distance of 1,200 miles, says that where the hard rock came to the -surface there were no traces of fissures, these being entirely confined -to the alluvium. The rumours of fire and smoke having appeared at some -of the fissures were without foundation, the presumed smoke being -nothing but dust.[47] - -In addition to flames lights appear often to have been observed, the -origin of which cannot be easily explained. - -The earthquake of November 22, 1751, at Genoa is said to have been -accompanied by a light like that of a prodigious fire which seemed to -arise out of the ground.[48] - -_Explanation of fissure phenomena._—The manner in which fissures are -formed has already been explained when referring to the want of support -in the face of hills (page 136). - -Similar remarks may be applied to the banks of rivers and all -depressions, whether natural or artificial, which have a steep slope. -At such places the wave of shock emerges on a free surface, which, -being unsupported in the direction of its motion, tends to tear itself -away from the material behind, and form a fissure parallel to the face -of the free surface. The distance of the fissure from the face of the -free surface will, theoretically, be equal to half the amplitude of the -wave of motion, one half tending to move forwards, and the other half -backwards. The reason that water and other materials rush forth from -fissures has been explained by Schüler as being due to cracks having -been opened through impervious strata, which, before the earthquake, -by their continuity prevented the rising of subterranean water under -hydrostatic pressure.[49] - -Kluge explains the coming up of the waters as being due to the same -causes which he considers may be the origin of disturbances in the sea. - -The most reasonable explanations of the eruption of water, mud, sand, -and gas through fissures are those given by Oldham and Mallet in their -account of the Cachar earthquake. - -In the case of a horizontal shock passing through a bed of ooze or -water-bearing strata, the elastic wave will tend to pack up the water -during the forward motion to such an extent that it will flow or -spout up through any aperture communicating with the surface. By the -repetition of these movements causing ejections, sand or mud cones, -like those produced by a volcanic eruption, may be formed, and by a -similar action water may be shot violently up out of wells, as was the -case in Jamaica in 1692. - -If an emergent wave acts through a water-bearing bed upon a -superincumbent layer of impervious material, this upper layer is, -during the upward motion, by its inertia suddenly pressed down upon the -latter. - -This pressure is equal to that which would raise the upper layer to -a height equal to the amplitude of the motion of an earth particle, -and with a velocity at least equal to the mean velocity of the earth -particle resolved in the vertical direction. - -For a moment the water-bearing strata receive an enormous squeeze, and -the water or mud starts up through any crevice which may be formed -leading to the surface. - -From this we see that liquids may rise far beyond the level due to -hydrostatic pressure.[50] - -Volger has attributed the origin of lights or flames appearing above -fissures to the friction which must take place between various rocky -materials at the time when the fissures are opened. As confirmatory of -this he refers to instances where similar phenomena have been observed -at the time of landslips. At the time of these landslips the heat -developed by friction has been sufficiently intense to convert water -into steam, the tension of which threw mud and earth into the air like -the explosion of a mine.[51] - -The gas eruptions which occasionally take place with earthquakes are -probably due to the opening of fissures communicating with reservoirs -or strata charged with products of natural distillation, or chemical -action, which previously had accumulated beneath impervious strata. Of -the existence of such gases we have abundant evidence. In coal mines we -have fire damp which escapes in increased quantities with a lowering of -the barometrical pressure. In volcanic regions we have many examples of -natural springs of carbon dioxide. - -These various gases sometimes escape in quantity, or erupt without -the occurrence of earthquakes. Rossi mentions an instance where a few -years ago quantities of fish were killed by the eruption of gas in the -Tiber, near Rome. Another instance is one which occurred at Follonica -on April 6, 1874. On the morning of that day many of the streets and -roads were covered with the dead bodies of rats and mice. It seemed as -if it had rained rats. From the facts that the bodies of the creatures -seemed healthy, that the destruction had happened suddenly, and not -come on gradually like an epidemic, it was supposed that they had been -destroyed by an emanation of carbon dioxide. The fact that many of them -lay in long lines suggested the idea that they had been endeavouring -to escape at the time of the eruption.[52] If we can suppose sudden -developments of gas like this to have occasionally accompanied -earthquakes, we may sometimes have the means of accounting for the -sickness which has been felt. - -_Disturbances in lakes._—It has often been observed that, at the time -of large earthquakes, lakes have been thrown into violent agitation, -and their waters have been raised or lowered. At the time of the great -Lisbon earthquake, not only were the waters of European lakes thrown -into a state of oscillation, but similar effects were produced in the -great lakes of North America. In some instances, as in the case of -small ponds, these movements may be produced by the horizontal backward -and forward motion of the ground. At other times they are probably due -to an actual tipping of a portion of their basins. Movements like these -latter will be again referred to in the chapter on Earth Pulsations. -On January 27, 1856, there was a shock of earthquake at Bailyborough, -Ireland, which occasioned an adjacent lough to overflow its banks -and rush into the town with great impetuosity. In returning it swept -away two men, leaving behind a great quantity of pike and eels of a -prodigious growth.[53] - -_Disturbances in rivers._—Just as lakes have been disturbed, so also -have there been sudden disturbances in rivers. Sometimes these have -overflowed their banks, whilst at other times they have been suddenly -dried up. In certain cases the reason that a portion of a river should -have become dry has been very apparent, as, for instance, at the time -of the Zenkoji earthquake in Japan in 1847, when the Shikuma-gawa -became partly dry in consequence of the large masses of earth which -had been shaken down from overhanging cliffs damming a portion of -its course, and thus forming, first, lakes, and subsequently, new -water-courses. As another example, out of the many which might be -quoted, may be mentioned the sudden drying up of the river Aboat, -a tributary of the Magat, in the Philippine Islands, on July 27, -1881, shortly after a severe shock of earthquake. The water of this -river ceased to flow for two hours, after which it reappeared with -considerable increase of volume and of a reddish colour. Signor E. A. -Casariego, who describes this, remarks that the phenomenon could easily -be explained through the slipping down of the steep banks in narrow -parts of its upper valley, by which means its flow had been obstructed -until the water had time to accumulate and pass over or demolish the -obstruction. - -After the earthquake of Belluno (June 29, 1873), the torrent Tesa, -which is ordinarily limpid, became very muddy.[54] Similar phenomena -have been observed even in Britain, as, for instance, in 1787, when, at -the time of a shock which was felt in Glasgow, there was a temporary -stoppage in the waters of the Clyde. Again, in 1110, there was a -dreadful earthquake at Shrewsbury and Nottingham, and the Trent became -so low at Nottingham that people walked over it. - -The earthquake of 1158, which was felt in many parts of England, was -accompanied by the drying up of the Thames, which was so low that it -could be crossed on foot even at London.[55] - -Facts analogous to these are mentioned in the accounts of many large -earthquakes. Sometimes rivers only come muddy or change their colour. -In an account of the Lisbon earthquake we read that some of the rivers -near Neufchâtel suddenly became muddy.[56] - -At other times large waves are formed. Thus the earthquake of Kansas -(April 24, 1867) apparently created a disturbance in the rivers at -Manhattan, which rolled in a heavy wave from the north to the south -bank.[57] - -Sometimes curious phenomena have happened with regard to rivers without -the occurrence of earthquakes. Thus, for instance, on November 27, -1838, there was a simultaneous stoppage of the Teviot, Clyde, and Nith. - -In these rivers similar phenomena have been observed in previous years. - -Again, on January 1, 1755, there was a sudden sinking of the river -Frooyd, near Pontypool. This appears to have been due to the water -sinking into chasms which were suddenly opened.[58] - -_Effects produced in springs, wells, fumaroles, &c._—Springs also are -often affected by earthquakes. Sometimes the character of their waters -change; those which were pure become muddy, whilst those which were hot -have their temperature altered. - -Sometimes springs have been dried up, whilst at other times new springs -have been formed. - -This latter was the case in New England (October 27, 1727). In some -places springs were formed, whilst at other places they were either -entirely or partly dried up.[59] - -At and near Lisbon, in 1755, some fountains became muddy, others -decreased, others increased, and others dried up. At Montreux, Aigle, -and other places, springs became turbid. - -The baths at Toplitz, in Bohemia, which were discovered in -A.D. 762, were seriously affected by the same earthquake. -Previous to the earthquake it is said that they had always given a -constant supply of hot water. At this time, however, the chief spring -sent up vast quantities of water and ran over. One hour before this -it had grown turbid and flowed muddy. After this it stopped for about -one minute, but recommenced to flow with prodigious violence, driving -before it considerable quantities of reddish ochre. Finally, it settled -back to its original clear state and flowed as before.[60] - -In 1855, at the earthquake of Wallis, many new springs burst forth, and -some of these in Nicolai Thale were so rich in iron that they quickly -formed a deposit of ochre. - -At the time of the Belluno earthquake (June 29, 1873), a hot spring, La -Vena d’Oro, suddenly became red.[61] - -The following examples of like changes are taken from the writings of -Fuchs.[62] - -In 1738 the hot springs of St. Euphema rose considerably in their -temperature. - -During the earthquake of October, 1848, the hot springs of Ardebil, -which usually had a temperature of from 44° to 46° C., rose so high -that their temperature was sufficient to cause scalding. - -At the time of the earthquake of Wallis, in 1855, the temperature of -hot springs rose 7°, and the quantity of water increased three times. - -During the earthquake of 1835 in Chili, the springs of Cauquenes fell -from 118° to 92° F. Subsequently, however, they again rose. - -Fumaroles are similarly disturbed. Thus, at the time of the earthquakes -of Martinique (September, 1875), the fumaroles there showed an abnormal -activity.[63] - -Wells often appear to be acted upon in the same manner as springs. - -At the time of the California earthquake (April, 1855), the level of -the water in certain wells was raised ten to twelve feet. - -A consequence of the earthquake at Neufchâtel, in 1749, was to fill -some of the wells with mud.[64] At Constantinople, on September 2, -1754, wells became dry.[65] - -_Explanation of the above phenomena._—That the water in springs and -wells should be caused to rise at the time of an earthquake, admits of -explanation on the supposition of compressions taking place similar -to those which cause the rise of water in fissures. That the water in -wells and springs should be rendered turbid, is partly explained on the -supposition of more or less dislocation taking place in the earthy or -rocky cavities in which they are contained or through which they flow. - -At the time of a large earthquake it is extremely probable that there -is a general disturbance in the lines of circulation of subterranean -waters and gases throughout the shaken area. By these disturbances, new -waters may be brought to the surface, two or more lines of circulation -may be united, and the flow of a spring or supply of a well be -augmented. Fissures, through which waters reached the surface, may be -closed, wells may become dry, or springs may cease to flow, hot springs -may have their temperature lowered by the additions of cold water -from another source, and, in a similar manner, waters may be altered -in their mineralisation. An important point to be remembered in this -consideration is the mutual dependence of various underground water -supplies, and the area over which any given supply may circulate. A -well in the higher part of Lincoln Heath is said to be governed by the -river Trent, which is ten miles distant; when the river rises the well -rises in proportion, and when the river falls the water in the well -falls.[66] - -The change which is usually observed in hot springs is, that before or -with earthquakes they increase in temperature, but afterwards sink back -to their normal state. This increase in temperature may possibly be due -to communication being opened with new or deeper centres of volcanic -activity, or a temporarily increased rate of flow. - -That the water issuing from newly formed fissures or springs should -be hot, might be explained on the supposition of its arising from a -considerable depth, or from some volcanic centre. It might also be -attributed to the heat developed by friction at the opening of the -fissures. These changes which earthquakes produce upon the underground -circulation of waters are phenomena deserving especial attention. -Although we know much about the circulation of surface water, it is but -little that we yet know about the movement of the streams hidden from -view, from which these surface waters have their sources. Earthquakes -may be regarded as gigantic experiments on the circulatory system of -the earth, which, if properly interpreted, may yield information of -scientific and utilitarian value. - -The sudden elevations, depressions, or lateral shifting of large tracts -of country at the time of destructive earthquakes are phenomena -with which all students of geology are familiar. In most cases these -displacements have been permanent; and evidences of many of the -movements which occurred within the memory of man, remain as witnesses -of the terrible convulsions with which they were accompanied. - -_Movements on coast lines and level tracts._—At the time of the -great earthquake of Concepcion, on February 20, 1835, much of the -neighbouring coast line was suddenly elevated four or five feet -above sea level. This, however, subsequently sank until it was only -two feet. A rocky flat, off the island of Santa Maria, was lifted -above high-water mark, and left covered with ‘gaping and putrefying -mussel-shells, still attached to the bed on which they had lived.’ The -northern end of the island itself was raised ten feet and the southern -extremity eight feet.[67] - -By the earthquake of 1839, the island of Lemus, in the Chonos -Archipelago, was suddenly elevated eight feet.[68] - -Of movements like these, especially along the western shores of South -America, Darwin, who paid so much attention to this subject, has given -many examples. In 1822, the shore near Valparaiso was suddenly lifted -up, and Darwin tells us that he heard it confidently asserted ‘that a -sentinel on duty, immediately after the shock, saw a part of a fort -which previously was not within the line of his vision, and this would -indicate that the uplifting was not vertical.’[69] That the large areas -of land should be shifted permanently in horizontal directions, as well -as vertically, we should anticipate from the observations which we are -able to make upon large fissures which are caused by earthquakes. - -Another remarkable example of sudden movement in the rocky crust is -that which took place during the earthquakes of 1811–12 in the valley -of the Mississippi, near to the mouth of the Ohio, which was convulsed -to such a degree, that lakes, twenty miles in extent, were formed -in the course of an hour. This country, which is called the ‘sunk -country,’ extends some seventy to eighty miles north and south, and -thirty miles east and west.[70] - -In the ‘Gentleman’s Magazine’ we read of the little territory of -Causa Nova, in Calabria, being sunk twenty-nine feet into the earth -by an earthquake, without throwing down a house. The inhabitants, -being warned by a noise, escaped into the fields, and only five were -killed.[71] - -Other examples of these permanent dislocations of strata are to be -found in almost every text-book on geology. - -_Geological changes produced._—Passing over the accounts of earth -movements which are more or less fictitious, and confining our -attention to the well authenticated facts, we see at once the important -part which earthquakes have played as agents working geological -changes. Even in the nineteenth century long tracts of coast, as in -Chili and New Zealand, have been raised, whilst other areas, like the -Delta of the Indus, have been sunk. Sir H. Bartle Frere, speaking about -the disturbance which took place in his latter region in 1819, remarks -that all the canals drawn from the Fullalee River ceased to run for -about three days, probably indicating a general upheaval of the lower -part of the canal. In consequence of the earthquakes in former times -it is not unlikely that water-courses have ceased to flow, water has -decreased in wells, and districts have been depopulated.[72] - -Sometimes these changes have taken place gradually and sometimes with -violence. Mountains have been toppled over, valleys have been filled, -cities have been submerged or buried. - -With the records of these convulsions before us, we see that seismic -energy yet exhibits a terrible activity in changing the features of the -globe. - -_Reason of these movements._—To formulate a single reason for these -catastrophes would be difficult. Where they are of the nature of -landslips, or materials have been dislodged from mountain sides, the -cause is evidently the sudden movement of this ground acting upon -strata not held together in a sufficiently stable condition. A similar -explanation may be given for the sudden elevations or depressions of -strata in a district removed from the centre where the disturbance had -its origin. The seismic effort exhibits itself in a certain area round -its origin as a sudden push, and by this push, strata are fractured and -caused to move relatively to each other. - -At or near to the origin of an earthquake it might be argued that it -was the sudden falling of rocky strata towards a position of stable -equilibrium that caused the shaking, and in such a case the movements -referred to may be regarded as the cause rather than the effect of an -earthquake. - -A subject closely connected with the sudden dislocation of strata, -is the production of secondary or consequent earthquakes, due to the -disturbance of ground in a critical state (see p. 248). - - - - - CHAPTER IX. - - DISTURBANCES IN THE OCEAN. - - Sea vibrations—Cause of vibratory blows—Sea waves: Preceding - earthquakes; Succeeding earthquakes—Magnitude of waves—Waves - as recorded in countries distant from the origin—Records on - tide gauges—Waves without earthquakes—Cause of waves—Phenomena - difficult of explanation—Velocity of propagation—Depth of the - ocean—Examples of calculations—Comparison of velocities of - earthquake waves with velocities which ought to exist from the - known depth of the ocean. - - -_Sea vibrations._—Whilst residing in Japan I have had many -opportunities of conversing with persons who had experienced -earthquakes when on board ships, and it has often happened that these -same earthquakes have been recorded on the shore. For example, at the -time of every moderately severe earthquake which has shaken Yokohama, -the same disturbance has been felt on board the ships lying in the -adjoining harbour. In some cases the effect had been as if the ship -was grounding; in others, as if a number of sharp jerks were being -given to the cable. The effect produced upon a man-of-war lying in the -Yokohama harbour on the evening of March 11, 1881, was described to me -as a ‘violent irresistible shaking.’ Vessels eighty miles at sea have -recorded and timed shocks which were felt like sudden blows. These were -accompanied by a noise described as a ‘dull rattle like thunder.’ - -In none of the cases here quoted was any disturbance of the water -observed. - -The great earthquake of Lisbon was felt by vessels on the Atlantic, -fifty miles away from shore. - -On February 10, 1716, the vessels in the harbour of New Pisco were so -violently shaken that both ropes and masts were broken, and yet no -motion in the water was observed. Some have described these shocks like -those which would be produced by the sudden dropping of large masses of -ballast in the hold of the vessel. Other cases are known where rigging -was damaged, and even cannon have been jerked up and down from the -decks on which they rested. - -_Cause of vibratory blows._—From the rattling sound which has -accompanied some of these submarine shocks, many of which, it may be -remarked, have never been recorded as earthquakes upon neighbouring -shores, it does not seem improbable that they may have been the result -of the sudden condensation of volumes of steam produced by submarine -volcanic eruptions. - -As confirmatory of this supposition we have the fact, that many of -the marine disturbances which might be called ‘sea-quakes,’ have been -observed in places which are close to, or in the line of, volcanic -vents. Thus, M. Daussy, who has paid special attention to this subject, -has collected evidence to show that a large number of shocks have been -felt by vessels in that portion of the Atlantic between Cape Palamas, -on the west coast of Africa, and Cape St. Roque, on the east coast of -South America.[73] - -Some of the vessels only felt shocks and tremblings, but others saw -smoke, and some even collected floating ashes. In considering the -submarine shocks of this particular area, we must bear in mind that it -lies in the line of Iceland, the west coast of Scotland, the Azores, -Canaries, Cape de Verd Islands, St. Helena, and other places, all of -which, if not at present in volcanic activity, shew evidence of having -been so within recent times. The connection between volcanic action and -earthquakes will be again referred to. - -_Sea waves._—Although in the above-mentioned instances sea waves have -not been noticed, it is by no means uncommon to find that destructive -earthquakes have been accompanied by waves of an enormous size, which, -if the earthquake has originated beneath the sea, have, subsequently -to the shaking, rolled in upon the land, to create more devastation -than the actual earthquake. It may, however, be mentioned that a few -exceptional cases exist when it is said that the sea wave has preceded -the earthquake, as, for example, at Smyrna, on September 8, 1852. - -Again, at the earthquake in St. Thomas, in 1868, it is said that the -water receded shortly _before_ the first shock. When it returned, after -the second shock, it was sufficient to throw the U.S. ship ‘Monagahela’ -high and dry.[74] - -Another American ship, the ‘Wateree,’ was also lost in 1868 by being -swept a quarter of a mile inland by the sea wave which inundated -Arequippa. - -Much of the great destruction which occurred at the time of the great -Lisbon earthquake was due to a series of great sea waves, thirty to -sixty feet higher than the highest tide, which swamped the town. These -came in about an hour after the town had been shattered by the motion -of the ground. - -The first motion in the waters was their withdrawal, which was -sufficient to completely uncover the bar at the mouth of the Tagus. -At Cadiz, the first wave, which was the greatest, is said to have been -sixty feet in height. Fortunately the devastating effect which this -would have produced was partially warded off by cliffs. - -At the time of the Jamaica earthquake (1692) the sea drew back for a -distance of a mile. - -In South America sea waves are common accompaniments of large -earthquakes, and they are regarded with more fear than the actual -earthquakes. - -On October 28, 1724, Lima was destroyed, and on the evening of that day -the sea rose in a wave eighty feet over Callao. Out of twenty-three -ships in the harbour, nineteen were sunk, and four others were carried -far inland. The first movement which is usually observed is a drawing -back of the waters, and this is so well known to precede the inrush of -large waves, that many of the inhabitants in South America have used it -as a timely warning to escape towards the hills, and save themselves -from the terrible reaction which, on more than one occasion, has so -quickly followed. - -At Caldera, near to Copiapo, on May 9, 1877, which was the time when -Iquique was devastated, the first motion which was observed in the sea -was that it silently drew back for over 200 feet, after which it rose -as a wave over five feet high. At some places the water came in as -waves from twenty to eighty feet in height. - -At Talcahuano, on the coast of Chili, in 1835, there was a repetition -of the phenomena which accompanied the destruction of Penco in 1730 -and 1751. About forty minutes after the first shock, the sea suddenly -retired. Soon afterwards, however, it returned in a wave twenty feet -high, the reflex of which swept everything towards the sea. These -phenomena were repeated three times.[75] - -When Callao and Lima were destroyed, in 1746, the sea first drew back, -then came in as waves, four or five minutes after the earthquake. -Altogether, between October 28 and February 24, 451 shocks were -counted. At one time the sea came in eighty feet above its usual -level. One account says that the large waves came in forty-one and a -half hours after the first shock, and seventeen and a half hours after -comparative tranquillity had prevailed.[76] - -At the eruption of Monte Nuovo, near Naples, in September 27, 1538, the -water drew back forty feet, so that the whole gulf of Baja became dry. - -In 1696, at the time of the Catanian earthquake, the sea is said to -have gone back 2,000 fathoms. Instances are recorded where the sea has -receded several miles. - -The time taken for the flowing back of the sea is usually very -different. Sometimes it has only been five or six minutes, whilst at -other times over half an hour, and there are records where the time is -said to have been still longer. - -Thus, at the earthquake of Santa (June 17, 1678), the sea is stated to -have gone as far back as the eye could reach, and did not rise again -for twenty-four hours, when it flooded everything. - -In 1690 at Pisco the sea went back two miles, and did not return for -three hours. When it returns it does so with violence, and examples of -the heights to which it may reach have been given. The greatest sea -wave yet recorded, according to Fuchs, is one which, on October 6, -1737, broke on the coast of Lupatka, 210 feet in height. - -There are, however, cases known where the sea has returned as gradually -as it went out. Thus, on December 4, 1854, when Acapulco was -destroyed, the sea is said to have returned as gently as it went out. - -When sea waves have travelled long distances from their origin, as, -for instance, whenever a South American wave crosses the Pacific to -Japan, the phenomena which are observed are like those which were -observed at Acapulco; the sea falls and rises, at intervals of from ten -minutes to half an hour, to heights of from six to ten feet, without -the slightest appearance of a wave. Its phenomenon is like that of an -unusually high tide, which repeats itself several times per hour. Even -if we watch distant rocks with a telescope, although the surface of the -ocean may be as smooth as the surface of a mirror, there is not the -slightest visible evidence of what is popularly called a wave. The sea -being once set in motion it continues to move as waves of oscillation -for a considerable time. In 1877, as observed in Japan, the motion -continued for nearly a whole day. The period and amplitude of the rise -and fall were variable, usually it quickly reached a maximum, and then -died out gradually. As observed in a self-recording tide gauge at San -Francisco, the disturbance lasted for about four days. A diagram of -this is here given. In its general appearance it is very similar to the -records of other earthquake waves. The large waves represent the usual -six hours rise and fall of the tides; usually these are fairly smooth -curves. Superimposed on the large waves are the smaller zigzag curves -of the earthquake disturbance, lasting with greater or less intensity -for several days. As these curves are drawn to scale—horizontally for -hours, and vertically one fifth inch to the foot, to show the extent of -the rise and fall—they will be easily understood. - -Sometimes, as in the present example, the first movement in the -waters is that of an incoming wave. In many instances, however, this -observation may be due to the slow and more gentle phenomena of the -previous drawing out of the water, which in a steep waste, or when the -water is rough, would be difficult to observe, not having been remarked. - -The distance to which these sea waves have extended has usually been -exceedingly great. - -[Illustration: FIG. 28.—Record of Tide Gauge at Port Point, San -Francisco. Showing Earthquake Waves of May 1877.] - -The sea wave of the Iquique earthquake of May 9, 1877, like many of its -predecessors, was felt across the basin of the whole Pacific, from New -Zealand in the south, to Japan and Kamschatka in the north. And but for -the intervention of the Eurasian and American continents would have -made itself appreciable over the surface of the whole of our globe. At -places on the South American coast, it has been stated that the height -of the waves varied from twenty to eighty feet. At the Samoa Islands -the heights varied from six to twelve feet. In New Zealand the sea rose -and fell from three to twenty feet. In Australia the heights to which -the water oscillated were similar to those observed in New Zealand. In -Japan it rose and fell from five to ten feet. In this latter country, -the phenomena of sea waves which follow a destructive earthquake on the -South American coast are so well known that old residents have written -to the local papers announcing the probability of such occurrences -having taken place some twenty-five hours previously in South America. -In this way news of great calamities has been anticipated, details of -which only arrived some weeks subsequently. Just as the destructive -earthquakes of South America have announced themselves in Japan, in -a like manner the destructive earthquakes of Japan have announced -themselves upon the tide gauges of California. Similarly, but not so -frequently, disturbances shake the other oceans of the world. - -For example, the great earthquake of Lisbon propagated waves to the -coasts of America, taking on their journey nine and a half hours. - -_Sea waves without earthquakes._—Sometimes we get great sea waves -like abnormal tides occurring without any account of contemporaneous -earthquakes. Although earthquakes have not been recorded, these ill -understood phenomena are usually attributed to such movements. - -Several examples of these are given by Mallet. Thus, at 10 A.M. on -March 2, 1856, the sea rose and fell for a considerable distance -at many places on the coast of Yorkshire. At Whitby, the tide was -described ebbing and flowing six times per hour, and this to such a -distance that a vessel entering the harbour was alternately afloat and -aground. - -In 1761, on July 17, a similar phenomena was observed at the same place. - -A like occurrence took place at Kilmore, in the county of Wexford, on -September 16, 1864, when the water ebbed and flowed seven times in the -course of two hours and a half. These tides, which appear to have taken -about five minutes to rise and five minutes to fall, were seen by an -observer approaching from the west as six distinct ridges of water. The -general character of the phenomena appears to have been very similar to -that which was produced at the same place by the Lisbon earthquake of -1755; and the opinion of those who saw and wrote about their occurrence -was that it was due to an earthquake disturbance. Such phenomena are -not uncommon on the Wexford coast, where they are popularly known as -‘death waves,’ probably in consequence of the lives which have been -lost by these sudden inundations. - -They have also been observed in other parts of Ireland, the north-east -coast of England, and in many parts of the globe. They will be again -referred to under the head of earth pulsations. - -_Cause of sea waves._—Mallet, who in his report to the British -Association in 1858, writes upon this last-mentioned occurrence at -considerable length, whilst admitting that many may have originated -from earthquakes, he thinks it scarcely probable that an earthquake -blow, sufficiently powerful to have produced waves like those observed -at Kilmore, should not have been felt generally throughout the south of -Ireland. He, therefore, suggests that sometimes waves like the above -might be produced by an underwater slippage of the material forming -the face of a submarine bank, the slope of which by degradation and -deposition, produced by currents, had reached an angle beyond the -limits of repose of the material of which it was formed. Mallet does -not insist upon the existence of these submarine landslips, but only -suggests their existence as a means of explaining certain abnormal sea -waves which do not appear to have been accompanied by earthquakes. - -In the generality of cases sea waves are accompanied by earthquakes, -but it may often happen that the connection between the two is -difficult to clearly establish. One simple explanation for the origin -of waves occurring with earthquakes, is, that in consequence of the -earthquake a large volume of water suddenly finds its way into cavities -which have been opened, the disturbance produced by the inrush giving -rise to waves. - -A second explanation is, that the land along a shore is caused by an -earthquake to oscillate upwards, the water running off to regain its -level. A supposition like this is negatived by the fact that these -disturbances are felt far away from the chief disturbance, on small -islands. Also, it may be added, that the whole disturbance appears to -approach the land from the sea, and not in the opposite direction. -Thus, in the earthquake of Oahu (February 18, 1871), it was remarked -that the shock was first felt by the ships farthest from the land.[77] - -Another suggestion is that the waves are due to a sudden heaving up -of the bottom of the ocean. If this lifting took place slowly, then -the first result would be that the water situated over the centres of -disturbance would flow away radially in all directions from above the -area of disturbance. - -If, however, the submarine upheaval took place with great rapidity, say -by the sudden evolution of a large volume of steam developed by the -entry of water into a volcanic vent, as the water was heaped above the -disturbed area, water might run in radially towards this spot. - -Supposing a primary wave to be formed in the ocean by any such causes, -then the falling of this will cause a second wave to be formed, -existing as a ring round the first one. The combined action of the -first and second wave will form a third one, and so the disturbance, -starting from a point, will radiate in broadening circles. During the -up and down motion of these waves, the energy which is imparted to any -particle of water will, on account of the work which it has to do in -displacing its neighbours, by frictional resistance, gradually grow -less and less, until it finally dies away. The waves which are the -result of this motion will also grow less and less. - -If a series of sea waves were produced by a single disturbance, we see -that these will be of unequal magnitude. Now, for small waves, the -velocity with which they travel depends upon the square root of their -lengths; but with large waves, like earthquake waves, the velocity -depends upon the square root of the depth of water, and these latter -travel more quickly than the former. - -If, therefore, we have a series of disturbances of unequal magnitude -producing sea waves, which, from the series of shocks which have -been felt upon shores subsequently invaded by waves, seems in all -probability often to have been the case, it is not unlikely that the -waves of an early disturbance may be overtaken and interfered with by a -series which followed. - -These considerations help us to understand the appearance of the -records on our tide gauges, and also the phenomena observed by those -who have recorded tidal waves as they swept inwards upon the land. For -instance, we understand the reason why sea waves, as observed at places -at different distances from the origin of a disturbance, should be of -different heights. We also see an explanation for the fact that small -waves should sometimes appear to be interpolated between large ones, -and that these should occur at varying intervals. - -The fact that whenever a wave is produced, a certain quantity of water -must be drawn from the level which surrounds it, in order that it -should be formed, explains the phenomena that the sea is often observed -first to draw back. Out in the open ocean it is drawn from the hollow -between two waves. As has been pointed out by Darwin, it is like the -drawing of the water from the shore of a river by a passing steamer. - -The difference in the height of waves, as observed at places lying -close to each other, is probably due to the configuration of the coast, -the interference of outlying islands, reefs, &c.—causes which would -produce similar effects in the height of tide. - -As a wave approaches shallow water it gradually increases in height, -its front slope becomes steep, and its rear slope gentle, until finally -it topples over and breaks. This increasing in height of waves is -no doubt connected with the destruction of Talcahuano and Callao, -which are situated at the head of shallow bays. Valparaiso, which is -on the edge of deep water, has never been overwhelmed.[78] Another -case tending to produce anomalies in the character of waves would be -their reflection and mutual interference, the reflections due to the -configuration of the ocean bed and coast lines. - -The complete phenomena which may accompany a violent submarine -disturbance are as follows:— - -By the initial impulse of explosion or lifting of the ground, a ‘great -sea wave’ is generated, which travels shorewards with a velocity -dependent upon its size and the depth of the ocean. At the same -instant, a ‘sound wave’ may be produced in the air, which travels at a -quicker rate than the ‘great sea wave.’ A third wave which is produced, -is an ‘earth wave,’ which will reach the shore with a velocity -dependent on the intensity of the impulse and the elasticity of the -rocks through which it is propagated. This latter, which travels the -fastest, may carry on its back a small ‘forced sea wave.’ On reaching -the shore and passing inland, this ‘earth wave’ will cause a slight -recession of the water as the ‘forced sea wave’ slips from its back. - -As these ‘forced sea waves’ travel they will give blows to ships -beneath which they may pass, being transmitted from the bottom of the -ocean to the bottom of the ships like sound waves in water. At the -time of small earthquakes, produced, for example, by the explosion of -small quantities of water entering volcanic fissures, or by the sudden -condensation of steam from such a fissure entering the ocean, aqueous -sound waves are produced, which cause the rattling and vibrating jars -so often noticed on board ships. - -_Phenomena difficult of explanation._—Although we can in this way -explain the origin and phenomena of sea waves, we must remember, as -Kluge has pointed out, that it is not the simple backward and forward -movement of the ground which produces sea waves, and that the majority -of earthquakes which have occurred in volcanic coasts have been -unaccompanied by such phenomena. Out of 15,000 earthquakes observed on -coast lines, only 124 were accompanied by sea waves.[79] Out of 1,098 -earthquakes catalogued by Perrey for the west coast of South America, -only nineteen are said to have been accompanied by movements in the -waters. According to the ‘Geographical Magazine’ (August 1877, p. 207), -it would seem that out of seventy-one severe earthquakes which have -occurred since the year 1500 upon the South American coast many have -been accompanied by sea waves. Darwin also remarks, when speaking of -South America, that almost every large earthquake has been accompanied -by considerable agitation in the neighbouring sea.[80] - -On April 2, 1851, when many towns in Chili were destroyed, the sea was -not disturbed. At the time of the great earthquake of New Zealand (June -23, 1855), although all the shocks came from the sea, yet there was no -flood. The small shock of February 14, however, was accompanied by a -motion in the sea. - -To these examples, which have been chiefly drawn from the writings of -Fuchs, must be added the fact that the greater number of disturbances -which are felt in the north-eastern part of Japan, although they -emanate from beneath the sea, do not produce any visible sea waves. -They are, however, sufficient to cause a vibratory motion on board -ships situated near their origin. - -Another point referred to by Fuchs, as difficult of explanation, -is, that the water, when it draws back, often does so with extreme -slowness, and farther, in some instances, it has not returned to its -original level. That the sea might be drawn back for a period of -fifteen or thirty minutes is intelligible, when we consider the great -length of the waves which are formed. Cases where it has retired for -several hours or days, and when its original level is altered, appear -only to be explicable on the assumption of more or less permanent -changes in the levels of the ground. For example, in the earthquake -of 1855 which shook New Zealand, the whole southern portion of the -northern island was raised several feet. - -These sudden alterations in the levels of coast lines have already -been referred to. - -Other points which are difficult to understand are the occurrence -of disturbances in the sea at the time of feeble earthquakes, and -with earthquakes occurring in distant places. As examples of such -occurrences, Fuchs quotes the following: ‘On May 16, 1850, at 4.28 -A.M., an earthquake took place in Pesth, and at 7.30 a motion was -observed in the sea at Livorno. Again, at the time of the earthquake -of December 19, 1850, which shook Heliopolis, a flood suddenly came -in upon Cherbourg.’ May not these phenomena be the result of an earth -pulsation, which produced an earthquake at one point, and a sea wave at -another? - -Equally difficult to understand are the observations when the -disturbance in the sea has occurred several hours after an earthquake; -as, for instance, at Batavia, in 1852, when there was an interval of -two hours; and to this must be added the observations where the motion -of the sea has preceded that of the earthquake—as, for instance, in -1852, at Smyrna. Whilst recognising the fact that it is possible to -suggest explanations for many of these anomalies, we must also bear -in mind that they are, generally speaking, exceptional, and, in some -instances, may possibly be due to errors in observations. - -_Velocity of propagation of sea waves, and depth of the ocean._—It -has long been known to physical science that the velocity with which -a given wave is propagated along a trough of uniform depth, holds a -relation to the depth of the trough. - -If _v_ is the velocity of the wave, and _h_ the depth of the trough, -this relation may be expressed as follows:— - - _v_^2 (_v_) - _h_ = ————— or _h_ = (———)^2 - _g_ (_k_) - - Where _g_ = 32·19 and _k_ = 5·671. - -It will be observed that these two formulæ (the first of which is -known as Russell’s formula, and the second as Airy’s) are practically -identical. - -The apparent difference is in the average value assigned to the -constant. - -For large waves such as we have to deal with, it would be necessary, -if we were desirous of great accuracy, to increase the value of _h_ -by some small fraction of itself. We might also make allowance for -the different values of _g_, according to our position on the earth’s -surface. With these formulæ at our disposal it is an easy matter, after -having determined the velocity with which a wave was propagated, to -determine the average depth of the area over which it was transmitted. - -In making certain earthquake investigations the reverse problem is -sometimes useful—namely, determining the velocity with which a sea wave -has advanced upon a shown line, from a knowledge of the depth of the -water in which it has been propagated. - -Calculations of the average depths of the Pacific, dependent on the -velocity with which earthquake waves have been propagated, have been -made by many investigators. - -In most cases, however, in consequence of having assumed the wave to -have originated on a coast line, when the evidence clearly showed it -to have originated some distance out at sea, the calculations which -have been made are open to criticism. The average depths which I -obtained for various lines across the Pacific appear to be somewhat -less than the average depth as given by actual soundings. We must, -however, remember that the common error in actual soundings is that -they are usually too great, it being difficult in deep-sea sounding -to determine when the lead actually reaches the bottom. Until oceans -have been more thoroughly surveyed with the improved forms of sounding -apparatus, we shall not be able to verify the truth of the results -which have been given to us by earthquake waves. - - - EXAMPLES OF CALCULATIONS ON SEA WAVES. - -1. _The wave of 1854._—This wave originated near Japan, and it was -recorded on tide gauges at San Francisco, San Diego, and Astoria. - -On December 23, at 9.15. A.M., a strong shock was felt at Simoda in -Japan, which, at 10 o’clock, was followed by a large wave thirty feet -in height. The rising and falling of the water continued until noon. -Half an hour after, the movement became more violent than before. At -2.15 P.M. this agitation decreased, and at 3 P.M. it was comparatively -slow. Altogether there were five large waves. - -On December 23 and 25, unusual waves were recorded upon the -self-registering tide gauges at San Francisco, San Diego, and Astoria. - -At San Francisco three sets of waves were observed. The average time of -oscillation of one of the first set was thirty-five minutes, whilst one -of the second and third sets was almost thirty-one minutes. - -At San Diego three series of waves were also shown, but with average -times of oscillation of from four to two minutes shorter than the waves -at San Francisco. - -The San Francisco waves appear to indicate a recurrence of the same -phenomena. - -The record at San Diego shows what was probably the effect of a -series of impulses, the heights increasing to the third wave, then -diminishing, then once more renewed, after which it died away. - -The result of calculations based on these data were:— - - +---------------+------------+------------+--------+--------------+ - | | Distance | Time |Velocity| Depth of | - | |geographical| of |in feet | ocean in | - | | miles |transmission|per sec.| fathoms | - +---- ----------+------------+------------+--------+--------------+ - | | | h. m. | | | - | Simoda to San | 4917 | 12 13 | 545 | 2100 | - | Diego | | | | | - | Simoda to San | 4527 | 12 39 | 528 | 2500 or 2230 | - | Francisco | | | | | - +---------------+------------+------------+--------+--------------+ - -The difference for the depths in the San Francisco path depends whether -the length of the waves is reckoned at 210 or 217 miles. The length of -the waves on the San Diego path were 186 or 192 miles.[81] - -_The wave of 1868._—On August 11, 1868, a sea wave ruined many cities -on the South American coast, and 25,000 lives were lost. This wave, -like all the others, travelled the length and breadth of the Pacific. - -In Japan, at Hakodate, it was observed by Captain T. Blakiston, R.A., -who very kindly gave me the following account: - -On August 15, at 10.30 A.M., a series of bores or tidal waves -commenced, and lasted until 3 P.M. In ten minutes there was a -difference in the sea level of ten feet, the water rising above high -water and falling below low water mark with great rapidity. The -ordinary tide is only two and a half to three feet. The disturbance -producing these waves originated between Iquique and Arica, in about -lat. 18.28 S. at about 5 P.M. on August 13. In Greenwich time this -would be about 13h. 9m. 40s. August 13. The arrival of the wave at -Hakodate in Greenwich time would be about 14h. 7m. 6s. August 14: -that is to say, the wave took about 24h. 57m. to travel about 8,700 -miles, which gives us an average rate of about 511 feet per second. -These waves were felt all over the Pacific. At the Chatham Islands they -rushed in with such violence that whole settlements were destroyed. At -the Sandwich Islands the sea oscillated at intervals of ten minutes for -three days. - -Comparing this wave with the one of 1877 we see that one reached -Hakodate with a velocity of 511 feet per second, whilst the other -travelled the same distance at 512 feet per second. - -An account of this earthquake wave has been given by F. von Hochstetter -(‘Über das Erdbeben in Peru am 13. August 1868 und die dadurch -veranlassten Fluthwellen im Pacifischen Ocean,’ Sitzungsberichte -der Kaiserl. Akademie der Wissenschaften, Wien 58. Bd., 2. Abth. -1868). From an epitome of this paper given in ‘Petermann’s Geograph. -Mittheil.’ 1869, p. 222, I have drawn up the following table of the -more important results obtained by F. von Hochstetter. - -The wave is assumed to have originated near Arica. - - +----------------+------------+----------+----------+----------+ - | | Distance | Time | Velocity | Depth of | - | | sea miles | taken by | in feet | ocean in | - | | from Arica | wave |per second| feet | - +----------------+------------+----------+----------+----------+ - | | | h. m. | | | - |Valdivia | 1,420 | 5 0 | 479 | 7,140 | - |Chatham Islands | 5,520 | 15 19 | 608 | 11,472 | - |Lyttleton | 6,120 | 19 18 | 533 | 8,838 | - |Newcastle | 7,380 | 22 28 | 538 | 9,006 | - |Apia (Samoa) | 5,760 | 16 2 | 604 | 11,346 | - |Rapa | 4,057 | 11 11 | 611 | 11,598 | - |Hilo | 5,400 | 14 25 | 555 | 9,568 | - |Honolulu | 5,580 | 12 37 | 746 | 17,292 | - +----------------+------------+----------+----------+----------+ - -Calculations on the same disturbance are also given by J. E. -Hilgard.[82] - -Assuming the origin of the wave to have been at Arica, his results are -as follows: - - +----------------+------------+------------+----------+----------+ - | | | | Nautical | | - | | Distance | Time of | miles |Mean depth| - | |from Africa |transmission| per hour | of ocean | - +----------------+------------+------------+----------+----------+ - | | miles | h. m. | | feet | - |San Diego | 4,030 | 10 55 | 369 | 12,100 | - |Fort Point | 4,480 | 12 56 | 348 | 10,800 | - |Astoria | 5,000 | 18 51 | 265 | 6,200 | - |Kodiak | 6,200 | 22 00 | 282 | 7,000 | - |Rapa | 4,057 | 10 54 | 372 | 12,200 | - |Chatham Islands | 5,520 | 15 01 | 368 | 12,100 | - |Hawaii | 5,460 | 14 10 | 385 | 13,200 | - |Honolulu | 5,580 | 12 18 | 454 | 18,500 | - |Samoa | 5,760 | 15 38 | 368 | 12,100 | - |Lyttelton | 6,120 | 19 01 | 322 | 9,200 | - |Newcastle | 9,380 | 22 10 | 332 | 9,800 | - |Sydney | 7,440 | 23 41 | 314 | 8,800 | - +----------------+------------+------------+----------+----------+ - -_The wave of 1877._—Two sets of calculations have been made upon the -wave of 1877 by Dr. E. Geinitz of Rostock.[83] - -The following table is taken from Dr. Geinitz’s second paper, in which -there are several modifications of his first results. The origin of the -disturbance is assumed to have been near Iquique. - - +---------------------+--------+----------+-------+--------+--------+ - | |Distance| | |Velocity| Mean | - | Observation | from | Arrival | Time |in feet |depth of| - | stations |Iquique | of wave | taken | per | ocean | - | | geol. | |by wave|second | in | - | | miles | | | |fathoms | - +---------------------+--------+----------+-------+--------+--------+ - | | | h. m. | h. m. | | | - |Taiohāc (Marquesa | | | | | | - | Islands) | 4,086 | 8 40 A.M.| 12 15 | 563·8 | 1,647 | - |Apia (Samoa) | 5,740 |12 0 „ | 15 30 | 610·4 | 1,930 | - |Hilo (Sandwich | 5,526 |10 24 „ | 14 0 | 667·9 | 2,310 | - | Islands) | | | | | | - |Kahuliu „ | 5,628 |10 30 „ | 14 5 | 675·2 | 2,361 | - |Honolulu „ | 5,712 |10 50 „ | 14 25 | 669·7 | 2,319 | - |Wellington (New | 5,657 | 2 40 P.M.| 18 15 | 524·2 | 1,430 | - | Zealand) | | | | | | - |Lyttelton „ | 5,641 | 2 48 „ | 18 23 | 519·8 | 1,400 | - |Newcastle (Australia)| 6,800 | 2 32 „ | 18 7 | 633·0 | 2,075 | - |Sydney „ | 6,782 | 2 35 „ | 18 10 | 631·4 | 2,065 | - |Kamieshi (Japan) | 8,790 | 7 20 „ | 22 55 | 649·0 | 2,182 | - |Hakodate „ | 8,760 | 9 25 „ | 25 0 | 592·5 | 1,818 | - |Kadsusa „ | 8,939 | 9 50 „ | 25 15 | 604·9 | 1,895 | - +---------------------+--------+----------+-------+--------+--------+ - -The mean depths represent a mean of two sets of calculations, one -made with the aid of Airy’s formula, and the other by Scott-Russell’s -formula. The result of my own investigation about this disturbance, the -origin of which, by several methods of calculation, is shown to have -been beneath the ocean, near 71° 5′ west long., and 21° 22′ south lat., -are given on next page. - -Dr. Geinitz considers that his calculated depths of the ocean and those -obtained by actual soundings are in accordance, a result which is -diametrically opposed to that which I have obtained. - -This difference between my calculations and those of Dr. Geinitz, -Hochstetter, and others, chiefly rests on the origin we have assigned -for the sea waves. Dr. Geinitz, for instance, although he says that -the origin of the 1877 earthquake was not on the continent but to the -west in the ocean, bases all his calculations on the assumption that -the _centrum_ was at or near to Iquique, and the time at which that -city was disturbed was the time at which the waves commenced to spread -across the ocean. This time is 8.25 P.M. At this time, however, it -appears that the waves must have been more than double the distance -between the true origin and Iquique, from Iquique on their way towards -the opposite side of the Pacific. Introducing this element into the -various calculations which have been made respecting the depth of the -Pacific Ocean as derived from observations on earthquake waves—which -element, insomuch as the waves appear to have come in to inundate the -land some time after the shock, needs to be introduced—we reduce the -velocity of transit of the earthquake wave and, consequently, the -resultant depths of the ocean. - - Key: - A Distance from the origin in miles (calculated in great circles) - B Velocity in feet per second - +-----------+---------+---------+-----+-----+---+-------+--------+---------+ - | | | Arrival |Time | | |Depth | |Interval | - | | | of wave |taken| | |of the | Height | between | - | |Longitude| in | by | A | B |ocean | of |waves in | - | | |Greenwich|wave | | |in feet| waves | minutes | - | | |mean time| | | | | | | - +-----------+---------+---------+-----+-----+---+-------+--------+---------+ - | | ° ′ |day h. m.|h. m.| | | | | | - |Origin | | | | | | | | | - | of wave | 71 5 W.| 9 12 59| | | | | | | - |San | | | | | | | | - | Francisco|122 32 |10 2 28|13 29|4,578|498| 7,721 | 9 in. | | - |Callao | 77 15 | 9 17 9| 4 10| 658|231| 1,657 | | | - |Iquique | 70 14½ | 9 13 21| 0 22| 87|348| 3,770 | 20 ft. |22 | - |Cobija | 70 21 | 9 13 19| 0 20| 80|352| 3,857 | 30 „ | | - |Mejillones | 70 35 | 9 13 27| 0 28| 108|339| 3,587 | 35 „ |15 or 45 | - |Chanaral | 71 34 | 9 15 26| 2 27| 455|272| 2,309 | |10 | - |Coquimbo | 71 24 | 9 15 15| 2 16| 508|328| 3,363 | |30 | - |Valparaiso | 71 38 | 9 16 16| 3 17| 695|310| 3,000 | | | - |Concepcion | 73 5 | 9 16 52| 3 53| 928|350| 3,824 | |12 to 15 | - |Honolulu |157 55 |10 3 52|14 53|5,694|561| 9,807 |34 to |25 | - | | | | | | | | 54 ft.| | - |Hilo |155 3 |10 3 5|14 6|5,506|563|10,217 |30 or |{3 or 15 | - | | | | | | | | 8 „ |{18 or 27| - |Kahuliu |156 43 |10 3 12|14 13|5,611|579|10,437 | | | - |Samoa |171 41 W.|10 3 57|14 58|5,773|566| 9,972 | 12 ft. |10 | - |Taurauga |176 11 E.|10 8 15|19 16|5,615|427| 5,697 | | | - |Wellington |174 30 |10 7 22|18 23|5,574|445| 6,168 | 11 „ |10 | - |Akaroa |172 59 |10 7 28|18 29|5,542|440| 6,031 | | | - |Lyttelton |172 45 |10 7 29|18 30|5,558|441| 6,055 | | | - |Kameishi |140 50 |10 12 37|23 38|8,844|549| 9,378 | 6 „ |15 | - |Hakodate |140 50 |10 14 7|25 8|8,778|512| 8,169 | 7 „ |20 | - +-----------+---------+---------+-----+-----+---+-------+--------+---------+ - -In Dr. Geinitz’s paper there are also some slight differences in the -times at which the earthquake phenomena were observed at various -localities. These, however, are but of minor importance. At the end -of the paper by Dr. Geinitz two interesting tide gauge records are -introduced, one from Sydney and the other from Newcastle. These appear -to show a marked difference in the periods of the sea waves at these -two places.[84] - -_Comparison of velocities of wave-transit which have been actually -observed, with velocities which ought to exist from what we know of -the depth of the Pacific by actual soundings._—From a chart given in -‘Petermann’s Geograph. Mittheilungen,’ Band xxiii. p. 164, 1877, it is -possible to draw approximate sections on lines in various directions -across the bed of the Pacific. - -From the origin of the shock to Japan (Kameishi) the line would be as -follows:— - - about 7,441 miles 15,000 feet deep - 1,100 „ 18,000 „ - 160 „ 27,000 „ - 80 „ 12,000 „ - 60 „ 6,000 „ - -On account of the Tuscarora and Belkap Deeps this would be the most -irregular line over which the wave had to travel. - -From the origin to New Zealand (Wellington) the line would be - - about 5,274 miles 15,000 feet deep. - „ 300 „ 12,000 „ - -From the origin to Samoa the line would be - - about 5,773 miles 15,000 feet deep. - -From the origin to the Sandwich Islands (Honolulu) the line would be - - almost 6,634 miles 15,000 feet deep - and 60 „ 12,000 „ - -By Scott-Russell’s rule, or, what is almost identically the same, by -Airy’s general formula, we can calculate how long it would take such -waves as we have been speaking about to travel over the different -portions of each of these lines, and by adding these times together we -obtain the time taken to travel across any one line. I have made these -calculations, but as I get in every case answers which are too small, I -think it unnecessary to give them. - -The actual times taken to travel the distances just referred to were, - - To Japan (Kameishi) 23 hr. 38 min. - „ New Zealand (Wellington) 18 „ 23 „ - „ Samoa 14 „ 58 „ - „ Sandwich Islands (Honolulu) 14 „ 53 „ - -From San Francisco to Simoda the line is almost 3,567 miles, 3,000 -fathoms deep, 840 miles, 2,500 fathoms deep, and 120 miles 1,000 -fathoms deep. This gives an average depth of about 2,854 fathoms. Bache -calculated the depth at 2,500 fathoms. - -If we are to consider that, because the sea wave at Simoda came in -some time after the land shock had been felt, the origin of this -earthquake, instead of being at Simoda, was some distance out at sea, -this calculated depth would be reduced. - - - - - CHAPTER X. - - DETERMINATION OF EARTHQUAKE ORIGINS. - - Approximate determination of an Origin—Earthquake-hunting - in Japan—Determinations by direction of motion—Direction - indicated by destruction of buildings—Direction determined by - rotation—Cause of rotation—The use of time observations—Errors - in such observations—Origin determined by the method of - straight lines—The method of circles, the method of hyperbolas, - the method of co-ordinates—Haughton’s method—Difference in time - between sound, earth, and water waves—Method of Seebach. - - -One of the most practical problems which can be suggested to the -seismologist is the determination of the district or districts in any -given country from which earthquake disturbances originate. With a -map of a country before us, shaded with tints of different intensity -to indicate the relative frequency of seismic disturbance in various -districts, we at once see the localities where we might dwell with -the least disturbance, and those we should seek if we wish to make -observational seismology a study. Before erecting observatories for -the systematic investigation of earthquakes in a country, it would -be necessary for us, in some way or other, to examine the proposed -country to find out the most suitable district. The special problem -of determining _approximately_ the origin or origins of a set of -earthquakes would be given to us. Having made this preliminary -investigation, the next point is, by means of observatories so -arranged that they could always work in conjunction with each other, -to determine the origins more accurately. By knowing the origin from -which a set of shocks spring we know the general direction in which -we may expect the most violent disturbances, and we can arrange our -seismometers accordingly. - -_Approximate determination of origins._—In 1880 I obtained a tolerably -fair idea of the distribution of seismic energy throughout Japan, by -compiling the facts obtained from some hundreds of communications -received from various parts of the country respecting the number of -earthquakes that had been felt. - -The communications were replies to letters sent to various residents -in the country and to a large number of public officers. By taking -these records, in conjunction with the records made by instruments, it -was ascertained that in Japan alone there were certainly 1,200 shocks -felt during the year, that is to say, three or four shocks per day. -The greater number of these shocks were felt along the eastern coast, -commencing at Tokio, in the south, and going northwards to the end of -the main island. These shocks were seldom felt on the west coast. It -appeared as if the central range of mountains formed a barrier to their -progress. Similarly, ranges of mountains to the south-west of Tokio -prevented the shocks from travelling southwards. Proceeding in this way -the conclusion was arrived at that the west coast, the southern part -of Japan, and the islands of Shikoku and Kiushiu, had their own local -earthquakes. - -_Earthquake-hunting._—These preliminary enquiries having shown that -the northern part of Japan was a better district for seismological -observations than the southern half, the next step was to subject the -northern half to a closer analysis. This analysis was commenced by -sending to all the important towns, from thirty to one hundred miles -distant from Tokio, bundles of postcards. These were entrusted to the -local government offices with a request that each week one of these -cards would be returned to Tokio stating the number of shocks felt. -In this way it was quickly discovered that the majority of shakings -emanated from the north and east, and seldom, if ever, passed a heavy -range of mountains to the south. The barricade of postcards was then -extended farther northwards, with the result of surrounding the origin -of certain shocks amongst the mountains, whilst others were traced to -the sea shore. By systematically pursuing earthquakes it was seen that -many shocks had their origin beneath the sea—they shook all the places -on the north-east coast, but it was seldom that they crossed through -the mountains, forming the backbone of the island, to disturb the -places on the west coast. - -The actual results obtained in three months by this method of working -are shown in the accompanying map, which embraces the northern half of -the main island of Nipon and part of Yezo. The shaded portion of the -map indicates the mountainous districts, which are traversed by ranges -varying in height from about 2,000 and 7,000 feet. The dotted lines -show the boundaries of the more important groups of earthquakes which -were recorded. - -I. is the western boundary of earthquakes, which at places to the -eastward are usually felt somewhat severely. Some of these have been -felt the most severely at or near Hakodadi, whilst farther south their -effects have been weak. Occasionally the greatest effect has been near -to Kameishi. Sometimes these earthquakes terminate along the western -boundaries of III. or IV., not being able to pass the high range of -mountains which separate the plain of Musashi from Kofu. - -[Illustration: FIG. 29.—Northern Japan. Mountainous districts shaded -with oblique lines.] - -II. is the boundary of a shock confined to the plain which surrounds -Kofu. These earthquakes are evidently quite local. Many of the -disturbances have evidently originated beneath the ocean, having come -in upon the land in the direction of the arrows A or B. - -III. This line indicates the boundary of a group of shocks which are -often experienced in Tokio. These may come in the directions D, E, -or F. It is probable that some of them originate to the eastward of -Yokohama, on or near to the opposite peninsula. - -IV. V. and VI. The earthquakes bounded by these lines probably -originate in the directions C or D. - -VII. The earthquakes bounded by this line probably come from the -direction E. - -VIII. This line gives us the boundary of earthquakes which may come -from the direction B. - -The above boundaries sometimes do not extend so far to the westward as -they are shown. At other times, groups like V. and VI. extend farther -to the south-west. These earthquake boundaries, which so clearly show -the effects of high mountains in preventing the extension of motion, -have been drawn up, not from single earthquakes, but from a large -series of earthquakes which have been plotted upon blank maps, and are -now bound together to form an atlas. To give an idea of the material -upon which I have been working, I may state that between March 1 and -March 10, 1882, I received records of no less than thirty-four distinct -shocks felt in districts between Hakodate and Tokio, and for each of -these it is quite possible to draw a map. In addition to the boundaries -of disturbances given in the accompanying map, other boundaries might -be drawn for shocks which were more local in their character. The -groups which contained the greatest number of shocks are III., IV., -V., VI., and VII. By work of this description it was found that a very -important group of earthquakes might be studied by a line of stations -commencing at Saporo in the north, passing through Hakodate, down the -east coast of the main island, to Tokio or Yokohama in the south. A -further aid to the study of this group, together with the study of -an important local group, might be effected with the help of a few -additional stations properly distributed on the plain of Musashi, which -surrounds Tokio. With this example before us it will be recognised that -the choice of sites for a connected set of seismological observatories -will often be more or less a special problem. If earthquake stations -were to be placed in different directions around Tokio without -preliminary investigation, it is quite possible that some of them might -be so situated that they would seldom if ever work in conjunction with -the remaining observatories, and therefore be of but little value. -And this remark must equally apply to districts in other portions -of the globe. The method is crude, and, so far as actual earthquake -origins are concerned, it only yields results which are approximate. -The crudeness and the want of absoluteness in the results is, however, -more than counterbalanced by the certainty with which we are enabled -to express ourselves with regard to such results as are obtained. Even -when working with the best instruments we have at our command, unless -we are employing some elaborate system, this method of working gives a -most valuable check upon our instrumental records, and enables us to -interpret them with greater confidence. - -_Determination of earthquake origins from the direction of motion._—If -we assume that an earthquake is propagated from a centre as a series -of waves, in which normal vibrations are conspicuous, and obtain at -two localities, not in the same straight line with the origin, and -sufficiently far separated from each other, the direction of movement -of these normal motions, by drawing lines parallel to these directions -through our two stations, the lines would intersect at a point above -the required origin. If instead of two points we had three, or, better -still, a large number, the results we should obtain ought to be -still more certain. Unfortunately, it seems that earthquakes seldom -originate from a given point, and, further, normal motions are not -always (sufficiently) prominent. Sometimes, as has already been shown -in the chapters on earthquake motion, they may be non-existent. It is -probable, however, that difficulties of this sort are more usually -associated with non-destructive earthquakes. Mallet regards the -destructive effects of an earthquake as almost solely due to normal -motions. If this be true, for destructive earthquakes, the problem is -shorn of many of its difficulties. In cases where normal vibrations -are not prominent, where we have only transverse vibrations, motions -due to the interference of normal or transverse motions, or directions -of motions due to the topographical or geological nature through which -the disturbance has passed, the determination of the origin of an -earthquake by observations on the direction in which the ground has -been moving appears to be a problem which is practically without a -solution. We will, therefore, only consider the determination of the -origin of those earthquakes which have predominating directions in -their movements, which directions we will consider as normal ones. -The question which is, then, before us, is the determination of the -direction of these normal movements. First of all we may take the -evidence of our senses. In exceptional cases these have given results -which closely approximate to the truth, but in the majority of cases -such results are not to be relied upon, as the inhabitants of a town -will, for the same shock, give directions corresponding to all points -of the compass. Much, no doubt, depends upon the situation of the -observer, and much, perhaps, upon his temperament. If he is sitting -in a room alone, and is accustomed to making observations on an -earthquake, on feeling the earthquake, if he concentrates his attention -on the direction in which he is being moved, his observations may be -of value. If, however, he is not so situated, and his attention is -not thus concentrated, his opinions, unless the motion has been very -decided in its character, are usually of but little worth. - -_Direction determined from destruction of buildings._—When an observer -first sees a town that has been partially shattered by an earthquake, -all appears to be confusion, and it is difficult to imagine that in -such apparent chaos we are able to discover laws. If, however, we take -a general view of this destruction and compare together similarly -built buildings, it is possible to discover that similar and similarly -situated structures have suffered in a similar manner. By carefully -analysing the destruction we are enabled to infer the direction in -which the destroying forces have acted. It was chiefly by observing the -cracks in buildings, and the direction in which bodies were overthrown -or projected, that Mallet determined the origin of the Neapolitan -earthquake. From the observations given in Chapter VII. it would appear -that, with destructive earthquakes, walls which are transverse to the -direction of motion are most likely to be overturned, whilst, with -small earthquakes, these walls are the least liable to be fractured. - -From a critical examination of the _general_ nature of the damage -done on the buildings of a town, earthquake observers have shown that -the direction of a shock may often be approximately determined. The -direction in which a body having a regular form like a prismatic -gravestone or a cylindrical column is overturned sometimes gives the -means by which we can determine the direction from which a movement -came. - -_The rotation of bodies._—It has often been observed that almost all -large earthquakes have caused objects like tombstones, obelisks, -chimneys, &c., to rotate. - -One of the most natural and at the same time most simple explanations -is to suppose that during the shock there had been a twisting, or -backward and forward screw-like motion in the ground. Amongst the -Italians and the Mexicans earthquakes producing an effect like this are -spoken of as ‘vorticosi.’ In the Calabrian earthquake, not only were -bodies like obelisks twisted on their bases, but straight rows of trees -seem to have been left in interrupted zigzags. These latter phenomena -have been explained upon the assumption of the interference of direct -waves and reflected waves, the consequence of which being that points -in close proximity might be caused to move in opposite directions. -Reflections such as these would be most likely to occur near to the -junction of strata of different elasticity, and it may be remarked that -it is often near such places that much twisting has been observed. - -Another way in which it is possible for twisting to have taken place -would be by the interference of the normal and transverse waves which -probably always exist in an earthquake shock, or by the meeting of -the parts of the normal wave itself, one having travelled in a direct -line from the origin, whilst the other, travelling through variable -material, has had its direction changed. - -Mallet, however, has shown that the rotation may have been in many -cases brought about without the supposition of any actual twisting -motion of the earth—a simple backward and forward motion being quite -sufficient. If one block of stone rests upon another, and the two are -shaken backwards and forwards in a straight line, and if the vertical -through the centre of gravity of the upper block does not coincide with -the point where there is the greatest friction between the blocks, -rotation must take place. If the vertical through the centre of gravity -falls on one side of the centre of friction, the rotation would be in -one direction, whilst, if on the other side, the rotation would be in -the opposite direction. - -Although the above explanation is simple, and also in many cases -probably true, it hardly appears sufficient to account for all the -phenomena which have been observed. - -Thus, for instance, if the stones in the Yokohama cemetery, at the time -of the earthquake of 1880, had been twisted in consequence of the cause -suggested by Mallet, we should most certainly have found that some -stones had turned in one direction whilst others had been twisted in -another. By a careful examination of the rotated stones, I found that -every stone—the stones being in parallel lines—had _revolved in the -same direction_, namely in a direction opposite to that of the hands of -a watch. - -As it would seem highly improbable that the centre of greatest friction -in all these stones of different sizes and shapes should have been at -the same side of their centres of gravity, an effect like this could -only be explained by the conjoint action of two successive shocks, the -direction of one being transverse to the other. - -Although fully recognising the sufficiency of two transverse shocks -to produce the effects which have been observed in Yokohama, I will -offer what appears to me to be the true explanation of this phenomenon: -it was first suggested by my colleague, Mr. Gray, and appears to be -simpler than any with which I am acquainted. - -[Illustration: FIG. 30.] - -If any columnar-like object, for example a prism which the basal -section is represented by A B C D (see fig. 30), receives a shock at -right angles to B C, there will be a tendency for the inertia of the -body to cause it to overturn on the edge B C. If the shock were at -right angles to D C, the tendency would be to overturn on the edge D C. -If the shock were in the direction of the diagonal C A, the tendency -would be to overturn on the point C. Let us, however, now suppose the -impulse to be in some direction like E G, where G is the centre of -gravity of the body. For simplicity we may imagine the overturning -effect to be an impulse given through G in an opposite direction—that -is, in the direction G E. This force will tend to tip or make the body -bear heavily on C, and at the same time to whirl round C as an axis, -the direction of turn being in the direction of the hands of a watch. -If, however, the direction of impulse had been E′ G, then, although the -turning would still have been round C, the direction would have been -_opposite_ to that of the hands of a watch. - -To put these statements in another form, imagine G E′ to be resolved -into two components, one of them along G C and the other at right -angles, G F. Here the component of the direction G C tends to make the -body tip on C, whilst the other component along G F causes revolution. -Similarly G E may be resolved into its two components G C and G F′, the -latter being the one tending to cause revolution. - -From this we see that if a body has a rectangular section, so long -as it is acted upon by a shock which is parallel to its sides or to -its diagonals, there ought not to be any revolution. If we divide -our section A B C D up into eight divisions by lines through these -directions, we shall see that any shock the direction of which passes -through any of the octants which are shaded will cause a _positive_ -revolution in the body—that is to say, a revolution corresponding in -its direction to that of the movements of the hands of a watch; whilst -if its direction passes through any of the remaining octants the -revolution will be _negative_, or opposite to that of the hands of a -watch. From the direction in which any given stone has turned, we can -therefore give two sets of limits between one of which the shock must -have come. - -Further, it will be observed that the tendency of the turning is to -bring a stone, like the one we are discussing, broadside on to the -shock; therefore, if a stone with a rectangular cross section has -turned sufficiently, the direction of a shock will be parallel to one -of its faces, but if it has not turned sufficiently it will be more -nearly parallel to its faces in their new position than it was to its -faces when in their original position. - -If a stone receives a shock nearly parallel with its diagonal, on -account of its instability it may turn either positively or negatively -according as the friction on its base or some irregularity of surface -bearing most influence. Similarly, if a stone receives a shock parallel -to one of its faces, the twisting may be either positive or negative, -but the probability is that it would only turn slightly; whereas in the -former case, where the shock was nearly parallel to a diagonal, the -turning would probably be great. - -_Determination of direction from instruments._—When speaking about -earthquakes it was shown, as the result of many observations, that -the same earthquake in the space of a few seconds, although it may -sometimes have only one direction of motion, very often has many -directions of motion. In certain cases, therefore, our records, if we -assume the most permanent motions to be normal ones, give definite and -valuable results. In other cases it is necessary to carefully analyse -the records, comparing those taken at one station with those taken at -another. - -One remarkable fact which has been pointed out in reference to -artificial earthquakes produced by exploding charges of gunpowder or -dynamite, and also with regard to certain earthquakes, is that the -greatest motion of the ground is _inwards_, towards the point from -which the disturbance originated. Should this prove the rule, it gives -a means of determining, not only the direction of earthquake, but the -side from which it came. - -_Determination of earthquake origins by time observations._—The times -at which an earthquake was felt at a number of stations are among the -most important observations which can be made for the determination of -an earthquake origin. The methods of making time observations, and the -difficulties which have to be overcome, have already been described. -When determining the direction from which a shock has originated, or -determining the origin of the shock by means of time observations, -it has been usual to assume that the velocity of propagation of the -shock has been uniform from the origin. The errors involved in this -assumption appear to as follows:— - -1. We know from observations on artificial earthquakes that the -velocity of propagation is greater between stations near to the origin -of the shock than it is between more remote stations; and also the -velocity of propagation varies with the initial force which produced -the disturbance. If our points of observation are sufficiently close -together as compared with their distance from the origin of the -disturbance, it is probable that errors of this description are small -and will not make material differences in the general results. - -2. We have reasons for believing that the transit velocity of an -earthquake is dependent on the nature of the rocks through which it -is propagated. Errors which arise from causes of this description -will, however, be practically eliminated if our observation points are -situated on an area sufficiently large, so that the distribution of the -causes tending to alter the velocity of a shock balance each other. It -must be remarked, that causes of this description may also produce an -alteration in the direction of our shock. - -Other errors which may sometimes enter into our results, when -determining the origin of shocks by means of observations on -velocities, are the assumptions that the disturbance has travelled -along the surface from the _epicentrum_ and not in a direct line from -the _centrum_. Again, it is assumed that the origin is a point, whereas -it may possibly be a cavity or a fissure. Lastly, if we desire extreme -accuracy, we must make due allowance for the sphericity of the earth -and the differences of elevation of the observing stations. - -I. _The method of straight lines._—Given a number of pairs of points -A_{0}, A_{1}, B_{0}, B_{1}, C_{0}, C_{1}, &c., at each of which the -shock was felt simultaneously, to determine the origin. - -Theoretically if we bisect the line which joins A_{0} and A_{1} by a -line at right angles to A_{0}, A_{1}, and similarly bisect the lines -B_{0}, B_{1}, C_{0}, C_{1}, all these bisecting lines _a__{0}, _a__{1}, -_b__{0}, _b__{1}, _c__{0}, _c__{1}, &c., ought to intersect in a point, -which point will be the _epicentrum_ or the point above the origin. - -This method will fail, first, if A_{0}, A_{1}, B_{0}, B_{0}, C_{0}, -C_{1} form a continuous straight line, or if they form a series of -parallel lines. - -Hopkins gives a method based on a principle similar to the one which -is here employed—namely, given that a shock arrives simultaneously -at _three_ points to determine, the centre. In this case, the -relative positions of the three points, where the time of arrival was -simultaneous, must be accurately known, and these three points must -not lie in a straight line, or the method will fail. For practical -application the problem must be restricted to the case of three points -which do not lie nearly in the same straight line. - -II. _The method of circles._—Given the times _t__{0}, _t__{1}, _t__{2}, -&c., at which a shock arrived at a number of places A_{0}, A_{1}, -A_{2}, &c., to determine the position from which the shock originated. - -Suppose A_{0} to be the place which the shock reached first, and that -it reached A_{1}, A_{2}, A_{3}, &c., successively afterwards. - - Let _t__{1} - _t__{0} = _a_ - _t__{2} - _t__{0} = _b_ - _t__{3} - _t__{0} = _c_, &c. - -With A_{1}, A_{2}, A_{3}, &c. as centres, describe circles with radii -proportional to the known qualities _a_, _b_, _c_, &c., and also a -circle which passes through A_{0} and touches these circles. The centre -of the last circle will be the _epicentrum_. The radii proportional to -_a_, _b_, _c_, &c. may be represented by the quantities _ax_, _bx_, -_cx_, &c., where _x_ is the velocity of propagation of the shock. - -It will be observed that the times at which the shock arrived at three -places might alone be sufficient. If, instead of taking the times of -arrival of a shock, the arrival of a sea wave be taken, the result -would be a closer approximate to the absolute truth. - -It will be observed that this method is not a direct one, but is -one of trial. If, however, an imaginary case be taken, and three -given points of observation, A_{0}, A_{1}, A_{2}, be plotted on a -piece of paper, it will be found that it is not a difficult matter to -determine two numbers proportional to _a_ and _b_ which will allow -you to draw two circles so that they may be touched by a third circle -drawn through A_{0}. This problem has practically been applied in the -case of the arrival of a sea wave at a number of places on the South -American coast, at the time of the earthquake of May 9, 1877. This is -illustrated as follows. The places which were chosen were Huanillos, -Tocopilla, Cobija, Iquique, Mejillones. - -In the following table the first column gives the times at which the -sea wave arrived at each of these places in Iquique time; in the second -column the difference between these times and the time at which it -reached Huanillos is given; in the third column the distances through -which a sea wave, propagated at the rate of 350 feet per second, could -travel during the intervals noted in the second column is given. - - +------------+----------+----------+------------+ - | | Arrival |Time after| Distance at| - | | of sea |arrival at|350 feet per| - | | wave | Huanillos| second | - +------------+----------+----------+------------+ - | | h. m. | minutes | miles | - | Huanillos | 8 30 | 0 | 0 | - | Tocopilla | 8 32 | 2 | 8 | - | Cobija | 8 38 | 8 | 32 | - | Iquique | 8 40 | 10 | 40 | - | Mejillones | 8 46 | 16 | 64 | - +------------+----------+----------+------------+ - -The distances marked in the third column are used as radii of the -circles drawn round the places to which they respectively refer. - -The centre of the circle drawn to touch the circles of the first -column, and at the same time to pass through Huanillos, is marked C. - -The position from which the shock originated appears therefore to have -occurred very near to a place lying in Long. 7° 15′ W. and Lat. 21° 22′ -S. - -[Illustration: FIG. 31.] - -The actual operations which were gone through in making the -accompanying map were as follows. First, the places with which we had -to deal were represented on a map in orthographical projection, the -centre of projection being near to the centre of the map. This was -done so that the measurements which were made upon the map might be -more correct than those we should obtain from an ordinary chart where -this portion of the world was not the centre of projection. Next, a -number was taken as equal to the velocity with which the sea wave had -travelled. The first velocity taken was about 400 feet per second—this -being about the velocity with which, theoretically, it must have -travelled in an ocean having a depth equal to that indicated upon the -charts—also it seemed to have travelled at this rate from the various -times of arrival as recorded at places along the coast. Circles were -then drawn round Tocopilla, Cobija, Iquique, and Mejillones with radii -equal to 2, 8, 10, and 15, each multiplied by (60 × 400). It was then -seen _by trial_ that it was impossible to draw a single circle which -should touch four circles and also pass through Huanillos. These four -circles were, in fact, too large. Four new but smaller circles, which -are shown in the map, were next drawn, their radii being respectively -equal to the numbers 2, 8, 10, and 16, each multiplied by (60 × 350), -and it was found that a circle, with a centre C, could be drawn which -would practically touch the four circles, and at the same time would -pass through Huanillos. - -III. _The method of hyperbolas._—The method which I call that of -hyperbolas is only another form of the method of circles. It is, -however, useful in special cases, as, for instance, where we have the -times of arrival of earthquakes at only two stations. Between Tokio -and Yokohama, at which places I frequently obtain tolerably accurate -time records, the method has been applied on several occasions with -advantage. In the preceding example let us suppose that the only time -records which we had were for Huanillos and Mejillones, and that the -wave was felt at the latter place sixteen minutes or 960 seconds -after it was experienced at the former. Calling these places H and M -respectively, round M draw a circle equal to the 960 multiplied by -the velocity with which the wave was propagated. It is then evident -that the origin of this disturbance must be the centre of a circle -which passes through H and touches the circle drawn round M. Join H -M, cutting the circle round M in Y. Bisect Y H in V. It is evident -that V is one possible origin for the disturbance. Next, from M, in -the direction of H, draw any line M Z P; join Z H; bisect Z H at right -angles by the line O P N. Because PH = PZ, it is evident that P is -a second possible origin. Proceeding in this way a series of points -lying to the right and left of V on the curve R V T may be found, and -we may therefore say that the origin lies somewhere in the curve R -V T. By increasing or decreasing our velocity we vary the position -of the curve R V T, and, instead of a line on which our origin may -be, we obtain a band. As the assumed velocity increases, the circle -round M becomes larger, and has its limit when it passes through H, -where the two arms of the curve R V T will close together and form a -prolongation of the line M Y H as the assumed velocity diminishes. -The circle round M becomes smaller until it coincides with the point -M. At such a moment the curve R V T opens out to form a straight line -bisecting M H at right angles. The curve R V T is a hyperbola with a -vertex V and foci H and M. Inasmuch as PM - PH = a constant quantity. -If we have the time given at which the shock or wave arrived at a third -station as at Iquique, it is evident that a second hyperbola R′ V′ T′ -might be drawn with Iquique and Huanillos as foci, and that the mutual -intersection of these two hyperbolas with a third hyperbola, having for -its foci Iquique and Mejillones, would give the origin of the wave. -The obtaining of a mutual intersection would depend on the assumed -velocity, and the accuracy of the result, like that of the method -of circles, would depend upon the trials we made. The method here -enunciated may be carried farther by describing hyperboloids instead -of hyperbolas, the mutual intersection of which surfaces would, in the -case of an earth wave, give the actual origin or _centrum_ rather than -the point above the origin or _epicentrum_. - -IV. _The method of co-ordinates._—Given the times at which a shock -arrived at five or more places, the position of which we have marked -upon a map, or chart, to determine the position on the map of the -centre of the shock, its depth, and the velocity of propagation. - -Commencing with the place which was last reached by the shock, call -these places _p_, _p__{1}, _p__{2}, _p__{3}, and _p__{4}, and let the -times taken to reach these places from the origin be respectively _t_, -_t__{1}, _t__{2}, _t__{3}, and _t__{4}. - -Through _p_ draw rectangular co-ordinates, and with a scale measure the -co-ordinates of _p__{1}, _p__{2}, _p__{3}, and _p__{4}, and let these -respectively be _a__{1}, _b__{1}; _a__{2}, _b__{2}; _a__{3}, _b__{3}; -_a__{4}, _b__{4}. Then if _x_, _y_, and _z_ be the co-ordinates of the -origin of the shock, _d_, _d__{1}, _d__{2}, _d__{3}, and _d__{4}, the -respective distances of _p_, _p__{1}, _p__{2}, _p__{3}, and _p__{4} -from this origin, and _v_ the velocity of the shock, we have - - 1. _x_^2 + _y_^2 + _z_^2 = _d_^2 = _v_^2 _t_^2 - 2. (_a__{1} - _x_)^2 + (_b__{1} - _y_)^2 + _z_^2 = _v_^2 _t__{1}^2 - 3. (_a__{2} - _x_)^2 + (_b__{2} - _y_)^2 + _z_^2 = _v_^2 _t__{2}^2 - 4. (_a__{3} - _x_)^2 + (_b__{3} - _y_)^2 + _z_^2 = _v_^2 _t__{3}^2 - 5. (_a__{4} - _x_)^2 + (_b__{4} - _y_)^2 + _z_^2 = _v_^2 _t__{4}^2 - -Because we know the actual times at which the waves arrived at the -places _p_, _p__{1}, _p__{2}, _p__{3}, _p__{4}, we know the values -_t_—_t__{1}, _t_—_t__{2}, _t_—_t__{3}, _t_—_t__{4}. Call these -respectively _m_, _p_, _q_, and _r_. Suppose _t_ known, then - - _t__{1} = _t_ - _m_ - _t__{2} = _t_ - _p_ - _t__{3} = _t_ - _q_ - _t__{4} = _t_ - _r_. - -Subtracting equation No. 1 from each of the equations 2, 3, 4, and 5, -we obtain, - - _a__{1}^2 + _b__{1}^2 - 2_a__{1} _x_ - 2_b__{1} _y_ - = _v_^2 (_t__{1}^2 - _t_^2) = _v_^2 (_m_^2 - 2_t_ _m_) - - _a__{2}^2 + _b__{2}^2 - 2_a__{2} _x_ - 2_b__{2} _y_ - = _v_^2 (_t__{2}^2 - _t_^2) = _v_^2 (_p_^2 - 2_t_ _p_) - - _a__{3}^3 + _b__{3}^2 - 2_a__{3} _x_ - 2_b__{3} _y_ - = _v_^2 (_t__{3}^2 - _t_^2) = _v_^2 (_q_^2 - 2_t_ _q_) - - _a__{4}^2 + _b__{4}^2 - 2_a__{4} _x_ - 2_b__{4} _y_ - = _v_^2 (_t__{4}^2 - _t_^2) = _v_^2 (_r_^2 - 2_t_ _r_) - -Now let _v_^2 = _u_, and 2_v_^2 _t_ = _w_. - -Then - - 1. 2_a__{1} _x_ + 2_b__{1} _y_ + _u_ _m_^2 - _n_ _m_ - = _a__{1}^2 + _b__{1}^2 - - 2. 2_a__{2} _x_ + 2_b__{2} _y_ + _u_ _p_^2 - _n_ _p_ - = _a__{2}^2 + _b__{2}^2 - - 3. 2_a__{3} _x_ + 2_b__{3} _y_ + _u_ _q_^2 - _n_ _q_ - = _a__{3}^2 + _b__{3}^2 - - 4. 2_a__{4} _x_ + 2_b__{4} _y_ + _u_ _r_^2 - _n_ _r_ - = _a__{4}^2 + _b__{4}^2 - -We have here four simple equations containing the four unknown -quantities _x_, _y_, _u_, and _w_. - -_x_ and _y_ determine the origin of the shock. The absolute velocity -_v_ equals √ _u_. From _v_ and _w_ we obtain _t_. Substituting -_x_, _y_, _v_, and _t_ in the first equation we obtain _z_. - -We have here assumed that the points of observation have all about the -same elevation above sea level. - -If it is thought necessary to take these elevations into account, a -sixth equation may be introduced. - -If the propagation of the wave is considered as a horizontal one, as -would be done when calculating the position of the _epicentrum_ or -point above the origin, by means of the times of arrival of a sea wave, -the ordinate _z_ of the first five equations would be omitted. Working -in this way the resulting four equations, viz. - - 2_a__{1} _x_ + 2_b__{1} _y_ + _u__m_^2 - _w__m_^2 = _a__{1}^2 + _b__{1}^2 - &c. &c. &c. - -remained unchanged. - -Applying this method to the same example as that used as illustration -for the two previous methods, we obtain for the co-ordinates of -Mejillones, Iquique, Cobija, Tocopilla, and Huanillos, measured in -geographical miles, and the times in Iquique time at which the wave -reached each, as given in the following table; _ox_ and _oy_ being, -drawn through Mejillones. - - +------------+--------------------------------+---------------+ - | | Co-ordinates |Time of arrival| - +------------+----------------+---------------+---------------+ - | | OX | OY | h. m. | - | Mejillones | _a_ or 0 | _b_ or 0 | 8 46 p. m. | - | Iquique | _a_{1}_ or 150 | _b_{1}_ or 96 | 8 40 „ | - | Cobija | _a_{2}_ or 36 | _b_{2}_ or 14 | 8 38 „ | - | Tocopilla | _a_{3}_ or 66 | _b_{3}_ or 31 | 8 32 „ | - | Huanillos | _a_{4}_ or 102 | _b_{4}_ or 58 | 8 30 „ | - +------------+----------------+---------------+---------------+ - -From this data we find the co-ordinates _x_ and _y_ of this origin to -be 85·8 and 56·7; whilst the velocity of propagation = 45 feet per -second. - -Measuring these ordinates upon the map, we obtain a centre lying very -near Long. 71° 5′ W. and Lat. 21° 22′ S., a position which is very near -to that which has already been obtained by other methods. - -If instead of Huanillos we substitute the ordinates and time of arrival -of the sea wave for Pabalon de Pica, another point for the origin will -be obtained lying farther out at sea. To obtain the best result, the -method to be taken will evidently be, first to reject those places -at which it seems likely that some mistake has been made with the -time observations, and then with the remaining places to form as many -equations as possible, and from these to obtain a mean value. This -is a long and tedious process, and as the time observations of this -particular earthquake are probably one and all more or less inaccurate, -it is hardly worth while to follow the investigation farther. - -In this example, as in the preceding ones, it will be observed that -it has been sea waves that have been dealt with, rather than earth -vibrations. It is evident, however, that these latter vibrations may be -dealt with in a similar manner. - -In these determinations it will also have been observed that it -is assumed that the disturbance has radiated from a centre, and, -therefore, approached the various stations in different directions. -If we assume that we have three stations very near to each other as -compared with their distances from the origin, so that we can assume -that the wave fronts at the various stations were parallel, the -determination of the direction in which the wave advanced appears to be -much simplified. The determination of the direction in which a wave has -passed across three stations was first given by Professor Haughton. - -_Haughton’s method._—Given, the time of an earthquake shock at three -places, to determine its horizontal velocity and coseismal line. - -The solution of this is contained in the formula - - _a_ (_t_{2}_ - _t_{1}_) sin β - tan φ = ——————————————————————————————————————————————————————. - _c_ (_t_{3}_ - _t_{2}_) + _a_ (_t_{2}_ - _t_{1}_ cos β - -When A, B, and C are three stations at which a shock is observed at -the times _t_{1}_, _t_{2}_, and _t_{3}_; _a_, _b_, and _c_ are the -distances between A, B, and C, and φ is the angle made by the coseismal -lines _x_ A _x_, _y_ B _y_, and the line A B, which are assumed to be -parallel. - -This I applied in the case of the Iquique earthquake, but owing to the -smallness of the angles between the three stations A, B, and C, the -result was unsatisfactory. The problem ought to be restricted, first, -to places which are a long distance away from a centre, and, secondly, -to places which are not nearly in a straight line. This problem may be -solved more readily by geometrical methods. Plot the three stations A, -B, and C on a map, join the two stations between which there was the -greatest difference in the time observation. Let these, for example, -be A and C. Divide the line A C at point D, so that A D : D C as the -interval between the shock felt at A and B is to the interval between -the shock felt at B and C. The line B D will be parallel to the -direction in which the wave advanced. - -_The difference in time of the arrival of two disturbances._—In the -various calculations which have been made to determine an origin -based on the assumption of a known or of a constant velocity, we have -only dealt with a single wave, which may have been a disturbance in -the earth or in the water. A factor which has not yet been employed -in this investigation is the difference in time between the arrival -of two disturbances; one propagated, for instance, through the -earth, and the other, for example, through the ocean. The difference -in the times of the arrival of two waves of this description is a -quantity which is so often recorded that it is well not to pass it -by unnoticed. To the waves mentioned we might also add sound waves, -which so frequently accompany destructive earthquakes, and, in some -localities, as, for instance, in Kameishi, in North Japan, are also -commonly associated with earthquakes of but small intensity. It was by -observing the difference in time between the shaking and the sound in -different localities that Signor Abella was enabled to come to definite -conclusions regarding the origin of the disturbances which affected the -province of Neuva Viscoya in the Philippines, in 1881; the places where -the interval of time was short, or the places where the two phenomena -were almost simultaneous, being, in all probability, nearer to the -origin than when the intervals were comparatively large. I myself -applied the method with considerable success when seeking for the -origin of the Iquique earthquake of 1877. The assumptions made in that -particular instance were, first, that the velocity of the disturbance -through the earth was known, and, secondly, that the velocity with -which a sea wave was propagated was also known. - -A method similar to the above was first suggested by Hopkins. It -depended on the differences of velocity with which normal and -transversal waves are propagated.[85] - -_Seebach’s method._—To determine the true velocity of an earthquake, -the time of the first shock, and the depth of the centre. - -[Illustration: FIG. 32.] - -Let the straight line M, _m__{1}, _m__{2}, _m__{3} represent the -surface of the earth shaken by an earthquake. For small earthquakes, to -consider the surface of the earth as a plane will not lead to serious -errors. - -If an earthquake originates at C, then to reach the surface at M it -traverses a distance _h_ in the time _t_. To reach the surface at M_{1} -it traverses a distance _h_ + _x__{1} in a time _t__{2}. If _v_ equals -the velocity of propagation, - - _h_ _h_ + _x__{1} - then _t_ = ———, _t_{1}_ = —————————————, - _v_ _v_ - - _h_ + _x__{2} - _t_{2}_ = —————————————, &c. - _v_ - -Seebach now says that _if we have given the position of_ M _or -epicentrum of the shock_, and draw through it rectangular axes like M -_m_{3}_ and M T_{3}, and lay down on M _m_{3}_ in miles the distances -from M of the various stations which have been shaken, and in equal -divisions for minutes lay down on M T_{3} the differences of time at -which M, _m_{1}_, _m_{2}_, &c. were shaken, then M_{1} T_{1}, M_{2} -T_{2}, &c. are the co-ordinates of points on an hyperbola. The degree -of exactness with which this hyperbola is in any given case constructed -is a check upon the accuracy of the time observations and the position -of the _epicentrum_. The apex of the hyperbola is the _epicentrum_. - -The intersection of the asymptote with the ordinate axis is the time -point of the first shock, which, because the scale for time and -for space were taken as equal, gives the absolute position of the -_centrum_. This intersection is shown by dotted lines. Knowing the -position of the _centrum_, we can directly read from our diagram how -far the disturbance has been propagated in a given time. - - - - - CHAPTER XI. - - THE DEPTH OF AN EARTHQUAKE CENTRUM. - - The depth of an earthquake centrum—Greatest possible depth of an - earthquake—Form of the focal cavity. - - -_Depth of centrum._—The first calculations of the depth at which an -earthquake originated were those made by Mallet for the Neapolitan -earthquake of 1857. These were made on the assumption that the earth -wave radiated in straight lines from the origin, and, therefore, at -points at different distances from the _epicentrum_ it had different -angles of emergence. These angles of emergence were chiefly calculated -from the inclination of fissures produced in certain buildings, which -were assumed to be at right angles to the direction of the normal -motion. If we have determined the _epicentrum_ of an earthquake and -the muzoseismal circle, and make either the assumption that the angle -of emergence in this circle has been 45° or 54° 44′ 9″ (see page -54), it is evidently an easy matter by geometrical construction to -determine the depth of the _centrum_. Höfer followed this method when -investigating the earthquake of Belluno. - -Other methods of calculation which have been employed are based on -time observations, as, for instance, the method of Seebach, the method -of co-ordinates, the method of hyperboloids or spheres (see pages -200–212). - -By means of a number of lines parallel to twenty-six angles of -emergence, drawn in towards the seismic vertical, Mallet found that -twenty-three of these intersected at a depth of 7⅛ geographical miles. -The maximum depth was 8⅛ geographical miles, and the minimum depth 2¾ -geographical miles. - -The mean depth was taken at a depth of 5¾ geographical miles where, -within a range of 12,000 feet, eighteen of the wave paths intersected -the seismic vertical. - -The point where these wave paths start thickest is at a depth not -greater than three geographical miles, and this is considered to be the -vertical depth of the focal cavity itself. - -For the Yokohama earthquake of 1880, from the indications of -seismometers, and by other means, certain angles of emergence were -obtained, leading to the conclusion that the depth of origin of that -earthquake might be between 1½ and 5 miles. - -Possibly, perhaps, the earthquake may have originated from a fissure -the vertical dimensions of which was comprised between these depths. - -A source of error in a calculation of this description is that the -vertical motions may have been a component of transverse motions or -perhaps due to the slope of surface waves. - -The following table of the depths at which certain earthquakes have -originated has been compiled from the writings of several observers. - - +---------------------------------+-----------------------------+ - | | In feet | - +----------------+----------------+---------+---------+---------+ - | | | Minimum | Mean | Maximum | - | Rhineland | 1846 (Schmidt) | | 127,309 | | - | Sillien | 1858 (Schmidt) | | 86,173 | | - | Middle Germany | 1872 (Seebach) | 47,225 | 58,912 | 70,841 | - | Herzogenrath | 1873 (Lasaulx) | 16,553 | 36,516 | 56,477 | - | Neapolitan | 1857 (Mallet) | 16,705 | 34,930 | 49,359 | - | Yokohama | 1880 (Milne) | 7,920 | 17,260 | 26,400 | - +----------------+----------------+---------+---------+---------+ - -A table similar to this has been compiled by Lasaulx.[86] - -With the exception of the determination for the two last disturbances -these calculations have been made with the assistance of the method -of Seebach, which depends, amongst other things, on the assumptions -of exact time determinations, direct transmission of waves from the -centrum, and a constant velocity of propagation. - -Admitting that our observations of time are practically accurate, it -appears that the other assumptions may often lead to errors of such -magnitude that our results may be of but little value. - -From what has been said respecting the velocity with which earth -disturbances are propagated, it seems that these velocities may vary -between large limits, being greatest nearest to the origin. - -If we refer to Seebach’s method, we shall see that a condition of this -kind would tend to make the differences in time between various places, -as we recede from the _epicentrum_, greater than that required for the -construction of the hyperbola. The curve which is obtained would, in -consequence, have branches steeper than that of the hyperbola, and the -resultant depth, obtained by the intersection of the asymptotes of this -curve with the seismic vertical, indicates an origin which may be much -too great. - -Another point worthy of attention, which is common to the method -of Mallet as well as to that of Seebach, is the question whether -the shock radiates directly from the origin, or is propagated from -the origin more or less vertically to the surface, and then spreads -horizontally. We know that earthquakes, both natural and artificial, -may be propagated as undulations on the surface of the ground, and -that the vertical motion of the latter, as testified by the records -of well-constructed instruments, has no practical connection with the -depth from which the disturbance originated. - -In cases like these, the direction of cracks in buildings, and other -phenomena usually accredited to a normal radiation, may in reality be -due to changes in inclination of the surface on which the disturbed -objects rested. When our points of observation are at a distance from -the _epicentrum_ of the disturbance which, as compared with the depth -of the same, is not great, calculations or observations based on the -assumption of a direct radiation of the disturbance may possibly lead -to results which are tolerably correct. The calculations of Mallet -for the Neapolitan earthquake appear to have been made under such -conditions. - -For smaller earthquakes, and for places at a distance from the seismic -vertical of a destructive earthquake, the results which are deduced -from the observations on shattered buildings, and all observations -based upon the assumption of direct radiation, we must accept with -caution. - -Another error which may enter into calculations of this description -is one which has been discussed by Mallet at some length. This is the -effect which the form and the position of the focal cavity may have -upon the transmission of waves. - -Should the impulse originate from a point or spherical cavity, then -we might, in a homogeneous medium perhaps, regard the isoseismals as -concentric circles, and expect to find that equal effects had been -produced at equal distances from the _epicentrum_. Should, however, -this cavity be a fissure, it is evident that even in a homogeneous -medium the inclination of the plane of such a cavity will have -considerable effect upon the form of the waves which would radiate from -its two walls. - -For example, let it be assumed that the first impulse of an earthquake -is due to the sudden formation of a fissure, rent open from its centre, -and that the waves leave the walls at all points normal to its surface. -Then, as Mallet points out, it is evident that the disturbance will -spread out in ellipsoidal waves, the greatest axis of which will be -perpendicular to the plane of the fissure. - -By taking a number of cases of fissures lying in various directions -and drawing the ellipsoidal waves which would result from an elastic -pressure, like that of steam suddenly admitted into such cavities, the -differences in effect which would be simultaneously produced by these -waves on reaching the surface can be readily understood. The following -example of an investigation on this subject will serve as an example to -illustrate the general nature of the many other cases which might be -taken. - -[Illustration: FIG. 33.] - -Let a disturbance simultaneously originate from all points of the -fissure _f_ _f_. This will spread outwards in ellipsoidal shells to -the surface of the earth _e_ _e_. The major axis of these ellipsoidal -shells will be the direction of greatest effect. In the direction _c_ -_d_ the waves will plunge into the earth, and places to the right side -of the fissure will, to use an expression due to Stokes, when speaking -of analogous phenomena connected with sound, be in _earthquake shadow_. -The same expression has been employed, somewhat differently, when -speaking of the effects produced on buildings. - -For places, like _s_ and _p_, situated at equal distances from the -seismic vertical, it is evident that the intensity of the shock will be -different, and also its time of arrival. It will also be observed that -the isoseismals will take the form of ovals or distorted ellipses, the -larger or fuller end of which being to the left of the fissure. - -Other cases, like those just given, which are discussed by Mallet in -his account of the Neapolitan earthquake, are where the fissure forms -the division between materials of different elasticities. In the hard -and more elastic material the waves will be more crowded, the velocity -of a wave particle will be greater, and the transit will be quicker -than in the less elastic medium. - -The result is that the distance of equal effect from the seismic -vertical will be greatest in the direction of the more compressible -material. - -Unless these considerations are kept carefully before the mind when -investigating the depth and, we may add, the position and form of the -centrum of an earthquake, serious errors may arise. - -_Greatest depth of an earthquake origin._—A curious but instructive -calculation which Mallet made was a determination of the greatest -possible depth at which an earthquake may occur. This calculation is -based upon the idea that the impulsive effect of an earthquake has an -intimate relationship with the height of neighbouring volcanoes, the -column of lava supported on a volcanic cone being a measure of the -internal pressure tending to rupture the adjacent crust of the earth. - -Mitchell, in 1700, virtually propounded this idea, when he suggested that -the velocity of propagation of an earthquake was related to the height -of such a column.[87] - -Mallet showed that there was probably considerable truth in such a -supposition by appealing to the results of actual observation. The -pressure gauge of the Neapolitan district would be Vesuvius, the height -of which has in round numbers varied between 3,500 to 4,000 feet. One -of the most destructive earthquakes in this district—namely, the one of -1857—projected bodies with an initial velocity of about fifteen feet -per second. The Riobamba earthquake, which projected bodies with an -initial velocity of eighty feet per second, appears to have been the -most violent earthquake, so far as its impulsive effort is concerned, -of which we have any record. It occurred amongst the Andes, where there -are volcanoes from 16,000 to 21,000 feet in height. - -Comparing these two earthquakes together, we see that the Riobamba -shock had a destructive power 5·33 times that of the Neapolitan shock, -and we also see that the Riobamba volcanoes were about 5·33 times -higher than Vesuvius. The accordance in these quantities is certainly -interesting, and tends to substantiate the idea that volcanoes are -barometrical-like pressure gauges of a district. - -Carrying the argument still further. Mallet says that if the depth of -origin of earthquakes were the same, then the _area of disturbance_ -would, for like formations and configuration of surface, be a measure -of the earthquake effort, and also some function of the velocity of the -wave. From this we may generally infer ‘that earthquakes, like that of -Lisbon, which have a _very great area_ of sensible disturbance, have -also a very deep seismal focus, and also the greatest depth of seismal -focus within our planet is probably not greater than that ascertained -for this Neapolitan earthquake, multiplied by the ratio that the -velocity of the Riobamba wave bears to that of its wave, or, what is -the same thing, by the ratio of the altitudes of the volcanoes of the -Andes to that of Vesuvius.’ - -Now, as the depth of the Neapolitan shock may be taken at 34,930 -feet, the greatest probable depth of origin of any earthquake impulse -occurring in our planet is limited to 5·333 × 4,930 feet, or 30·64 -geographical miles. - -Ingenious as this argument is, we can hardly admit it without certain -qualifications. - -First, we are called upon to admit the identity of the originating -cause of the volcano and the earthquake—as to what may be the -originating cause of earthquakes we have yet to refer, but certainly in -the case of particular earthquakes, as, for instance, those which occur -in countries like Scotland, Scandinavia, and portions of Siberia, the -direct connection between these phenomena are not at first sight very -apparent. - -Secondly, even if we admit the identity of the origin of these -phenomena, it is not difficult to imagine that the fluid pressure -brought to bear upon certain portions of the crust of the earth may -possibly in many instances be infinitely greater than that indicated -by the height of the column of liquid lava in the throat of a volcano, -the true height of which we are unable to obtain. Further, in certain -instances such a column only appears to be a measure of the pressure -upon the crust of the earth in the immediate vicinity of the cone. - -Thus, in the Sandwich Islands, we have lava standing in the throat -of the volcano of Mauna Loa 10,000 feet higher than it stands in the -crater Kilauea, only twenty miles distant. That these columns should be -measures of the same pressure, originating in a general subterranean -liquid layer with which they are connected, is a supposition difficult -to satisfactorily substantiate. - -Another measure of the impulsive efforts which subterranean forces -may exert upon the crust above them is evidently the height to which -volcanoes eject materials. Cotopaxi is said to have hurled a 200-ton -block of stone nine miles. Sir W. Hamilton tells us that in 1779 -Vesuvius shot up a column of ashes 10,000 feet in height; and Judd -tells us that this same mountain in 1872 threw up vapours and rock -fragments to the enormous height of 20,000 feet. This would indicate an -initial velocity of 1,131 feet per second. - -Notwithstanding Mallet’s calculation that thirty miles is the limiting -depth for the origin of an earthquake, the origin of the Owen’s -Valley earthquake of March 1872 was estimated as being at least fifty -miles.[88] - -_Form of the focal cavity._—Among the various problems which are put -before those who study the physics of the interior of our earth it -would at first sight appear that there was none more difficult than the -attempt to determine the form of the cavity, if it be a cavity, from -which an earthquake originates. Almost all investigators of seismology -have recognised that the birthplace of an earthquake is not a point, -and have made suggestions about its general nature. The ordinary -supposition is that the earthquake originates from a fissure, and if -the focus of a disturbance could be laid bare to us it would have the -appearance of a fault such as we so often see exposed on the faces of -cliffs. - -A strong argument, tending to demonstrate that some of the shakings -which are felt in Japan are due to the production of such fissures, -is the fact that the vibrations which are recorded are transverse to -a line joining the point of observation and the district from which, -by time observations, we know the shock to have originated. The most -probable explanation of this phenomena appears to be that one mass of -rock has been sliding across another mass, giving rise to shearing -strains, and producing waves of distortion. - -The first seismologist who attacked the problem of finding out the -dimensions and position of such a fissure was Mallet, when working on -the Neapolitan earthquake of 1857. The reasons that the origin should, -in the first place, have been a fissure, rather than any other form of -cavity, was that such a supposition seemed to be _a priori_ the most -probable, and, further, that it afforded a better explanation of the -various phenomena which were observed, than that obtained from any -other assumption. - -The method on which Mallet worked to determine the form and position of -the assumed fissure, which method was subsequently more or less closely -followed by other investigators, was as follows:— - -From an observation of the various phenomena produced upon the surface -of the disturbed area, a map of isoseismals was constructed. These were -seen, as has been the case with many earthquakes, not to distribute -themselves in circles round the _epicentrum_, but as distorted oval or -elliptical figures, the major axes of which roughly coincided with each -other. Further, the _epicentrum_, did not lie in the centre of these -ovals, but was near to the narrow end where they converged. - -This at once showed, if the reasoning respecting the manner in which -waves are propagated from an inclined fissure be correct, that the -fissure was at right angles to the major axis of the curves, dipping -from their narrow end downwards, in the direction of their larger -widespread ends. - -The next weapon which Mallet employed to attack this problem was the -sound which was heard at different points round about the focus. These -sounds appear to have been of the nature of sudden explosive reports -accompanied by rushing, rolling sounds. The form of the area in which -these sounds were heard was closely similar to that of the first two -isoseismals. Except in the central area of great disturbance, no sound -was heard to accompany the shock. - -Those at the northern and southern extremity of the sound area all -described what they heard as a ‘low, grating, heavy, sighing rush, of -twenty to sixty seconds’ duration.’ Those in the middle and towards the -east and west boundaries of this area described a sound of the same -tone, but shorter and more abrupt, and accompanied with more rumbling. - -The nature of the arguments which were followed in discussing the sound -observations will be found in the chapter relating to these phenomena. - -A portion of the argument which it is difficult to follow relates to -the maximum rate at which it can be supposed possible for a fissure to -be rent in rocks, which rate depends on the density and elasticity of -these rocks and other constant factors. - -Next it was observed that the paths of the waves drawn on the surface, -although generally intersecting in a point, did not do so absolutely, -but along a line passing through the main focus some 7½ miles in -length. This, coupled with the observations of sounds, led to the -supposition that the centre of disturbance, considered horizontally, -originated along a curved line passing through the chief focus and the -various intersections of the wave paths. - -The last phenomena brought forward to assist in the solution of this -interesting problem were a study of the tremulous movements that -preceded and followed the shock, and their relation to the sound -phenomena. - -If the earthquake originated by the formation of a fissure, after the -rending has gone on for a certain time the focal cavity is enlarged -to a certain extent, and the great shock takes place. This would be -followed by concluding tremulous waves. A succession of phenomena like -those accompanied the shock about which Mallet writes. - -By observations such as these, coupled with what has been said about -the maximum and mean depths of the focal cavity, Mallet came to the -conclusion that the focal cavity was a fissure, the rending open of -which produced the earthquake. The vertical dimensions of this cavity -were not more than 5·3 miles, but were probably limited to three miles. - -From the intersection of the wave paths upon the surface and the -observed emergences, this fissure followed horizontally a curve of -double flexure, about nine geographical miles in length. The area of -this fissure was twenty-seven geographical miles. The time of rending -it open in Apennine limestone would be about 7½ seconds, which -should be the same as the period during which tremors were felt. The -time actually recorded was six or eight seconds. - -Briefly, this is, then, the line of reasoning which was followed by -Mallet in an investigation the results of which are as interesting -as they are startling. Since the line of investigation has been -opened, and the existence of new problems has been indicated, other -investigators, although not exactly following Mallet’s method in -all their details, have, when endeavouring to attain the same ends, -employed similar weapons. - -Thus, for example, Seebach, when determining the depth and nature of -the origin of the earthquake of Middle Germany, reasoned somewhat as -follows:— - -Had the origin been more or less of a spherical cavity, then the region -of most violent disturbance upon the surface would, according to a -theorem we have already mentioned, have been upon or near a circle of -about 8·8 miles in radius round the _epicentrum_. This region, however, -was found by observation to lie along a curved band about forty miles -in length, altogether on one side of the _epicentrum_. - -To explain this anomaly Seebach followed Mallet, and assumed that the -origin was not a spherical cavity, but a fissure. - -The depth and strike of this fissure was determined by the observation -that the area of greatest disturbance was along a curved line lying -radial to the _epicentrum_. Such a condition it was assumed indicated -that the fissure of origin must be inclined towards this area of -greatest disturbance. A line was then drawn from this area to the -_centrum_. A second line at right angles to this one gave the dip of -the fissure. - -Höfer, when working on the earthquake of Belluno, came to the -conclusion that the disturbance originated from two faults meeting -each other at an angle of 60°. In this determination he was chiefly -influenced by the form of a certain homoseist which was of the form of -an elongated ellipse met on one side by a second ellipse, the principal -axes of the two ellipses giving the strike of the two faults. - - - - - CHAPTER XII. - - DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME. - - General distribution of earthquakes—Occurrence along lines—Examples - of distribution—Italian earthquake of 1873—In Tokio—Extension - of earthquake boundaries—Seismic energy in relation to - geological time; to historical time—Relative frequency of - earthquakes—Synchronism of earthquakes—Secondary earthquakes. - - -_General distribution of earthquakes._—The records of earthquakes -collected by various seismologists lead us to the conclusion that -at some time or other every country and every ocean in the world -has experienced seismic disturbances. In some countries earthquakes -are felt daily, and from what will be said in the chapter on earth -pulsations it is not unlikely that every large earthquake might with -proper instrumental appliances be recorded at any point on the land -surfaces of our globe. The area over which any given earthquake extends -is indeterminate. The area over which an earthquake is sensible is -sometimes very great. The Lisbon shock of 1755 is estimated as having -been sensible over an area of 3,300 miles long and 2,700 miles wide, -but in the form of tremors and pulsations it may have shaken the whole -globe. - -The regions in which earthquakes are frequent are indicated in the -accompanying map, which, to a great extent, is a reproduction of a map -drawn by Mallet. The regions coloured with the darkest tint are those -where great earthquakes are the most frequent. The actual number of -earthquakes which have been felt in the differently coloured areas are -given, when speaking of the relation of seismic energy to season. - -When looking at this chart it must be remembered that if we were -to make a detailed map of any one of the different countries where -earthquakes are frequent, we should find in it all the differences that -we observe in the general chart. For instance, one portion of Japan, -where perhaps sixty shocks are felt per year, would be coloured with -a dark tint, whilst other portions of the same country, where there -is only one slight shaking felt every few years, would be left almost -uncoloured. The black dots indicating the position of volcanic vents -are even more general in their signification than the tinted areas. -Professor Haughton gives for the world a list of 407 volcanoes, 225 of -which are active. These numbers are the same as those given by A. von -Humboldt. Of the active volcanoes 172 are on the margin of the Pacific, -and of the total number eight are in Japan. From my own observations -in Japan independently of the Kurile Islands, I have enumerated -fifty-three volcanoes which are either active or have been active -within a recent period. In a few years’ time this list will probably be -increased. I mention this fact to show how very imperfect our knowledge -is respecting the number of volcanic vents existing on our globe. If we -were in a position to indicate the volcanoes which had been in eruption -during the last 4,000 years, the probability is that they would number -several thousands rather than four or five hundred. - -An inspection of the map shows that earthquakes chiefly occur in -volcanic and mountainous regions. The most earthquake-shaken regions of -the world form the boundaries of the Pacific ocean. It may be remarked -that these boundaries slope beneath the neighbouring ocean at a much -steeper angle than the boundaries of countries where earthquakes occur -but seldom. The coasts of South America, Kamschatka, the Kuriles, -Japan, and the Sandwich Islands, for example, have slopes beneath the -Pacific from one in twenty to one in thirty. The coasts of Australia, -Scandinavia, and the eastern parts of South America, where earthquakes -are practically unknown, have slopes from one in fifty to one in two -hundred and fifty. Many earthquakes have taken place in mid-ocean. In -the Atlantic Ocean M. Perrey has given about 140 instances of such -occurrences. - -The majority of the earthquakes which shake Japan appear to have their -origin in the neighbouring ocean. If we could draw a map of earthquake -origins, it is probable that the greater number of the marks indicating -these origins would be found to be suboceanic and along lines parallel -to the shores of continents and islands which rise steeply from the -bed of deep oceans. In countries like Switzerland and India, our marks -would hold a relationship to the mountains of these countries.[89] -Looking at the broad features of the globe, we see on its surface -many vast depressions. Some of these saucer-like hollows form land -surfaces, as in central Asia. The majority of these, however, are -occupied by the oceans. Active volcanoes chiefly occur near the rim of -the hollows which have the steepest slopes. The majority of earthquakes -probably have their origin on or near the bottom of these slopes. To -these, however, there are exceptions, as for instance the earthquakes -in the Alps, in the hills of Scotland, and the shakings which are -occasionally felt in countries like Egypt. The earthquakes which shake -the borders of the Pacific have their origins in, and their effects are -almost exclusively felt on, the sides of the bounding ridge facing this -ocean. In Japan it is the eastern sides of the islands which suffer, -the western side being almost as free from these convulsions as England. - -Similar remarks may be made about the eastern side of South America, -especially the southern portion of the continent. At Buenos Ayres, for -example, there has been no disturbance since Mendoza was destroyed, -some twenty years ago. In British Guiana slight shocks are occasionally -felt in the low delta which forms the settled portion of the colony, -but they are extremely rare. - -_Disturbances in lines or zones._—It has often been observed that -disturbances are propagated along the length of mountains or valleys, -and it is but seldom that earthquakes cross them transversely. Thus the -valleys of the Rhone, the Rhine, and the Danube are lines along which -disturbances travel. - -The major axes of the elliptical areas of disturbances which have -shaken India have a general direction parallel to the valley of the -Ganges along the flanks of the Himalayas. - -The disturbances which have shaken London appear to have been chiefly -east and west, or along the valley of the Thames. In South America the -line of disturbance is along the western sides of the Andes. Another -line is along the northern coast of the continent through Andalusia -and Caraccas towards the Antilles and Trinidad. The shocks of the -Pyrenees are chiefly felt along the southern side of these mountains. -In the middle and on the northern side they are but seldom felt. This -propagation in lines or zones may in certain cases be apparent rather -than real. Thus the north and south ranges of mountains in Japan are -mountains almost simultaneously shaken along their eastern flanks, -giving the impression that an earthquake had originated simultaneously -from a fissure parallel to this line, or else, starting at one end, -had run down their lengths. Time observations have, however, shown -that such disturbances had their origin at some distance in the ocean, -and, travelling inwards, had reached all points on the flanks of these -mountains almost simultaneously. The same explanation will probably -hold for the so-called linear disturbances of western South America. - -All earthquake disturbances have probably a tendency to radiate from -their source, and are only prevented from doing so by meeting with -heavy mountainous districts, which by their mass and structure absorb -the energy communicated to them. Much energy is also lost by emergence -on the open flanks of a range of mountains. Rather than say that -high mountains often bound the extension of an earthquake, or that -earthquakes appear to run along the flanks of such mountains, we might -say that earthquakes have boundaries parallel to the strike of the -rocks in a given district, that such a direction is the one in which -the propagation is the easier. - -Rossi is of opinion that volcanic fractures play an important part in -governing the distribution of seismic disturbances. When a volcano is -formed, a series of starlike fractures are formed round the central -crater. Secondary craters may indicate the line of these fissures. The -mountains about Rome are regarded as typical of this radial structure. -The more distant the secondary craters are from the centre of the -system, the smaller will they be, and also the younger. If two fissures -intersect we get a larger crater at the junction. Earthquakes are -propagated along the direction of these fissures, whilst the rising -and falling of these lips throw off transverse waves. Rossi adduces -observations which appear to meet with explanation on such suppositions. - -Suess, who has written upon the earthquakes of lower Austria, shows how -the majority of the disturbances have had their origin along certain -lines which form a break in the continuity of the Alps. One line runs -north-east from Bruck towards Vienna. Near Wiener Neustadt, where the -greatest number and heaviest shocks have occurred, this line is met by -a north-north-west line crossing the Danube and following the valley -of the river Kamp.[90] Hoeffer has drawn similar lines from the head -of the Adriatic, one set running north-north-east to intersect near -Litschau, and the other north-north-west to intersect near Frankfort in -the valley of the Rhine.[91] - -_Examples of distribution._—A curious example of the distribution of -seismic movement is that of the earthquake of March 12, 1873, worked -out by Professor P. A. Serpieri. This earthquake appears to have been -simultaneously felt on the Dalmatian coast and in central Italy, in a -region lying north-east from Rome and south-east from Florence. In both -of these areas the motion was from south-east to north-west. The shock -then radiated from the central Italian regions, so that at places on -the western shore of the Adriatic it was felt after it had been felt on -the Dalmatian coast. - -Many explanations might be offered for this peculiar distribution -of seismic activity. Possibly the shock originated at a great depth -beneath the bed of the southern part of the Adriatic, and by following -existing lines of weakness simultaneously reached the surface of the -earth in central Italy and Dalmatia. - -In Tokio, which is built partly on a flat plain, partly in valleys -denuded from a low tableland, and partly on the spurs of the tableland -itself, the distribution of earthquakes is a subject yet requiring -attention. Sometimes it has happened that persons in one house have -been sufficiently alarmed to escape into the open air, whilst others, -not more than a mile distant, have not been aware that the city had -been shaken. - -[Illustration: FIG. 34. - Areas almost simultaneously struck from S.E. to N.W. [graphic] - - Subsequent radial disturbance [graphic]] - -_Extension of earthquake boundaries._—Natural obstructions which may be -sufficient to retard small earthquakes may in certain instances not be -found sufficient to retard the larger disturbances. Thus the shocks of -Calabria are usually only felt on the western side of the Apennines, -but instances have occurred when they have crossed this barrier. In -1801 the earthquake of Cumana crossed a branch of the coast range. - -Sometimes earthquake boundaries give way, and countries which they -sheltered subsequently become exposed to all disturbances. The true -explanation of this is probably in a shifting of the centre of seismic -activity. Thus up to December 14, 1797, although Cumana was often -devastated, the peninsula of Araya was not hurt. On this date Araya -commenced to suffer, and has continued to suffer ever since. - -Fuchs gives an example of the movement of a seismic centre in the case -of the Calabrian earthquake. The first shock commenced near Oppiedo, -the second shock commenced four or five miles farther to the north, and -the third shock had its origin five or six miles still farther, near to -Girifalco. - - - - - CHAPTER XIII. - - DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_). - - -_Seismic energy in relation to geological time._—If we admit that -seismic energy is only a form of volcanic energy, it must also be -admitted that any cause tending to produce a general decrease in the -amount of the latter will also produce an alteration in the amount of -the former. - -The nebular hypothesis of Laplace tells us that the solar system is the -result of the whirling of a heated gaseous mass, which as it cooled -continually contracted and consequently whirls the faster. With this -hypothesis before us, we understand why all the planets and their -satellites have a similarity in the directions of their movements, why -they revolve nearly in the same plane, in orbits nearly circular, why -some have a flattened figure and are surrounded by rings or belts, why -the exterior planets should have a greater velocity of rotation, a -greater number of satellites, and a less density as compared with the -interior planets, the similarity of the elements in meteoric stones, -the sun, the stars, and those found upon our earth, and lastly why -there should be an increase in temperature as we descend into our -earth.[92] This increase in temperature as we descend into the earth -as deduced from many observations appears to be about 1° F. for every -fifty or sixty feet of descent. - -To explain this and other kindred phenomena it is assumed that the -earth was once very much hotter than it is at present, and to reach its -present stage it has been gradually cooling. As the laws of cooling are -perfectly known, to calculate how many years it must have taken a body -like our earth to cool down to its present temperature is a definite -problem. Sir William Thomson, starting with the temperature of 7,000° -F., when all the rocks of the earth must have been molten and a skin or -crust upon the surface, such as is so quickly produced upon the surface -of molten lava, finds by calculation that the time taken to reach the -present temperature must have been about one hundred million years. -Into this period he and other physicists desire to compress the history -of all the stratified deposits. Geologists find this period too short. -Others seeking to reconcile the views of physicists and geologists -endeavour to show that the various agencies engaged in degrading rocks -and accumulating sediments in former ages are not to be judged of by -the agencies we now see around us; in former times they were more -active. At one period the elastic tides in the earth may have been so -great that they resulted in the fracturing off from our planet its -satellite the moon, and subsequently the moon, acting on the waters of -the earth, may, even as late as 150,000 years ago, have produced every -three hours tides 150 feet in height. - -Whatever may be the value of the figures here quoted, reasonings like -these bring us to the conclusions that the various agencies which we -now know to be acting upon our earth were once far more potent than -they are at present, and if the moon, as a producer of elastic tides, -has any influence upon the occurrence of earthquakes, it must have had -a much greater influence in bygone times. - -We might speak similarly with regard to the internal heat of the earth. - -From the present heat gradient of our globe it is possible to calculate -how much heat flows from the earth every year. - -This is equivalent to a quantity which would raise a layer of water ·67 -centimetres thick, covering the whole of our globe, from a temperature -of 0° to 100° C. - -Similarly, we might calculate the quantity of heat which would be lost -when the average heat gradient, instead of being 1° F. for fifty feet -of descent, was 1° F. for twenty-five feet of descent. - -We might also calculate how many years ago it was since such a gradient -existed. - -The general result which we should arrive at would be that in past -ages the loss of heat was more rapid than it is at present. Now the -contraction of a body as it cools is for low temperatures proportional -to its loss of heat, and this law is also probably true for contraction -as it takes place from high temperatures. - -Contraction being more rapid, it is probable that phenomena like -elevations and depressions would be more rapid than they are at -present, and generally all changes due to plutonic action, as has -already been pointed out by Sir William Thomson, must have been more -active. - -We have, therefore, every reason to imagine that earthquakes which -belong to the category of phenomena here referred to were also numerous -and occurred on a grander scale during the earlier stages of the -world’s history than they do at present, and seismic and volcanic -energy, when considered in reference to long periods of time, is -probably a decreasing energy. - -In making this statement we must not overlook the fact that in -geological time, as testified by the records of our rocks, volcanic -action, and with it probably seismic action, has been continually -shifting, first appearing in one area and then in another, and even -in the same area we have evidence to show that these have periods -of activity and repose successively succeeding each other. Thus in -Britain, during the Palæozoic times, we have many evidences of an -intense volcanic activity. During the Mesozoic or Secondary period -volcanic energy appears to have subsided, to wake up with renewed -vigour in the Cainozoic or Tertiary period. - -During this latter period it is not at all improbable that Scotland -was in past times as remarkable for its earthquakes as Japan is at the -present day. - -Later on it will also be shown that earthquakes are concomitant -phenomena, with those elevatory processes which we have reason to -believe are slowly going on in certain portions of the earth’s crust. -If, therefore, we are able by the examination of the rocks which -constitute the accessible portions of our globe to determine which -periods were characterised by elevation, we may assume that such -periods were also periods of seismic activity. - -Amongst these periods we have those in which various mountain ranges -appeared. Thus the Grampians, and the mountains of Scandinavia, were -probably produced before the deposition of the Old Red sandstone. The -Urals were upheaved prior to Permian times. The chief upheaval in the -Alps took place after Eocene times. The Rigi and other sub-Alpine -mountains were formed after the deposition of the Miocene beds. About -this same time the Himalayas were upheaved.[93] - -The earthquakes which from time to time shake those newer mountains -apparently indicate that the process of mountain-making is hardly ended. - -_Seismic energy in relation to historical time._—The distribution -of seismic energy with regard to historical time is a subject which -has been very carefully examined by Mallet, who collected together -a catalogue of between six and seven thousand earthquakes, embraced -between the periods B.C. 1606 and A.D. 1850. The earthquake of B.C. -1606 was on the occasion of the delivery of the law at Mount Sinai. -Between B.C. 1604 and B.C. 1586 an earthquake probably occurred in -Arabia, when Korah, Dathan, and Abiram were swallowed up. Another -biblical record is that of B.C. 1566, when the walls of Jericho were -overthrown. - -The earliest records from China is in B.C. 595; in Japan B.C. 285; in -India A.D. 894. - -By using the number of earthquakes which have been recorded in each -century as ordinates, Mallet constructed a curve, which apparently -shows a continual increase in seismic energy, especially during recent -times. This, Mallet remarks, contradicts all the analogies of the -physics of the globe, and points out that the rapid increase in the -number of earthquakes in latter years is chiefly due to the greater -number of records which have been made, and the increase of the area -of observation. No doubt many of the records made by the ancients have -been lost. - -If we compare Mallet’s records, as he invites us to do, with the -great outlines of human progress, we see that the two increase -simultaneously, and we come to the conclusion that, taken as a whole, -during the historical period the seismic activity of the world has been -tolerably constant. - -These conclusions, based on the evidence at our command, are not to -be confuted. If, however, instead of considering the seismic energy of -the whole world, we consider the seismic energy of particular areas, -it seems reasonable to expect that in certain instances sometimes a -decrease and sometimes an increase in this energy might be discovered, -especially, perhaps, in areas which are highly volcanic. - -In France we know that volcanic activity ceased at a period closely -bordering on historical times, and it is not unlikely that seismic -activity may have ceased at a corresponding time. - -In a country like Japan, it is possible that in one district seismic -energy may be on the increase, whilst in another upon the decrease. - -In a country like England, it is probable that the seismic state is -constant, and, whatever changes may be now occurring, they are taking -place at so slow a rate that, even if our records of the historical -period were complete, we could hardly be expected to find these changes -sufficiently marked to be observable. - -For purposes of reference, and also for examining the present question, -the table, page 240, has been compiled. The earthquakes given are -chiefly those which have been recorded in histories as being more or -less destructive. - -In the second column of this table will be seen the number of -earthquakes which have occurred in Japan during each century, the -centuries being marked in the first column. In columns 3 to 18 -inclusive are given the number of earthquakes which have occurred -during different centuries in the various countries and districts -mentioned at the head of each column. These latter, which are taken -from the writings of Mallet, are given for the sake of comparison with -the Japanese earthquakes. If we commence with the seventh century in -the column for Japan, we see that a great increase in the number of -earthquakes, as we come towards the present time, is not so observable -as it is in the other columns. - - Key: - 1 Centuries - 2 Japan - 3 Scandinavia and Iceland - 4 British Isles and Northern Isles - 5 Spanish Peninsula - 6 France, Belgium, Holland - 7 Rhine Basin - 8 Switzerland and Rhine Basin - 9 Danube Basin - 10 Italy, Sicily, Sardinia, and Malta - 11 Supplemental table for Italy, Sardinia, and Malta - 12 Turco-Hellenic Territory, Syria, Ægean Isles, and Levant - 13 United States and Canada - 14 Mexico and Central America - 15 Antilles - 16 Cuba - 17 Chili and La Plata Basin - 18 Northern Zone of Asia - 19 Approximate Intensity in the Kioto District of Japan - +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+ - | 1 |2 | 3 | 4 |5 | 6 |7 | 8 | 9 | 10|11| 12|13|14| 15|16| 17|18|19| - +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+ - |I. | 1| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--| - |II. |--| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--| - |III. | 1| --| --|--| --|--| --| --| --|--| --|--|--| --|--| --|--|--| - |IV. |--| --| --|--| --|--| --| --| 6|--| 23|--|--| --|--| --|--|--| - |V. | 1| --| --|--| 1|--| --| --| 5|--| 19|--|--| --|--| --|--|--| - |VI. | 1| --| --|--| 6|--| --|} | 3|--| 27|--|--| --|--| --|--|--| - |VII. |12| --| --|--| --|--| --|} | 1|--| 8|--|--| --|--| --|--|15| - |VIII. |11| --| --|--| --|--| --|} | 2| 1| 12|--|--| --|--| --|--|17| - |IX. |40| --| --|--| 21|--| 19|} | 6|--| 7|--|--| --|--| --|--|60| - |X. |17| --| --|--| 2|--| 2|}19| 3| 3| 5|--|--| --|--| --|--|24| - |XI. |20| --| 8| 3| 16|--| 9|} | 7| 5| 18|--|--| --|--| --|--|28| - |XII. |18| --| 11| 4| 12|--| 8|} | 18|22| 23|--|--| --|--| --|--|20| - |XIII. |16|} | 15| 3| 9|--| 3|} | 15|26| 13|--|--| --|--| --|--|16| - |XIV. |19|} | 4| 8| 21|--| 18|} | 20|51| 8|--|--| --|--| --|--|25| - |XV. |36|}28| 1| 4| 14|--| 12|} | 18|47| 11|--|--| --|--| --|--|29| - |XVI. |17|} | 8|10| 61|10| 52| 35| 32| 5| 22|--| 6| 1| 4| 5|--|17| - |XVII. |26|} | 14|10| 91|29|120| 31|121| 9| 53|10| 7| 16| 4| 9|--|11| - |XVIII.|31|111| 63|93|237|71|141| 88|438|20|124|88|24| 85| 2| 10|32| 8| - |XIX. |27|113|110|85|211|81|173|145|390|88|194|51|30|145|50|170|57| 8| - +------+--+---+---+--+---+--+---+---+---+--+---+--+--+---+--+---+--+--+ - -The explanation for this probably lies in the fact that Japan has -practised civilised arts for a longer period than many of the European -and other countries mentioned in the table. - -In Japan, no doubt, the records of later years have been more perfect -than they were in early times, but this, although so potent an effacer -of what was probably the true state of natural phenomena in the case of -Europe, has not quite obliterated the truth in Japan; for instead of an -apparent increase of seismic energy since early times it shows a slight -decrease. - -To draw up a table of earthquakes such as the one which has just been -given, and then, after the inspection of it, draw conclusions as to -whether there has been an increase or decrease in seismic energy, is, -however, hardly a just method of reasoning. The earthquakes, taken as -they are, for the whole of Japan, represent a collection of places some -of which are 1,000 miles apart. When we consider that many earthquakes -which occurred at one end of this line were never felt at the other -end, in order to justly estimate the periodicity of seismic phenomena -it would seem that we ought either to take some particular seismic area -or else the whole world. - -The particular area which has been taken is that of Kioto in Central -Japan, and the earthquakes which have been felt there are enumerated in -the table. - -In order to show the variation in seismic activity of this district a -curve has been plotted, fig. 35, with ordinates equal to the values -given for the Kioto earthquakes during succeeding centuries. The upper -points of these ascending and descending lines are joined by a free -curve. The lower points are similarly joined. The points of bisection -of ordinates drawn between these two curves are taken as points in a -curve to show the true secular change in seismic energy. - -[Illustration: FIG. 35.—Curve of Seismic Intensity for Kioto.] - -By looking at this wavy line it will be seen that the intervals between -maxima and minima are closer together in early times than they are -later on. - -Thus, between the eighth century and the ninth century, points of -maximum and minimum seismic efforts occurred at times a century apart, -whilst later on, from the eleventh to the fifteenth century, they were -at intervals of 300 years apart. - -By inspecting either the wavy line or the resultant curve, it will be -seen that since the ninth century down to the present time there has -been a decided decrease in seismic energy. From the ninth century down -to the fifteenth century this decrease is represented by a regular -curve. At this point, however, the decrease becomes slightly more -rapid, and is represented by a second curve. If, instead of calculating -ordinates for my curve, in which intensity has been considered, simply -the number of earthquakes are counted, a similar result is obtained. -From this it appears that the rate at which seismic energy decreased -during the last 500 years was about the same as that at which it -decreased during the 500 years previous to this period. - -If the lists for the Italian and Turco-Hellenic districts could be -similarly analysed, and the earthquakes of any particular district -picked out from the others, it is very probable that a similar decrease -or alteration in seismic energy might be observed. - -Provided that we have at our disposal records of the various -earthquakes which have occurred in any given district during a -sufficiently long period of time, one conclusion that we may expect -to arrive at is that we shall be able to trace some variation in -the seismic activity of that district. For the Kioto area, it has -been shown that there is a diminution in seismic activity, In other -districts, however, there may possibly be an increase.[94] - -_Relative frequency of earthquakes._—A question which is of great -interest to those who dwell in shaken districts is as to how often -disturbances may be expected to occur. - -From a general examination of this question, considering the -earthquakes of the whole world. Mallet arrived at the following -conclusions:— - -1. While the smallest or minimum paroxysmal intervals may be a year -or two, the average interval is from five to ten years of comparative -repose. - -2. The shorter intervals are in connection with periods of fewer -earthquakes—not always with those of least intensity, but usually so. - -3. The alternations of paroxysm and of repose appear to follow no -absolute law deducible from these curves. - -4. Two marked periods of extreme paroxysm are observable in each -century, one greater than the other—that of greatest number and -intensity occurring about the middle of each century, the other towards -the end of each. - -The form of the curves which Mallet has drawn seem to indicate that -seismic energy came in sudden bursts, and then subsided, gradually -gathering strength for another exhibition. This is continually seen -in the shocks experienced in various seismic areas—a large shock, or -the maximum of the activity dying out by repeated small shocks on -succeeding days. - -Mr. I. Hattori, writing on the large earthquakes of Japan, remarks that -on the average there has been one large earthquake every ten years. -They, however, occur in groups, as shown in the following table. - - +------+---------------+----------+ - |No. of| | | - |shocks| Period | Interval | - +------+---------------+----------+ - | 6 | A.D. 827–836 | 10 years | - | 6 | „ 880–890 | 10 „ | - | 4 | „ 1040–1043 | 4 „ | - | 5 | „ 1493–1507 | 5 „ | - | 4 | „ 1510–1513 | 4 „ | - | 5 | „ 1645–1650 | 6 „ | - | 5 | „ 1662–1664 | 3 „ | - | 4 | „ 1853–1856 | 4 „ | - +------+---------------+----------+ - -Dr. E. Naumann, who has also written on the earthquakes in Japan, -remarks that if periods of seismic activity do not occur every 490 -years, there is a repetition of the cycle after 980 years, but there -is much variability. A period of 68 years is very marked. On the -average, large earthquakes have occurred every 5·9 years. Fuchs -gives some interesting examples of the repetition of earthquakes at -definite intervals, of which the following are examples. Sometimes -earthquakes appear to have repeated themselves after 100 years. One -remarkable example of this is that of Lima, on June 17, 1578, which was -repeated on the same day in the year 1678. In Copiapo it is believed -that earthquakes occur every twenty-three years, and examples of such -repetitions are found in the years 1773, 1796, and 1819. In Canada, -near to Quebec, earthquakes lasting forty days are said to occur every -twenty-five years. The plateau of Ardebil is said to be regularly -shaken by earthquakes every two years. - -A. Caldcleugh, writing on the earthquake of Chili, in 1835,[95] remarks -that the Spaniards first had the idea that a great earthquake occurs -every century. Afterwards they thought the period was every fifty -years. As a matter of fact, however, there were large earthquakes in -1812, at Caracas; in 1818, at Copiapo; in 1822, at Santiago; in 1827, -at Bogota; in 1828, at Lima; in 1829, at Santiago; and in 1832, at -Huasco. - -The average period of seismic disturbances in any country probably -depends upon the subterranean volcanic activity of that country. -When the activity is great the large earthquakes may occur at short -intervals; but when the activity is small, as in England, shocks of -moderate intensity may not be felt more than once or twice per century. -A general idea of the relative frequency of the large earthquakes in -various parts of the world may be easily obtained by an inspection of -the table on page 240. - -Between the years 1850 and 1857 Kluge found that in the world there had -been 4,620 earthquakes, which is, upon the average, nearly two per day. -This estimate of the frequency of earthquakes of sufficient intensity -to be recorded without the aid of instruments is, however, much below -the truth. In Japan alone there probably occurs, as a daily average, a -number at least equal to that which has been just given for the whole -world. Boussingault considered that, in the Andes, earthquakes were -occurring every instant of time.[96] - -To state definitely how many earthquakes are felt in the world on the -average every day is, from the data which we have at our command, an -impossibility. Perhaps there may be ten, perhaps there may be 100. The -question is one which remains to be decided by statistics which have -yet to be compiled. - -After a large earthquake, smaller shocks usually occur at short -intervals. At first the succession of disturbances are separated from -each other by perhaps only a few minutes or hours. Later on, the -intensity of these shocks usually decreases, and the intervals between -them become greater and greater, until, finally, after perhaps a few -months, the seismic activity of the area assumes a quiescent state. - -The great earthquake which overtook Concepcion on February 20, 1835, -was followed by a succession of shocks like those just referred to, -there being registered, between the date of the destructive shock and -March 4, 300 smaller disturbances. - -During the twenty-four hours succeeding the destruction of Lima -(October 28, 1746), 200 shocks were counted, and up to the 24th of -February in the following year 451 shocks were felt. - -At St. Thomas, in 1868, 283 shocks were counted in nine and a quarter -hours. - -Similar examples might be taken from the description of almost -all destructive earthquakes of which we have records. For a large -earthquake to occur, and not to be accompanied by a train of succeeding -earthquakes, is exceptional. Sometimes we find that a large number of -small earthquakes have occurred without a large one being felt. Seismic -storms of this description have happened, even in England—for instance, -in the year 1750, which appears to have been a year of earthquakes for -many portions of the globe. - -In this year, which is known as the ‘earthquake year,’ shocks were felt -in England as follows: On March 14, in Surrey; March 18, in south-west -of England; April 2, at Chester; June 7, at Norwich; August 23, in -Lincolnshire; September 30, Northamptonshire. - -_Synchronism of earthquakes._—One of the first writers who drew -attention to the fact that two shocks of earthquakes have been felt -simultaneously at distant places was David Milne, who published a list -of these occurrences.[97] - -In two instances, February and March 1750, shocks were simultaneously -felt in England and Italy. In September 1833 shocks appear to have been -simultaneously felt in England and Peru. These and many other similar -examples are discussed by Mallet, who thinks with Milne that these -coincidences are in every probability matters of accident. According to -Fuchs, Calabria and Sicily appear often to have had earthquakes at the -same time, as for instance in 1169, 1535, 1638, when the town Euphemia -sank, and in the years 1770, 1776, 1780, and 1783. - -A remarkable example of coincidence occurred on November 16, 1827, when -a terrible earthquake was felt in Columbia, and at the same time a -shock occurred on the Ochotsk plains, nearly antipodal to each other. - -Kluge also gives a large number of instances of simultaneous -earthquakes; thus, on January 23, 1855, on the same day that -Wellington, New Zealand, so severely suffered, there was a heavy -earthquake in the Siebengeberge, and also in North America. To this -might be added the fact that the last destructive earthquake in Japan -occurred within a few days of this time. - -Sometimes neighbouring countries where earthquakes are common are -equally remarkable by their utter want of synchronism. For example, -Southern Italy and Syria are said never to be shaken simultaneously. - -_Secondary earthquakes._—Although it is possible that the simultaneous -occurrence of earthquakes in distant regions may sometimes be a -matter of chance, it must also be remarked that the shaking produced -by one earthquake may be sufficient to cause ground which is in a -critical state to give way, and thus the first disturbance becomes the -originator of a second earthquake. Admitting that an earthquake, as -it radiates from its centre, may act in such a manner, we see that a -feeble disturbance might be the ultimate cause in the production of a -destructive earthquake, just as the disturbance of a stone upon the -face of a scarp might, by its impact upon other stones, cause many tons -of material to be dislodged. - -It is also easy to conceive how the seismic activity of two districts -may be dependent upon each other. Inasmuch as these secondary shocks -are direct effects of primary disturbances, they might have been -treated in a previous chapter. - -As examples of consequent or secondary earthquakes Fuchs tells us that -when small earthquakes take place in Constantinople and Asia Minor, -earthquakes are felt in Bukharest, Galazy, and Kronstadt. - -The great Lisbon earthquake also appears to have given rise to several -consequent disturbances. One was in Derbyshire, occurring at 11 a.m. -It was sufficiently violent to cause plaster to fall from the sides -of a room and a chasm to open on the surface of the ground. Some -miners working underground were so alarmed that they endeavoured to -escape to the surface. During twenty minutes there were three distinct -disturbances. - -Another shock was felt at Cork.[98] - -Although these disturbances own a consequence of the Lisbon earthquake -they might properly perhaps be attributed to the pulsations produced by -the shock at Lisbon, which spread through England and other countries -without being felt. - -The shocks which men felt in New Zealand and New South Wales in 1868 -were probably secondary shocks, due to the disturbance at Arequipa and -other places on the South American coast. - -These so-called secondary earthquakes, although in many instances -they may be due to earth pulsations produced by earthquake, or to the -immediate sensible shaking of a large earthquake, may perhaps, in -certain instances, be attributed to some widespread disturbance beneath -the crust of the earth. The occurrence of periods where all earthquake -countries suffer, unusual disturbances indicate the probability of such -underground phenomena. - - - - - CHAPTER XIV. - - DISTRIBUTION OF EARTHQUAKES IN TIME (_continued_). - - The occurrence of earthquakes in relation to the position of the - heavenly bodies—Earthquakes and the moon—Earthquakes and the sun; - and the seasons; the months—Planets and meteors—Hours at which - earthquakes are frequent—Earthquakes and sun spots—Earthquakes - and the aurora. - - -_The position of the heavenly bodies and the occurrence of -earthquakes._—Since the earliest times, in searching for the cause of -various natural phenomena, man has turned his energies towards the -heavens. One of the earliest observations was the connection that -exists between the season of the year and the motions of the heavenly -bodies. Tides were seen to be influenced by the moon. In later times -it has been discovered that periods of maximum magnetic disturbances -occur every ten or eleven years with the sun spots, and Herr Kreil, of -Vienna, tells us our satellite, the moon, has also an influence upon -the magnet. - -From day to day we see the bond connecting our planet with the sun, the -moon, and other heavenly bodies which are outside us gradually becoming -closer. - -Inasmuch as many phenomena, like the motion of the tides, the rise and -fall of the barometer, fluctuations in temperature, are all more or -less directly connected with the relative position of our planet with -regard to the sun and moon, any coincidence between the phases of -these bodies and the occurrence of earthquakes more or less involves a -time relationship with the other phenomena resultant on lunar and solar -influences. - -_Earthquakes and the position of the moon._—Many earthquake -investigators have attempted to show the connection between earthquakes -and the phases of the moon. - -The first and most successful worker in this branch of seismology was -Professor Alexis Perrey, of Dijon, who, after many years of arduous -labour in tabulating and examining catalogues of earthquakes, showed -that earthquakes were more likely to occur at the following periods -than at others. - -1. They are more frequent at new or full moon (syzygies) than at half -moon (quadratures). - -2. They are more frequent when the moon is nearest the earth (perigee) -than when she is farthest off (apogee). - -3. They are more frequent when the moon is on the meridian than when -she is on the horizon. - -These results were obtained by Perrey after analysing his catalogues -by three different and independent methods, and they were confirmed -by the report of a committee appointed by the Academy of Sciences. It -must, however, be remarked that in several instances anomalies occur, -and also that the difference between the number of earthquakes at any -two periods is not a very large one. Thus, for instance, the annual -catalogues compiled by Perrey from 1844 to 1847, the earthquakes in -perigee are to those in apogee as 47 : 39. Between the years 1843 and -1872 Perrey finds that 3,290 shocks occurred at the moon’s perigee, and -3,015 at the apogee.[99] - -Between 1761 and 1800 earthquakes occurred as follows:— - - In Perigee 526 - Apogee 465 - -The following table shows the results which enabled Perrey to deduce -his first law. - -Dividing the period of lunation into quarters, with the time of -syzygies and quadratures as the centres of these quarters, he found -that the earthquakes were distributed as follows. - - +-----------+--------+----------+-------------+---------------+ - | | | | | Difference in | - | | Totals | Syzygies | Quadratures | favour of the | - | | | | | Syzygies | - +-----------+--------+----------+-------------+---------------+ - | 1843–1847 | 1,604 | 850·48 | 753·52 | 69·96 | - | 1848–1852 | 2,049 | 1,053·53 | 995·47 | 58·06 | - | 1853–1857 | 3,018 | 1,534·13 | 1,483·87 | 50·26 | - | 1858–1862 | 3,140 | 1,602·99 | 1,537·41 | 65·98 | - | 1863–1867 | 2,845 | 1,463·42 | 1,381·58 | 81·84 | - | 1868–1872 | 4,593 | 2,333·48 | 2,259·52 | 73·96 | - | 1843–1872 | 17,249 | 8,838·03 | 8,410·97 | 427·06 | - +-----------+--------+----------+-------------+---------------+ - -The reported earthquakes between 1751 and 1843 are shown to conform -with the same rule.[100] Julius Schmidt, astronomer at Athens, found -for the earthquakes of Eastern Europe and adjacent countries for the -years 1776 to 1873 that there were more earthquakes when the moon was -in perigee. Other maxima were at new moon, and two days after the first -quarter. There was a diminution at full moon, and a minimum on the day -of the last quarter. As one example of results which are antagonistic -to the general results obtained by Perrey may be quoted the results of -an examination by Professor W. S. Chaplin of the earthquake recorded at -the meteorological observatory in Tokio. The list of earthquakes, 143 -in number, extending over a period of three years, was recorded by one -of Palmieri’s instruments. The results were as follows:— - -1. There have been maxima of earthquakes when the moon was two and -nine hours east and seven hours west. At the upper transit there is a -minimum. - -2. Considering the moon’s position with regard to the sun, at -conjunction there were 32, at opposition 37, and at quadrature 74. East -of the meridian the maximum was at least four hours. - -3. When the moon was north of the equator these were 68, when south 82. - -4. A maximum of earthquakes seven and eleven days after the moon’s -perigee. The fact that these results were obtained for the earthquakes -of a special small seismic area renders them more interesting.[101] - -_Frequency of earthquakes in relation to the position of the sun._—The -question as to whether there is a connection between the frequency of -earthquakes and the relative position of the sun is to a great extent -identical with the question as to the relative frequency of earthquakes -in the various seasons. It is a subject which we find referred to -by writers in the earliest ages. Pliny and Aristotle thought that -earthquakes occurred chiefly in spring and autumn. In later times it -has been a subject which has been most carefully considered by Merian, -von Hoff, Perrey, Mallet, Volger, Kluge, and others who have devoted -attention to seismology. In a résumé of the earthquakes of Europe, and -of the adjacent parts of Asia and Africa, from A.D. 306–1843, Mallet -gives the following results:— - - +------------------+------------------------+----------------------+ - | | For Nineteenth Century | For the whole period | - +------------------+------------------------+----------------------+ - | Winter Solstice | 177 } Solstices | 253 } Solstices | - | Spring Equinox | 151 } } 306 | 170 } } 403 | - | Summer Solstice | 129 } } Equinoxes | 150 } } Equinoxes | - | Autumnal Equinox | 164 } 315 | 159 } 329 | - +------------------+------------------------+----------------------| - -The above periods were called by Perrey _critical epochs_, because -as a general result of his researches he found that at such periods -there was a greater frequency of earthquakes. Fuchs, quoting from -Kluge’s tables, extending from 1850–1857, tells us that the recorded -earthquakes occurred as follows:— - - In the Northern Hemisphere— - Equinoxes =1324= - Solstices 1202 - In the Southern Hemisphere— - Equinoxes =301= - Solstices 261 - -Earthquakes are, therefore, more frequent at the equinoxes, and this -especially at the autumnal equinox. In the northern hemisphere, at -the solstices, the greater number of shocks occur about the winter -solstices, whilst in the southern hemisphere, about the summer -solstices. - -Exceptions, however, are found in Central America and the West Indies, -in the Caucasus, and the Ægean Sea. - -The idea that earthquakes had a periodicity dependent upon the position -of the heavenly bodies is by no means confined to Europe. In a Japanese -work called ‘Jishin Setsu’ (an opinion about earthquakes) by a priest -called Tensho, it is stated that the relative positions and movements -of the twenty-eight constellations with respect to the moon cause -earthquakes. This Tensho asserts after careful calculation, and Falb -tells us that all future earthquakes can be predicted. - -In the Kuriles and Kamschatka, Sicily, and in parts of South America, -it is said that the equinoxes are regarded as dangerous seasons. - -_Frequency of earthquakes in relation to the seasons and months._—What -is here said respecting the relative frequency of earthquakes at the -different seasons and months is little more than an extension and -critical examination of the results which have been given respecting -the frequency of earthquakes in regard to the position of the sun. - -That there is a difference between the number of earthquakes which are -felt at one season of the year as compared with those felt at another -is a fact which, as seismoscopic observations are extended, is becoming -more and more recognised. - -Some of the more important results which were arrived at by Mallet from -5,879 observations made in the northern hemisphere, and 223 in the -southern hemisphere, may be expressed as follows:— - - +---------------------+------------------------+-------------------+ - | | Maxima | Minima | - +---------------------+------------------------+-------------------+ - | Northern Hemisphere | January, also a slight | May, June, and | - | | rise in August and | July | - | | October | | - | Southern Hemisphere | November, also May | March, extending | - | | and June | over one month, | - | | | also August | - +---------------------+------------------------+-------------------+ - -Julius Schmidt, of Athens, who so carefully examined the earthquakes of -eastern Europe, came to the following conclusions:— - -For the earthquakes between 1200 and 1873, a maximum on September 26 -and January 17; a minimum on December 3 and June 13. - -For the earthquakes between 1873 and 1874, a maximum on March 1 and -October 1; a minimum on July 7 and December 15. - -For all the earthquakes of eastern Europe, a maximum on January 3; a -minimum on July 8, or there was a maximum at perihelion and aphelion. - -When the months are grouped together according to the seasons, spring, -summer, autumn, and winter, we find that in the northern hemisphere the -minimum is in summer and the maximum in winter, whilst in the southern -hemisphere (giving the proper months corresponding to its seasons) we -find two maxima, one at the commencement of winter, and the other at -midsummer, whilst the minima are in spring and autumn. - -[Illustration: FIG. 36.—Curves of Monthly Seismic Intensity (Mallet).] - -In the following table the difference in the number of earthquakes felt -at different seasons is given more in detail. - -In examining this table, we must remember that for countries like Peru, -Chili, and New Zealand, lying in the southern hemisphere, the records -given for the months April to September correspond to the winter months -of those countries. The Roman numerals indicate the centuries between -which the records date. - - +----------+----------------------------------------+-------+---------+ - | | |October| April | - | | | to | to | - | | | March |September| - +----------+----------------------------------------+-------+---------+ - | {| 1. Scandinavia and Iceland, xii–xix | =129= | 91 | - | {| 2. British and Northern Isles, xi–xix | =123= | 94 | - | {| 3. Belgium, France, and Holland, iv–xix| =395= | 272 | - | {| 4. Rhone Basin, xvi–xix | =115= | 69 | - | {| 5. Switzerland and Rhine Basin, ix–xix | =327= | 205 | - | {| 6. Danube Basin, v–xix | =147= | 128 | - | {| 7. Spanish Peninsula, xi–xiv | =114= | 87 | - | {| 8. Italy, Sicily, Sardinia, and Malta, | | | - |Northern {| iv–xix | 650 | 581 | - | Regions {| 9. Turco-Hellenic Territory, Syria, | | | - | {| Ægean Isles, and Levant, iv–xix | 214 | =222= | - | {|10. Northern Zone of Asia, xviii–xix | =46= | 36 | - | {|11. Japan (Tokio area), 1872–1880 | | | - | {| (small earthquakes) | =213= | 157 | - | {|12. Japan B.C. 295-A.D. 1872 (large | | | - | {| earthquakes) | 165 | =188= | - | {|13. Algeria and Northern Africa | =26= | 20 | - | {|14. United States and Canada, xvii–xix | =86= | 48 | - | { |15. Java, Sumatra, and neighbouring | | | - | { | Islands, 1873–4–7–8 | =194= | 182 | - |Central { |16. Mexico and Central America, xvi–xix | 26 | 26 | - |Regions { |17. West Indies (Mallet), xvi–xix | 108 | =114= | - | { |18. West Indies, xvi–xix | 296 | =343= | - | { |19. Cuba, xvi–xix | =28= | 23 | - | {|20. Chili, and La Plata Basin, xvi–xix | 89 | 89 | - |Southern {|21. Peru, Columbia, Basin of Amazons, | | | - |Regions {| xvi–xix | 506 | =541= | - | {|22. New Zealand, 1869–1879 | 166 | =176= | - +----------+----------------------------------------+-------+---------+ - -Neglecting those records which show as many earthquakes for the winter -months as for the summer months, we see at a glance that generally -the greater number of shocks have happened during the colder seasons. -In the southern hemisphere, so far as the records go, this is not -true. In the northern regions, out of fourteen examples there are -two exceptions. In the central regions there are two cases where the -greatest number of earthquakes have been recorded in the winter months, -and two cases where the greatest number have been recorded for the -summer. - -Altogether, out of twenty-two examples, there are only six exceptions -to the rule. These exceptions altogether occur among records many of -which are ancient, and are, therefore, more open to error than lists -which have been compiled in modern times. - -Because small earthquakes are seldom noticed by persons out in the -open air, it might be expected that the number of earthquakes observed -in warm countries at one portion of the year would be equal to those -observed in any other season. Such an argument, however, would hardly -apply to most of the records which are quoted, as they refer to -destructive disturbances. - -If, however, we take the records made in tropical countries from the -table just given, we see that in such countries there have been almost -as many observations of earthquakes at one season as at any other. - -Another fact which might be adduced against the rule that the -greater number of earthquakes occur during the winter months would -be the comparison of a table of earthquakes recorded previous to the -nineteenth century. By doing this we see that for certain countries the -winter rule is inverted, and that the greater number of shocks are felt -during the summer. - -Notwithstanding these objections to Perrey’s conclusions, the balance -of evidence is in favour of his general result, and we may conclude -that during the colder portions of the year we may expect more shakings -than during the warmer portions. Comparing the number of earthquakes of -winter and autumn to those of summer and spring, they are to each other -in the proportion of 4 : 3. - -A fairer way to examine this question, and to determine what is -probably the present state of seismic activity in our globe, would -be only to consider the earthquakes which have taken place in -comparatively recent times, laying especial stress upon those -observations which have been made with the assistance of automatic -instruments, or those which have been collected by persons interested -in these investigations. - -For this purpose the following table, showing the distribution of -earthquakes in different countries during the nineteenth century, has -been compiled. - -The arrangement is mensual. Where the number of earthquakes in any -month is above the average, the number is printed in large type; where -below the average, in small type. - - - EARTHQUAKES OF THE NINETEENTH CENTURY, CHIEFLY FROM PERREY. - - Key: - Jan January - Feb February - Mar March - Apr April - May May - Jun June - Jul July - Aug August - Sep September - Oct October - Nov November - Dec December - Ave Average per month - +------------------------------------------------------------------------------+ - | |Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec|Ave | - |-------------------------+---+---+---+---+---+---+---+---+---+---+---+---+----| - |Scandinavia and Iceland | 17| 11| 11| 7| 7| 6| 8| 8| 10| 10| 11| 6| 9·3| - |British Isles and | | | | | | | | | | | | | | - | Northern Isles | 9| 9| 10| 7| 8| 6| 5| 11| 12| 8| 11| 12| 9 | - |France, Belgium, Holland | 27| 17| 21| 13| 13| 8| 15| 17| 15| 17| 21| 25|17 | - |Basin of the Rhone | 12| 12| 8| 3| 3| 2| 2| 4| 6| 6| 8| 14| 6·6| - |Basin of the Rhine and | | | | | | | | | | | | | | - | Switzerland | 15| 17| 13| 12| 11| 6| 12| 11| 10| 17| 24| 25|14 | - |Basin of the Danube | 14| 15| 9| 8| 12| 8| 16| 11| 11| 16| 10| 12|11·8| - |Spanish Peninsula | 10| 5| 6| 7| 4| 6| 10| 5| 9| 11| 7| 5| 7 | - |Italian Peninsula, | | | | | | | | | | | | | | - | Sicily, Sardinia, | | | | | | | | | | | | | | - | and Malta | 44| 44| 48| 43| 40| 34| 41| 46| 27| 45| 26| 39|39 | - |Turco-Hellenic Territory,| | | | | | | | | | | | | | - | Syria, Ægean Islands, | | | | | | | | | | | | | | - | and Levant | 22| 20| 10| 10| 16| 15| 14| 22| 14| 17| 12| 14|16 | - |Northern Zone of Asia | 4| 6| 6| 4| 4| 3| 5| 7| 6| 3| 4| 5| 4·7| - |1876–1881, Japan (Tokio | | | | | | | | | | | | | | - | area) | 39| 41| 41| 30| 33| 30| 27| 21| 10| 28| 34| 43|31·4| - |Japan (large earthquakes)| --| 5| 3| 3| 1| --| 5| 4| --| --| 1| 5| 2 | - |Algeria and North’rn | | | | | | | | | | | | | | - | Africa | 5| 2| 6| 7| 3| 2| 2| 5| 1| 4| 8| 1| 3·8| - |United States and Canada | 4| 4| 3| 3| 3| --| 4| 6| 3| 2| 7| 5| 3·8| - |Java, Sumatra, &c., | | | | | | | | | | | | | | - | 1873–4–5–7 and 9 | 35| 30| 38| 33| 22| 36| 27| 40| 24| 35| 30| 26|31 | - |Mexico and Central | | | | | | | | | | | | | | - | America | 3| 2| 2| 2| 6| 2| 2| 1| 1| 3| 2| 3| 2·5| - |Antilles | 9| 8| 19| 12| 12| 10| 9| 16| 12| 10| 13| 12|11·8| - |Cuba | 4| 3| 2| 3| 3| 4| 5| 2| 6| 5| 6| 4| 4 | - |Chili and La Plata | 14| 10| 14| 8| 19| 11| 16| 15| 16| 9| 27| 8|13·9| - |Peru, Columbia, Basins | | | | | | | | | | | | | | - | of Amazons, xvi–xix | 92| 83| 92| 27|106| 79| 94| 93| 97| 77| 72| 90|87 | - |New Zealand, 1869–79 | 31| 27| 37| 23| 22| 31| 27| 36| 37| 21| 27| 23|28·5| - | Jan. 1850, Dec. 1857 | | | | | | | | | | | | | | - |Northern Hemisphere |153|162|143|161|126|124|141|156|154|171|151|168|150 | - |Southern Hemisphere | 75| 43| 61| 66| 46| 42| 53| 39| 54| 55| 57| 46|53 | - | 1821–1830 | | | | | | | | | | | | | | - |Northern Hemisphere | 31| 36| 31| 29| 33| 33| 20| 31| 24| 41| 26| 34|30 | - |Southern Hemisphere | 2| --| 1| 1| 3| 1| 3| 2| 3| 2| 1| 1| 1·6| - +-------------------------+---+---+---+---+---+---+---+---+---+---+---+---+----+ - -A glance at this table shows that for most countries in the northern -hemisphere the rule that there are generally more earthquakes during -the winter months—that is, from October to March—holds good. For -countries which lie comparatively near to the Equator, and also for -those countries in the southern hemisphere, the rule is not so clear. -When examining this table it must be remembered that it does not enable -us to judge of the relative frequency of earthquakes in different -countries, inasmuch as the periods over which the records were taken -are different in different cases. - -To the above table might be added the records of P. Merian, who -examined the earthquakes felt in Basle up to 1831. As a result he -found that during the winter months eighty shocks had been felt, -whilst during the summer only forty. Taking the records for the two -hemispheres from 1850–1857, compiled by Kluge,[102] in the northern -hemisphere we have in the months between October and March 948 shocks -against 862 in the remainder of the year. In the same months in the -southern hemisphere we have for the corresponding periods the numbers -337 and 300, and thus both hemispheres would appear to follow the same -rule. If, however, we examine the table we see that the two seasons -are not so pronounced for the southern hemisphere as they are for -the northern, and that there may be two or three periods of maximum -disturbance as has been previously indicated. - -_Earthquakes and the planets and meteors._—Just as the moon and the sun -may exert an attractive influence upon the earth and cause earthquakes -to predominate at certain seasons rather than at others, several -investigators of seismic phenomena have thought that the planets might -act in a similar manner. - -M. J. Delauney, from a study of Perrey’s tables of earthquakes from -1750–1842, found two groups of maxima each with a period of about -twelve years, one commencing in 1759 and the other in 1756. Two -other groups with twenty-eight year periods respectively commence in -1756 and 1773. These groups coincide with the times when Jupiter and -Saturn reach the mean longitudes of 265° and 135°. From this Delauney -concludes that earthquakes have a maximum when the planets are in the -mean longitudes just mentioned. - -The increased number of earthquakes, especially in November, are -attributed to the passage of the earth through swarms of meteors, and -in like manner supposes the influence of Jupiter and Saturn to be due -to their passing through meteor streams situated in mean longitudes -135° and 265°. - -As a consequence of this he predicts an increase of earthquakes in the -years 1886, 1891, 1898, 1900, &c.[103] - -Dr. E. Naumann, who critically examined the large earthquakes of Japan, -showed that there was an approximate coincidence between many of the -disturbances and the thirty-three year period of meteoric showers.[104] - -Humboldt states that a great shower of meteors was seen at Quito before -the great earthquake of Riobamba (Feb. 4, 1797). The earthquakes of -1766 and 1799 at Cumana are also said to have been accompanied with -meteoric showers. Mallet gives a list of large earthquakes which -occurred at the times when meteors were observed.[105] - -_The hours at which earthquakes are most frequent._—From the -examination of a catalogue of over 2,000 earthquakes which occurred in -various parts of the world between the years 1850 and 1857, made by -Kluge, it is found that both for the northern and southern hemispheres -the observations which were made during the night generally exceed -those which were made during the day. - - +---------------------------+-----------------------+ - | | Number of Earthquakes | - +---------------------------+-----------------------+ - | | Day | Night | - |In the Northern Hemisphere | 938 | 1592 | - |In the Southern Hemisphere | 292 | 357 | - +---------------------------+---------+-------------+ - -In the northern hemisphere the greatest number were observed between -10 P.M. and 12 P.M. (360 shocks), and the fewest between 12 and 2 P.M. -(139 shocks). In the southern hemisphere, the greatest number were -observed at night between 12 and 1, and the smallest number between 1 -and 2 and 4 and 5 in the afternoon.[106] These distinctions, however, -are less distinctly marked as we approach the Equator. Schmidt found -for the earthquakes of the Orient between 1774 and 1873, that shocks -had been most frequent about half-past two A.M., and less frequent -about 1 P.M. With regard to these conclusions, which have been reached -with much labour, we might be inclined to think that they are partially -to be explained on the supposition that more observations are made -during the night than during the day—the personal experience of -residents in an earthquake country being, that many earthquakes which -occur during the day are passed by unnoticed, whilst those which occur -during the night are recorded by thousands of observers. Such a view is -certainly confirmed by the instrumental records obtained in Japan. From -1872 to 1880 inclusive there were 261 shocks recorded, 132 of which -occurred between the hours of 6 P.M. and 6 A.M. - -_Earthquakes and sun spots._—Of late years considerable attention -has been drawn to a coincidence between the occurrence of sun spots, -magnetic disturbances, rainfall, and other natural phenomena. - -These periods of sun spots occur about every eleven years, and appear -to be coincident with the periodical return of the planet Jupiter. In -Japan, Dr. E. Naumann sought for a coincidence between these periods of -sun spots and earthquakes, but without any marked results. - -Schmidt, who carefully compared his lists of earthquakes with the -appearance of sun spots, came to the conclusion that there was no -marked coincidence. The occurrence of earthquakes had sometimes -synchronised with sun spots, whilst at other times there had been a -maximum of sun spots and no earthquakes. - -M. R. Wolf[107] apparently considers that earthquakes, like volcanic -eruptions and the appearance of the aurora, are coincident with sun -spots. - -Kluge, however, came to the conclusion that when there are few sun -spots, earthquakes, like volcanic eruptions and magnetic disturbances, -have been at a maximum. - -M. A. Poey, who examined a catalogue of the earthquakes of Mexico -and the Antilles, extending from 1634 to 1870, shows by a table that -earthquakes have come in groups, first at the maxima and then at the -minima period of sun spots. Out of thirty-eight groups, seventeen -being at the maximum and seventeen at the minimum, the remaining four -are exceptions to the rule, being between the maximum and minimum. -Phenomena which are dependent upon heat occur with the minima of sun -spots, and those dependent upon cold with the maxima.[108] - -_Earthquakes and the aurora._—The possible connection between -earthquakes and the aurora is a subject which has attracted some -attention. Boué has especially made a careful examination of this -subject.[109] - -He comes to the conclusion that if we compare the monthly periods of -earthquake frequency and the aurora there is an agreement between -the two. Comparing Perrey’s tables of earthquakes from the fourth to -the nineteenth century, with tables of the aurora, one-third of both -phenomena have occurred, not only in the same day, but often at the -same hour. Between 1834 and 1847, 457 earthquakes are given and 351 -notices of the aurora. - -Out of these:— - 48 occur on the same day, - 5 occur in the same hour, - 30 approximate to the same time. - -The nearer together that these phenomena have occurred the stronger -have they been. - -Professor M. S. di Rossi brings forward many examples where there -has been a coincidence between the appearance of the aurora and -earthquakes. On 139 nights out of 211 days the aurora was seen in -some parts of Italy, and ninety-three times earthquakes were felt. On -forty-six occasions earthquakes and aurora took place together.[110] In -considering the probability of a connection existing between these two -phenomena, we must bear in mind that the aurora is at no great height -above the surface of our earth, and, further, that it can be partially -imitated. The fact that in earthquake countries, like Japan, the aurora -is practically never seen, would indicate that we can neither regard -this imperfectly understood phenomenon either as an effect or cause -of earthquakes. That earthquakes and the appearance of the aurora in -certain countries should not sometimes coincide is an impossibility. - -Dr. Stukeley, who, it must be remembered, attempted to correlate -the phenomena of earthquakes and electricity, when writing of the -disturbances which shook England in 1849 and 1850, says that the -weather had been unusually warm, the aurora borealis frequent and of -unusually bright colours, whilst the whole year was remarkable for its -fire-balls, lightnings, and corruscations.[111] - -The aurora was observed before the commencement of the Maestricht -earthquakes in 1751[112]; whilst at the time of the shock flashes of -light like lightning were observed in the sky. - -Glimmering lights were seen in the sky before the New England -earthquakes (Nov. 18, 1755), and again, before the disturbances which -occurred in the same region in 1727, peculiar flashes of light were -seen. - -Preceding the Sicilian earthquake of 1692 strange lights were seen -in the sky. Ignis fatui have also been observed with earthquakes. At -the time of auroral displays Bertelli has observed microseismical -disturbances, and M. S. di Rossi, who has made similar observations, -thinks that there is an intimate connection between the aurora and -earthquakes; the aurora either occurring in a period of earthquakes, or -else taking the place of earthquakes. - - - - - CHAPTER XV. - - BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE - AND EARTHQUAKES. - - -_Changes in the barometer and earthquakes._—Mallet, who collected -together a number of examples of earthquakes which have occurred with -a fall of the barometer, and a number which have happened with a rise, -concludes that there are as many instances of the one as of the other. -At the great earthquake of Calabria, in 1783, the barometer was very -low. The earthquake of the Rhine (February 23, 1828) was preceded by -a gradual fall of the barometer, which reached its lowest point upon -that day. After the earthquake the barometer again rose. The earthquake -of February 22, 1880, in Japan, was accompanied by exactly similar -phenomena. Caldcleugh, who observed the heavy shocks in Chili (February -20, 1835), noticed that on February 17 and 18 the barometer fell 5/10 -inches. Similar phenomena were observed before the succeeding smaller -shocks. After the shocks the barometer again rose. Principal Dawson, -speaking of the earthquakes of Canada, observes that some of the shocks -have been accompanied with a low barometer. - -P. Merian, who examined the connection between the Swiss earthquakes -and atmospheric pressure, found that out of twenty-two earthquakes -observed in Basle between 1755 and 1836, thirteen of these were -local shocks, of which eight were accompanied with sudden changes of -pressure. Of the remaining nine, which were only felt slightly in -Basle, no change in atmospheric pressure was observed. Of thirty-six -earthquakes which, between 1826 and 1836, were felt in Switzerland, -thirty were chiefly confined to Switzerland, and ten of these occurred -with a low or falling barometer. - -Humboldt is of opinion that earthquakes only occur with changes in -barometric pressure in those countries where earthquakes are few; and -he gives examples where the regular variations of the barometer have -gone on without interruption at the time of earthquakes. - -Frederick Hoffmann, who examined fifty-seven earthquakes which occurred -at Palermo between 1788 and 1838, came to the following result:— - - The barometer was sinking in 20 cases - „ „ rising in 16 „ - „ „ at a minimum in 7 „ - „ „ „ maximum in 3 „ - „ „ undetermined in 11 „[113] - -The observations of M. S. di Rossi apparently show that the earthquakes -in Italy chiefly occur with a barometrical depression and with sudden -jumps in atmospheric pressure. - -Schmidt, who examined the earthquakes of the Orient, which occurred -between 1858 and 1873, says that they were rare with a high barometer, -but numerous when the barometer was low. - -From an examination of a table of 396 earthquakes (May 8, 1875-Dec. -1881) felt in Tokio, furnished to me by Mr. Arai Ikunosuke, the -director of the meteorological department, I obtained the following -results:— - - The barometer was rising in 169 cases - „ „ falling in 154 „ - „ „ steady in 73 „ - „ „ below the monthly mean in 189 „ - „ „ above „ in 192 „ - -From this it would appear that in Japan at least the movements of the -barometer do not show any marked connection with the occurrence of -earthquakes. - -When considering this question we must remember the marked effects -which a lowering of the barometer produces upon certain volcanoes and -solfataras. The volumes of steam emitted from Stromboli and from some -of the solfataras in Tuscany hold a marked connection with atmospheric -pressure as the quantity of fire damp given off from coal seams—these -being greatest when the barometer is low. At certain changes of -the weather it is said that the volcano of Vulture, near Melfi, -emits noises. These phenomena at once place volcanic phenomena and -barometrical pressure in direct relationship. - -_Changes in temperature._—If, with an earthquake, it should happen -that there is a change in the height of the barometer, we should -naturally expect that this might be accompanied with the changes in the -temperature, in the wind, and in other atmospheric phenomena which are -more or less connected with the height of the barometer. - -Many times it has been observed that after an earthquake there has been -a sudden fall in the temperature. Such was the case with the Yokohama -earthquakes of 1880. - -Cotte endeavours to show that the earthquakes of Lisbon produced a -change upon the temperature of all Europe. In the year which followed -this earthquake storms were more common than usual. - -Kluge has collected together a large number of examples when there has -been a fall of temperature at the time of an earthquake.[114] - -At Kiachta, in Siberia, at the time of the earthquake of December -27, 1856, the thermometer fell from 12° to 25° R. We must, however, -remember that there are many cases known where the thermometer rose. - -M. S. di Rossi remarks that we have the highest records of temperature -in the years richest in earthquakes. Thus, in 1873, at the time of the -earthquakes in Central and Northern Italy, an abnormal high temperature -was remarked. Japanese writers have remarked upon the unusual heat -which has shaken their countries. The temperature of subterranean -waters have been known to increase before earthquakes. - - - - - CHAPTER XVI. - - RELATION OF SEISMIC TO VOLCANIC PHENOMENA. - - Want of synchronism between earthquakes and volcanic - eruptions—Synchronism between earthquakes and volcanic - eruptions—Conclusion. - - -_Connection between earthquakes and volcanic eruptions._—Insomuch as -it is a recognised fact that regions which are characterised by their -seismic activity are chiefly those which are also characterised by -the number of their volcanoes, it is generally assumed that these -two phenomena have an intimate relation. The residents in a volcanic -country, when seeking for the origin of an earthquake, invariably turn -towards the volcanoes which surround them. If a neighbouring volcano -is in a state of activity, it is often regarded as a safeguard against -seismic convulsions, in other cases it is looked upon as being the -cause of such disturbances. In certain instances both of these views -have apparently been corroborated. When we consider that an earthquake -and a volcanic eruption may both be the result of some great internal -convulsion, and that first one and then the other may take place in the -same neighbourhood, it is natural to expect that when these internal -forces have expended themselves in the production of one of these -phenomena, it is not so likely that they should exhibit themselves in -the other. The inhabitants of Sicily and Naples, we are told, regard -eruptions of Etna and Vesuvius as safeguards against earthquakes. A -similar belief is to be found in portions of South America with regard -to the volcanoes for which that country is so celebrated. - -From an examination of the records of the large earthquakes and the -volcanic eruptions which have taken place in Japan during the last -2,000 years, Dr. Naumann found that there was often an approximate -coincidence between the times of the occurrence of these phenomena, -suggesting the idea that the efforts which had been sufficient to -establish the volcano had at the same time been sufficient to shake the -ground. - -Of destructive earthquakes which have occurred at the time of volcanic -eruptions, and of examples when these phenomena have occurred at widely -separated intervals, the records are extremely numerous. - -_Want of synchronism between earthquakes and volcanic eruptions._—Many -of the great earthquakes of South America do not appear to have been -connected with volcanic eruptions. - -The great earthquakes of the world, like those of Calabria and Lisbon, -which took place in regions which are not volcanic, have not, Fuchs -tells us, taken place in conjunction with volcanic outbursts. - -In Japan, as in the Sandwich Islands and in many other parts of the -globe, the small earthquakes which occur almost daily do not appear to -show any marked connection with volcanic disturbances. - -In 1881, during the eruption of Natustake, a volcano lying about a -hundred miles north of Tokio, there was neither an increase nor a -decrease in the earthquakes which were felt in Tokio. Similar remarks -apply to the state of seismic activity of 1876–77, when Oshima, a -volcanic island about seventy miles to the south of Tokio, was in -eruption. In the Sandwich Islands Mauna Loa seems to have its eruptions -independently of the disturbances which shake these islands.[115] - -_Synchronism of earthquakes and volcanic eruptions._—Although many -examples like the above may be quoted, which apparently show an utter -want of connection between earthquakes and volcanoes, we must not -overlook that class of earthquakes which almost invariably accompany -all great volcanic disturbances. In fact the sudden explosions which -take place at volcanic foci, as, for instance, at the commencement -of an eruption, are enumerated as one of the causes which produce -earthquakes. Earthquakes like these usually continue until the pressure -of the steam and lava have found for themselves an opening. As compared -with the total number of earthquakes which are recorded, they form but -an insignificant portion. - -The direct connection which exists between these phenomena has, no -doubt, done very much to spread the popular belief that all earthquakes -may be connected with volcanic eruptions. As examples where this -connection has existed we might quote from almost all the volcanic -countries in the world. - -Thus, Fuchs tells us that on October 6, 1737, almost the whole of -Kamschatka and the Kurile Islands were disturbed by movements which -were simultaneous with the outbreak of the great volcano Klutschenskja -of North Kamschatka. - -One of the earliest records of a severe earthquake and a volcanic -eruption occurring simultaneously is found in the accounts of the -destruction of Herculaneum and Pompeii. The throwing up of Monte Nuovo -in the neighbourhood of Pozzuoli was accompanied with a dreadful -earthquake.[116] - -In 1868 the earthquake of Arequipa was accompanied by the opening of -the volcano Misti, on its north side. The distance to the volcano is -about fourteen miles. - -At the time of the eruptions of Kilauea in 1789 the ground shook and -rocked so that persons could not stand. - -The first eruption of the volcano Irasu, in Costa Rica (1783), was -accompanied by violent earthquakes.[117] The smoke and flames which -are said to have issued from the side of Mount Fojo at the time of the -Lisbon earthquake are regarded by some as having been volcanic. Others -thought that the phenomena, rather than being on the side of Fojo, -which showed no traces of volcanic action, had taken place in the ocean. - -At the time of the great earthquake at Concepcion (1835), whilst the -waves were coming in, two great submarine eruptions were observed. One, -behind the Isle of Quiriquina, appeared like a column of smoke. The -other, in the bay of San Vicente, appeared to form a whirlpool. The -sea-water became black, and had a sulphurous smell, there being a vast -eruption of gas in bubbles. Many fish were killed.[118] - -With this same earthquake, near to Juan Fernandez, about one mile from -the shore, the sea appeared to boil, and a high column of smoke was -thrown into the air. At night flames were seen. - -In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a -volcano at the foot of which Mendoza is situated burst into eruption. - -The earthquake of 1822 at Valdivia was accompanied by eruptions of the -neighbouring mountains, which only lasted a few minutes. - -At the time of the Leghorn shocks (January 16–27, 1742) some fishermen -observed a part of the sea to rage violently, to raise itself to a -great height, and then rush landwards.[119] - -In 1797, when Riobamba was destroyed, the neighbouring volcanoes were -not affected, but Mount Pasto, 120 miles distant, suddenly ceased to -throw out its usual column of water. - -On the night of December 10, 1874, a strong shock was felt in New -England, whilst at 4.45 A.M. on December 11 a shock was felt in the Pic -du Midi, in the Pyrenees. In the middle of December there were volcanic -outbursts in Iceland.[120] - -It is possible that these occurrences might be the results of some -widespread disturbance beneath the crust of the earth, or perhaps even -of widely extended earth pulsations. The probability, however, is that -these coincidences are accidental. When we remember that in a small -area like the northern half of Japan alone there are periods when there -are at least two shocks per day on the average, it is impossible for -these coincidences not to exist. Less frequently coincidences between -the larger disturbances must occur. Over and above these accidental -coincidences, it would appear that in the world’s history periods -have occurred when earthquakes were unusually frequent, and at such -times distant countries have suffered simultaneously. This approximate -coincidence in period, which has been referred to when speaking of the -distribution of destructive earthquakes in historical time, does not -imply an exact synchronism in the single shocks. - -Small earthquakes, or, more properly speaking, local tremblings, are a -necessary accompaniment of almost all volcanic eruptions. Tremors of -this description are seldom, however, felt beyond the crater, or at the -most upon the flanks of the mountain where the eruption is going on. - -They are due to the explosive action of steam bursting through the -molten lava. - -_Volcanic eruption succeeding earthquakes._—Sometimes it has happened -that an earthquake, or a series of earthquakes, have terminated with -the formation of volcanic vents. - -As an example of a volcanic outburst terminating a seismic disturbance, -may be mentioned the appearance of a new volcano in the centre of -Lake Ilopango, as a sequel to the shocks which had disturbed that -neighbourhood in 1879.[121] - -In 1750 there were continuous shakings lasting over three months at -Manilla. These terminated with an eruption of a small island in the -middle of a neighbouring lake. Three days after the commencement of -this eruption, four other small islands rose in the same lake.[122] - -Antonio d’Ulloa, when speaking of the Andes, remarks that after a -volcanic eruption the shocks cease.[123] - -_Conclusion._—Looking at this question generally, insomuch as the -greatest number of volcanic eruptions appear, according to Fuchs, -to have taken place in summer, whilst the greatest number of its -earthquakes have apparently taken place in winter, it would seem that -the two phenomena are without any direct connection, unless it be that -both are different effects of a common cause. - -Regarded in this manner, an earthquake may be looked upon as an -uncompleted effort to establish a volcano. To use the words of Mallet, -‘The forces of explosion and impulse are the same in both; they differ -only in degree of energy, or on the varying sorts and degrees of -resistance opposed to them.’[124] - -Although we have many examples of earthquakes having occurred without -volcanic eruptions, and, on the other hand, of volcanic eruptions -without earthquakes, volcanoes may still be regarded as ‘safety-valves -of the earth’s crust,’ which, by giving relief to internal stresses, -guard us against the effects of earthquakes. - -That many earthquakes are felt at Copiapo is attributed to the fact -that in the neighbouring mountains there are no volcanic vents. - -We must not, however, overrate the protective influence of volcanoes. -In the Sandwich Islands we see the columns of liquid lava in -neighbouring mountains standing at different heights, indicating a -want of subterranean connection between these vents. In consequence -of this it would seem that enormous pressures might be generated in -the neighbourhood of one of these mountains without finding relief at -the other. When we have conditions like these, it would seem that the -eruption of a volcano may have little or no influence in protecting -neighbouring districts. - -This may possibly be the explanation of the fact that in 1835 -Concepcion was destroyed, notwithstanding there being an unusual -activity in the volcanic vents of the neighbouring mountains. - - - - - CHAPTER XVII. - - THE CAUSE OF EARTHQUAKES. - - Modern views respecting the cause of earthquakes—Earthquakes due - to faulting—To explosions of steam—To volcanic evisceration—To - chemical degradation—Attractive influence of the heavenly - bodies—The effect of oceanic tides—Variation in atmospheric - pressure—Fluctuation in temperature—Winds and earthquakes—Rain - and earthquakes—Conclusion. - - -As the results of modern inquiries respecting the cause of earthquakes, -we see many investigators chiefly attributing these phenomena to -special causes. A few attribute them to several causes. It seems to us -that they might be attributed to very many causes which often act in -a complex manner. The primary causes are telluric heats, solar heat, -and variations in gravitating influences. These may be the principal, -and sometimes the immediate, cause of an earthquake. The secondary -causes are those dependent upon the primary causes, such as expansions -and contractions of the earth’s crust, variations in temperature, -barometrical pressure, rain, wind, the attractive influences of the -sun and moon in producing tides in the ocean or the earth’s crust, -variations in the distribution of stress upon the earth’s surface -caused by processes of degradation, the alterations in the position of -isogeothermal surfaces, &c. - -The part which may be played by these various causes in the production -of oscillations, pulsations, and tremors will be referred to. - -_Earthquakes consequent on faulting._—In the chapter on Earth -Oscillations, the causes producing the phenomena of elevation and -depression are briefly indicated. - -By the variations in stress accompanying elevations and depressions, -cracks are produced. Inasmuch as compression would crush the rocks -constituting the earth’s crust, we must conclude with Captain Dutton -that these cracks are formed by tension. By elevation, the upper rigid -crust of the earth is stretched, and fissures are produced. The sudden -formation of these fissures or faults gives rise to earthquakes, and -perhaps also to volcanic vents. That earthquake and volcanic regions -are situated on areas where there is evidence of rapid elevation is -strikingly illustrated round the shores of the Pacific. - -Lasaulx considered that the earthquake of Herzogenrath was more -or less intimately connected with the great mountain fissure—the -_Feldbiss_—which crosses the coal region of the Wurm.[125] The sudden -elevation or sinking of large areas at the time of an earthquake may be -a consequence of these dislocations. - -It has already been pointed out that the earthquake region of Japan -is the one where we have evidence of recent and rapid elevation. That -certain earthquakes of this region may possibly be the result of -faulting we have the evidence of our senses and of our instruments. The -sudden blows and jolts which are sometimes felt are indicative of the -sliding of one mass of rock across another. - -Should the ground be simply torn asunder, this tearing would give rise -to a series of waves of distortion, vibrating in directions parallel -to the plane of the fissure. Supposing this motion to be propagated -to a number of surrounding stations, it would be recorded at each of -these as having the same direction. To those situated on a line forming -a continuation of the strike of the fissure, the vibrations would -advance so to speak _end on_, whilst to those stations lying in a line -perpendicular to the strike of the fissure, the motion would advance -_broadside on_. - -Motions like these latter have been recorded in Tokio, where -earthquakes which from time observations were known to have come from -the faulted and rising region to the south have been registered as a -series of east and west motions, or vibrations transverse to this line -of propagation. - -It must, however, be here mentioned that the registration of only -transverse motion may possibly be due to the extinction of normal -motion, although this is not generally regarded as probable. - -It would therefore appear that certain earthquakes and faults are -closely related phenomena, the former being an immediate effect of -the latter. Faults are due to earth oscillations, and to a variety of -causes producing disturbances in the equilibrium of the earth’s crust; -the principal cause of all these phenomena being alterations in the -distribution of heat, and the attractive force of gravity. - -_Earthquakes consequent on the explosion of steam._—Humboldt regarded -volcanoes and earthquakes as the results of a common cause, which he -formulated as ‘the reaction of the fiery interior of the earth upon -its rigid crust.’ Certain investigators, who have endeavoured to -reduce Humboldt’s explanation to definite limits, have suggested that -earthquakes may be due to sudden outbursts of steam beneath the crust -of the earth, and its final escape through cracks and fissures. - -Admitting that steam may accumulate by separating out from the cooling -interior of our globe, its sudden explosion might be brought about by -its own expansive force, or by the movements in the bubbling mass from -which it originated. - -Others, however, rather than regard the steam as being a primeval -constituent of the earth’s interior, imagine it arises from the gradual -percolation of water from the surface of the earth down to volcanic -foci, into which it is admitted against opposing pressures, by virtue -of capillary action. - -Mallet, in his account of the Neapolitan earthquake, shows that the -whole of the observed phenomena can be accounted for by the admission -of steam into a fissure, which by the expansive force exerted on its -walls was rent open. Just as at the Geysers we hear the thud and -feel the trembling produced by the sudden evolution and condensation -of steam, so may steam by its sudden evolution and condensation in -the ground beneath us give rise to a series of shocks of varying -intensity, accompanied by intermediate vibratory motions—that is to -say, a motion which, as judged of by our feelings, is not unlike many -earthquakes. Often it may happen that the result of the explosion may -be the production of a fault, or at least a fissure; and thus in the -resulting movements we may have a variety of vibrations, some being -those of compression and distortion, produced by the blow of the -explosion, and others being those of distortion alone, produced by the -shearing action which may have taken place by the opening of the fault. -Sometimes one set of these vibrations may be prominent, and sometimes -the other. Thus, when we say that an earthquake has shown evidence by -the nature of its vibrations that it was produced by a fault, this -by no means precludes the possibility that an explosion of steam may -also have been connected with the production of the disturbance. -Mallet threw out the suggestion that the opening of fissures beneath -the ocean might admit water to volcanic foci. During the time that the -water was in the spheroidal state, the preliminary tremors, so common -to many earthquakes, would be produced. These would be followed by the -explosion, or series of explosions, constituting the shock or shocks of -the earthquakes. - -The chief reasons for believing that the earthquakes of North-Eastern -Japan are partly due to explosive efforts are:— - -1. That the greater number of disturbances, perhaps ninety per cent., -originate beneath the sea, where we may imagine that the ground, -under the superincumbent hydrostatic pressure, is continuously being -saturated with moisture. - -2. Many of the diagrams show that the prominent vibrations, of which -there are usually from one to three, in a given disturbance have the -same character as those produced by an explosive like dynamite, the -greatest and probably the most rapid motions being inwards towards the -origin. - -It may here be remarked that a very large proportion of the destructive -earthquakes of the world have originated beneath the sea, as has often -been testified by the succeeding sea waves. Also, it must be observed, -that earthquake countries, like volcanic countries, are chiefly those -which have a coast line sloping at a steep angle beneath the sea—that -is to say, earthquakes are frequent along coasts bordered by deep water. - -The earthquakes which occur at volcanic foci constitute another class -of disturbances which may be accredited to the explosive efforts of -steam. - -_Earthquakes due to volcanic evisceration._—By the ejection of ashes -and lava from volcanic vents, there is an extensive evisceration of -the neighbouring ground. When we look at a volcano like Fujiyama, -13,000 feet in height, and at least fifty miles in circumference, and -remember that the mass of cinders and slag of which it is composed -came from beneath the area on which it rests, the point to be wondered -at is, that earthquakes, consequent on the collapse of subterranean -hollows, are not more frequent than they are. At the time of a single -eruption of a volcano, the quantity of lava ejected amounts to many -thousand millions of cubic feet. In 1783 the quantity of lava ejected -from Skaptas Joknee, in Iceland, was estimated as surpassing ‘in -magnitude the bulk of Mont Blanc.’[126] Admitting that hollow spaces -are the results of these eruptions, and that in consequence of this -evisceration the ground is rendered unstable, the instability being -increased by the additional load placed above the eviscerated area, it -would seem that from time to time earthquakes are inevitable. - -Facts, however, teach us that volcanoes act as safety valves, and -that, as a rule, at or shortly after an eruption, earthquakes cease. -The relationship of earthquakes to volcanic eruptions would therefore -indicate, notwithstanding the arguments put forward to show that an -area loaded by a volcano has in consequence of the evisceration and the -load a quaquaversal dip, that evisceration does not take place beneath -volcanoes as is usually supposed, and we may conclude that it is but -few earthquakes which have an origin due to these causes. - -_Earthquakes and evisceration by chemical degradation._—A powerful -agent, which tends to the formation of subterranean hollows, is -chemical degradation. The effects of this have been often measured by -quantitative analysis of the solid materials which are daily carried -away by many of our springs. In limestone districts this is very great. -Prof. Ramsay estimates that the mineral matter discharged annually by -the hot springs of Bath is equivalent in bulk to a column 140 feet in -height and 9 feet in diameter. At San Filippo, in Tuscany, the solid -matter discharged from the springs has formed a hill a mile and a -quarter long, a third of a mile broad, and 250 feet in thickness.[127] -Many other examples of subterranean chemical degradation will be found -in text-books of geology. - -By this chemical action large cavernous hollows are produced. Beneath -a volcano it is probable that liquid material immediately takes the -place of that which is ejected, and that hollows are not formed as in -the case of chemical degradation. If a cavern becomes too large, it -eventually collapses. - -Of the falling in of large excavations we have examples in large mines. -As a consequence, not only is a trembling produced, but also a noise, -which is so like that produced by certain earthquakes that the South -American miners have but one word, ‘bramido,’ to express both.[128] - -Boussingault, who was an advocate for the theory that many earthquakes -are produced by the sinking of the ground, calls attention to the fact -that we have evidences of the subsidence of great mountains, like -the Andes, the districts around which are so well known for their -earthquakes. Capac Urcu is one of these mountains which legends tell us -has decreased in height. - -The variation in the height of mountains is a subject which deserves -attention. That mountains may possibly be hollow, we have the -remarkable results attained by Captain Herschel, who found that the -attractive force of gravity in the neighbourhood of the Himalayas was -not so great as it ought to have been had these mountains been solid. -The Rev. O. Fisher gives another explanation of this phenomenon. -Palmieri considers that the terrible earthquake which devastated -Casamicciola (1881) was due to the hot springs having gradually eaten -out cavernous spaces beneath the town. The extremely local character of -this shock was certainly favourable to such a view. - -The earthquake which, in 1840, caused Mount Cernans, in the Jura, to -fall, is also attributed to the solvent action of waters in undermining -its foundations. This undermining action was in great measure probably -due to a large spring, which, twenty-five years previously, had -disappeared, and which subsequently may possibly have been slowly -disintegrating the foundations of the mountain. Earthquakes of this -order would be principally confined to districts where there are rocks -which are more or less soluble, as, for instance, rock salt, gypsum, -and limestone. - -_Earthquakes and the attractive influences of the heavenly bodies._—The -most important attractions exercised upon our planet are those due to -the sun and moon. To these influences we owe the tides in our ocean, -and possibly elastic tides in the earth’s crust. Some theorists would -also insist upon liquid tides in the fluid interior of our earth. The -nature of the earth’s interior is, however, a question on which there -is a diversity of opinion. - -One doctrine, which, until recent years, received much support, was -that the interior of the earth was a reservoir of molten matter -contained within a thin crust. Hopkins showed that the least -possible thickness of such a crust must be from 800 or 1,000 miles, -otherwise the motions of precession and nutation would be subject to -interference. - -M. Delauney objected to the views of Hopkins, on the supposition that -the fluid interior of the earth had a certain viscosity. - -Sir William Thomson arrives at the conclusion that the earth on the -whole must be more rigid than a continuous solid globe of glass. Mr. -George H. Darwin’s investigations on the bodily tides of viscous or -semi-elastic spheroids tend to strengthen the arguments of Sir William -Thomson. - -Some philosophers hold the view that the central portion of the earth, -although intensely hot, is solid by pressure, whilst the outer crust is -solid by cooling. Between the two there is a shell of liquid or viscous -molten matter. - -Another argument is, that although the interior of the globe may be -solid, it is only retained in that condition by an immense pressure, on -the relief of which it is liquefied—it is potentially liquid. - -As these views, and the arguments for and against them, are to be -found in all modern text-books of geology, we will at once proceed -to consider the effect of solar and lunar attractive influences in -producing earthquakes upon a globe which is either solid, partially -solid, or which has an interior wholly liquid. - -_Effect of the attractive influences of the sun and moon._—In 1854 -M. F. Zantedeschi put forward the view—that it is probable there is -a continual tendency of the earth to protuberance in the direction -of the radii vectores of the two luminaries which attract it. In -consequence of these protuberances, pendulums ought at one time to -swing more slowly than at others. Zantedeschi remarks that the periods -of earthquakes appear to confirm such a view, insomuch as they occur -more often at the syzygies, or epoch of the spring tides, than at neap -tides—an observation found in the works of Georges Baglivi (1703) and -Joseph Toaldo (1770).[129] - -Prof. Perrey, of Dijon, who did so much for seismology, held the view -that the preponderance in the number of earthquakes felt at particular -seasons was possibly due to the attractive influence of the sun and -moon producing a tide in the fluid interior of the earth, which, acting -on the solid crust, produced fractures. - -Rudolf Falb, whose writings have of late years attracted considerable -attention, brings forward views which may be regarded as amplifications -of those suggested by Perrey. - -According to Falb, the inner portion of the earth must be regarded as -fluid. In the crust above this fluid reservoir are cracks and channels, -into which, by the attraction of the moon and sun, the fluid is drawn. -On entering these cracks cooling takes place, together with explosions -of gas and subterranean volcanic disturbances. The attractions -producing the internal tides required by Falb are chiefly dependent -upon the following factors:— - -1. The nearness and distance of the sun from the earth (January 1 and -July 1). - -2. The position of the moon with regard to the earth, which in every -twenty-seven days is once near and once distant. - -3. The phases of the moon—whether full or new moon (syzygies), or -whether first or last quarter (quadratures). - -4. The equinoxes, the position of the sun in the equator, and the -relative position of the earth. - -5. The position of the moon relative to the equator. - -6. The concurrence of the ‘centrifugal force’ of the earth with the -last quarter of the moon. - -7. The entrance of the moon on the ecliptic—the so-called nodes. - -Assuming that earthquakes are wholly consequent on these attractions, -it at once becomes possible to predict their occurrence. This Falb -does, and when his predictions have been fulfilled he has certainly -gained notoriety. - -He commenced by the predictions of great storms. In 1873 he predicted -the destructive earthquake of Belluno, which earned for himself a -eulogistic poem, which he has republished in his ‘Gedanken und Studien -über Vulkanismus.’ After this, in 1874, he predicted the eruption of -Etna. He also explained why, in B.C. 4000, there should have been a -great flood, and for A.D. 6400 he predicts a repetition of such an -occurrence. - -When we approach the question of the extent to which the attraction -of the sun and moon may influence the production of earthquakes, a -question which we have to answer is, whether it is likely that the -attractive power of the moon is so great that it could draw up the -crust the earth beyond its elastic limits. We know what it can do with -water. It can lift up a hemispherical shell 8,000 miles in diameter -about two or three feet higher at its crown than it lifts the earth. -Even supposing the solid crust to be lifted 100 times the apparent -rise of the tide, is it likely that a hemispherical arch 8,000 miles -in diameter when it is raised 200 feet at its crown could by any -possibility suffer fracture? If an arch is 12,000 miles in length, all -that we here ask is, whether the materials which compose the arch are -sufficiently elastic to allow themselves to be so far stretched that -the crown may be raised 200 feet. The result which we should arrive -at is apparently so obvious that actual calculation seems hardly -necessary. If we regard the earth as being solid, the question resolves -itself into the inquiry as to whether a column of rock, which is equal -in length to the diameter of the earth, or about 8,000 miles, can be -elongated 200 feet without a fracture. This is equivalent to asking -whether a piece of rock one yard in length can be stretched one seventy -thousandth of a foot. Considering that this is a quantity which is -scarcely appreciable under the most powerful of our microscopes, we -must also regard this as a question which it is hardly necessary to -enter into calculations about before giving it an answer. To vary -the method of treating such a question, may we not ask what is the -utmost limit to which it would be possible to raise up or stretch the -crust of the earth without danger of a fracture? Thus, for instance, -to what extent might a column of rock be elongated without danger -of its being broken? From what we know of the tenacity of materials -like brick and their moduli of elasticity, it would seem possible to -stretch a bar of rock 8,000 miles in length for approximately half -a mile before expecting it to break. As to whether there is a wave, -the height of which is equal to half this quantity, running round our -earth as successive portions of its surface pass beneath the attracting -influences of the sun and moon, is a phenomenon which, if it exists, -would probably long ago have met with a practical demonstration. - -The deformation which a solid globe or spherical shell would -experience under the attractive influences of the sun and moon has -been investigated by Lamé, Thomson, Darwin, and other physicists and -mathematicians. - -A conclusion that we are led to as one result of these valuable -investigations is, that if the interior of the earth be fluid, and -covered with a thin shell, then enormous elastic tides must be -produced. A consequent phenomenon, dependent on the existence of these -tides, would be a marked regularity in the occurrence of earthquakes. -As this marked regularity does not exist, we must conclude that -earthquakes are not due to the attractive influences of the sun and -moon acting upon the thin crust of the earth covering a fluid interior. -The periodicity of earthquakes corroborates the conclusions of Sir -William Thomson, who remarks that if the earth were not extremely rigid -the enormous elastic tides which must result would be sufficient to -lift the waters of the ocean up and down so that the oceanic tide would -be obliterated. - -Assuming that the earth has the rigidity assigned to it by mathematical -and physical investigators, we nevertheless have travelling round our -earth, following the attractions of the moon and sun, a tidal stress. -This stress, imposed upon an area in a critical state, may cause it to -give way, and thus be the origin of an earthquake. Earthquakes ought -therefore to be more numerous when these stresses are the greatest. - -The periods of maximum stress or greatest pull exerted by the moon and -sun will occur when these bodies are nearest to our planet—that is, in -perigee and perihelion, and again when they are acting in conjunction -or at the syzygies. That earthquakes are _slightly_ more numerous -at these particular periods than at others is a strong reason for -believing that the attractions of the moon and sun enter into the list -of causes producing these phenomena. - -Had there been a strongly marked distinction in the number of -earthquakes occurring at these particular seasons as compared with -others, we might have attributed earthquakes to the existence of -elastic tides of a sensible magnitude. As the facts stand, it appears -that the maximum pulls exerted by the moon and sun are only sufficient -to cause a slight preponderance in the number of earthquakes felt at -particular seasons, and therefore that these pulls only result in -earthquakes when the distorting effort has been exerted on an area -which, by volcanic evisceration, the pressure of included gases, and -other causes, is on the verge of yielding. - -_Earthquakes and the tides._—If we assume that earthquakes are in many -cases due to the overloading of an area and its consequent fracture, -such loading may occur by the rising of the tide. A belief that the -earthquakes of Japan were attributable to the tides may be found in the -diary of Richard Cocks under the date November 7, 1618, who remarks:— - -‘And, as we retorned, about ten aclock, hapned a greate earthquake, -which caused many people to run out of their howses. And about the lyke -hower the night following hapned an other, this countrey being much -subject to them. And that which is comunely markd, they allwais hapen -at a hie water (or full sea); so it is thought it chauseth per reason -is much wind blowen into hollow caves under ground at a loe water, and -the sea flowing in after, and stoping the passage out, causeth these -earthquakes, to fynd passage or vent for the wind shut up.’[130] - -Although we may not acquiesce in Cock’s views respecting the imprisoned -wind, it would seem that a comparison of the occurrence of earthquakes -and the state of the tide would be a legitimate research. Inasmuch -as the stresses which are brought to bear upon an area by the rising -of the tide are so very much greater than those due to barometrical -changes, it is not unlikely that a marked connection would be found. -But it must be remembered that because researches, so far as they have -gone, tend to show that earth movements are more frequent when an area -is relieved of a load, it is not unlikely that the greatest number of -earthquakes may be found to occur at low water. Prof. W. S. Chaplin -attempted to make this investigation in Japan, but not being able to -obtain the necessary information respecting the tides, was compelled to -relinquish this interesting work. - -Every foot of rise in a tide is equivalent to a load being placed on -the area over which the tide takes place of sixty-two pounds to the -square foot. This load is not evenly distributed, but stops abruptly at -a coast line. Lastly, it may be observed that many coast lines are not -simultaneously subjected to stresses consequent upon this load. Japan, -for instance, may be regarded as an arch placed horizontally. The -area near the crown of this arch is loaded by the tidal wave crossing -the Pacific before the areas near the abutment, and farther there is -a horizontal pressure at the crown which, if Japan were like a raft, -would tend, as the tide advanced, to straighten its bow-like form, but -as the wave passed its abutments to increase its curvature. - -Prof. G. Darwin has calculated the amount of rise and fall of a shore -line due to tidal loads (see p. 336, ‘Earth Pulsations’). The result -of these calculations apparently indicates that these loads may have a -considerable influence upon the stability of an area in a more or less -critical condition. - -Mr. J. Carruthers suggests that tidal action may hold a general but -indirect relationship to volcanic and seismic action by the retardation -it causes on the earth’s rotation. By this retardation the polar axis -tends to lengthen, and tensile stresses are induced, resulting in -fracture. The fluid interior of the earth, being no longer restrained, -would move polewards, and, leaving equatorial portions unsupported, -this would gradually collapse. The primary fractures would be north and -south, while the secondary fractures would be east and west.[131] - -That the rise of the tide is accompanied by a greater percolation -of water to volcanic foci, which, in consequence, assume a greater -state of activity, is a theory which was advanced many years ago. To -determine how far tides may directly be connected with earthquakes, the -necessary records have yet to be examined. - -_Variations in atmospheric pressure._—When we consider the immense -load which, by a sudden rise of the barometer, is placed upon the area -over which this rise takes place, it is not difficult to imagine that -this rise may occasionally be the final cause which makes the crust of -the earth to give way. A barometric rise of an inch is equivalent to a -load of about seventy-two pounds being put upon every square foot of -area over which this rise takes place. On the other hand, a fall in the -barometric column indicates that a load has been removed, and whatever -elastic effort may be exerted by subterranean forces in endeavouring to -escape, being met by less resistance, they may burst these bonds, and -an earthquake will result. For reasons such as these the final cause -of earthquakes has often been attributed to variations in atmospheric -pressure. In Japan there are practically as many earthquakes with a -high barometer as with a low one. - -The extent to which barometric fluctuations have acted as final causes -in the production of earthquakes may be judged of by a comparison of -the times of barometric variation and the times at which earthquakes -have occurred. - -Three important laws of barometric variation are the following:— - -1. In the world generally the average barometric pressure is highest in -winter. (Exceptions occur near Iceland and in the North Pacific.) - -2. The summer and winter monthly mean barometer differs least near the -equator and over the great oceans. They differ most over the great -continents and generally with increasing latitude. - -3. The greatest number of barometrical fluctuations usually take place -in winter. - -Inasmuch as there are generally more earthquakes in winter than in -summer, the first of these laws would indicate that this might be due -to the greater load which acts upon the crust of the earth at that -season. The second law would indicate that the distinction between -the winter and summer earthquakes ought to be most marked in high -latitudes, which, if we refer to the table on p. 257, we observe to be -borne out by the results of observation. The countries where there are -as many earthquakes in winter as in summer are chiefly those in low -latitudes. The number of these countries from which we have records -are, however, few. - -Facts opposed to the idea that earthquakes may be caused by an increase -of barometric pressure are the results of observations like those of -Schmidt and Rossi, which show that earthquakes chiefly occur with a low -barometer. - -Assuming that these latter observations will be found by future -investigators to be generally true, we must conclude that the relief -of atmospheric pressure has an influence upon the occurrence of -earthquakes. Such a conclusion would partially accord with the third -barometrical law, or the fact that there are more occasions on which we -get a low barometer during the winter months. - -Other writers who have examined this question are Volger, Kluge, -Andrès, and Poly. The latter investigator sought a connection between -earthquakes and revolving storms, in the centres of which there is -usually an abnormal decrease of atmospheric pressure. If an area over -which such a sudden change in pressure took place was in a critical -state, it is not difficult to see that storms such as Poly refers to -might sometimes be accompanied by earthquakes. - -_Fluctuations in temperature._—Inasmuch as fluctuations in temperature -are governed by the sun, it may at once be said that there is a -connection between earthquakes and readings of the thermometer. -Certainly earthquakes occur mostly during the cold months or in -winter. Similarly, as changes in temperature are so closely connected -with barometric fluctuations, and these are said to have a direct -influence upon the yielding of the earth’s crust, seismic phenomena are -indirectly linked to fluctuations in temperature. A rise in temperature -is usually accompanied by a fall in the barometer, and this in turn may -be a condition favourable for the occurrence of an earthquake. - -If we regard solar heat as an agent causing expansions or contractions -in the earth’s crust, then fluctuations in temperature become an -immediate cause of earthquakes. The probability, however, is that -solar heat has little or no connection with the final cause producing -earthquakes, although at the same time coincidences between the -occurrence of earthquakes and unusual fluctuations in temperature may -from time to time be observed. - -_Winds and earthquakes._—Although it may be admitted that high winds -exert enormous pressures upon mountain ranges, and might occasionally -give rise to stresses causing rocky masses in unstable equilibrium to -give way, the coincidences which have been established between the -occurrence of storms and earthquakes can usually only be regarded as -occurrences which have synchronised by chance. - -Storms are usually accompanied with a barometric depression, and the -relation of diminutions in atmospheric pressure to earthquakes has been -discussed. - -_Rain and earthquakes._—It has already been shown that earthquakes -have occasionally been found to coincide with rain and rainy seasons. -Whether the saturation of the ground with moisture or the percolation -of the same to volcanic foci may be a direct effect producing -earthquakes it is difficult to say. The probability, however, is -that, rain being dependent on phenomena like changes in temperature, -barometric fluctuations, and winds, we must regard it and the -earthquakes which happen to coincide with these precipitations of -moisture as congruent effects of more general causes. - -_Conclusion._—Although it would be an easy matter to discuss the -relationship of earthquakes and other phenomena, we must conclude -that the primary cause of earthquakes is endogenous to our earth, and -that exogenous phenomena, like the attraction of the sun and moon and -barometric fluctuations, play but a small part in the actual production -of these phenomena, their greatest effect being to cause a slight -preponderance in the number of earthquakes at particular seasons. They -may, therefore, sometimes be regarded as final causes. The majority of -earthquakes are due to explosive efforts at volcanic foci. The greater -number of these explosions take place beneath the sea, and are probably -due to the admission of water through fissures to the heated rocks -beneath. A smaller number of earthquakes originate at actual volcanoes. -Some earthquakes are produced by the sudden fracture of rocky strata or -the production of faults. This may be attributable to stresses brought -about by elevatory pressure. Lastly, we have earthquakes due to the -collapse of underground excavations. - - - - - CHAPTER XVIII. - - PREDICTION OF EARTHQUAKES. - - General nature of predictions—Prediction by the observation of - unusual phenomena (alteration in the appearance and taste of - springs; underground noises; preliminary tremors; earthquake - prophets—warnings furnished by animals, &c.)—Earthquake warning. - - -_General nature of predictions._—Ever since seismology has been -studied, one of the chief aims of its students has been to discover -some means which would enable them to foretell the coming of an -earthquake, and the attempts which have been made by workers in various -countries to correlate these occurrences with other well-marked -phenomena may be regarded as attempts in this direction. - -Ability to herald the approach of these calamities would unquestionably -be an inestimable boon to all who dwell in earthquake-shaken countries, -and the attempts which have been made both here and in other places are -extremely praiseworthy. In almost all countries where earthquakes are -of common occurrence these movements of the earth have been more or -less connected with certain phenomena which, in the popular mind, are -supposed to be associated with an approach of an earthquake. - -If predictions were given in general terms, and they only referred -to time, inasmuch as on the average there are in the world several -shakings per day, we should always find that predictions were -verified. We might even go further and predict that on certain days -earthquakes would occur in certain countries, and still find that in -many instances our supposed power of foresight had not deceived us. -Thus, for instance, in Japan, where on the average there are probably -one or two shakings every day, if prognostications were never correct -there would be a violation of the laws of chance. - -What is required from those who undertake to forewarn us of an -earthquake is an indication not only of the time at which the -disturbance will happen, but also an indication of the area in which -it is to occur. Those who dwell in an area where there are certain -well-defined periods during which seismic activity is at a maximum—if -ten or fourteen days should have passed without a shock—might, in many -instances, find that a prophecy that there would be an earthquake -within the next few days would prove itself correct. Also, if a severe -shock had taken place, a prophecy that there would be a second or -third smaller disturbance within a short period would also meet with -verification. - -Certain persons with whom I am intimate appear to have persuaded -themselves that they can foretell the coming of an earthquake by the -sultry state of the atmosphere or a certain oppressiveness they feel, -and an instinctive feeling arises that an earthquake is at hand. - -It is said that a few minutes before many of the shocks which shook -New England between 1827 and 1847 people could foretell the coming -disturbance by an alteration in their stomach.[132] No doubt many who -dwell in earthquake countries, and have been alarmed by earthquakes, -are at times subject to nervous expectancy. - -The author has had such sensations himself, due, perhaps, to a -knowledge that it was the earthquake season, that there had been no -disturbance for some weeks, and a consequent increasing state of -nervous presentments. In consequence of this, not only has he carefully -prepared his instruments for the coming shock, but he has written and -telegraphed to friends to do the same. - -Sometimes these guesses have proved correct. One remarkable instance -was a few hours prior to the severe shock of February 22, 1880, when -he communicated with his friends in Yokohama and asked them to see -that their instruments were in good order. Oftener, however, his -prognostications have been incorrect. The point in connection with -this subject which he wishes to be remarked is, that the instances -where earthquakes occurred shortly after the receipt of his letters -are carefully remembered, and often mentioned, but the instances in -which earthquakes did not occur appear to be entirely forgotten. He is -led to mention these facts because they appear to be an experimental -proof of what has taken place in bygone times, and what still takes -place, especially amongst savages—namely, that the record of that which -is remarkable survives, whilst that which is of every-day occurrence -quickly dies. Had the records of all prognostications been preserved, -the probability is that we should find that they had, in the majority -of cases, been incorrect, whilst it would have been but in very few -instances they had been fulfilled. - -_Prediction by the observation of natural phenomena._—The above remarks -may perhaps help us to understand the prognostications of the ancient -philosophers about which Professor Antonio Favaro, of Padua, has -written.[133] Cicero in the ‘De Divinatione,’ speaking on this subject, -says that ‘God has not predicted so much as the divine intelligence of -man.’—‘Non Deus prævidet tantum, sed et divini in genii viri.’ Favaro -regards these predictions, however, as the result of observations of -nature which show it is possible that indications of coming earthquakes -had been announced by variations in the gas given out from subterranean -sources, the change in colour, taste, level, temperature of the water -in springs, &c. - -In 1843 a bishop of Ischia forewarned his people of a conning -earthquake, and thus was instrumental in the saving of many lives. -Naturally, in an age of superstition, the bishop would be regarded as -a prophet, but Favaro considers that the prognostication was probably -due to a knowledge of premonitory signs as exhibited in changes in the -characters of mineral waters. - -The shock of 1851, at Melfi, was in this way predicted by the Capuchin -fathers, who observed that a lake near their door became frothy and -turbulent. - -Underground noises have led persons to the belief that an earthquake -was at hand. It was in this way that Viduari, a prisoner at Lima, -predicted the destruction of that city. - -Before the earthquake of 1868, so severely felt at Iquique, the -inhabitants were terrified by loud subterranean noises. - -That underground noises have preceded earthquakes by considerable -intervals appears to be a fact, but, at the same time, it must -be remembered that similar noises have often occurred without an -earthquake having taken place. - -Farmers predicted the earthquake of St. Remo, in 1831, by underground -noises. - -On the day before the earthquake which, in 1873, shook Mount Baldo, -the inhabitants of Puos, a village north of Lake Santa Croce, heard -underground noises. - -Before the earthquakes which, in 1783, shook Calabria and Sicily, fish -are said to have appeared in great numbers on the coast of Sicily, and -the whirlpool of Charybdis assumed an unusual excited state. - -It is said that Pherecydes predicted the earthquakes of Lacedemon and -Helm out, by the taste of the water in the very deep well at the castle -of Lovain.[134] - -The writer of an article on the Lisbon earthquake says that ‘after the -24th I felt apprehensive, as I observed the same prognostics as on the -afternoon of October 31, that is, the weather was severe, the wind -northerly, a fog came from the sea, the water in a fountain ran of a -yellow clay colour, and’ he adds, ‘from midnight to the morning of the -25th I felt five shocks.’[135] - -At the present time Rudolf Falb, following a theory based upon the -attractive influences of the sun and moon, tells us the time at which -we are to expect earthquakes. - -That occasionally there are signs attendant on earthquakes, although -we cannot give them a physical explanation, we cannot doubt. Also we -know that in certain areas earthquakes are more likely to occur at -one season than at another. Should earthquakes be foretold with the -assistance of knowledge of this description, the predictions at once -become the result of the application of certain natural laws, and are -not to be regarded as predictions in the popularly accepted sense of -that term, any more than the arrival of a friend is predicted by the -previous receipt of a telegram announcing his coming. - -Rather than accredit the ancients and those of more modern times who, -in consequence of their feelings, have recorded the coming of an -earthquake, with a knowledge of premonitory signs, we might in many -instances regard the records of those prognostications as the survival -of accidental guesses, and, as such, examples of the survival of the -useless. - -The effect of accidental occurrences of this description upon an -uneducated mind, in engendering superstition, is a subject which has -often been dwelt upon, and the difficulty of eradicating the same—as -may be judged of by the following accident which came under the -observation of Mr. T. B. Lloyd and the author, in 1873, when travelling -in Newfoundland—will be easily appreciated. - -At the time to which I refer, my companion was bringing a canoe down -the rapids below the Grand Pond in a country which is practically -uninhabited, and where an Indian trapper would perhaps be the only -person met with, and this not more than once a year. Whilst shooting -the rapids one of the Indians, Reuben Soulian, shot at a deer passing -up one bank of the river. That the deer had been hit was testified by a -trail of blood which bespattered the rocks. Subsequently several more -shots were fired, and it was believed by all that the deer was killed. -Soulian quickly followed the animal to the spot where it was supposed -to have fallen. Some time after he returned, having failed to find any -trace of the animal. He was greatly agitated, but eventually became -melancholy, saying that the sudden disappearance of the animal was a -sure sign that some of his relations had suddenly died. About two hours -afterwards Mr. Lloyd’s party met with a party of Indians coming up -the river, the first they had seen for four weeks, who told them that -Soulian’s sister had just died on the coast. - -In the northern part of South America certain shocks are anticipated by -preliminary vibrations which cause a little bell attached to a T-shaped -frame (cruz sonante) to ring. There are, however, persons (trembloron) -who are supposed to be endowed with seismic foresight, whose verdicts -are much relied upon. - -In Caraccas it is said that nearly every street in the river suburb has -an earthquake Cassandra or two. Some of these go so far not only as -to predict the coming seisms, but also the vicissitudes of particular -streets.[136] Earthquake prophets are, however, by no means confined to -the new world, and many examples of them may be found in the histories -of countries where earthquakes have been felt. - -The story of the crazy lifeguardsman who prophesied an earthquake to -take place in London on April 4, 1691, is an example. The Rev. Sig. -Pasquel E. Perdini, writing on the earthquakes at Leghorn in 1742, -says that ‘a Milanese astrologer predicted this earthquake for January -27, by which “misfortune” the faith and credit given to the astrologer -gained him more reverence and honour than the prophets and holy -gospel.’ Before the time at which he predicted a second shock, people -removed away from Leghorn. - -_Warnings furnished by animals._—A study of the warnings furnished by -animals is also interesting. Several of the natives in Caraccas possess -oracular quadrupeds, such as dogs, cats, and jerboas, which anticipate -coming dangers by their restlessness. - -Before the catastrophe of 1812, at Caraccas, a Spanish stallion broke -out from its stable and escaped to the highlands, which was regarded -as the result of the prescience of a coming calamity. Before the -disturbances of 1822 and 1835, which shook Chili, immense flocks of sea -birds flew inland, as if they had been alarmed by the commencement of -some suboceanic disturbance. Before this last shock it is also related -that all the dogs escaped from the city of Talcahuano. - -_Earthquake warning._—What has here been said respecting the prediction -of earthquakes is necessarily imperfect—many of the signs which are -popularly supposed to enable persons to foretell the coming of an -earthquake having already been mentioned in previous chapters. That -we shall yet be able to prepare ourselves against the coming of -earthquakes, by the applications of laws governing these disturbances, -is not an unreasonable hope. - -With an electric circuit which is closed by a movement of the ground, -we are already in a position to warn the dwellers in surrounding -districts that a movement is approaching. - -An earthquake which travelled at the rate of four seconds to the -mile might, if it were allowed to close a circuit which fired a gun -at a station fifteen miles distant, give the inhabitants at that -place a minute’s warning to leave their houses. The inhabitants of -Australia and the western shores of the Pacific might, by telegraphic -communication, receive eighteen to twenty-five hours’ warning of the -coming of destructive sea waves resulting from earthquakes in South -America. - -Although warnings like these might have their value, that which is -chiefly required is to warn the dwellers at and near an earthquake -centre of coming disturbances. - -What the results of the observations on earth tremors will lead to is -problematical. - -Should microseismic observation enable us to say when and where the -minute movements of the soil will reach a head, a valuable contribution -to the insurance of human safety in earthquake regions will have been -attained. - -As to whether the movements of tromometers are destined to become -barometric-like warnings of increased activity beneath the earth -crust, or whether they are only due to vibrations of the earth -crust produced by variations in atmospheric pressure, has yet to be -investigated. - -Other phenomena which may probably forewarn us of the coming of an -earthquake are phenomena resultant on the stresses brought to bear upon -the rocky crust previous to its fracture, or phenomena due to changes -in the position and condition of heated materials beneath the earth’s -surface. Amongst these may be mentioned electrical disturbances, which -appear to be so closely related to seismic phenomena. - -At the time of earthquakes telegraph lines have been disturbed, but -as to what may happen before an earthquake we have as yet but little -information. The subject of earthquake warning is of importance to many -countries, and is deserving of attention. - -As our knowledge of earth movements, and their attendant phenomena, -increases, there is but little doubt that laws will gradually be -formulated, and in the future, as telluric disturbances increase, a -large black ball gradually ascending a staff may warn the inhabitants -on the land of a coming earthquake, with as much certainty as the ball -upon a pole at many seaports warns the mariner of coming storms. - - - - - CHAPTER XIX. - - EARTH TREMORS. - - Artificially produced tremors—Observations of Kater, Denman, - Airy, Palmer, Paul—Natural tremors—Observations of Zöllner, - M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With - seismoscopes, microphones, pendulums—Work in Italy—Bertelli, - Count Malvasia, M. S. di Rossi—Instruments employed in - Italy—Tromometers, microseismographs, microphones—Results - obtained in Italy—in Japan—Cause of microseismic motion. - - -During the past few years considerable attention has been drawn towards -the study of small vibratory motions of the ground, which to the -unaided senses are usually passed by without recognition. These motions -are called _earth tremors_. Their discovery appears to have been due -to accident, and not to the results of inductive reasoning. No sooner -had philosophers contrived astronomical and other instruments for the -purpose of making refined measurements and observations than they at -once discovered that they had an enemy to contend against in the form -of microscopic earthquakes. - -_Artificially produced tremors._—Artificial disturbances of this -description exist in all our towns, and near a railway line they are -perceptible with every passing train. Those who have used microscopes -of high power will readily appreciate how small a disturbance of -the ground is visible in the apparent movement of the object under -examination. - -Captain Kater found that he could not perform his pendulum experiments -in London on account of the vibrations produced by the rolling of -carriages. Captain Denman, who made some observations on artificially -produced tremors, found that a goods train produced an effect 1,100 -feet distant in marshy ground over sandstone. Vertically, however, -above a tunnel through the sandstone, the effects only extended 100 -feet. - -A remarkable example of the trouble which artificially produced earth -vibrations have occasioned those who make astronomical observations -occurred some twenty years ago at the Greenwich Observatory. When -determining the collimation error of the transit circle by means of the -reflexion of a star in a tray of mercury, it was found that on certain -nights the surface of the mercury was in such a state of trembling -that the observers were unable to complete their observations until -long after midnight. After obtaining a series of dates on which these -disturbances occurred, it was found that they coincided with public and -bank holidays, on which days crowds of the poorer classes of London -flocked to Greenwich Park, and there amused themselves with running and -rolling down the hill on which the observatory is situated. On these -occasions it was found that the disturbances in the mercury were such -that observations could not be made until two or three hours after the -crowds had been turned out of the neighbouring park.[137] - -To obviate this difficulty Sir George Airy suspended his dish of -mercury in a system of india-rubber bands, and in this way succeeded in -eating the intruders up. - -Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit of -Venus expedition in New Zealand, in 1874, was troubled with vibrations -produced from a neighbouring railway. To escape the enemy he intrenched -his instruments by placing them in pits. With pits 3½ feet deep he -found himself sufficiently protected. The distance from the line was -about 400 yards, and the soil through which the disturbances were -propagated was a coarse pebbly gravel.[138] - -Before the United States Naval Observatory was established at -Washington, Professor H. M. Paul was deputed to make a tremor survey -to discover stable ground. The results of these experiments were -exceedingly interesting. By watching the reflected image of a star in -a dish of mercury a passing train would be noticed at the distance of -a mile. Its approach could be detected by the trembling of the image -before its coming could be heard. At one point of observation the -disturbance appeared to be cut off by a ravine. The strata was gravel -and clay.[139] - -These few examples of artificially produced tremors, to which many -more might be added, have been given because they teach us something -respecting their nature. Hitherto earth tremors have only been regarded -as intruders, which it was necessary to escape from or destroy. From -what has been said they appear to be a superficial disturbance which -is propagated to an enormous distance. This distance appears to -depend upon the propagating medium, upon the intensity of the initial -disturbance, and upon its duration. In the observation of these -artificial disturbances, which are accessible to every one, and which -hitherto have been so neglected, we have undoubtedly a fruitful source -of study. - -_Natural tremors._—Next let us turn to those microscopical -disturbances of our soil which are due to natural causes. Thus far they -seem to have been recorded wherever instruments suitable for their -detection have been erected, and it is not improbable that they are -common to the surface of the whole globe. - -Some of the more definite observations which have been made upon earth -tremors were those made in connection with experiments on the deviation -of the vertical due to the attractive influence of the moon and sun. - -Professor Zöllner, who invented the horizontal pendulum which he used -in the attempt to measure the change in level due to lunar and solar -attraction, found his instruments so sensitive that the readings were -always changing. - -The most interesting observations which were made upon small -disturbances of the soil were those of M. d’Abbadie, who carried on his -experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant -from the Atlantic, and 62 mètres above sea level. The soil was a loamy -rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height, -which was pierced down the centre by a vertical hole or well, which was -continued two mètres below the cone into the solid rock. At the bottom -of this hole or well a pool of mercury was formed which reflected -the image of cross wires placed at the top of the hole. These cross -wires and their reflection were observed by means of a microscope. -The observations consisted in noting the displacement and azimuth of -the reflected image relatively to the real image of the wires. After -allowing this structure five years to settle, M. d’Abbadie commenced -his observations. To find the surface of the mercury tranquil was -a rare occurrence. Sometimes the mercury appeared to be in violent -motion, although both the air and neighbouring sea were perfectly calm. -At times the reflected image would disappear as if the mercury had -been disturbed by a microscopic earthquake. - -The relative positions of the images were in part governed by the state -of the tide. Altogether the movements were so strange that M. d’Abbadie -did not venture any speculations as to their cause, but he remarks -that the cause of the changes he observed were sometimes neither -astronomical nor thermometrical. These observations, the principal -object of which was to determine changes in level rather than earth -vibrations, were carried on between the years 1868 and 1872.[140] - -_Observations at Cambridge._—Another instructive set of observations -were those which were made in the years 1880–1882, by George and Horace -Darwin, in the Cavendish Laboratory, at Cambridge. The main object -in these experiments was to determine the disturbing influence of -gravity produced by lunar attraction. The result which was obtained, -however, showed that the soil at Cambridge was in such an incessant -state of vibration that whatever pull the moon may have exerted upon -the instrument which was employed was masked by the magnitude of the -effects produced by the earth tremors, and the experiments had, in -consequence, to be abandoned. - -The principle of this instrument was similar to one devised by Sir -William Thomson, and put up by him in his laboratory at Glasgow. -As erected by the brothers Darwin, at Cambridge, it was briefly as -follows: A pendulum, which was a massive cylinder of pure copper, -was hung by a copper wire, about four feet long, inside a hollow -cylindrical tube rising from a stone support. A small mirror was then -hung by two silk fibres, one of which was fastened to the bob and the -other to the stone basement. A ray of light sent from a lamp on to -the mirror was reflected to a scale seven feet distant, and by this -magnification any motion of the bob relatively to the stone support -was magnified 50,000 times. In several ways the apparatus was insulated -from all accidental disturbances. The spot of light was observed from -another room by means of a telescope. This instrument was so delicate -that even at the distance of sixteen feet the shifting of your weight -from one foot to the other caused the spot of light to run along the -scale. So sensitive was the instrument that, notwithstanding its being -cut off from the surrounding soil by a trench filled with water and -the whole instrument being immersed in water to damp out the small -vibrations, it would seem that the ground was in a constant state of -tremor; in fact, so persistent and irregular were these movements that -it seemed impossible to separate them from the perturbations due to the -attraction of the moon.[141] - -As a result of observations like these, the world had gradually -forced upon it the fact that the ground on which we live is probably -everywhere in what is practically an incessant state of vibration. - -This led those who were interested in the study of earth movements to -establish special apparatus for the purpose of recording these motions -with the hope of eventually discovering the laws by which they were -governed. - -_Experiments in Japan._—The simpler forms of apparatus which have been -used in Japan may be described as delicate forms of seismoscopes, -which, in addition to recording earth tremors, also record the -occurrence of small earthquakes. - -A simple contrivance which may be used for the purpose of recording -small earthquakes can be made with a small compass needle. - -If a light, small sensitive compass needle be placed on a table, it -will be found that a small piece of iron like a nail may be pushed so -near to it that the needle assumes a position of extremely unstable -equilibrium. If the table now receives the slightest tap or shake this -condition is overcome, and the needle flies to the iron and there -remains. By making the support of the needle and the iron the poles of -an electric circuit it is possible to register the time at which motion -took place with considerable accuracy. - -With crude apparatus like this a large number of small earth -disturbances have been recorded. - -Another form of apparatus, employed in Japan, has been a delicately -constructed _circuit closer_. The motions of this instrument were -recorded by causing an electro-magnet to deflect a pencil which was -tracing a circle on a revolving dial. The revolving dial was a disc of -wood covered with paper fixed to the hour-hand axle of a common clock. - -A third form of apparatus used in Japan consisted of a small piece of -sheet lead about the size of a threepenny piece suspended by a short -loop from a rigid support. Projecting from the lead a fine wire, about -two inches in length, passed freely through a hole in a metallic plate. -By the slightest motion of the support the small pendulum of lead was -set into a state of tremor and caused its pointer to come in contact -with one or other side of the hole in the metal plate and thus to close -an electric circuit. - -A more refined kind of apparatus which has been employed in Japan was -similar to that used by the Darwins at Cambridge. This was so arranged -that any deflection of the mirror was permanent until the instrument -was reset, and in this way the maximum disturbance which had taken -place between each observation was recorded. - -In addition to these and other contrivances, experiments were made with -microphones. - -The microphones used were small doubly pointed pencils of carbon -about three centimetres long, saturated with mercury, and supported -vertically in pivot holes bored in other pieces of carbon, which were -the terminals of an electric circuit. These microphones were screwed -down on the top of stakes driven deeply into the ground. They were -covered with a glass shade thickly greased at its base. The stakes were -in the ground at the bottom of a small pit—about two feet square and -two feet deep—which was lined with a box. The box was covered with a -lid, and earth to the depth of nine inches or one foot. One of these -pits was in the middle of a lawn in the front of my house, and the -other was at the foot of a hill at the back of the house. The wires -from the microphone passed through the side of the box into a bamboo -tube and thence up to my dining-room and bed-room. In one of the -circuits there were three Daniell’s cells, a telephone, and a small -galvanometer. I used the galvanometer because I found that when there -was sufficient motion in the microphone to produce a sound in the -telephone a motion in the needle of a galvanometer was produced. If in -any case motion took place in the magnetic needle during my absence, it -was held deflected by a small piece of iron with which it was brought -into contact by the movement. - -The sensitiveness of the arrangement may be judged of from the fact -that if a person walked on the grass within six feet of the microphone, -each step caused a creak in the telephone, and the needle of the -galvanometer was caused to swing and come in contact with the iron. -Dogs running on the grass had no effect. A small stone one or two -inches in diameter thrown from the house so that it fell near to the -microphone pit caused a sharp creak in the telephone and a movement in -the needle. - -The nature of the records I received from this contrivance may be -judged of from the following extract from my papers. - - h. m. - 1879. Nov. 12th 7 0 P.M. contact of needle - 7 2 „ difficult to set the needle - 7 3 „ needle swings and telephone creaks - 7 4 „ „ „ „ - 7 5 „ „ „ „ - 7 6 „ „ „ „ - 7 10 „ 3 more swings - 7 11 „ again „ - -Here I went out, took away the covering, and examined the microphone. -Nothing wrong was to be observed. All that I saw was one small ant. I -do not think that this could have caused the disturbance, because it -could not get near the instrument. - -On the succeeding nights I experienced similar disturbances, and it -seemed as if they might possibly have been the prelude to several small -shocks which occurred about this time (November 15, 16, and 17). On -November 17, at 8 A.M., the needle was found in contact, and again at -5 P.M., and at 6 P.M. the shock of a small earthquake was felt _which -caused a rattling sound in the telephone for about one minute after the -motion had appeared to cease_. The needle swung considerably, but did -not come in contact. - -The great objection to these observations is that it is possible -that the movements and sounds which I have recorded might, with the -exception of one case when the shaking was actually felt, possibly have -been produced by causes other than that of the movement of the ground. -To determine this I subsequently put up two distinct sets of apparatus -to determine whether the motions of each were synchronous. So far as -I went this appeared only to be sometimes the case:—but this is a -question difficult to determine, unless a recorder of time be added to -the apparatus. - -The greatest objection to observations of this sort is that the -sensibility of the instrument is not constant. After a current has been -running for several days it is no longer sensible to slight shocks, it -appears as if its resistance had been increased. To overcome this it is -necessary to resharpen the carbon points and bore out the pivot holes -every three or four days. Farther, the battery varies. This might to -some extent be overcome by using a battery with large plates. These -two causes tend to reduce the sensitiveness of the galvanometer-like -recorder—the deflection of the needle gradually becoming less and less, -and therefore day by day needing a greater swing to bring it into -contact with the iron. For reasons such as these this instrument, to be -used successfully, appears to require considerable attention. - -Another form of microphone employed by the author consisted of an -aluminium wire standing vertically on a metallic plate, its upper end -passing loosely through a hole in an aluminium wire standard. - -The upper end of the vertical wire was loaded with lead. This -contrivance possesses all the sensitiveness of an ordinary microphone, -whilst, if it receives a sudden impulse, there is a sudden break in the -current, and the vertical wire is thrown from one side to the other of -the hole in the standard. - -After many months of tiresome observation with instruments of this -description, and after eliminating all motions which might have been -produced by accidental causes, the general result obtained showed that -in Tokio there were movements of the soil to be detected every day, -and sometimes many times per day, which to ordinary persons were passed -by unnoticed. - -_Work in Italy._—The most satisfactory observations which have been -made upon microseismic disturbances are those which have been made -during the last ten years in Italy. The father of systematical -microseismical research appears to have been Father Timoteo Bertelli, -of Florence. - -In 1870 Father Bertelli suspended a pendulum in a cellar, and observed -it with a microscope. As the result of his observations it was -announced that he had perceived the earthquakes which shook Romagna, -although to the ordinary observer in Florence these shakings had not -been perceptible. - -In 1873 Bertelli, by means of microscopes fixed in several azimuths, -made 5,500 observations on free pendulums. He also made observations on -reflections from the surface of mercury.[142] - -One result of these observations was to show that microseismic motions -increased with a fall of the barometer. Similar observations were made -at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near -Rome. On January 14, 15, and February 25 these three observers at their -respective stations simultaneously observed great disturbances. - -Similar investigations were made at Nice by M. le Baron Prost. - -Although doubt was cast upon Bertelli’s observations they appear to -have been the origin of a series of microseismical observations, a -distinguished leader in which is Professor Rossi, who, in 1874, found -that large earthquakes were almost always preceded or accompanied with -microseismical storms. In 1878 Professor Rossi worked upon these small -disturbances with the assistance of the microphone and telephone, and -his first results were published by Professor Palmieri. - -Many of Professor Rossi’s observations were made in the grotto of Roca -de Papa, 700 mètres high and eighteen mètres under the soil. Here over -6,000 observations were made by means of microscopes, on pendulums of -different lengths, suspended in tubes cut in the solid rock. - -_Instruments employed in Italy._—It is impossible to describe in detail -the various forms of apparatus which have been used by the Italian -investigators. A description of one or two of the more important -instruments may not, however, be out of place, inasmuch as they will -assist the reader to understand the manner in which the various results -respecting the laws governing microseismic movements have been arrived -at. - -The most important of these instruments is the _Normal Tromometer_ of -Bertelli and Rossi. - -This consists of a pendulum 1½ metres long, carrying, by means of -a very fine wire, a weight of 100 grammes. To the base of the bob -a vertical stile is attached, and the whole is enclosed in a tube -terminated, at its base, by a glass prism of such a form that when -looked through horizontally the motion of the stile can be seen in -all azimuths. In front of this prism a microscope is placed. Inside -the microscope there is a micromatic scale, so arranged that it can -be turned to coincide with the apparent direction of oscillation of -the point of the stile. In this way not only can the amplitude of the -motion of the stile be measured, but also its azimuth. The extent of -vertical motion is measured by the up and down motion of the stile due -to the elasticity of the supporting wire. This instrument is shown in -the accompanying drawing. - -[Illustration: FIG. 37.—Normal Tromometer. - -B, bob of pendulum; P, prism; M, microscope; S, rim of scale.] - -Another apparatus is the _Microseismograph_ of Professor Rossi. Here we -have an arrangement which gives automatic records of slight motions. -It consists of four pendulums, each about three feet long, suspended -so that they form the corners of a square platform. In the centre -of this platform a fifth, but rather longer, pendulum is suspended. -The four pendulums are each connected just above their bobs to the -central pendulum with loose silk threads. Fixed to the centre of each -of these threads, and held vertically by a light spring, is a needle, -so adjusted that each thread is depressed to form an obtuse angle of -about 155°. These needles form the terminals of an electric circuit, -the other termination of which is a small cup of mercury placed just -below the lower end of the needle. By a horizontal swing of one of the -pendulums this arrangement causes the needle to move vertically, but -with a slightly multiplied amplitude. By this motion the needle comes -in contact with the mercury, and an electro-magnet with a lever and -pencil is caused to make a mark on a band of paper moved by clockwork. -The five pendulums being of different lengths allows the apparatus to -respond ‘to seismic waves of different velocities.’[143] - -Lastly, we have Professor Rossi’s _Microphone_. This consists of a -metallic swing arranged like the beam of a balance. By means of a -movable weight at one end of the beam this is so adjusted that it -falls down until it comes in contact with a metallic stop. This can be -so adjusted that a slight tap will cause the beam to slightly jump from -the stop. The beam and the stop form two poles of an electric circuit, -in which there is a telephone. The slightest motion in a _vertical_ -direction causes a fluctuation in the current passing between the stop -and the beam, and a consequent noise is heard in the telephone. - -With instruments analogous to these, observations have been made by -various observers in all portions of Italy, extending over a period -of ten years. Every precaution appears to have been taken to avoid -accidental disturbances, and the experiments have been repeated in a -variety of forms. - -_Results obtained in Italy._—The results which from time to time have -been announced are of the greatest interest to those who study the -physics of the earth’s crust, and they appear to be leading to the -establishment of laws of scientific value. - -It would seem that the soil of Italy is in incessant movement, there -being periods of excessive activity usually lasting about ten days. -Such periods are called seismic storms. These storms are separated by -periods of relative calm. These storms have their greater regularity -in winter, and sharp maximums are to be observed in spring and autumn. -In the midst of such a period or at its end there is usually an -earthquake. Usually these storms are closely related to barometric -depressions. To distinguish these movements from those which occur -under high pressure, the latter are called baro seismic movements, and -the former _vulcano seismic_ movements. The relation of these storms -to barometric fluctuation has been observed to have been very marked -during the time of a volcanic eruption. - -At the commencement of a storm the motions are usually small, and one -storm, lasting two or three days, may be joined to another storm. -In such a case the action may be a local one. It has been observed -that a barometrical depression tended to bring a storm to a maximum, -whilst an increase of pressure would cause it to disappear. Sometimes -these actions are purely local, but at other times they may affect a -considerable tract of land. - -If a number of pendulums of different length are observed at the same -place, there is a general similarity in their movements, but it is also -evident that the free period of the pendulum more or less disturbs -the character of the record. The greatest amplitude of motion in a -set of pendulums is not reached simultaneously by all the pendulums, -and at every disturbance the movement of one will predominate. From -this Rossi argues that the character of the microseismical motions is -not constant. Bertelli observed that the direction of oscillation of -the pendulums is different at different places, but each place will -have its particular direction dependent upon the direction of valleys -and chains of mountains in the neighbourhood. Rossi shows that the -directions of movement are perpendicular to the direction of lines of -faults, the lips of these fractures rising and falling, and producing -two sets of waves, one set parallel to the line of fracture, and the -other perpendicular to such a direction. These movements, according -to Bertelli, have no connection with the wind, rain, change of -temperature, and atmospheric electricity. - -[Illustration: FIG. 38.] - -The disturbances, as recorded at different towns, are not always -strictly synchronous, but succeed each other at short intervals. If, -however, we take monthly curves of the disturbances as recorded at -different towns in Italy, we see that these are similar in character. -The maximum of disturbance occurs about the winter solstice, and the -minimum about the summer solstice, and in this respect they exhibit -a perfect accordance with the curves drawn by Mallet to show the -periodicity of earthquakes. The accompanying curves taken from one -of Bertelli’s original memoirs not only show this general result but -also show the close accord there is between the results obtained at -different towns during successive months. - -At Florence, before a period of earthquakes there is an increase in the -amplitude and frequency of vertical movements. These vertical movements -do not appear to coincide with the barometrical disturbances, but they -appear to be connected with the seismic disturbances. - -They are usually accompanied with noises in the telephone, but as the -microphone is so constructed as to be more sensitive to vertical motion -than to horizontal motion, this is to be expected. This vertical motion -would appear to be a local action, inasmuch as the accompanying motions -of an earthquake which originates at a distance are horizontal. - -Storms of microseismical motions appear to travel from point to point. - -Sometimes a local earthquake is not noticed in the tromometer, -whilst one which occurred at a distance, although it may be small, -is distinctly observed. To explain this, Bertelli suggests the -existence of nodes. Similar conclusions were arrived at by Rossi when -experimenting on different portions of the sides of Vesuvius. Galli -noticed an augmentation in microseismic activity when the sun and moon -are near the meridian. Grablovitz found from Bertelli’s observations -a maximum two or three days before the syzygies, and a minimum -three days after these periods. He also found that the principal -large disturbances occurred in the middle of periods separating -the quadrature from the syzygies, the apogee from perigee, and the -lunistigi period from the nodes, whilst the smallest disturbances -happened in the middle of periods opposed to these. - -P. C. Melzi says that the curves of microseismical motions, -earthquakes, lunar and solar motions, show a concordance with each -other. - -With the microphone Rossi hears sounds which he describes as roarings, -explosions, occurring isolated or in volleys, metallic and bell-like -sounds, ticking, &c., which, he says, revealed natural telluric -phenomena. Sometimes these have been intolerably loud. At Vesuvius the -vertical shocks corresponded with a sound like volleys of musketry, -whilst the undulatory shocks gave the roaring. Some of these sounds -could be imitated artificially by rubbing together the conducting wires -in the same manner in which the rocks must rub against each other in -an earthquake. Other sounds were imitated by placing the microphone -on a vessel of boiling water, or by putting it on a marble slab and -scratching and tapping the under side of it. - -These, then, are some of the more important results which have been -arrived at by the study of microseismic motions. One point which seems -worthy of attention is that they appear to be more law-abiding than -their more violent relations, the earthquakes, and as phenomena in -which natural laws are to be traced they are certainly deserving our -attention. As to whether they will ever become the means of forewarning -ourselves against earthquakes is yet problematical. Their systematic -study, however, will enable us to trace the progress of a microseismic -storm from point to point, and it is not impossible that we may yet -be enabled to foretell where the storm may reach its climax as an -earthquake. These, I believe, are the views of Professor di Rossi, who -is at the present time engaged in the establishment of a system of -microseismic observations throughout Italy. - -Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer -was in a state of agitation, and similar disturbances were observed at -Livorno, Florence, and Bologna. - -Since February 1883 I have observed a tromometer in Japan, and such -results as have been obtained accord with results obtained in Italy. -The increase in microseismical activity with a fall of the barometer -is very marked. Other peculiarities in the behaviour of the instrument -will be referred to under ‘Earth Pulsations.’ - -_Cause of microseismic movements._—As to the cause of tromometric -movements, we have a field for speculation. Possibly they may be -due to slight vibratory motions produced in the soil by the bending -and crackling of rocks produced by their rise upon the relief of -atmospheric pressure. If this were so we should expect similar -movements to be produced at the time of an increase of pressure. Rossi -suggests that they may be the result of an increased escape of vapour -from the molten materials beneath the crust of the earth, consequent -upon a relief of pressure. The similarity of some of the sounds which -are heard with the microphone to those produced by boiling water are -suggestive of this, and Rossi quotes instances when underground noises -like those which we should expect to hear from a boiling fluid have -been heard before earthquakes without the aid of microphones. One -instance was that of Viduari, a prisoner in Lima, who, two days before -the shock of 1824, repeatedly predicted the same in consequence of the -noises he heard. - -A possible cause of disturbances of this order may be small but -sudden fluctuations in barometric pressure, which are visible during -a storm. During a small typhoon on September 15, 1881, when in the -Kurile Islands, I observed that the needle of an aneroid worked back -and forth with a period of from one to three seconds. This continued -for several hours. With every gust of wind the needle suddenly rose -and then immediately fell. At times it trembled. These movements were -observed in the open air. The extent of these sudden variations -was approximately from ·03 to ·05 inches. Beckoning an increase of -barometrical pressure of one hundredth of an inch as equivalent to a -load of twenty million pounds on the square mile, during this storm -there must have been the equivalent of loads of from 60 to 100 million -pounds to the square mile continually placed on and removed from a -considerable tract of the earth’s surface. If the period of application -of these stresses approximately coincide with the natural vibrational -period of the area affected, it would surely seem, especially when -we reflect upon the effect of an ordinary carriage, that tremors of -considerable magnitude ought to be produced. - -An inspection of the following few observations taken from my note-book -for the same typhoon will suggest that even the large and slower -variations are capable of producing tremulous motions. - - Time Barometer - h. m. reading - 12 5 P.M. 29·02 - 12 10 „ 29·05 - 12 12 „ 29·07 - 12 13 „ 29·05 - 12 25 „ 29·10 - 12 50 „ 29·00 - 1 10 „ 29·00 - 1 20 „ 29·07 - - - - - CHAPTER XX. - - EARTH PULSATIONS. - - Definition of an earth pulsation—Indications of - pendulums—Indications of levels—Other phenomena indicating - the existence of earth pulsations—Disturbances in lakes and - oceans—Phenomena resultant on earth pulsations—Cause of earth - pulsations. - - -The object of the present chapter is to show that from time to time it -is very probable that slow but large wave-like undulations travel over -or disturb the surface of the globe. - -These movements, which have escaped our attention on account of their -slowness in period, for want of another term I call earth pulsations. - -The existence of movements such as these may be indicated to us by -changes in the level of bodies of water like seas and lakes, by the -movements of delicate levels, by the displacement of the bob of a -pendulum relatively to some point on the earth above which it hangs, -and by other phenomena which will be enumerated. - -_Indication of pendulums._—Pendulums which have been suspended for the -purposes of seismometrical observations have, both by observers in -Italy and Japan, been seen to have moved a short distance out from, and -then back to, their normal position. - -This motion has simply taken place on one side of their central -position, and is not due to a swing. The character of these records -is such that we might imagine the soil on which the support of the -pendulum had rested to have been slowly tilted, and slowly lowered. -They are the most marked on those pendulums provided with an index -writing a record of its motions on a smoked glass plate, which index is -so arranged that it gives a multiplied representation of the relative -motion between it and the earth. As motions of this sort might be -possibly due to the action of moisture in the soil tilting the support -of the pendulum, and to a variety of other accidental causes, we cannot -insist on them as being certain indications that there are slow tips in -the soil, but for the present allow them to remain as possible proof of -such phenomena. - -Evidence of displacement of the vertical, which are more definite than -the above, are those made by Bertelli, Rossi, Count Malvasia, and other -Italian observers, who, whilst recording earth tremors, have spent so -much time in watching the vibrations of stiles of delicate pendulums -by means of microscopes. As a result of these observations we are -told that the point about which the stile of a pendulum oscillates -is variable. These displacements take place in various azimuths, and -they appear to be connected with changes of the barometer. I have made -similar observations in Japan. - -From this, and from the fact that it is found that a number of -different pendulums differently situated on the same area give similar -evidence of these movements, it would hardly seem that these phenomena -could be attributed to causes like changes in temperature and moisture. -M. S. di Rossi lays stress on this point, especially in connection with -his microseismograph, where there are a number of pendulums of unequal -length which give indications of a like character. The direction in -which these tips of the soil take place—which phenomena are noticeable -in seismic as well as microseismic motions—Rossi states are related to -the direction of certain lines of faulting. - -_Indications of levels._—Bubbles of delicate levels can be easily seen -to change their position with meteorological variations; but Rossi also -tells us that they change their position, sometimes not to return for -a long time, during a microseismic storm. Here again we have another -phenomenon pointing to the fact that microseismic disturbances are the -companions of slow alterations in level. - -One of the most patient observers of levels has been M. Plantamour, -who commenced his observations in 1878, at Sécheron, on the Lake of -Geneva. He used two levels, one placed north and south, and the other -east and west. During the summer of 1878 the east end rose, but at the -end of September a depression set in. The diurnal movements had their -maximum and minimum at 6 and 7.45 A.M. and P.M. The total amplitude was -4·89″. The variations of the east and west level appeared to be due to -the temperature, but the movements of the north and south level were -dependent upon an unknown cause. - -Between October 1, 1879, and September 30, 1880, the east end fell -rapidly, from the middle of November up to December 26, amounting to -88·71″. It then rose 6·55″ to January 5, and then fell again. On -January 28 it reached 89·95″, after which it rose. - -Between October 4, 1879, and January 28, 1880, the movement was 95·8″, -against 28·08″ of the previous year. - -These movements were not due alone to temperature. The north and south -level, which was not influenced by the cold of the winter, moved -4·56″. In the previous year 4·89″.[144] - -From February 17 to June 5, 1883, the author observed in Tokio the -bubbles of two delicate levels, one placed north and south, and the -other east and west. They were placed under glass cases on the head of -a stone column. The column, which is inside a brick building, rests -on a concrete foundation, and is about ten years old. It is in no way -connected with the building. The temperature of the room has a daily -variation of about 1° Fahr. - -In both these levels diurnal changes are very marked. Occasionally they -are enormously great. Thus, on March 25, the readings of the south end -of the north south level were as follows:— - - Time Readings. - h. m. - 25th. 4 00 P.M. 104·5 - 4 5 „ 103 - 4 10 „ 102 - 4 25 „ 101 - 4 30 „ 100 - 4 40 „ 98 - 4 42 „ 99·5 - 4 45 „ 100 - 4 50 „ 101 - 4 55 „ 101 - 5 00 „ 100 - 26th. 7 00 A.M. 105 - -Usually this level moves through about three divisions per day. - -From March 25 to May 4 it travelled from 98 to 127. Since then, to June -5, it has descended to 116. During this period the east west level -has been _comparatively_ quiet. One division of the north south level -equals about 2″ of arc. - -Many of these changes may be due to changes in temperature, variations -in moisture, and other local actions. Some of them, however, are hardly -explicable on such assumptions. The fact that the general direction in -change of the vertical, as indicated by a tromometer standing on the -same column with the levels, showed that the change which was taking -place was rather in the column than in the instruments. - -The fact also that at the time of a barometrical depression a -_pulse-like surge_ can be seen in the levels, having a period averaging -about three seconds and sometimes amounting to about one second of -arc, is a phenomenon hardly to be attributed to sudden fluctuations in -moisture or temperature, but indicates real changes in level.[145] - -In addition to variation in the bubbles of levels which come on more or -less gradually, we have many recorded instances of _sudden_ alterations -taking place in these instruments. - -Examples of what may have been a slow oscillating motion of the earth’s -crust are referred to by Mr. George Darwin in a Report to the British -Association in 1882. - -One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged -in levelling the axis of a telescope, observed spontaneous oscillation -in the bulb of the level. - -This was on May 10 (April 28), 1877. The complete period was about -20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen -minutes before this he observes that there had been a severe earthquake -at Iquique, the distance to which in a straight line was 10,600 -kilomètres, and on an arc of a great circle, 12,500 kilomètres. -On September 20 (8), in 1867, Mr. Wagner had observed at Pulkova -oscillations of 3″, seven minutes before which there had been an -earthquake at Malta. On April 4 (March 23), 1868, an agitation of the -level had been observed by Mr. Gromadzki, five minutes before which -there had been an earthquake in Turkestan. Similar observations had -been made twice before. These, however, had not been connected with any -earthquakes—at least, Mr. Darwin remarks—with certainty. - -_Phenomena analogous to the pendulum and level observations._—As -examples of phenomena which are analogous to those made on pendulums -and levels, the following may be noticed. On March 20, 1881, at 9 P.M. -a watchmaker in Buenos Ayres observed that all his clocks oscillating -north and south suddenly began to increase their amplitude, until some -of them became twice as great as before. Similar observations were -made in all the other shops. No motion of the earth was detected. -Subsequently it was learnt that this corresponded with an earthquake in -Santiago and Mendoza.[146] - -Another remarkable example illustrating the like phenomena is furnished -by the observations which were made on December 21, 1860, by means of a -barometer in San Francisco, which oscillated, with periods of rest, for -half an hour. No shock was felt, nor is it likely that it was a local -accident, as it could not be produced artificially. On the following -day, however, a violent earthquake was experienced at Santiago.[147] - -At the time or shortly after the great Lisbon earthquake, curious -phenomena were observed in distant countries, which only appear to be -explicable on the assumption of the existence of earth pulsations. - -Thus at Amsterdam and other towns, chandeliers in churches were -observed to swing. At Haarlem water was thrown over the sides of tubs, -and it is expressly mentioned that no motion was perceived in the -ground. - -At the Hague a tallow chandler was surprised at the clashing noise -made by his candles, and this the more so because no motion was felt -underfoot. - -_Unusual disturbances in bodies of water._—At the time of large -earthquakes it would appear that earth pulsations are produced, which -exhibit themselves in countries where the actual shaking of the -earthquake is not felt, by disturbances in bodies of waters like lakes -and seas. - -Some remarkable examples of these disturbances are to be found in the -records of the great Lisbon earthquake. This earthquake, as a violent -movement of the ground, was chiefly felt in Spain, Portugal, northern -Italy, the south of France and Germany, northern Africa, Madeira, and -other Atlantic islands. In other countries further distant, as, for -instance, Great Britain, Holland, Scandinavia, and North America, -although the records are numerous, the only phenomena which were -particularly observed was the slow oscillations of the waters in lakes, -ponds, canals, &c. In some instances the observers especially remark -that there was no motion in the soil. - -Pebley Dam, in Derbyshire, which is a large body of water covering -some thirty acres, commenced to oscillate from the south. A canal near -Godalming flowed eight feet over the walk on the north side. - -Coniston Water, in Cumberland, which is about five miles long, -oscillated for about five minutes, rising a yard up its shores. Near -Durham a pond, forty yards long and ten broad, rose and fell about one -foot for six or seven minutes. There were four or five ebbs and flows -per minute. - -Loch Lomond rose and fell through about two and a half feet every -five minutes, and all the other lochs in Scotland seem to have been -similarly agitated. - -At Shirbrun Castle, in Oxfordshire, where the water in some moats and -ponds was very carefully observed, it was noticed that the floods -began gently, the velocity then increased, till at last with great -impetuosity they reached their full height. Here the water remained for -a little while, until the ebb commenced, at first gently, but finally -with great rapidity. At two extremities of a moat about 100 yards long, -it was found that the sinkings and risings were almost simultaneous. -The motions in a pond a short distance from the moat were also -observed, and it was found that the risings and sinkings of the two did -not agree. - -During these motions there were several maxima. - -These few examples of the motions of waters, without any record of the -motions of the ground, at the time of the Lisbon earthquake, must be -taken as examples of a very large number of similar observations of -which we have detailed accounts. - -Like agitations, it must also be remembered, were perceived in North -America and in Scandinavia, and if the lakes of other distant countries -had been provided with sufficiently delicate apparatus, it is not -unlikely that similar disturbances would have been recorded. - -Besides these movements in the waters of seas and lakes, at or about -the time of great earthquakes, we have records of like movements, which -take place as independent phenomena. - -Thus we read that on October 22, 1755, the waters of Lake Ontario rose -and fell five and a half feet several times in the course of half an -hour.[148] On March 31, 1761, Loch Ness rose suddenly for the period -of three-quarters of an hour.[149] - -As another example of the disturbance of water at the time of a great -earthquake in districts where the earthquake was not felt, may be -mentioned the swelling of the waters of the Marañon, in 1746, on the -night when Callao was overwhelmed. - -Sudden variations in the level of the water have been many times -observed in the North American lakes. The changes in level which -sometimes take place in the Genfer and Boden lakes are supposed to have -some relation to the condition of the atmosphere. A rising and falling -of especial note took place on April 18, 1855. - -In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’ - -From the observations and calculations of Prof. Forel it would seem -that the period of the ‘seiches’ depends upon the dimensions of the -lakes; the calculated periods dependent on the depths of the lakes -being approximately equal to the observed periods.[150] - -W. T. Bingham, writing on the volcanoes of the Hawaiian Islands, -remarks that it is not unusual for the sea to be agitated by great and -unusual tides, and that such sea waves have not been attended with -volcanic eruptions or seismic disturbances. Thus in May 1819 the tide -rose and fell thirteen times. On November 7, 1837, there was an ebb and -flow of eight feet every twenty-eight minutes. Again, on May 17, 1841, -like phenomena, unaccompanied by any other unusual occurrences, were -recorded.[151] - -Phenomena which may possibly hold a relationship to earth pulsations -are the periodical swellings of the ocean on the coast of Peru. Dr. C. -F. Winslow, who made a long period observation upon the coast of Peru, -found ‘the highest tides to prevail at Callao and Paita in December -and January,’ and ‘also a series of enormous waves or sea-swells to -be thrown from time to time upon the coast, varying from twenty-four -to twenty-seven hours in continuance, accompanied by unusual height -of the tide during the same period.’ During June and July the ocean -was unusually tranquil. These phenomena do not appear to be connected -with great atmospheric storms, nor do they hold any relation to the -prevailing wind. They increase with and accompany the swelling of the -tides, and occur generally, but not always, about full moon. - -Sometimes they break suddenly upon the coast. ‘_They are annual and -constant in their periodicity._’ - -The periodical swellings are most noticeable between Tumbez 3° S.L. and -the Chincha Islands 14° S.L. - -These oceanic phenomena synchronise with the periodic intensity of -earthquake phenomena in that part of the globe, and these with tidal -movements.[152] - -_Other phenomena possibly attributable to earth pulsations._—If we -assume that earth pulsations have an existence, these many phenomena -which are otherwise difficult to understand meet with an explanation. -The curious effects which were produced in the springs at Toplitz at -the time of the Lisbon earthquake may have been due to a pulse-like -wave. The flow of the principal spring was greatly increased. Before -the increase it became turbid and at one time stopped. Subsequently it -became clear and flowed as usual, but the water was hotter and more -strongly mineralised. Sudden changes in the flow of underground waters -which from time to time are observed may be attributed to like causes. -Secondary earthquakes such as occurred after the Lisbon earthquake, -as for instance in Derbyshire, may have been produced by pulsations -disturbing the equilibrium of ground in a critical state. - -The falling in of subterranean excavations is also possibly connected -with these phenomena. - -_Possible causes of earth pulsations._—Mr. George Darwin, in a report -to the British Association (1882), has shown that movements of -considerable magnitude may occur in the earth’s crust in consequence of -fluctuations in barometrical pressure. (A rise of the barometer over an -area is equivalent to loading that area with a weight, in consequence -of which it is depressed. When the barometer falls, the load is removed -from the area, which, in virtue of its elasticity, rises to its -original position. This fall and rise of the ground completes a single -pulsation.) - -On the assumption that the earth has a rigidity like steel, Mr. Darwin -calculates that if the barometer rises an inch over an area like -Australia, the load is sufficient to sink that continent two or three -inches. - -The tides which twice a day load our shores cause the land to rise and -fall in a similar manner. On the shores of the Atlantic, Mr. Darwin -has calculated that this rise and fall of the land may be as much as 5 -inches. By these risings and fallings of the land the inclination of -the surface is so altered that the stile of a plummet suspended from a -rigid support ought not always to hang over the same spot. There would -be a deflection of the vertical. - -In short, calculations respecting the effects of loads of various -descriptions, which we know are by natural operations continually -being placed upon and removed from the surface of various areas of the -earth’s surface, indicate that slow pulsatory movements of the earth’s -surface must be taking place, causing variations in inclination of one -portion of the earth’s crust relatively to another. - -Although it is possible that phenomena like the surging of levels may -be attributable to causes like these, we can hardly attribute the other -phenomena to such agencies. - -Rather than seek an explanation from agencies exogenous to our earth, -we might perhaps with advantage appeal to the endogenous phenomena of -our planet. When the barometer falls, which we have shown corresponds -to an upward motion of the earth’s crust, we know, from the results of -experiments, that microseismic motions are particularly noticeable. - -As a pictorial illustration of what this really means, we may imagine -ourselves to be residing on the loosely fitting lid of a large -cauldron, the relief of the external pressure over which increases the -activity of its internal ebullition—the jars attendant on which are -gradually propagated from their endogenous source to the exterior of -our planet. This travelling outwards would take place much in the same -way that the vibrations consequent to the rattle and jar of a large -factory slowly spread themselves farther and farther from the point -where they were produced. - -Admitting an action of this description to take place, it would then -follow that this extra liberation of gaseous material beneath the -earth’s crust would result in an increased upward pressure from within, -and a tendency on the part of the earth’s crust to elevation. If we -accept this as an explanation of the increased activity of a tremor -indicator, then such an instrument may be regarded as a barometer, -measuring by its motions the variations in the internal pressure of our -planet. - -The relief of external pressure and the increase of internal pressure, -it will be observed, both tend in the same direction—namely, to an -elevation of the earth’s crust. - -This explanation of the increased activity of earth tremors, which -has also been suggested by M. S. di Rossi, is here only advanced as a -speculation, more probable perhaps than many others. - -We know how a mass of sulphur which has been fused in the presence of -water in a closed boiler gives up in the form of steam the occluded -moisture upon the relief of pressure. In a similar manner we see steam -escaping from volcanic vents and cooling streams of lava. We also -know how gas escapes from the pores and cavities in a seam of coal -on the fall of the barometrical column. We also know that certain -wells increase the height of their column under like conditions. The -latter of these phenomena, resulting in an increase in the rate of -drainage of an area by its tendency to render such an area of less -weight, facilitates its rise. If we follow the views of Mr. Mallet in -considering that the pressures exerted on the crust of our earth may in -volcanic regions be roughly estimated by the height of a column of lava -in the volcanoes of such districts, we see that in the neighbourhood of -a volcano like Cotopaxi the upward pressures must be enormously great. -Further, the phenomena of earthquakes and volcanoes indicate that these -pressures are variable. Before a volcano bursts forth we should expect -that there would be in its vicinity an upward bulging of the crust, and -after its formation a fall. Further, it is not difficult to conjecture -other possible means by which such pressures may obtain relief. - -Should these pressures then find relief without rupturing the -surface, it is not difficult to imagine them as the originators of -vast pulsations which may be recorded on the surface of the earth as -wave-like motions of slow period. - -As an explanation of the strange movements observed on seas and lakes, -Kluge brings forward the following strange and remarkable theory. The -oxygen of the air is magnetic, whilst water is diamagnetic and the -earth magnetic: we have, therefore, in our seas and lakes a diamagnetic -body lying between and being, consequently, repelled from two magnetic -bodies. By variation in temperature, the balance of repulsions -exerted by the air and the earth is destroyed. Thus, by an elevation -of temperature the air expands and flows away from the heated area, -where, in consequence, there is less oxygen. The result of this is, -that the repulsion of the air upon the waters is less than that of the -earth upon the waters, and the waters are in consequence raised up. By -a falling of temperature the waters may be depressed, and by either -of these actions waves may be produced without the intervention of -earthquakes or earth pulsations. - -The more definite kinds of information which we have to bring forward, -tending to prove the existence of earth pulsations, too slow in period -to be experienced by ordinary observers, are those which appear to be -resultant phenomena of great earthquakes. - -The phenomena that we are certain of in connection with earth -vibrations, whether these vibrations are produced artificially by -explosions of dynamite in bore-holes, or whether they are produced -naturally by earthquakes, are, first, that a disturbance as it dies out -at a given point often shows in the diagrams obtained by seismographs a -decrease in period; and, secondly, a similar decrease in the period of -the disturbance takes place as the disturbance spreads. - -As examples of these actions I will quote the following. - -The diagram of the disturbance of March 1, 1882, taken at Yokohama, -shows that the vibrations at the commencement of the disturbance had a -period of about three per second, near the middle of the disturbance -the period is about 1·1, whilst near the end the period has decreased -to ·46. That is to say, the backward and forward motion of the ground -at the commencement of the earthquake was six times as great as it was -near the end, when to make one complete oscillation it took between two -and three seconds. Probably the period became still less, but was not -recorded owing to the insensibility of the instruments to such slow -motions.[153] - -We have not yet the means of comparing together diagrams of two or more -earthquakes, one having been taken near to the origin, and the other -at a distance. The only comparisons which I have been enabled to make -have been those of diagrams taken of the same earthquake, one in Tokio -and the other in Yokohama. As this base is only sixteen miles, and the -earthquake may have originated at a distance of several hundreds of -miles, comparisons like these can be of but little value. - -Other diagrams illustrating the same point are those obtained at three -stations in a straight line, but at different distances from the -origin of a disturbance produced by exploding a charge of dynamite in -a bore-hole. A simple inspection of these diagrams shows that at the -near station the disturbance consisted of backward and forward motions, -which, as compared with the same disturbance as recorded at a more -distant station, were very rapid. Further, by examining the diagram -of the motions, say, at the near station, it is clearly evident that -the period of the backward and forward motion rapidly decreased as the -motion died out. - -These illustrations are given as examples out of a large series of -other records, all showing like results. - -An observation which confirms the records obtained from seismographs -respecting the increase in period of an earthquake as it dies out -I have had opportunities of twice making with my levels. After all -perceptible motion of the ground subsequent upon a moderately severe -shock had died away, I have distinctly seen the bubble in one of these -levels slowly pulsating with an irregular period of from one to five -seconds. - -Although we must draw a distinction between earth waves and water -waves, we yet see that in these points they present a striking -likeness. Let us take, for example, any of the large earthquake waves -which have originated off the coast of South America, and then radiated -outwards, until they spread across the Pacific, to be recorded in Japan -and other countries perhaps twenty-five hours afterwards, at a distance -of nearly 9,000 miles from their origin. Near this origin they appeared -as walls of water which were seen rapidly advancing towards the coast. -These have been from twenty to two hundred feet in height, and they -succeeded each other at rapid intervals, until finally they died out as -a series of gentle waves. By the time these walls of water traversed -the Pacific, to, let us say, Japan, they broadened out to a swell so -flat that it could not be detected on the smoothest water excepting -along shore lines where the water rose and fell like the tide. Instead -of a wall of water sixty feet in height, we had long flat undulations -perhaps eight feet in height, but with a distance from crest to crest -of from one to two hundred miles. - -If we turn to the effects of large earthquakes as exhibited on the -land, I think that we shall find records of phenomena which are only -to be explained on the assumption of an action having taken place -analogous to that which takes place so often in the ocean, or an action -similar to that exhibited by small earthquakes, and artificially -produced disturbances, if greatly exaggerated. - -The only explanation for the phenomena accompanying the Lisbon -earthquake appears to be that the short quick vibrations which had -ruined so many cities in Portugal had, by the time that they had -radiated to distant countries, gradually become changed into long flat -waves having a period of perhaps several minutes. In countries like -England these pulse-like movements were too gentle to be perceived, -except in the effects produced by tipping up the beds of lakes and -ponds. - -The phenomenon was not unlike that of a swell produced by a distant -storm. It would seem possible that in some cases pulsations producing -phenomena like the ‘seiches’ of Switzerland might have their origin -beneath the ocean, or deep down beneath the earth’s crust. Perhaps, -instead of commencing with the ‘snap and jar’ of an earthquake, they -may commence as a heaving or sinking of a considerable area, which -may be regarded as an uncompleted effort in the establishment of an -earthquake or a volcano. - -From what has now been said it would seem that earth pulsations -are phenomena with a real existence, and that some of these are -attributable to earthquakes. On the other hand, certain earthquakes -are attributable to earth pulsations. Some of the phenomena which -have been brought forward have only a possible connection with these -movements, and they yet require investigation. Elastic tides in the -earth’s crust have for long been realities in the minds of physicists. -These, however, are due to lunar and solar influences, and are regular -in their action. The tidal-like movements called pulsations are of -greater magnitude, and their goings and comings are irregular. - - - - - CHAPTER XXI. - - EARTH OSCILLATIONS. - - Evidences of oscillation—Examples of oscillation—Temple of Jupiter - Serapis—Observations of Darwin—Causes of oscillation. - - -_Evidences of oscillation._—By earth oscillations are meant those slow -and quiet changes in the relative level of the sea and land which -geologists speak of as elevations or subsidences. These movements are -especially characteristic of volcanic and earthquake-shaken countries. - -As evidences of elevations we appeal to phenomena like raised beaches, -sea-worn caves, raised coral reefs, and the remains of other dead -organisms like barnacles, and the borings of lithodomous shells in and -on the rocks of many coasts high above the level of the highest tides. -As a proof that subsidence has taken place, there is the evidence -afforded by submerged forests, the prolongation of certain valleys -beneath the bed of the ocean, the formation of coral islands, the -peculiar distribution of the plants and animals which we find in many -countries, and the submergence of works of human construction. Inasmuch -as these phenomena are discussed so fully in many treatises on physical -geology, the references to them here will be made as brief as possible. -Elevations and depressions which have taken place at the time of large -earthquakes in a paroxysmal manner have already been mentioned. The -movements referred to in this chapter, although generally taking place -with extreme slowness, in certain instances, by an increase in their -rapidity, have approached in character to earth pulsations. In most -instances it would appear that the upward movement of the ground, which -may be likened to a process of tumefaction, goes on so gently that it -only becomes appreciable after the lapse of many generations. - -_Examples of movements._—Lyell estimated that the average rate of rise -in Scandinavia has been about two and a half feet per century. At the -North Cape the rise may have been as much as five or six feet per -century. Observations made at the temple of Jupiter Serapis, between -October 1822 and July 1838, showed that the ground was sinking at the -rate of about one inch in four years. Since the Roman period, when this -temple was built, the ground has sunk twenty feet below the waves. Now -the floor of the temple is on the level of the sea. Lyell remarks that -if we reflect on the dates of the principal oscillations at this place -there appears to be connection between the movements of upheaval and a -local development of volcanic heat, whilst periods of depression are -concurrent with periods of volcanic quiescence.[154] - -As examples of movements even more rapid than those at the Temple of -Jupiter Serapis we refer to an account of the earthquakes in Vallais -(November 1755), when the ground about a mountain at a small distance -from Brigue sank about a thumb’s-breadth every twenty-four hours. This -took place between December 9 and February 26.[155] - -Another remarkable example of earth movement is given in the account -of the earthquake at Scarborough, on December 29, 1737, when the head -of the spa water well was forced up in the air about ten yards high. At -this time the sands on the shore are said to have risen so slowly that -people came out to watch them.[156] - -Two other examples of rapid earth movement are taken from Professor -Rossi’s ‘Meteorologia Endogena.’ Professor D. Seghetti, writing to -Professor Rossi, says that a few lustres ago (one lustre = twenty -years) Mount S. Giovanni hid the towns Jenne and Subiaco from each -other. From Subiaco the church at Jenne is now visible, which a few -years ago was invisible. The people at Jenne also can see more than -formerly. The supposition is that the side of Mount S. Giovanni is -lowered. This fact corresponds to a fact stated by Professor Carina, -who says that forty or fifty years ago from Granaiola you could not see -either the church of S. Maria Assunta di Citrone or the church of S. -Pietro di Corsena. Now you can see both.[157] - -For a remarkable example illustrating the connection between seismic -activity and elevation we are indebted to the patient labours of -Darwin, who carefully investigated the evidences of elevation which -are visible upon the western coasts of South America. These evidences, -consisting of marks of erosion, caves, ancient beaches, sand dunes, -terraces of gravel, &c., were traced between latitudes 45° 35′ to 12° -5′, a distance north and south of 2,075 geographical miles, and there -is but little doubt that they extend much farther. As deduced from -observations upon upraised shells alone, a summary of Mr. Darwin’s -observations are contained in the following table:— - - Feet - At Chiloe the recent elevation has been 350 - „ Concepcion „ „ 625 to 1,000 - „ Valparaiso „ „ 1,300 - „ Coquimbo „ „ 252 - „ Lima „ „ 85 - -Shells, similar to those clinging to uplifted rocks, which are -evidences of these elevations, still exist in the neighbouring seas, -and in the same proportionate numbers as they are found in the upraised -beds. In addition to this, Mr. Darwin shows us that at Lima, during the -Indo-human period, the elevation has been at least eighty-five feet. -At Valparaiso, during the last 220 years, the rise was about nineteen -feet, and in the seventeen years subsequent to 1817 the rise has been -ten or eleven feet, a portion only of which can be attributed to -earthquakes. In 1834 the rise there was apparently still in progress. - -At Chiloe there has been a gradual elevation of about four feet in four -years. These, together with numerous other examples, testify to the -gradual but, as compared with other parts of the globe, exceedingly -rapid rise of the ground upon the western shores of South America.[158] -The most important point to be noticed is that this district of rapid -elevation is one of the most earthquake-shaken regions of the world. -And further, judging from Darwin’s remarks, in those portions of it -where the movements have been the most extensive, and at the same time -probably the most rapid, the seismic disturbances appear to have been -the most noticeable. - -Similar remarks may be applied to Japan, it being in those districts -where evidences of recent elevation are abundant that earthquakes -are numerous. Thus, in the bay of Yedo, where we have borings of -lithodomi in the tufaceous cliffs ten feet above high-water mark, -which, inasmuch as the rock in which they are found is soft and easily -weathered, indicate an exceedingly rapid elevation, earthquakes are of -common occurrence. - -From the evidences of elevation which we have upon the South American -coast, Japan, and in other countries, it appears that these movements -are intermittent, there being periods of rest, when sea cliffs are -denuded, and perhaps even periods of subsidence. There is also evidence -to show that, although these movements have been gradual from time to -time, they have been aided by starts occasioned by earthquakes. - -As to whether earthquakes are more numerous during periods of -elevation, or of subsidence, or during the intermediate periods of -rest, we have no evidence. - -Sudden displacements which occasionally accompany earthquakes might, it -was said, sometimes be regarded as the _cause_ of an earthquake, and -sometimes as the _effect_. - -The slow elevations here referred to may be looked upon as being one of -the more important factors in the production of earthquakes. By various -causes the rocky coast is bent until, having reached the limit of its -elasticity, it snaps, and, in flying back like a broken spring, causes -the jars and tremors of an earthquake. - -If this is the case, then the number of earthquakes felt in a district -which is being elevated may possibly be a function of the rate of -elevation. - - - - - APPENDIX. - - LIST OF THE PRINCIPAL BOOKS, PAPERS, PERIODICALS, WHICH ARE - REFERRED TO IN THE PRECEDING PAGES. - - • • • • • - - _For a more complete bibliography of earthquakes refer to Mallet’s - catalogue of works given in his report to the British Association - in 1858._ - - • • • • • - - A True and Particular Relation of the Dreadful Earthquake which - happened at Lima, &c. (1746). 1768. - Abbot, Gen. H. L. On the Velocity of Transmission of Earth Waves. - _Am. Jour. Sci._ XV., March 1878. - — Shock of the Explosion at Hallet’s Point, Nov. 14, 1876. - _Battalion Press._ - Alexander, Prof. T. See _Trans. Seis. Soc. of Japan_. - American Journal of Science. - Annali del reale osservatorio meteorologico Vesuviano. - Annual Register, The. - Anonymous, A Chronological and Historical Account of the most - Memorable Earthquakes in the World, &c. 1750. - — A Vindication of the Bishop of London’s Letter occasioned by the - Late Earthquake. 1750. - — Phenomena of the Great Earthquake of Nov. 1, 1755. - — Serious Thoughts occasioned by the Late Earthquake at Lisbon. - 1755. - Asiatic Society of Japan, Transactions of. - Ayrton, Prof. W. E. _See_ Perry, J. - - Bárceno, M. Estudio del Terremoto (May 17, 1879) Mexico. 1879. - Beke, Dr. C. T. Mount Sinai a Volcano. - Bissett, Rev. J. A Sermon (on account of the Earthquake at Lisbon, - Nov. 1, 1755). 1757. - Bittner, A. Beiträge zur Kenntniss des Erdbebens von Belluno vom - 29. Juni 1873. - — Sitzungsb. der K. Akad. d. Wissensch., lxix. II. Abth., 1874. - Bollettino del Vulcanismo Italiano. - Boué, Dr. A. Ueber das Erdbeben welches Mittel-Albanien im - October d. J. so schrecklich getroffen hat. _Die K. Akad. d. - Wissenschaften_, Nov. 1851. - — Parallele der Erdbeben, des Nordlichtes und des Erdmagnetismus. - — Ueber die Nothwendigkeit die Erdbeben und vulkanischen - Erscheinungen genauer als bis jetzt beobachten zu lassen. _Die - K. Akad. d. Wissenschaften_, 1851 and 1857. - Bouguer, M. Of the Volcanoes and Earthquakes in Peru. - British Association, Reports of. - Brunton, R. H. Constructive Art in Japan. _Trans. Asiatic Soc. of - Japan_, II. and III., Pt. 2. - Bryce, J. Report to British Association, 1841. - Buffour, M. The Natural History of Earthquakes and Volcanoes. - - C. H. A Physical Discussion of Earthquakes, &c. 1693. - Canterbury, Thomas, Lord Archbishop of, The Theory and History of - Earthquakes. - Casariego, E. A. See _Trans. Seis. Soc. of Japan_. - Cawley, G. Some Remarks on Construction in Brick and Wood, &c. - _Trans. Asiatic Soc. of Japan_, VI. Plate ii. - Chaplin, Prof. W. S. An Examination of the Earthquakes recorded - at the Meteorological Observatory, Tokio. _Trans. Asiatic Soc. - of Japan_, VI. Part ii. - Comptes Rendus. - Credner, H. Das Dippoldiswalder Erdbeben vom Oktober 1877. - — Zeitschr. f. d. Naturwiss. f. Sachsen u. Thüringen. - — Das Vogtländisch-erzgebirgische Erdbeben, 23. Nov. 1875. - — Zeitschr. f. d. gesammt. Naturwissenschaften, xlviii., Oktober. - - Dan, T. See _Trans. Seis. Soc. of Japan_. - Darwin, Charles. Researches on Geology and Natural History. - — Geological observations. - Darwin, G. H. Reports on Lunar Disturbance of Gravity to British - Association, 1881. 1882. - Diffenbach, F. Plutonismus und Vulkanismus in der Periode von - 1868–1872, und ihre Beziehungen zu den Erdbeben im Rheingebiet. - Doelter, C. von. Ueber die Eruptivgebilde von Fleims, nebst einigen - Bemerkungen über den Bau älterer Vulcane. - — lxxiv. Band d. Sitzungsb. d. K. Akad. d. Wissensch., I. Abth., - Dec. Heft, Jahrg. 1876. - Doolittle, Rev. T. Earthquakes Explained and Practically Improved, - &c. 1693. - Doyle, P. See _Trans. Seis. Soc. of Japan_. - - Emerson, Prof., B.A. Review of Von Seebachs’ Earthquake of March 6, - 1872. _Am. Jour. Sci._, Series III. - Ewing, Prof. J. A. Earthquake Measurement. A memoir published by - the Tokio University. 1883. - — See _Trans. Seis. Soc. of Japan_. - - Falb, R. Gedanken und Studien über den Vulkanismus, &c. 1875. - — Grundzüge zu einer Theorie der Erdbeben und Vulkanausbrüche. - — Das Erdbeben von Belluno. ‘Sirius,’ Bd. VI., Heft ii. - Flamstead, J. A Letter concerning Earthquakes. 1693. - Forel, F. A. Les Tremblements de Terre (Suisse). _Arch. des - Sciences Physiques et Naturelles_, VI. p. 461. - — Tremblement de Terre du 30 Décembre 1879. - Fuchs, Karl. Vulkane und Erdbeben. - — _Die Vulkanischen Erscheinungen der Erde._ - - Garcia, J. C. See _Trans. Seis. Soc. of Japan_. - Geinitz, Dr. E. Das Erdbeben von Iquique am 9. Mai 1877, &c. _Die - K. Leop.-Carol.-Deutschen Akademie der Naturforscher_, Band - xl., Nr. 9. - Gentleman’s Magazine, The. - Geographical Society, Proceedings of. - Geological Society, Proceedings of. - Girard, Dr. H. Ueber Erdbeben und Vulkane. 1845. - Gray, T. See _Trans. Seis. Soc. of Japan_. - — On Instruments for Measuring and Recording Earthquake Motions. - Phil. Mag. Sept. 1881. - — On Recent Earthquake Investigation. _The Chrysanthemum_, 1881. - Guiscardi, Prof. G. Notizie del Vesuvio. 1857. - — Il terremoto di Casamicciola del 4 Marzo. 1881. - - Hales, S., D.D., F.R.S. Some Considerations on the Causes of - Earthquakes. 1750. - Hamilton, Sir W. Observations on Mount Vesuvius, Mount Etna, &c. - 1774. - Hattori, I. Destructive Earthquakes in Japan. _Trans. Asiatic Soc. - of Japan_, V. Plate i. - Heim, Prof. A. Les Tremblements de Terre et leur Etude - Scientifique. 1880. - — Prof. A. Die Schweizerischen Erdbeben in 1881–1882. - Hoeffer, Prof. H. Die Erdbeben Kärntens und deren Stosslinien. - _Die Kais. Akademie d. Wissenschaften_, Band xlii. - Höfer, Prof. H. Das Erdbeben von Belluno, am 29. Juni 1873. - _Sitzungsb. der K. Akad. d. Wissensch._, I. Abth., Band lxxiv. - Hoff, K. E. A. von. Geschichte der durch Ueberlieferung - nachgewiesenen natürlichen Veränderungen der Erdoberfläche. - 1822. - Hooke, R., M.D., F.R.S. Discourses concerning Earthquakes. - Hopkins, William. Report to the British Association on the - Geological Theories of Elevation and Earthquakes. 1847. - Horton, Rev. Mr. An Account of the Earthquake which happened at - Leghorn in Italy (Jan. 1742). 1750. - Humboldt, Alexander von. Cosmos. - — Travels. - - Jeitteles, L. A. Bericht über das Erdbeben am 15. Januar 1858. - — Sitzungsberichte der mathem.-naturw. Classe d. K. Akad. d. - Wissensch., xxxv. S. 511. - Judd, J. W., Prof. Volcanoes, What they Are and What they Teach. - - Knipping, E. Verzeichniss von Erdbeben wahrgenommen in Tokio, - &c. _Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde - Ostasiens_, Heft 14. - — See _Trans. Seis. Soc. of Japan_. - - Lasaulx, A. von. Das Erdbeben von Herzogenrath am 22. October 1873. - Lemery, M. A Physico-Chemical Explanation of Subterranean Fires, - Earthquakes, &c. - Lescasse, M. J. Etude sur les Constructions Japonaises, &c. - _Mémoires de la Société des Ingénieurs Civils_. - Lister, M., M.D., F.R.S. Of the Nature of Earthquakes. - Little, Rev. J. Conjectures on the Physical Causes of Earthquakes - and Volcanoes. 1820. - - Mallet, R. The Neapolitan Earthquake, Vol. II. _Reports to the - British Association_, 1850, 1851, 1852, 1854, 1858, 1861. - — Secondary Effects of the Earthquake of Cachar. _Proc. Geolog. - Soc._, 1872. - — Dynamics of Earthquakes. _Trans. Royal Irish Acad._ 1846. - Milne, David. Reports to British Association, 1841, 1843, 1844. - Milne, John. See _Trans. Seis. Soc. of Japan_. - — On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) _Trans. - Royal Soc._ 1882. - — On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) _Proc. - Royal Soc._ No. 217, 1881. - — Earthquake Observations and Experiments in Japan (with T. Gray, - B.Sc., F.R.S.E.) _Phil. Mag._, Nov. 1881. - — On the Elasticity and Strength Constants of certain Rocks (with - T. Gray, B.Sc., F.R.S.E.) _Jour. Geolog. Soc._, 1882. - — A Visit to the Volcano of Oshima. _Geolog. Mag._, Dec. 2, Vol. - IV., pp. 193–197, 255. - — On the Form of Volcanoes. _Geolog. Mag._, Dec. 2, Vol. V., and - Dec. 2, Vol. VI. - — Note upon the Cooling of the Earth, &c. _Geolog. Mag._, Dec. 2., - Vol. VII., p. 99. - — Investigation of the Earthquake Phenomena of Japan. _Rep. Brit. - Assoc._, 1881 and 1882. - — A Large Crater. _Popular Science Review._ - — The Volcanoes of Japan (a series of Articles). _Japan Gazette._ - — Earthquake Literature of Japan (a series of Articles). _Japan - Gazette._ - — The Earthquake of Dec. 23, 1880. _The Chrysanthemum_, 1881. - — Earthquake Motion. _The Chrysanthemum_, 1882. - — Seismology in Japan. _Nature_, Oct. 1882. - — Earth Movements. _The Times_, Oct. 12, 1882. - Mitchell, Rev. J. Conjectures Concerning the Cause and Observations - upon the Phenomena of Earthquakes. 1760. - Mohr, Dr. F. Geschichte der Erde. 1875. - - Naturkundig Tijdschrift voor Nederlandsch Indie. 1875–1880. - Naumann, Dr. E. Ueber Erdbeben und Vulkanausbrüche in Japan. - _Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde - Ostasiens._ Heft 15. - Noggerath, Dr. J. Die Erdbeben vom 29. Juli 1846 im Rheingebiet, - &c. - — Die Erdbeben im Vispthale (1855). - — Die Erdbeben im Rheingebiet in den Jahren 1868, 1869, 1870. - — Jahrgänge d. Verbandlungen d. Natur. Vereins für 1870. _Rheinland - u. Westphalen_, xxvii. - - Oldham, Dr. Secondary Effects of the Earthquake of Cachas. _Proc. - Geolog. Soc._ 1872. - — Thermal Springs of India. _Memoirs of Geolog. Survey of India_, - XIX. Plate 2. - — A Catalogue of Indian Earthquakes. _Memoirs of Geolog. Survey of - India_, XIX. Plate 3. - — The Cachas Earthquake. _Memoirs of Geolog. Survey of India_, XIX. - Plate 1. - - Palmer, Col. H. S. See _Trans. Seis. Soc. of Japan_. - Palmieri, Prof. L., e Scacchi, A. Della Regione Volcanica del Monte - Vulture, e del Tremuoto ivi avvenuto nel dì 14 Agosto 1851, 1852. - — Annali del reale Osservatorio Meteorologico Vesuviano. - — Il Vesuvio, il Terremoto d’ Isernia e l’eruzione sottomarina di - Santorino. _R. Accad. d. Sci. Fis. e Mat. di Napoli_, iv. 1866. - — Sul recente Terremoto di Corleone. _R. Accad. d. Sci. Fis. e - Mat._, v. 1876. - — Il Terremoto di Scio del dì 4 Aprile. _R. Accad. d. Sci. Fis. e - Mat. di Napoli_, v. 1881. - — Sul Terremoto di Casamicciola del 4 Marzo 1881. _R. Accad. d. - Sci. Fis. e. Mat. di Napoli_. 1881. - Paul, Prof. H. M. See _Trans. Seis. Soc. of Japan_. - Perrey, Prof. A. Earthquake Catalogue and Memoirs. (For list see - Mallet’s Report to British Association. 1858.) - — See _Trans. Seis. Soc. of Japan_. - Perry, J., and W. E. Ayrton. On a Neglected Principle that may be - Employed in Earthquake Measurement. - — See _Trans. Seis. Soc. of Japan_. - Philosophical Magazine. - Pickering, Rev. R. An Address to those who have either retired - or intend to leave Town under the Imaginary Apprehension of - the Approaching Shock of another Earthquake. 1750. - - Ray, J., F.R.S. A Summary of the Causes of the Alterations which - have happened to the Face of the Earth. - Rockstroh, E. Informe de la Comision Científica del Instituto - Nacional de Guatemala, nombrada por el Sr. Ministro de - Instruccion Pública para el Estudio de los Fenómenos - Volcánicos en el Lago de Ilopango. 1880. - Rockwood, Prof. C. G. Notes on Earthquakes. Annually in the _Am. - Jour. Sci._ - — Japanese Seismology. _Am. Jour. Sci._, XXII. Dec. 1881. - Romaine, W. A Discourse occasioned by the Late Earthquake. 1755. - Rossi, Prof. M. S. di. Intorno all’ odierna fase dei Terremoti in - Italia, e segnatamente sul Terremoto in Casamicciola del 4 - Marzo 1881. _Società Geografica Italiana._ 1881. - — La Meteorologia Endogena, 2 vols. - Royal Society, Transactions of. - - Scacchi, A. _See_ Palmieri. - Schmidt, Dr. J. F. Untersuchungen über das Erdbeben am 15. Januar - 1858. - — Studien über Erdbeben. 1879. - — Die Eruption des Vesuv (1855). 1856. - Scrope, G. P. Volcanoes. - Seebach. Das mittle Deutsche Erdbeben (1872). _Mitt. der K.K. - geograph. Gesellsch._, II. Jahrg., 2. Heft, 1873. - Serpieri, Prof. A. C. S. Nuove Osservazioni sul Terremoto avvenuto - in Italia il 12 Marzo 1873. _Istituto Lombardo._ 1873. - — Il Terremoto di Rimini della notte 17–18 Marzo 1875. - — Documenti nuove e Riflessioni sul Terremoto della notte 17–18 - Marzo 1875. _Meteorologia Italiana_, iv. 1875. - — Determinazione delle fasi e delle leggi del grande Terremoto - avvenuto in Italia nella notte 17–18 Marzo 1875. _Istituto - Lombardo._ 1875. - — Dell’ influenza del Lume Solare sui Terremoti. _Istituto - Lombardo._ 1882. - Sherlock, T., D.D. (Lord Bishop of London). A Letter on the - occasion of the late Earthquakes. 1750. - Shower, Rev. J., D.D. Practical Reflections on the Earthquakes that - have happened in Europe and America, &c. 1750. - Stübel, A. (see Reiss, W.) - Stukeley, Rev. W., M.D., F.R.S. The Philosophy of Earthquakes, - Natural and Religious, &c. Plates 1, 2, and 3. 1756. - Sturmius, J. C. A Methodical Account of Earthquakes. - Suess, E. Die Erdbeben Niederösterreiches. _Die Kais. Akademie der - Wissenschaften_, Bd. xxxiii. - — Die Erdbeben des südlichen Italiens. _Die Kais. Akademie der - Wissenschaften_, Bd. xxxiv. - - Volger, Dr. G. H. Untersuchungen über das Phänomen der Erdbeben. - 1857. - - Wagener, Dr. G. Bemerkungen über Erdbebenmesser und Vorschläge zu - einem neuen Instrumente dieser Art. _Mitt. d. Deutsch. - Gesellsch. für Natur- und Völkerkunde Ostasiens_, Heft 15. - — See _Trans. Seis. Soc. of Japan_. - Winchilsea, The Earl of. A True and Exact Relation of the late - Prodigious Earthquake and Eruption of Mount Etna. 1669. - Woodward, J., M.D., F.R.S. Earthquake caused by some Accidental - Obstruction of a Continual Subterranean Heat. - - - SEISMOLOGICAL SOCIETY OF JAPAN. - - The following are a list of the papers published by this Society:— - - - VOL. I. - - Milne, J. Seismic Science in Japan. 35 pages. - Ewing, J. A. New Form of Pendulum Seismograph. 6 pages, 3 plates. - Gray, T. Seismometer and Torsion Pendulum Seismograph. 8 pages, 2 - plates. - Mendenhall, T. C. Acceleration of Gravity at Tokio (abstract). 2 - pages. - Wagener, G., and E. Knipping. New Seismometer and Observations with - same. 18 pages, 1 plate. - Milne, J. Earthquake in Japan of Feb. 22, 1880. 116 pages, 5 - plates, 8 woodcuts. - - - VOL. II. - - Milne, J. Recent Earthquakes of Yeddo, Effects on Buildings, &c. 38 - pages, 2 plates, and many tables. - Mendenhall, T. C. Gravity on Summit of Fujiyama (abstract). 2 pages. - Paul, H. M. Earth Vibrations from Railroad Trains (abstract). 4 - pages. - Ewing, J. A. Astatic Horizontal Lever Seismograph (abstract). 5 - pages, 1 plate. - Milne, J. Peruvian Earthquake of May, 9, 1877. 47 pages, 2 plates, - tables. Constitution, Rules, Officers and Members of the - Society, Dec., 1881. - - - VOL. III. - - Gray, T. Steady Points for Earthquake Measurements. 11 pages, 3 - plates. - Milne, J. Experiments in Observational Seismology. 53 pages, 1 - plate, tables. - — The Great Earthquakes of Japan. 38 pages, 1 plate, many tables. - Perry, J. Theory of a Rocking Column. 4 pages. - Knipping, E. Earthquake of July 25, 1880, with Dr. Wagener’s - Seismometer. 4 pages. - Ewing, J. A. Earthquake Observation at three or more Stations, &c. - 4 pages. - — Records of three recent Earthquakes. 6 pages, 3 plates. - — Earthquake of March 8, 1881. 8 pages, 1 plate. - Milne, J. Horizontal and Vertical Motion in Earthquake of March 8, - 1881. 8 pages, 3 plates. - Gray, T. Seismograph for Registering Vertical Motion. 3 pages, 1 - plate. - Ewing, J. A. Seismometer for Vertical Motion. 3 pages, 1 plate. - Gray, T. Seismograph for Large Motions. 2 pages. - — Compensating a Pendulum to make it Astatic. 3 pages. - Palmer, H. S. Note on Earth Vibrations. 3 pages. - Kuwabara, M. The Hot Springs of Atami. 2 pages. - - - VOL. IV. - - Milne, J. Distribution of Seismic Activity in Japan. 30 pages, 1 - plate. - Wada, T. Notes on Fujiyama. 7 pages. - Casariego, E. Abella y. Earthquakes of Nueva Vizcaya in 1881. 23 - pages, 2 maps. - Milne, J. Utilisation of Earth’s Internal Heat. 12 pages. - Ewing, J. A. Earthquake of March 11, 1882. 5 pages. - Doyle, P. Note on an Indian Earthquake. 6 pages. - Milne, J. Systematic Observation of Earthquakes. 31 pages, 5 plates. - - - VOL. V. - - Naumann, Dr. E. Notes on Secular Changes of Magnetic Declination in - Japan, p. 1–18. - Casariego, Don E. Abella y. Monografía Geológica del Volcan de - Albay ó El Máyon. p. 19–43. - Garcia, Don J. Centeno y. Abstract of a Memoir on the Earthquakes - on the Island of Luzon in 1880. p. 43–89. - Ewing, Prof. J. A. Seismological Notes. - — A Duplex Pendulum Seismometer. - — The Suspension of a Horizontal Pendulum. - — A Speed Governor for Seismograph Clocks, p. 89–95. - Dan, T., S.B. Notes on the Earthquake at Atami, in the Province of - Idzu, on September 29, 1882. p. 95–105. - - - VOL. VI. - - Alexander, Prof. T. The Development of the Record given by a - Bracket Machine. - Milne, J. Earth Pulsations. - Ewing, J. A. Note on a Duplex Pendulum with a Single Bob. - Gergens, F. Note on an Iron Casting, Supposed to have been - Disturbed whilst Cooling by an Earthquake. - West, C. D. On a Parallel Motion Seismograph. - Ewing, J. A. Certain Methods of Astatic Suspension. - Alexander, T. Ball and Cup Seismometer. - Knipping, Messrs. Paul and. Report on a System for Earthquake - Observation. - Catalogues of Earthquakes. - - - - - INDEX. - - - Abbadie, M. d’, on earth tremors, 309 - Abbot, General H. L., on the transmission of vibrations, 62 - Abella, M., on the earthquake in the Philippines in 1881, 77 - Activity, on seismic, 6 - Aristotle, on the classification of earthquakes, 41 - Artificial earthquakes, experiments on, 57 - — — intensity of, 61 - Aurora, on the occurrence of, with earthquakes, 264 - Ayrton and Perry, on the effect of soft foundations, 130 - — — on the period of vibration of buildings, 115 - — — on the principle of, 31 - - Barometer, effect of changes of, on earthquakes, 266 - Bertelli, on aurora and earthquakes, 265 - — on earth tremors, 316, 320 - — on the normal tromometer of, 317 - Bittner, A., on the buildings of Belluno, 100 - Bridges, on earthquake, 140 - Brunton, R. H., on buildings in earthquake countries, 123 - Buckle, on the history of civilisation, 1 - Builders, interest of the study of earthquakes to, 3 - Buildings, on cracks in, 98, 108 - — the effect of earthquakes on, 96 - — on the irregular destruction of, 96 - — — effect on the end house in a row, 112 - — — church of St. Augustin at Manilla, 113 - — — relation of destruction of, to earthquake motion, 103 - — — protection of, 143 - — — pitch of the roof of, 110 - — — position of openings in the walls of, 111 - — — swing of, 115 - — — period of vibration of, 115 - — — principle of relative periods in, 116 - — — types of, for earthquake countries, 121 - — — effect of underlying rocks on, 130 - — general conclusions regarding, 144 - - Cacciatore, definition of the, 18 - Caldcleugh, A., on earthquake frequency, 245 - — on barometric height and earthquakes, 266 - Carruthers, J., on earthquakes and tides, 291 - Centrum, definition of, 9 - — on the depth of, 213 - — on the maximum depth of, 218 - Centrum, determination of position of, _see_ origins - Chaplin, W. S., on the bracket seismometer of, 27 - — on earthquakes and the position of the moon, 252 - Coast line, on the movement of, 160 - Cocks, R., on earthquakes and tides, 290 - Coseismic lines, definition of, 10 - Curves, on microseismic, 321 - - Darwin, Charles, on the movement of coast lines, 160 - — George H., on earth tides, 285 - — on tidal loads, 291 - — on earth pulsation, 330 - — on the effect of fluctuations of barometric pressure, 336 - — experiments at Cambridge, 310 - Delauney, M. J., on the influence of the planets on earthquakes, 261 - Diagonic, definition of, 11 - Diastrophic, definition of, 11 - Direction of motion, from instrumental records, 198 - Distribution, on earthquake, 226 - — examples of, 231 - Disturbance, on the propagation of, 50 - Douglas, J., on South American houses, 126 - - Earth particle, on the velocity and acceleration of, 79 - Earthquake motion, nature of, as deduced from the feelings, 67 - — direction of, derived from instrumental records, 69 - — duration of, 71 - — period of vibration in, 74 - — examples of extent of, 75–77 - — absolute intensity of force in, 83 - — radiation of, 85 - — velocity of propagation of, 87 - Earthquake at Lisbon, velocity of propagation of, 88 - Earthquakes, general examples of effects of, 142 - — geological changes produced by, 161 - — hunting, 187 - — distribution of, 226 - — maps, 189 - — secondary, 248 - — table of, for nineteenth century, 259 - — on the course of, 277–281 - — and tides, 290 - — prediction of, 297–304 - Elastic waves, nature of, 44 - Emergence, angle of, 9 - Energy, dissipation of, in earthquakes, 52 - — seismic, in relation to geological time, 234 - — — table of, 240 - Epicentrum, definition of, 9 - Euthutropic, definition of, 11 - Ewing, J. A., pendulum seismograph of, 25 - — astatic pendulum of, 26 - — bracket seismograph of, 26 - - Falb, R., on the influence of the sun and moon on earthquakes, 286 - Fissures, on the material discharged from, 148 - — on the explanation of, 151 - Focal cavity, definition of, 9 - — on the form of, 221 - Forbes, D., on an earthquake in Mendoza, 151 - Frequency of earthquakes, 243 - Frere, Sir H. Bartle, on geological changes produced by earthquakes, 161 - Fuchs, on sea waves, 176 - — on the movement of the seismic centre, 233 - — on earthquakes and volcanic outbursts, 271 - — on hot springs, 157 - Fumaroles, the effect of earthquakes on, 156 - - Geinitz, Dr., on sea waves, 182 - Geologists, on the interest of seismology to, 2 - Gray, T., astatic pendulum of, 26 - — bracket seismometer of, 27 - — conical pendulum of, 29 - — dead heat pendulum of, 22 - — on the rotation of bodies, 196 - — rolling spheres and cylinders of, 29 - — torsion pendulum seismometer of, 25 - — vertical motion seismometers of, 32, 33 - — and Milne, seismograph of, 38 - - Hattori, I., on the large earthquakes of Japan, 244 - Haughton, Prof., list of active volcanoes of, 227 - — method of finding earthquake origins of, 209 - Hills, on the want of support on the face of, 136 - Höfer, on an earthquake at Belluno, 225 - Hoffmann, F., on the barometer and earthquakes, 267 - Hooke, on earthquake motion, 42 - Hopkins, on the thickness of the earth’s crust, 284 - Humboldt, on meteors and earthquakes, 261 - — on the barometer and earthquakes, 267 - — on volcanoes and earthquakes, 279 - - Imagination, effect of earthquakes on the, 2 - Instruments, direction of motion derived from, 198 - Intensity, on earthquake, 51, 71 - — seismic curve of, for Kioto, 242 - Isoseismic circles, definition of, 10 - — areas, definition of, 10 - - Kluge, on sea waves, 175 - — on earthquake frequency, 246 - — on simultaneous earthquakes, 248 - — on earthquakes and sun spots, 263 - — on earth pulsations, 339 - Kreil, pendulum seismometer of, 25 - - Lakes, on disturbances in, 154 - Land, effect of earthquakes on, 146–162 - — on the reason of movements of, 162 - — on cracks and fissures formed in, 146 - Level, on the use of for earth pulsations, 328 - Literature, on seismic, 6 - — on Japanese earthquake, 7 - - Mallet, R., on area of disturbance as a test of seismic energy, 78 - — on clock stopping, 36 - — list of works on earthquakes of, 5 - — curve of seismic energy of, 238 - — definition of earthquake of, 43 - — on earthquake frequency, 243 - — on the influence of the heavenly bodies on earthquakes, 253 - — on maximum depth of origin, 218 - — on pendulum seismometers, 20 - — projection seismometer of, 17 - — on the Neapolitan earthquake, 69, 77, 83, 97, 103, 132, 142, 218, 280 - — on sea waves, 170 - — on the swing of mountains, 135 - — on propagation from a fissure, 217 - Mallet on the temperature of focal cavity, 84 - Malvasia, M. le Conte, on earth tremors, 316 - Martin, D. S., on the New England earthquake of 1874, 142 - Meizoseismic area, definition of, 10 - Melzi, on curves of microseismic motion, 322 - Meteors, on earthquakes and, 260 - Microseismic movements, on cause of, 324 - Milne, D., on the Lisbon earthquake, 87 - — on earthquake synchronism, 247 - Mitchell, on earthquake motion, 42 - Moon, effect of, on earthquakes, 251, 285 - Mountains, on the swing of, 135 - - Naumann, E., on meteors and earthquakes, 261 - — on sun spots and earthquakes, 263 - - Ocean, on disturbances in, 163–186 - Origin, definition of, 9 - — on the determination of, 187 - — position of, deduced from direction of motion, 192 - — — from destruction of buildings, 194 - — — from rotation of bodies, 195 - — — from time of occurrence, 199 - — — examples of methods of calculating, 200–212 - Oscillations, on earth, 344 - Overturning moment, on the area of greatest, 53 - - Palmer, Col. H. S., on earth tremors, 307 - Palmieri, on clock stopping, 36, 62 - Paul, H. M., on earth tremors, 308 - Perrey, A., on the influence of the moon on earthquakes, 251 - Perrey on the periodicity of earthquakes, 8 - Perry, J., on position of openings in walls, 111 - Physicists, on the interest of earthquakes to, 2 - Planets, influence of, on earthquakes, 260 - Plantamour, M., on earth pulsations, 328 - Pleistoseists, definition of, 10 - Poly, M. A., on earthquakes and sun spots, 263 - — on earthquakes and revolving storms, 294 - Prost, M. le Baron, on earth tremors, 316 - Pulsation, on earth, 4, 326–343 - - Records, on receivers of, 33 - Rivers, on disturbances in, 154 - Rockwood, Prof., on American earthquakes, 6 - Ronaldson, T., on San Francisco houses, 129 - Rossi, M. S. di, on an eruption of gas in the Tiber, 153 - — aurora and earthquakes, 264 - — earth tremors, 317, 320 - — earth oscillations, 346 - — earth pulsations, 327 - — microseismograph of, 318 - — microphonic observations of, 319, 323 - — normal tromometer of, 317 - - Schmidt, on the influence of barometric pressure on earthquakes, 267 - Sea waves, on nature of, 165 - — on cause of, 171 - — seldom produced by earthquakes which originate inland, 175 - — on velocity of propagation of, 177 - — examples of, 179 - Seasons, frequency of earthquakes at different, 254 - Seebach, on the determination of origins, 211 - — on the focal cavity, 224 - Seismic vertical, definition of, 9 - Seismic and volcanic phenomena, relation of, 270 - — — conclusions regarding, 275, 295 - Seismology, definition of, 9 - Seismometers, on various forms of, 17–40 - Seismoscopes, on various forms of, 13–20 - Serpieri, P. A., on distribution of seismic movement, 231 - Shadows, on earthquake, 137 - Spring, on frequency of earthquakes during, 156 - Stukeley, on earthquake motion, 42 - — earthquakes and aurora, 265 - Succussatore, definition of, 10 - Sun, on the effect of, on earthquakes, 253, 285 - - Temperature, effect of changes of, on earthquakes, 268, 294 - Terremoto, definition of, 10 - Thomson, Sir W., on the rigidity of the earth, 285 - Time, on recording apparatus for, 35 - Travagini, F., on earthquake motion, 42 - Trembelores, definition of, 10 - Tremors, on earth, 3, 306–325 - - Understanding, effects of earthquakes on, 2 - - Verbeck, on the ball and plate seismometer of, 31 - Vibration, on the nature of earthquake, 12 - Vorticose motion, on, 70 - — definition of, 10 - - Wagener, on the pendulum seismometer of, 25 - — vertical motion seismometer of, 33 - — list of earthquakes, 76 - Wave paths, definition of, 9 - Waves, on the nature of earthquake, 55 - — on the interference of, 138 - Wells, on the effect of earthquakes on, 156 - Wenthrop, on the New Zealand earthquake of 1855, 79 - West, on the parallel motion seismometer of, 28 - Winslow, on pulsations of the ocean, 334 - Wolf, R., on earthquakes and sun spots, 263 - Woodward, on earthquake motion, 42 - - Young, Dr. T., on earthquake motion, 43 - - Zantedeschi, M. F., on the influence of the sun and moon on earthquakes, 285 - Zöllner, on the bracket seismometer of, 27 - — on earth tremors, 309 - - - - - +--------------------------------------------------------------------+ - | | - | FOOTNOTES: | - | | - | [1] _Mémoires de l’Académie Imp. de Dijon_, vols. xiv. and xv., | - | 2nd Series, 1855–56. | - | | - | [2] _Trans. Seis. Soc. of Japan_, vol. iii. p. 65. | - | | - | [3] _Gentleman’s Magazine_, 1753. | - | | - | [4] 1 Kings xix. 11, 12. | - | | - | [5] ‘Notes on the Great Earthquake of Japan.’ J. Milne, _Trans. | - | Seis. Soc. of Japan_, vol. iii. | - | | - | [6] See Mallet’s List of Works on Earthquakes, _Report of the | - | British Association_, 1858, p. 107. | - | | - | [7] _Quarterly Review_, vol. lxiii. p. 61. | - | | - | [8] _De Mundo_, c. iv. | - | | - | [9] See _Phil. Trans. R. S._, Part III. 1882. | - | | - | [10] _Report of the British Association_, 1851. | - | | - | [11] ‘On the Velocity of Transmission of Earth Waves,’ by General | - | H. L, Abbot, _American, Journal of Science and Arts_, vol. xv. | - | March 1878; ‘Shock of the Explosion at Hallet’s Point,’ by | - | Bvt. Brig.-Gen. Henry L. Abbot, read before the Essayons Club | - | of the Corps of Engineers, Nov. 1876. | - | | - | [12] _West. Rev._, July 1849. | - | | - | [13] _Phil. Trans._, L., 1755. | - | | - | [14] The solution is taken from Mallet’s _Account of the | - | Neapolitan Earthquake_, vol. i. p. 155. | - | | - | [15] _Neapolitan Earthquake_, ii. p. 300. | - | | - | [16] See _Edinburgh Phil. Trans._, vol. xxxi. | - | | - | [17] See _Report of British Association_, 1858, p. 10. | - | | - | [18] _Meteorologia Endogena_, i. p. 306. | - | | - | [19] See remarks on the Earthquake ‘Push,’ p. 162. | - | | - | [20] See _Researches in Geology and Natural History_, p. 374. | - | | - | [21] ‘The City of Earthquakes,’ H. D. Warner, _Atlantic Monthly_, | - | March, 1883. | - | | - | [22] Mallet, _Dynamics of Earthquakes_. | - | | - | [23] Stud Mill at Haywards. | - | | - | [24] See ‘Constructive Art in Japan,’ by R. H. Brunton, C.E., | - | F.R.G.S., F.G.S., _Transactions of Asiatic Society of Japan_, | - | December 22, 1873, and January 13, 1875. | - | | - | [25] _Journal of the American Geographical Society_, vol. x. | - | | - | [26] _Phil. Trans._, li. 1760. | - | | - | [27] _Ibid._, xviii. | - | | - | [28] ‘The City of Earthquakes,’ H. D. Warner, _Atlantic Monthly_, | - | March 1883. | - | | - | [29] T. Ronaldson, _A Treatise on Earthquake Dangers &c._ | - | | - | [30] _Principles of Geology_, Lyell, vol. ii. p. 106. | - | | - | [31] _The Neapolitan Earthquake of 1857_, R. Mallet, vol. ii. p. | - | 359. | - | | - | [32] _Am. J. Sci._ x. 191. | - | | - | [33] _Reports of British Association_, 1858, p. 106. | - | | - | [34] See chapter ‘Causes of Earthquakes’ for details of this myth. | - | | - | [35] _Am. Jour. Sci._ vol. x. p. 191. | - | | - | [36] _The Earth_, p. 599. | - | | - | [37] Lyell, _Principles of Geology_, vol. ii. chap. xxix. | - | | - | [38] _Gent. Mag._ vol. xx. p. 212. | - | | - | [39] _Trans. Seis. Soc._ vol. v. p. 67–68. | - | | - | [40] _Am. Jour. Sci._ vol. iv. | - | | - | [41] _Phil. Trans._ vol. xviii. | - | | - | [42] Oldham and Mallet, ‘Cachar Earthquake,’ _Proc. Geolog. Soc._ | - | 1872. | - | | - | [43] _Phil. Trans._ vols. li. and xviii.; _Gent. Mag._ vol. xx. | - | 212. | - | | - | [44] _Trans. Royal Geog. Soc._ vol. vi. | - | | - | [45] _Phil. Trans._ vols. xxxvi. and xxxix. | - | | - | [46] _Am. Jour. of Sci._ 1865, vol. xl. p. 365. | - | | - | [47] _Proc. Geolog. Soc. Ap._ 1875, p. 270. | - | | - | [48] _Gent. Mag._ vol. xxi. p. 569. | - | | - | [49] _Jahrb. f. Min._ 1840, p. 173. | - | | - | [50] Oldham and Mallet, ‘Cachar Earthquake,’ _Trans. Geolog. Soc. | - | Ap._ 1872. | - | | - | [51] O. Volger, _Unters üb. d. Phän. d. Erdb._ vol. iii. p. 414. | - | | - | [52] _Meteorologia Endogena_, vol. i. p. 166. | - | | - | [53] _Gent. Mag._ vol. xxvi. p. 91. | - | | - | [54] _Compte Rendu_, 1873, p. 66. | - | | - | [55] _An Historical Account of Earthquakes_, p. 46. | - | | - | [56] _Phil. Trans._ vol. xlix. p. 436. | - | | - | [57] _Am. Jour. Sci._ vol. xlv. p. 129. | - | | - | [58] _Phil. Trans._ vol. xlix. p. 547. | - | | - | [59] _Ibid._ vols. xlii. and xxxix. | - | | - | [60] _Phil. Trans._ vol. xlix, part i. | - | | - | [61] _Compte Rendu_, 1873, part ii. p. 66. | - | | - | [62] _Die Vulcan. Ers. d. Erde_, C. W. C. Fuchs. | - | | - | [63] _Comte Rendu_, 1875, p. 693. | - | | - | [64] _Gent. Mag._ vol. xix. p. 190. | - | | - | [65] _Phil. Trans._ vol. xlix. p. 115. | - | | - | [66] _Gent. Mag._ vol. xxi. 1751. | - | | - | [67] _Jour. Royal Geo. Soc._ vol. vi. p. 319. | - | | - | [68] Darwin, _Geolog. Observations_, p. 232. | - | | - | [69] _Ibid._ p. 245. | - | | - | [70] Lyell, _Principles of Geology_, vol. ii. pp. 107–8. | - | | - | [71] _Gent. Mag._ 1733, vol. iii. p. 217. | - | | - | [72] ‘Earthquakes of Cutch,’ _Jour. Royal Geo. Soc._ vol. xl. | - | | - | [73] M. Daussy, ‘Sur l’existence probable d’un volcan sousmarin | - | situé ar environ 0° 20′ de lat. S., et 22° 0′ de long, ouest,’ | - | _Comptes Rendus_, vol. vi. p. 512. | - | | - | [74] _Am. Jour. Sci._ vol. xlv. p. 133. | - | | - | [75] _Am. Jour. Sci._ vol. xiv. p. 209. | - | | - | [76] D. C. F. Winslow, ‘Tides at Tahiti,’ _Am. Jour. Sci._ 1865, | - | p. 45; also Mallet’s _Catalogue of Earthquakes_. | - | | - | [77] _Am. Jour. Sci._ vol. i. p. 469. | - | | - | [78] Darwin, _Researches in Geology, &c._, p. 378. | - | | - | [79] Kluge, _Jahrb. f. Min._ 1861, p. 977. | - | | - | [80] Darwin, _Voyage of a Naturalist_, p. 309. | - | | - | [81] Prof. A. D. Bache, _United States Coast Survey Report_, | - | 1855, p. 342. | - | | - | [82] _United States Coast Surrey Report_, or _Am. Jour. Sci._ vi. | - | p. 77. | - | | - | [83] _Petermann’s Mittheilungen_, 1877, Heft xii. S. 454, | - | and _Nova Acta der Ksl. Leop. Carol. Deutschen Acad. d | - | Naturforscher_, Band xl. No. 9. | - | | - | [84] J. Milne: ‘Peruvian Earthquake of May 9, 1877.’ See _Trans. | - | Seis. Soc. of Japan_, vol. ii. | - | | - | [85] _Report of British Association_, 1847, p. 84. | - | | - | [86] _Das Erdbeben von Herzogenrath, &c._, p. 134. | - | | - | [87] _Phil. Trans._ vol. li. | - | | - | [88] See _Am. Jour. Sci._ 1872. | - | | - | [89] David Milne says that ‘out of 110 shocks recorded in | - | England, thirty-one originated in Wales, thirty-one along the | - | south coast of England, fourteen on the borders of Yorkshire | - | and Derbyshire, and five or six in Cumberland.’ | - | | - | [90] E. Suess, _Die Erdbeben Niederösterreiches_. | - | | - | [91] H. Hoeffer, _Die Erdbeben Kärntens_. | - | | - | [92] _Six Lectures on Physical Geography_, by Rev. S. Haughton, | - | F.R.S., chap. i. | - | | - | [93] Ramsay, ‘Geological History of Mountain Chains,’ _Mining | - | Journal_. | - | | - | [94] A notable example of a rapid diminution in the number of | - | earthquakes felt at a place is that of Comrie in Scotland. In | - | 1839–40, no less than sixty shocks were felt in eleven months. | - | In 1842–43, about thirty shocks were felt, and in the | - | following year thirty-seven. Since this time the number of | - | shocks has decreased until they are almost of as rare | - | occurrence at Comrie as in other portions of the British | - | Isles. | - | | - | [95] _Phil. Trans._ vol. i. 1836. | - | | - | [96] _Am. Jour. of Sci._ vol. xxxvii. p. 1. | - | | - | [97] Milne, ‘British Earthquakes,’ _Edin. Phil. Jour._ vol. xxxi. | - | | - | [98] _Phil. Trans._ vol. xlix. pt. i. | - | | - | [99] _Compte Rendus_, 1875, p. 690. | - | | - | [100] _Am. Jour. Sci._ vol. xi. p. 233. | - | | - | [101] _Transactions of the Asiatic Society of Japan_, vol. vi. | - | pt. i. p. 353. | - | | - | [102] Kluge, _Ueber die Ursachen_, &c., p. 74. | - | | - | [103] _Am. Jour. Sci._ vol. xix. p. 162. | - | | - | [104] _Mitt. d. Deutsch._ Ges., Aug. 1878. | - | | - | [105] _Report to British Association_, 1850, p. 74. | - | | - | [106] Fuchs, _Die Vulkanischen Erscheinungen der Erde_, p. 424. | - | | - | [107] _Bern. Naturf. Gesellschaft_, 1852. | - | | - | [108] _Comptes Rendus_, 1874, Jan. to June, p. 51. | - | | - | [109] Boué, _Parallele der Erdbeben, Nordlichter und | - | Erdmagnetismus, in Sitz. der K. A. d. Wissensch_. 1856, vol. | - | iv. p. 395. | - | | - | [110] _Meteorologia Endogena_, vol. i. p. 107, &c. | - | | - | [111] _Phil. Trans._ vol. lxviii. p. 221. | - | | - | [112] _Gent. Mag._ vol. xxvii. p. 508. | - | | - | [113] _Die Vulkanischen Erscheinungen der Erde_, p. 419. | - | | - | [114] Petermann’s _Geogr. Mitth._ 1858, sec. 246. | - | | - | [115] _Notes on volcanoes of the Hawaiian Islands_, W. T. | - | Brigham, Mem. Boston Soc. of Nat. Hist., 1868. | - | | - | [116] _Gent. Mag._ vol. xxiii., 1753. | - | | - | [117] _Jour. Royal Geog. Soc._ vol. vi. | - | | - | [118] _Ibid._ vol. vi. | - | | - | [119] _Phil. Trans._ vol. xlii. | - | | - | [120] _Am. Jour. Sci._ vol. x. p. 191. | - | | - | [121] ‘Earthquakes of San Salvador, December 21–30, 1879.’ _Am. | - | Jour. Sci._ vol. xix. p. 415. | - | | - | [122] _Gent. Mag._ 1757, p. 323. | - | | - | [123] _Phil. Trans._ vol. li., 1760. | - | | - | [124] Mallet, _Report to Brit. Ass._, 1858, p. 67. | - | | - | [125] Von Lasaulx, _Earthquakes of Herzogenrath_. | - | | - | [126] Lyell, _Principles_, vol. ii. p. 51. | - | | - | [127] Lyell, _Principles_, vol. i. p. 402. | - | | - | [128] Fuchs, p. 464. | - | | - | [129] _Comptes Rendus_, August 1854. | - | | - | [130] _Nature_, April 26, 1883. | - | | - | [131] _Phil. Soc._, Wellington, New Zealand, 1875. | - | | - | [132] _Phil. Trans._, vol. xlii. | - | | - | [133] M. S. di Rossi, _Earthquakes of Casamicciola_. | - | | - | [134] _Phil. Trans._, vol. xviii. 1683–5. | - | | - | [135] _Ibid._ vol. xlix. | - | | - | [136] H. D. Warner, ‘The City of Earthquakes,’ _Atlantic | - | Monthly_, March 1833. | - | | - | [137] Palmer, _Trans. Seis. Soc. of Japan_, vol. iii. p. 148. | - | | - | [138] Palmer, _Trans. Seis. Soc. of Japan_, vol. iii. p. 148. | - | | - | [139] Paul, _Trans. Seis. Soc. of Japan_, vol. ii. p. 41. | - | | - | [140] G. H. and H. Darwin, _Reports of British Association_, 1881. | - | | - | [141] _Reports of British Association_, 1881. | - | | - | [142] _Comptes Rendus_, 1875, January to June, p. 685. | - | | - | [143] _Tel. Jour._, November 15, 1881. | - | | - | [144] _Minutes and proceedings of the Institute of Civil | - | Engineers_, vol. lx. p. 412, and vol. lxiv. p. 343. | - | | - | [145] _See_ ‘Earth Tremors,’ p. 309, experiments of M. d’Abbadie, | - | &c. | - | | - | [146] _Meteorologia Endogena._ | - | | - | [147] _Ibid._ | - | | - | [148] _Phil. Trans._ vol. xlix. p. 544. | - | | - | [149] _Annual Register_, vol. iv. 1761, p. 92. | - | | - | [150] _Phil. Mag._, May 1876, p. 447. | - | | - | [151] _Boston Soc. Nat. Hist._, 1868. | - | | - | [152] ‘Notes on Tides at Tahiti,’ &c., _Am. Jour. Sci._ 1866, | - | vol. xlii. p. 45. | - | | - | [153] _Trans. Seis. Soc. of Japan_, vol. iv. Milne, _Systematic | - | Observation of Earthquakes_. | - | | - | [154] _Principles of Geology_, vol. ii. 177. | - | | - | [155] _Gent. Mag._, vol. xxvii. p. 448. | - | | - | [156] _Phil. Trans._, vol. xli. p. 805. | - | | - | [157] _Meteorologia Endogena_, vol. i. pp. 186, 187. | - | | - | [158] Darwin, _Geological Observations_, p. 275 _et seq._ | - | | - +--------------------------------------------------------------------+ - - - - - THE INTERNATIONAL SCIENTIFIC SERIES. - - • • • • • - EACH BOOK COMPLETE IN ONE VOLUME, 12MO, AND BOUND IN CLOTH. - • • • • • - - 1. FORMS OF WATER: A Familiar Exposition of the Origin and - Phenomena of Glaciers. By J. TYNDALL, LL. D., F. R. S. With 25 - Illustrations. $1.50. - - 2. PHYSICS AND POLITICS; Or, Thoughts on the Application of the - Principles of “Natural Selection” and “Inheritance” to Political - Society. By WALTER BAGEHOT. $1.50. - - 3. FOODS. By EDWARD SMITH, M. D., LL. B., F. R. S. 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THE FORMATION OF VEGETABLE MOULD, THROUGH THE ACTION OF WORMS. - With Observations on their Habits. By CHARLES DARWIN, LL. - D., F. R. S. With Illustrations. $1.50. - - 38. THE CONCEPTS AND THEORIES OF MODERN PHYSICS. By J. B. STALLO. - $1.75. - - 39. THE BRAIN AND ITS FUNCTIONS. By J. LUYS. $1.50. - - 40. MYTH AND SCIENCE. By TITO VIGNOLI. $1.50. - - 41. DISEASES OF MEMORY: An Essay in the Positive Psychology. By - TH. RIBOT, author of “Heredity.” $1.50. - - 42. ANTS, BEES, AND WASPS. A Record of Observations of the Habits - of the Social Hymenoptera. By Sir JOHN LUBBOCK, Bart., F. - R. S., D. C. L., LL. D., etc. $2.00. - - 43. SCIENCE OF POLITICS. By SHELDON AMOS. $1.75. - - 44. ANIMAL INTELLIGENCE. By GEORGE J. ROMANES. $1.75. - - 45. MAN BEFORE METALS. By N. JOLY, Correspondent of the - Institute. With 148 Illustrations. $1.75. - - 46. THE ORGANS OF SPEECH AND THEIR APPLICATION IN THE FORMATION - OF ARTICULATE SOUNDS. By G. H. VON MEYER, Professor in - Ordinary of Anatomy at the University of Zürich. With 47 - Woodcuts. $1.75. - - 47. FALLACIES: A View of Logic from the Practical Side. By - ALFRED SIDGWICK, B. A., Oxon. $1.75. - - 48. ORIGIN OF CULTIVATED PLANTS. By ALPHONSE DE CANDOLLE. $2.00. - - 49. JELLY-FISH, STAR-FISH, AND SEA-URCHINS. Being a Research on - Primitive Nervous Systems. By GEORGE J. ROMANES. $1.75. - - 50. THE COMMON SENSE OF THE EXACT SCIENCES. By the late WILLIAM - KINGDON CLIFFORD. $1.50. - - 51. PHYSICAL EXPRESSION: Its Modes and Principles. By FRANCIS - WARNER, M. D., Assistant Physician, and Lecturer on Botany to - the London Hospital, etc. With 51 Illustrations. $1.75. - - 52. ANTHROPOID APES. By ROBERT HARTMANN, Professor in the - University of Berlin. With 63 Illustrations. $1.75. - - 53. THE MAMMALIA IN THEIR RELATION TO PRIMEVAL TIMES. By OSCAR - SCHMIDT. $1.50. - - 54. COMPARATIVE LITERATURE. By HUTCHESON MACAULAY POSNETT, - M. A., LL. D., F. L. S., Barrister-at-Law; Professor of Classics - and English Literature, University College, Auckland, New - Zealand, author of “The Historical Method,” etc. $1.75. - - 55. EARTHQUAKES AND OTHER EARTH MOVEMENTS. By JOHN MILNE, - Professor of Mining and Geology in the Imperial College of - Engineering, Tokio, Japan. With 38 Figures. - - - New York: D. APPLETON & CO., 1, 3, & 5 Bond Street. - - - - - VOLCANOES: - - _WHAT THEY ARE AND WHAT THEY TEACH_. - - =By J. W. JUDD=, - Professor of Geology in the Royal School of Mines (London). - • • • • • - With Ninety-six Illustrations. 12mo. Cloth, $2.00. - • • • • • - -“The volume before us is one of the pleasantest science manuals we have -read for some time.”—_Athenæum._ - -“Mr. Judd’s summary is so full and so concise, that it is almost -impossible to give a fair idea in a short review.”—_Pall Mall Gazette._ - -“Professor Judd discusses the nature of volcanic action, the internal -structure of volcanic mountains, the distribution of volcanoes upon -the surface of the globe, their activity in different periods of the -earth’s existence, the use of volcanoes in the economy of nature, the -various theories that have been made to explain volcanic action. He has -abbreviated in this volume a vast amount of information, which has a -fascinating interest for many minds by reason of its relation to the -past history and future destiny of this little bubble of earth upon -which we sail through the infinite spaces of ether.”—_New York Home -Journal._ - -“The book gives an exhaustive statement of the phenomena of volcanoes, -and of the facts in the formation of mountain-chains, and relates a -mass of interesting observations and facts, the results of patient and -extensive personal investigation and study, mostly in different places -in Southern Europe, but not neglecting the world’s larger volcanoes in -other regions.”—_Hartford Times._ - -“A fascinating example of patient observation, sound judgment, and -acute reasoning. Under Professor Judd’s skillful treatment the volcano -is forced not only to tell its own history, but also to solve a number -of earth problems seemingly disconnected with it; and the story is -told in strong, nervous language, and with an earnestness and subdued -enthusiasm that are delightfully stimulating.”—_Boston Gazette._ - -“Professor Judd first points out the errors in the old definition of -a volcano. The volcanic hole is very often not on the summit, but -on the side, sometimes at the base of the mountain or hill, and it -sends forth steam rather than smoke, and the supposed raging flames -are nothing more than the glowing light of a mass of molten material -reflected from those vapor-clouds. So our old ignorance vanishes, and -in this admirable work the internal structure of volcanic mountains, -the nature and products of volcanic action, and the distribution of the -materials rejected from volcanic vents, the succession of operations -taking place at volcanic centers, are all very ably and clearly -discussed.”—_Philadelphia Times._ - -“A succinct and excellent treatise on a very interesting -subject.”—_Philadelphia North American._ - - • • • • • -_For sale by all booksellers; or sent by mail, post paid, on receipt of - price._ - • • • • • - New York: D. APPLETON & CO., 1, 3, & 5 Bond Street. - - - THE CONCEPTS AND THEORIES OF MODERN PHYSICS. - - =By J. B. STALLO=. - • • • • • - 12mo, cloth $1.75. - • • • • • - -“Judge Stallo’s work is an inquiry into the validity of those -mechanical conceptions of the universe which are now held as -fundamental in physical science. He takes up the leading modern -doctrines which are based upon this mechanical conception, such as -the atomic constitution of matter, the kinetic theory of gases, the -conservation of energy, the nebular hypothesis, and other views, to -find how much stands upon solid empirical ground, and how much rests -upon metaphysical speculation. Since the appearance of Dr. Draper’s -‘Religion and Science,’ no book has been published in the country -calculated to make so deep an impression on thoughtful and educated -readers as this volume.... The range and minuteness of the author’s -learning, the acuteness of his reasoning, and the singular precision -and clearness of his style, are qualities which very seldom have been -jointly exhibited in a scientific treatise.”—_New York Sun._ - -“Judge J. B. Stallo, of Cincinnati, is a German by birth, and came to -this country at about the age of seventeen. He was early familiar with -science, and he lectured for some years in an Eastern college; but -at length he adopted the profession of law. He is also remembered by -many as an author, having a number of years ago written a metaphysical -treatise of marked ability for one of his youthful years. His present -book must be read deliberately, must be studied to be appreciated; but -the students of science, as well as those of metaphysics, are certain -to be deeply interested in its logical developments. It is a timely and -telling contribution to the philosophy of science, imperatively called -for by the present exigencies in the progress of knowledge. It is to be -commended equally for the solid value of its contents and the scholarly -finish of its execution.”—_The Popular Science Monthly._ - -“The book is of vital interest to a much larger class than -specialists—to all, in fact, who value clear thinking or are interested -in the accuracy more than the progress of scientific thought. It deals -with the results and theories of physical science, and in no sense -with the processes of the laboratory. It is written with a clearness -that is uncommon in philosophic works and with a desire to find truth, -conscious of the fact that a prime prerequisite of finding it is to -clear the way of accumulated and fast-settling untruths. It is a -scientific rebuke, as severe as it is lucid, of the scientists who -leave their apparatus and go star-gazing: here is the pit into which -they have fallen.”—_New York World._ - -“The volume is an important contribution to scientific discussion, -and is marked by closeness of reasoning, and clearness and cogency of -statement.”—_Boston Journal._ - - • • • • • -_For sale by all booksellers; or sent by mail, post paid, on receipt of - price._ - • • • • • - New York: D. APPLETON & CO., 1, 3, & 5 Bond St. - - - • • • • • - -Transcriber’s Notes: - - Text enclosed by underscores is in italics (_italics_). - - Text enclosed by equals is in bold (=bold=). - - Blank pages have been removed. - - Silently corrected typographical errors. - - Spelling and hyphenation variations made consistent. - - Front publication list moved to the back. - - Tables pages 77, 89: removed unneeded right braces. - - Tables pages 240, 257: changed to use cell borders instead of right braces. - - Table page 259: Northern Hemisphere average 15·0 corrected to 150. - - - - - -End of the Project Gutenberg EBook of Earthquakes and Other Earth Movements, by -John Milne - -*** END OF THIS PROJECT GUTENBERG EBOOK EARTHQUAKES, OTHER EARTH MOVEMENTS *** - -***** This file should be named 60007-0.txt or 60007-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/0/0/0/60007/ - -Produced by deaurider, Robert Tonsing, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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