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- } - } - - @media handheld { - body { margin: 0; } - - /* page break instead of rule for chapter heading */ - hr.page { display: none; } - - h2 { page-break-before: always; } - - /* repeating these here seems to override epubmaker 'float' treatment */ - .figright { - float: right; - margin: 1% 0 1% 2%; - page-break-inside: avoid; - } - - .figleft { - float: left; - margin: 1% 2% 1% 0; - page-break-inside: avoid; - } - } - - /* === Transcriber's notes === */ - .transnote { - background-color: #E6E6FA; - color: black; - font-size: smaller; - padding: 0.5em; - margin-bottom: 5em; - font-family: sans-serif, serif; - } - - </style> -</head> - -<body> - - -<pre> - -Project Gutenberg's Earthquakes and Other Earth Movements, by John Milne - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. 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) - - - - - - -</pre> - - - <div class="figcenter"> - <img id="coverpage" src="images/cover.jpg" width="529" height="800" alt="" /> - </div> - - <hr class="page" /> - - <div class="center xlarge"> - <b>THE INTERNATIONAL SCIENTIFIC SERIES<br /> - VOLUME LV</b> - </div> - - <hr class="page" /> - - <div class="chapter"> - <div class="center mb1"><b>THE INTERNATIONAL SCIENTIFIC SERIES</b></div> - <hr class="r15" /> - <h1><span class="gespertt">EARTHQUAKES</span><br /> - <span class="small">AND</span><br /> - <span class="xlarge gespertt">OTHER EARTH MOVEMENTS</span> - </h1> - </div> - - <div class="center mt10"> - <span class="small">BY</span><br /> - <span class=" large gespertt">JOHN MILNE</span><br /> - <span class="xsmall">PROFESSOR OF MINING AND GEOLOGY IN THE IMPERIAL COLLEGE OF ENGINEERING,<br /> - TOKIO, JAPAN</span> - </div> - - <div class="center small mt10"><i>WITH THIRTY-EIGHT FIGURES</i></div> - - <div class="center mt10">NEW YORK<br /> - <span class="large gespertt">D. APPLETON AND COMPANY</span><br /> - <span class="smcap small">1, 3, and 5 BOND STREET</span><br /> - 1886 - </div> - - <hr class="page" /> - <div class="chapter" id="PREFACE"> - <h2 class="gespertt xxlarge">PREFACE</h2> - </div> - - <div class="figcenter"> - <img src="images/divider.png" width="80" height="9" alt="" /> - </div> - - <p class="noindent"><span class="smcap">In</span> the following pages it has been my object to give a systematic - account of various Earth Movements.</p> - - <p>These comprise <em>Earthquakes</em>, or the sudden violent movements of the - ground; <em>Earth Tremors</em>, or minute movements which escape our attention - by the smallness of their amplitude; <em>Earth Pulsations</em>, or movements - which are overlooked on account of the length of their period; and - lastly, <em>Earth Oscillations</em>, or movements of long period and large - amplitude which attract so much attention from their geological - importance.</p> - - <p>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.</p> - - <hr class="tb" /> - - <p>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.</p> - - <p>As very much of what might be said about the other Earth Movements is - common to what is said about Earthquakes, - <span class="pagenum">vi</span>it has been possible to make - the description of these phenomena comparatively short.</p> - - <p>The scheme which has been adopted will be understood from the following - table:—</p> - - <table class="ssmall" summary="Book scheme"> - <tr> - <td colspan="3" class="tdc"><div>I. <span class="smcap">Earthquakes</span>.</div></td> - </tr> - <tr> - <td colspan="3" class="tdl">1. Introduction.</td> - </tr> - <tr> - <td colspan="3" class="tdl">2. Seismometry.</td> - </tr> - <tr> - <td class="tdl">3. Earthquake Motion.</td> - <td><div class="x300">{</div></td> - <td class="tdl0"> - <div>(<i>a</i>) Theoretically.</div> - <div>(<i>b</i>) As deduced from experiments.</div> - <div>(<i>c</i>) As deduced from actual Earthquakes.</div> - </td> - </tr> - <tr> - <td class="tdl">4. Earthquake Effects.</td> - <td><div class="x200">{</div></td> - <td class="tdl0"> - <div>(<i>a</i>) On land.</div> - <div>(<i>b</i>) In the ocean.</div> - </td> - </tr> - <tr> - <td colspan="3" class="tdl">5. Determination of Earthquake origins.</td> - </tr> - <tr> - <td class="tdl">6. Distribution of Earthquakes.</td> - <td><div class="x300">{</div></td> - <td class="tdl0"> - <div>(<i>a</i>) In space.</div> - <div>(<i>b</i>) In time (geological time, historical time, annual, seasonal, diurnal, &c.)</div> - </td> - </tr> - <tr> - <td colspan="3" class="tdl">7. Cause of Earthquakes.</td> - </tr> - <tr> - <td colspan="3" class="tdl">8. Earthquake prediction and warning.</td> - </tr> - <tr> - <td colspan="3" class="tdc"><div>II. <span class="smcap">Earth Tremors</span>.</div></td> - </tr> - <tr> - <td colspan="3" class="tdc"><div> III. <span class="smcap">Earth Pulsations</span>.</div></td> - </tr> - <tr> - <td colspan="3" class="tdc"><div> IV. <span class="smcap">Earth Oscillations</span>.</div></td> - </tr> - </table> - - <p class="noindent">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.</p> - - <p>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 <i xml:lang="la">vice versâ</i>, or they may both - be the effect of a third phenomenon.</p> - - <p><span class="pagenum">vii</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum">viii</span> - 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.</p> - - <p>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.</p> - - <p class="rightinset"><span class="smcap">John Milne.</span></p> - - <p class="small"><span class="smcap">Tokio, Japan</span>; <i>June</i> 30, 1883</p> - - <hr class="page" /> - <div class="chapter"> - <span class="pagenum">ix</span> - <h2 class="gespertt xxlarge">CONTENTS.</h2> - </div> - - <div class="figcenter"> - <img src="images/divider.png" width="80" height="9" alt="" /> - </div> - - <table class="mt0" summary="Contents"> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_I">CHAPTER I.</a><br /> - <span class="subhead">INTRODUCTION.</span> - </div> - </td> - </tr> - <tr> - <td></td> - <td class="tdpage"><div>PAGE</div></td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>1</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_II">CHAPTER II.</a><br /> - <span class="subhead">SEISMOMETRY.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>12</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <span class="pagenum">x</span> - <a href="#CHAPTER_III">CHAPTER III.</a><br /> - <span class="subhead">EARTHQUAKE MOTION DISCUSSED THEORETICALLY.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>41</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_IV">CHAPTER IV.</a><br /> - <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>57</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_V">CHAPTER V.</a><br /> - <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION - ON EARTHQUAKES.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>67</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <span class="pagenum">xi</span> - <a href="#CHAPTER_VI">CHAPTER VI.</a><br /> - <span class="subhead">EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>96</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_VII">CHAPTER VII.</a><br /> - <span class="subhead">EFFECTS PRODUCED UPON BUILDINGS (<i>continued</i>).</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>122</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_VIII">CHAPTER VIII.</a><br /> - <span class="subhead">EFFECTS OF EARTHQUAKES ON LAND.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>146</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_IX">CHAPTER IX.</a><br /> - <span class="subhead">DISTURBANCES IN THE OCEAN.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - Sea vibrations—Cause of vibratory blows—Sea waves: preceding - earthquakes; succeeding earthquakes—Magnitude of waves - <span class="pagenum">xii</span>—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 - </td> - <td class="tdrb"><div>163</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_X">CHAPTER X.</a><br /> - <span class="subhead">DETERMINATION OF EARTHQUAKE ORIGINS.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>187</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XI">CHAPTER XI.</a><br /> - <span class="subhead">THE DEPTH OF AN EARTHQUAKE CENTRUM.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - The depth of an earthquake centrum—Greatest possible depth of - an earthquake—Form of the focal cavity - </td> - <td class="tdrb"><div>213</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XII">CHAPTER XII.</a><br /> - <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>226</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <span class="pagenum">xiii</span> - <a href="#CHAPTER_XIII">CHAPTER XIII.</a> - </div> - </td> - </tr> - <tr> - <td><div class="center small">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</div></td> - <td class="tdrb"><div>234</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XIV">CHAPTER XIV.</a><br /> - <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>250</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XV">CHAPTER XV.</a><br /> - </div> - </td> - </tr> - <tr> - <td class="tdl small"> - BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE AND EARTHQUAKES - </td> - <td class="tdrb"><div>266</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XVI">CHAPTER XVI.</a><br /> - <span class="subhead">RELATION OF SEISMIC TO VOLCANIC PHENOMENA.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - Want of synchronism between earthquakes and volcanic eruptions—Synchronism - between earthquakes and volcanic eruptions—Conclusion - </td> - <td class="tdrb"><div>270</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XVII">CHAPTER XVII.</a><br /> - <span class="subhead">THE CAUSE OF EARTHQUAKES.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>277</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <span class="pagenum">xiv</span> - <a href="#CHAPTER_XVIII">CHAPTER XVIII.</a><br /> - <span class="subhead">PREDICTION OF EARTHQUAKES.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>297</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XIX">CHAPTER XIX.</a><br /> - <span class="subhead">EARTH TREMORS.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>306</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XX">CHAPTER XX.</a><br /> - <span class="subhead">EARTH PULSATIONS.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - 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 - </td> - <td class="tdrb"><div>326</div></td> - </tr> - <tr> - <td colspan="2"> - <div class="tdh"> - <a href="#CHAPTER_XXI">CHAPTER XXI.</a><br /> - <span class="subhead">EARTH OSCILLATIONS.</span> - </div> - </td> - </tr> - <tr> - <td class="tdl"> - Evidences of oscillation—Examples of oscillation—Temple of Jupiter - Serapis—Observations of Darwin—Causes of oscillation - </td> - <td class="tdrb"><div>344</div></td> - </tr> - <tr> - <td><div class="tdl mt3"><a href="#APPENDIX">APPENDIX</a></div></td> - <td class="tdrb"><div>349</div></td> - </tr> - <tr> - <td><div class="tdl mt3"><a href="#INDEX">INDEX</a></div></td> - <td class="tdrb"><div>359</div></td> - </tr> - </table> - - <div class="chapter" id="CHAPTER_I"> - <span class="pagenum" id="Page_1">1</span> - <div class="xxlarge center"><b>EARTHQUAKES</b>.</div> - <div class="figcenter"> - <img src="images/divider.png" width="80" height="9" alt="" /> - </div> - <h2>CHAPTER I.<br /> - <span class="subhead">INTRODUCTION.</span> - </h2> - </div> - - <div class="summary">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.</div> - - <p class="noindent"><span class="smcap">In</span> - 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.</p> - - <p>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<span class="pagenum" id="Page_2">2</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_3">3</span> - thermometer, the quantity of rainfall, - and like phenomena to which he devotes his attention.</p> - - <p>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.</p> - - <p>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.</p> - - <p>A study of the effects which earthquakes produce on the lower animals - will not fail to interest the student of natural history.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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,<span class="pagenum" id="Page_4">4</span> - it would appear that the ground on which we dwell is - incessantly in a state of tremulous motion.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_5">5</span> - on this subject, gave, in 1856, a - catalogue of 1,837 works devoted to seismology.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a> - 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 <span class="smcap">a.d.</span> 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.<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a> - 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.</p> - - <p>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 <span class="pagenum" id="Page_6">6</span> - 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.’<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a> - As might be expected, - these occurrences gave rise to many articles and notes directing - attention to the subject of earthquakes.</p> - - <p>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.</p> - - <p>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 (<span class="smcap">b.c.</span> 918–897).<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a> - One of the most terrible earthquakes mentioned in the Bible is that which took - place in the days of Uzziah, king of Judah (<span class="smcap">b.c.</span> 811–759), - <span class="pagenum" id="Page_7">7</span> - 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.</p> - - <p>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.</p> - - <p>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;<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a> 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.<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a></p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_8">8</span></p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_9">9</span> - 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.</p> - - <p>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 <em>earthquake</em>, the German - <i xml:lang="de">erdbeben</i>, the French <i xml:lang="fr">tremblement de terre</i>, the Spanish <i xml:lang="es">terremoto</i>, - the Japanese <i xml:lang="ja">jishin</i> &c., all mean, when literally translated, - <em>earth-shaking</em>, and are popularly understood to mean a sudden and more - or less violent disturbance.</p> - - <p>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.’</p> - - <p>The source from which an earthquake originates is called the ‘origin,’ - ‘focal cavity,’ or ‘centrum.’</p> - - <p>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.’</p> - - <p>The radial lines along which an earthquake may be propagated from the - centrum are called ‘wave paths.’</p> - - <p>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 <i>e</i>.</p> - - <p>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<span class="pagenum" id="Page_10">10</span> - 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.</p> - - <p>The isoseismic area in which the greatest disturbance has taken place - is called the ‘meizoseismic area.’ Seebach calls the lines enclosing - this area ‘pleistoseists.’</p> - - <p>These last-mentioned lines are wholly due to Mallet and Seebach.</p> - - <p>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.</p> - - <p>In South America small earthquakes, consisting of a series of rapidly - recurring vibratory movements not sufficiently powerful to create - damage, are spoken of as <i xml:lang="es">trembelores</i>.</p> - - <p>The <i xml:lang="es">terremotos</i> 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 <i xml:lang="es">terremoto</i> - would at another and more distant place probably be described as - <i xml:lang="es">trembelores</i>.</p> - - <p>The <i xml:lang="es">succussatore</i> 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.</p> - - <p>The <i xml:lang="es">vorticosi</i> are shocks which have a twisting or rotatory motion.</p> - - <p>Another method of describing earthquakes would be - <span class="pagenum" id="Page_11">11</span> 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 <em>transverse</em> 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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_II"> - <span class="pagenum" id="Page_12">12</span> - <h2>CHAPTER II.<br /> - <span class="subhead">SEISMOMETRY.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p class="noindent"><span class="smcap">Before</span> - 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<span class="pagenum" id="Page_13">13</span> - 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.</p> - - <p>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 <em>seismoscopes</em>. 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 <em>seismometers</em> or <em>seismographs</em>.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Eastern Seismoscopes.</em>—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 <span class="smcap">a.d.</span> 136. A description is - given in the Chinese<span class="pagenum" id="Page_14">14</span> - history called ‘Gokanjo,’ and the translation of - this description runs as follows:—</p> - - <div class="figcenter"> - <img src="images/i_p014.jpg" width="400" height="473" alt="" /> - <div class="caption"><span class="smcap">Fig. 1.</span></div> - </div> - - <p>‘In the first year of Yōka, <span class="smcap">a.d.</span> 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<span class="pagenum" id="Page_15">15</span> - 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.’</p> - - <p>‘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.’</p> - - <p>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.</p> - - <p>‘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.’</p> - - <p>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.</p> - - <p>Another earthquake instrument also of Eastern origin is the magnetic - seismoscope of Japan.</p> - - <p>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<span class="pagenum" id="Page_16">16</span> - 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.</p> - - <p><em>Columns.</em>—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<span class="pagenum" id="Page_17">17</span> - of the earthquake seldom remains in - the same azimuth throughout the whole disturbance.</p> - - <p>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.</p> - - <p><em>Projection Seismometers.</em>—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 - <img class="iglyph-a" src="images/i_p017.png" alt="" width="48" height="25" /> - 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, - <span class="pagenum" id="Page_18">18</span> 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.</p> - - <p>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.</p> - - <p><em>Vessels filled with liquid.</em>—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.’<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a></p> - - <p><span class="pagenum" id="Page_19">19</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_20">20</span> - of apparatus forming the well-known seismograph of Palmieri.</p> - - <p><em>Pendulum instruments.</em>—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.</p> - - <p>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.’</p> - - <p>To obtain an absolutely ‘steady point’ at the time of an earthquake, - has been one of the chief aims of all recent seismological - investigations.</p> - - <p>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<span class="pagenum" id="Page_21">21</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_22">22</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="smcap">b b b b</span> is a box 113 cm. high and 30 cm. by 18 cm. square. - Inside this box a lead ring <span class="smcap">r</span>, 17 cm. in diameter and 3 cm. - thick, is suspended as a pendulum from the screw <span class="smcap">s</span>. This screw - passes through a small brass plate <span class="smcap">p p</span>, 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.</p> - - <div class="figright"> - <img src="images/i_p023.jpg" width="300" height="539" alt="" /> - <div class="caption"><span class="smcap">Fig. 2.</span></div> - </div> - - <p>Projecting over the top of the pendulum there is a - <span class="pagenum" id="Page_23">23</span> wooden arm - <span class="smcap">w</span> carrying two sliding pointers <span class="smcap">h h</span>, 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 <em>large</em> 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 - <span class="smcap">r</span> there is a brass bar perforated with a small conical hole at - <span class="smcap">m</span>. A stiff wire passes through <span class="smcap">m</span> and forms the upper - portion of the index <span class="smcap">i</span>, 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 <span class="smcap">o o</span> crossing the - box.</p> - - <p>If at the time of an earthquake the upper part of the index <span class="smcap">i</span> - remains steady at <span class="smcap">m</span>, then by the motion at <span class="smcap">o</span>, the - lower end of the index which carries a sliding needle at <span class="smcap">g</span>, - will magnify the motion of the earth in the ratios <span class="smcap">m o</span> : - <span class="smcap">o g</span>. In this instrument <span class="smcap">o g</span> is about 17 cm.</p> - - <p>The needle <span class="smcap">g</span> works upon a piece of smoked glass. In - <span class="pagenum" id="Page_24">24</span> order to - bring the glass into contact with the needle without disturbance, the - glass is carried on a strip of wood <span class="smcap">k</span>, hinged at the back of - the box, and propped up in front by a loose block of wood <span class="smcap">y</span>. - When <span class="smcap">y</span> is removed the glass drops down with <span class="smcap">k</span> out of - contact with the needle. The box is carried on bars of wood - <span class="smcap">c c</span>, which are fixed to the ground by the stakes <span class="smcap">a</span> - <span class="smcap">a</span>.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_25">25</span> short - distance, and then allowing it to fall back towards its normal position.</p> - - <p>In connection with this subject we may mention the pendulum - seismographs of Kreil, Wagener, Ewing, and Gray.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_26">26</span> - arrangement, so that the pendulum, for small - displacements, shall be in neutral equilibrium, and the errors due to - swinging shall be avoided.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Another important class of instruments are <em>inverted pendulums</em>. 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<span class="pagenum" id="Page_27">27</span> - 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.</p> - - <p><em>Bracket Seismographs.</em>—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.</p> - - <p>In the accompanying sketch <span class="smcap">b</span> is a heavy weight pivoted at the - end of a small bracket <span class="smcap">c a k</span>, which bracket is free to turn - on a knife-edge, <span class="smcap">k</span>, above, and a pivot <span class="smcap">a</span>, below, - in the stand <span class="smcap">s</span>. At the time of an earthquake <span class="smcap">b</span> - remains steady, and the index <span class="smcap">p</span>, forming a continuation - <span class="pagenum" id="Page_28">28</span> of - the bracket, magnifies the motion of the stand, in the ratio of - <span class="smcap">a c</span> : <span class="smcap">c n</span>.</p> - - <div class="figcenter"> - <img src="images/i_p028a.jpg" width="500" height="225" alt="" /> - <div class="caption"><span class="smcap">Fig. 3.</span></div> - </div> - - <p>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.</p> - - <p>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.</p> - - <div class="figleft"> - <img src="images/i_p028b.jpg" width="266" height="175" alt="" /> - <div class="caption"><span class="smcap">Fig. 4.</span></div> - </div> - - <p><em>Parallel motion Instrument.</em>—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 <span class="smcap">a</span> is suspended from both sides on pivots at - <span class="smcap">c c</span> by a system of light arms hinging with each other at - <span class="pagenum" id="Page_29">29</span> the - black dots, between the upper and lower parts of the rigid frame - <span class="smcap">b c</span>. The arms are of such a length that for small displacements - parallel to the length of the bar, <span class="smcap">c c</span> practically move - in a straight line, and the bar is in neutral equilibrium. A light - prolongation of the bar <i>d</i> works the upper end of the light index <i>e</i>, - passing as a universal joint through the rigid support <span class="smcap">f</span>. A - second index <i>e′</i> from the bar at right angles also passes through - <span class="smcap">f</span>. The multiplying ends of these indices are coupled together - to write a resultant motion on a smoked glass plate <span class="smcap">s</span>.</p> - - <p><em>Conical Pendulums.</em>—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.</p> - - <p><em>Rolling Spheres and Cylinders.</em>—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.</p> - - <p>The general arrangement and principle of one of these instruments will - be readily understood from the<span class="pagenum" id="Page_30">30</span> - accompanying figure. <span class="smcap">s</span> is a - segment of a large sphere with a centre near <span class="smcap">c</span>. Slightly below - this centre a heavy weight <span class="smcap">b</span>, which may be a lead ring, is - pivoted. At the time of an earthquake <span class="smcap">c</span> is steady, and the - earth’s motions are magnified by the pointer <span class="smcap">c a n</span> in the - proportion of <span class="smcap">c a</span> : <span class="smcap">a n</span>. The working of this pointer - or index is similar to that of the pointer in the pendulum.</p> - - <div class="figcenter"> - <img src="images/i_p030.jpg" width="400" height="356" alt="" /> - <div class="caption"><span class="smcap">Fig. 5.</span></div> - </div> - - <p>Closely connected with the rolling sphere seismographs, are Gray’s - rolling cylinder seismographs.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_31">31</span></p> - - <p><em>Ball and Plate Seismograph.</em>—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.</p> - - <p><em>The Principle of Perry and Ayrton.</em>—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.</p> - - <p><em>Instruments to record Vertical Motion.</em>—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<span class="pagenum" id="Page_32">32</span> - 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.</p> - - <div class="figleft"> - <img src="images/i_p032.jpg" width="280" height="496" alt="" /> - <div class="caption"><span class="smcap">Fig. 6.</span></div> - </div> - - <p>The most satisfactory instrument which has yet been devised for - recording vertical motion is Gray’s horizontal lever spring seismograph.</p> - - <p>This instrument will be better understood from the accompanying sketch. - A vertical spring <span class="smcap">s</span> is fixed at its upper end by means of a - nut <i>n</i>, which rests on the top of the frame <span class="smcap">f</span>, and serves to - raise or lower the spring through a short distance as a last adjustment - for<span class="pagenum" id="Page_33">33</span> - the position of the cross-arm <span class="smcap">a</span>. The arm <span class="smcap">a</span> rests - at one end on two sharp points, <i>p</i>, one resting in a conical hole - and the other in a <span class="smcap">v</span>-slot; it is supported at <span class="smcap">b</span> by - the spring <span class="smcap">s</span>, and is weighted at <span class="smcap">c</span> with a lead ring - <span class="smcap">r</span>. Over a pin at the point <span class="smcap">c</span> a stirrup of thread is - placed which supports a small trough, <i>t</i>. The trough <i>t</i> is pivoted at - <i>a</i>, has attached to it the index <i>i</i> (which is hinged by means of a - strip of tough paper at <i>h</i>, and rests through a fine pin on the glass - plate <i>g</i>), and is partly filled with mercury.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Record Receivers.</em>—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,<span class="pagenum" id="Page_34">34</span> - 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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_35">35</span> - 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.</p> - - <p>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.</p> - - <p><em>Time-recording Apparatus.</em>—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<span class="pagenum" id="Page_36">36</span> - 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.</p> - - <div class="figleft"> - <img src="images/i_p036.jpg" width="270" height="357" alt="" /> - <div class="caption"><span class="smcap">Fig. 7.</span></div> - </div> - - <p>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.</p> - - <p><span class="smcap">s</span> is the segment of a sphere about 4·5 cm. radius, with a - centre slightly above <span class="smcap">c</span>. <span class="smcap">l</span> is a disc of lead about 7 - cm. in diameter resting upon the segment. Above this there is a light - pointer, <span class="smcap">p</span>, about 30 cm. long. On the top of the pointer a - small cylinder of iron, <span class="smcap">w</span>, is balanced, and connected by a - string with the catch to be relieved. When the table on which <span class="smcap">w p s</span> - rests is shaken, rotation takes place near to <span class="smcap">c</span>, the - motion of the base <span class="smcap">s</span> is magnified at the upper end of the - pointer, and the weight overturned. This catch may be used to relieve - a<span class="pagenum" id="Page_37">37</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_38">38</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>The Gray and Milne Seismograph.</em>—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, <span class="smcap">d</span>, - kept continuously in motion by clockwork, <span class="smcap">w</span>, by means of two - conical pendulum seismographs, <span class="smcap">c</span>. The vertical motion is - recorded on the same sheet of paper by means of a compensated-spring - seismograph, <span class="smcap">s l m b</span>.</p> - - <p>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<span class="pagenum" id="Page_39">39</span> - 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 - <span class="pagenum" id="Page_40">40</span> 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.</p> - - <div class="figcenter"> - <img src="images/i_p039.jpg" width="625" height="699" alt="" /> - <div class="caption"><span class="smcap">Fig. 8.</span></div> - </div> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_III"> - <span class="pagenum" id="Page_41">41</span> - <h2>CHAPTER III.<br /> - <span class="subhead">EARTHQUAKE MOTION DISCUSSED THEORETICALLY.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>Ideas of Early Writers.</em>—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.<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a> - It is as follows:—</p> - - <p>1. Epiclintæ, or earthquakes which move the ground obliquely.</p> - - <p>2. Brastæ, with an upward vertical motion like boiling water.</p> - - <p>3. Chasmatiæ, which cause the ground to sink and form hollows.</p> - - <p>4. Rhectæ, which raise the ground and make fissures.</p> - - <p>5. Ostæ, which overthrow with one thrust.</p> - - <p>6. Palmatiæ, which shake from side to side with a sort of tremor.</p> - - <p><span class="pagenum" id="Page_42">42</span></p> - - <p>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.’</p> - - <p>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.</p> - - <p>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 <em>genus</em> producing - elevations, a <em>genus</em> producing sinkings, a <em>genus</em> producing - conversions and transportations, and a <em>genus</em> which produces what, in - modern language, we should term metamorphic action.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_43">43</span> - 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.</p> - - <p>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.</p> - - <p>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:—</p> - - <p>An earthquake is ‘<em>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</em>.’</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_44">44</span></p> - - <p>To obtain a true idea of earthquake motion is a matter of cardinal - importance, as it forms the key-stone of many investigations.</p> - - <p>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.</p> - - <p><em>Nature of Elastic Waves and Vibrations.</em>—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.</p> - - <p>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.</p> - - <p>To understand what is meant by elastic waves, it is - <span class="pagenum" id="Page_45">45</span> 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.</p> - - <p>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.</p> - - <p>Here, then, we have two distinct things to observe—one being the - transmission of motion up to the ceiling, - <span class="pagenum" id="Page_46">46</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>All that has thus far been considered has been a backward and forward - kind of motion, where there is a rectilinear <em>compression</em> and - <em>extension</em> amongst the particles on which we stand.</p> - - <p>We might, however, imagine our rock, which for the - <span class="pagenum" id="Page_47">47</span> moment we will - consider to be a square column, to be twisted, and thus have its - <em>shape</em> 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 - <em>volume</em> may be very different from that which it offers against a - change of <em>form</em>.</p> - - <p>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.</p> - - <p>The following are examples of possible causes which might give rise to - these different orders of disturbance:—</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_48">48</span> in which - they are produced, we should expect to find waves of compression and - extension.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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:</p> - - <p>If <span class="smcap">t</span> be the time of vibration, or the time taken by a particle - to make one complete backward and forward swing, <span class="smcap">d</span> the density - of the material of which this particle forms a part, and <span class="smcap">e</span> the - proper modulus of elasticity of the material, then,</p> - - <div class="center"> - <span class="smcap">t</span> = 2π <span class="x200a">√</span><span class="sqrt2"><span class="frac"><sup><span class="smcap">d</span></sup><span>/</span><sub><span class="smcap">e</span></sub></span></span> - </div> - - <p>From this formula, - <span class="smcap">t</span> = 2π <span class="x200a">√</span><span class="sqrt2"><span class="frac"><sup><span class="smcap">d</span></sup><span>/</span><sub><span class="smcap">e</span></sub></span></span>, - we see that the<span class="pagenum" id="Page_49">49</span> - 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.</p> - - <p><em>Velocity and Acceleration of an Earth Particle.</em>—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, - <span class="smcap">t</span> = 2π <span class="x200a">√</span><span class="sqrt2"><span class="frac"><sup><span class="smcap">d</span></sup><span>/</span><sub><span class="smcap">e</span></sub></span></span>, - 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.</p> - - <p>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.</p> - - <p>Thus, if an earth particle takes one second to complete a - semi-oscillation, half of which, or the amplitude of the motion, equals - <i>a</i>, the maximum velocity equals <i>π</i> × <i>a</i>.</p> - - <p>Again, assuming the earth vibrations to be simple harmonic, the maximum - acceleration or rate of change in<span class="pagenum" id="Page_50">50</span> - velocity will come about at the ends - of each semi-oscillation; and if <span class="smcap">v</span> be the maximum velocity of - the particle, and <i>a</i> the amplitude or half semi-oscillation, then the - maximum acceleration equals - <span class="frac"><sup><span class="smcap">v</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>.</p> - - <p>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.</p> - - <p><em>Propagation of a Disturbance.</em>—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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_51">51</span> - 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.</p> - - <p>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.</p> - - <p>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—</p> - - <blockquote> - <p class="noindent"> - For slate and quartz transverse to lamination, 9,691 feet per second.<br /> - „ - „ in line of lamination, 5,415 - „ „ - </p> - </blockquote> - - <p>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.</p> - - <p><em>The Intensity of an Earthquake.</em>—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<span class="pagenum" id="Page_52">52</span> - 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 - motion, or as - <span class="frac"><sup><i>v</i><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span> - where <i>v</i> indicates velocity and <i>a</i> amplitude.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_53">53</span> - the energy in a particle of the - first shell at any particular phase of the motion be <span class="smcap">k</span><sub>1</sub>, - and the energy in a particle of the second shell <span class="smcap">k</span><sub>2</sub>, 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.</p> - - <p>In symbols, - <span class="icenter"><span class="frac"><sup><span class="smcap">k</span><sub>2</sub></sup><span>/</span><sub><span class="smcap">k</span><sub>1</sub></sub></span> - = <span class="frac"><sup><i>r</i><sub>1</sub><sup>2</sup></sup><span>/</span><sub><i>r</i><sub>2</sub><sup>2</sup></sub></span></span> - <span class="right">(1)</span> - </p> - - <p>Assuming that energy is dissipated,</p> - - <div class="center"> - <span class="frac"><sup><span class="smcap">k</span><sub>2</sub></sup><span>/</span><sub><span class="smcap">k</span><sub>1</sub></sub></span> - > <span class="frac"><sup><i>r</i><sub>1</sub><sup>2</sup></sup><span>/</span><sub><i>r</i><sub>2</sub><sup>2</sup></sub></span> - = <i>f</i> <span class="frac"><sup><i>r</i><sub>1</sub><sup>2</sup></sup><span>/</span><sub><i>r</i><sub>2</sub><sup>2</sup></sub></span> - <span class="right">(2)</span> - </div> - - <p class="noindent">where <i>f</i> < 1 is the rate of dissipation of energy which is assumed to - be constant.</p> - - <p><em>Area of greatest Overturning Moment.</em>—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.</p> - - <p>At the <em>epicentrum</em> 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.</p> - - <p>This will be rendered clear by the following diagram.</p> - - <p>In the accompanying diagram let <span class="smcap">o</span> be the origin of a shock, - and <span class="smcap">o c</span> the seismic vertical equal to <i>r</i>. Let the direct or - normal shock emerge at <span class="smcap">c</span>, <span class="smcap">c</span><sub>1</sub>, <span class="smcap">c</span><sub>2</sub>, and - <span class="pagenum" id="Page_54">54</span> - at the angles <i>θ</i><sub>1</sub>, <i>θ</i><sub>2</sub>, &c.</p> - - <p>Assuming that the displacement of an earth particle at <span class="smcap">c</span> - equals <span class="smcap">c b</span>, and at <span class="smcap">c</span><sub>1</sub> equals <i>c</i><sub>1</sub> <i>b</i><sub>1</sub>, - and at <span class="smcap">c</span><sub>2</sub> equals <i>c</i><sub>2</sub> <i>b</i><sub>2</sub>, &c., and let these - displacements <span class="smcap">c b</span>, <i>c</i><sub>1</sub> <i>b</i><sub>1</sub>, <i>c</i><sub>2</sub> <i>b</i><sub>2</sub>, &c., for - the sake of argument, vary inversely as <i>r</i>, <i>r</i><sub>1</sub>, <i>r</i><sub>2</sub>, &c.</p> - - <div class="figcenter"> - <img src="images/i_p054.jpg" width="460" height="164" alt="" /> - <div class="caption"><span class="smcap">Fig. 9.</span></div> - </div> - - <p>The question is to determine where the horizontal component <span class="smcap">c a</span> - of these normal motions is a maximum.</p> - - <p>First observe that the triangle <span class="smcap">o c</span> <i>c</i> is similar to <i>a</i>, - <i>b</i>, <i>c</i>.</p> - - <p>Also <i>r</i> = <span class="frac"><sup><i>h</i></sup><span>/</span><sub>sin <i>θ</i></sub></span>, - and therefore the normal component <i>c</i><sub>1</sub> - <i>b</i><sub>1</sub> at <span class="smcap">c</span><sub>1</sub> is equal to - <span class="smcap">c</span> <span class="frac"><sup>sin <i>θ</i></sup><span>/</span><sub><i>h</i></sub></span>.</p> - - <p>Also <i>c</i><sub>1</sub> <i>a</i><sub>1</sub> = <i>c</i><sub>1</sub> <i>b</i><sub>1</sub>, cos <i>θ</i>.</p> - - <div class="center"> - ∴ <span class="smcap">c</span><sub>1</sub> <i>a</i> = <span class="smcap">c</span> - <span class="frac"><sup>sin <i>θ</i> cos <i>θ</i></sup><span>/</span><sub><i>h</i></sub></span> - = <span class="frac"><sup><i>c</i></sup><span>/</span><sub><i>h</i></sub></span> - ∙ <span class="frac"><sup>sin 2<i>θ</i></sup><span>/</span><sub>2</sub></span>, - </div> - - <p class="noindent">and sin 2<i>θ</i> is greatest when 2<i>θ</i> = 90° or <i>θ</i> = 45°.</p> - - <p>That is to say, the horizontal component reaches a maximum where the - angle of emergence equals 45°.</p> - - <p>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″. - <span class="pagenum" id="Page_55">55</span> Both of these methods are - referred to by Mallet, but the first is considered as probably the more - correct.</p> - - <p><em>Earthquake Waves.</em>—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.</p> - - <p>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.</p> - - <p>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.<span class="pagenum" id="Page_56">56</span> - This approaches more nearly to the actual motions we feel - as earthquakes.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_IV"> - <span class="pagenum" id="Page_57">57</span> - <h2>CHAPTER IV.<br /> - <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM EXPERIMENT.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>Experiments with Falling Weights.</em>—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.</p> - - <p><span class="pagenum" id="Page_58">58</span></p> - - <p>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.</p> - - <p>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.</p> - - <div class="figcenter"> - <img src="images/i_p058.jpg" width="514" height="283" alt="" /> - <div class="caption"><span class="smcap">Fig. 10.</span></div> - </div> - - <p>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<span class="pagenum" id="Page_59">59</span> - 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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a></p> - - <p>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 <span class="pagenum" id="Page_60">60</span> - 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.</p> - - <div class="figcenter"> - <img src="images/i_p060.jpg" width="563" height="458" alt="" /> - <div class="caption"><span class="smcap">Fig. 11.</span>—Records obtained at three stations - of the motion of the ground produced by the explosion of 2 lbs. of - dynamite.</div> - </div> - - <p><span class="pagenum" id="Page_61">61</span></p> - - <p><em>The Intensity of Artificial Disturbances.</em>—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 <a href="#Page_60">p. 60</a>. 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.</p> - - <p>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 - acceleration being equal to - <span class="frac"><sup><span class="smcap">v</span><sup>2</sup></sup><span>/</span><sub>2<span class="smcap">a</span></sub></span>.</p> - - <div class="figright"> - <img src="images/i_p061.jpg" width="349" height="265" alt="" /> - <div class="caption"><span class="smcap">Fig. 12.</span></div> - </div> - - <p>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 - <span class="pagenum" id="Page_62">62</span> represent distance from the origin - in feet, and the vertical measurements mean acceleration in thousands - of millimetres per second.</p> - - <p>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.</p> - - <p>The only other investigations which have been made in this interesting - branch of observational seismology are those by Mr. Robert Mallet,<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a> - and those by General Henry L. Abbot.<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a></p> - - <p><em>Mallet’s Results.</em>—The velocity with which earth vibrations were - transmitted as deduced by Mr. Mallet were as follows:—</p> - - <table summary="Mallets results"> - <tr> - <th> </th> - <th>Feet per second</th> - </tr> - <tr> - <td class="tdhang">In sand</td> - <td class="tdr"><div>824·915</div></td> - </tr> - <tr> - <td class="tdhang">In contorted stratified rock, quartz, and slate at Holyhead</td> - <td class="tdr"><div>1,088·669</div></td> - </tr> - <tr> - <td class="tdhang">In discontinuous and much shattered granite</td> - <td class="tdr"><div>1,306·425</div></td> - </tr> - <tr> - <td class="tdhang">In more solid granite</td> - <td class="tdr"><div>1,664·574</div></td> - </tr> - </table> - - <p>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.</p> - - <p><span class="pagenum" id="Page_63">63</span></p> - - <p><em>Abbot’s Results.</em>—The important results obtained by General Abbot are - contained in the following table:—</p> - - <table id="AbbotsResults" summary="Abbots results"> - <tr> - <th>No. of Observation</th> - <th>Date</th> - <th>Cause of Shock</th> - <th>Distance to<br />Station in miles</th> - <th>Type of Seismometer</th> - <th>Velocity in<br />feet per second</th> - </tr> - <tr> - <td class="tdr"><div>1</div></td> - <td class="tdc">Aug. 18, 1876</td> - <td class="tdc">200 lbs. of dynamite</td> - <td class="tdr"><div>5 ± </div></td> - <td class="tdc">B</td> - <td class="tdc">5,280</td> - </tr> - <tr> - <td class="tdr"><div>2</div></td> - <td class="tdc">Sep. 24, 1876</td> - <td class="tdc">Hallet’s Point Explosion</td> - <td class="tdr"><div>5·134</div></td> - <td class="tdc">A</td> - <td class="tdc">3,873</td> - </tr> - <tr> - <td class="tdr"><div>3</div></td> - <td class="tdc">„</td> - <td class="tdc">„ „ „ </td> - <td class="tdr"><div>8·330</div></td> - <td class="tdc">B</td> - <td class="tdc">8,300</td> - </tr> - <tr> - <td class="tdr"><div>4</div></td> - <td class="tdc">„</td> - <td class="tdc">„ „ „ </td> - <td class="tdr"><div>9·333</div></td> - <td class="tdc">A</td> - <td class="tdc">4,521</td> - </tr> - <tr> - <td class="tdr"><div>5</div></td> - <td class="tdc">„</td> - <td class="tdc">„ „ „ </td> - <td class="tdr"><div>12·769</div></td> - <td class="tdc">B</td> - <td class="tdc">5,309</td> - </tr> - <tr> - <td class="tdr"><div>6</div></td> - <td class="tdc">Oct 10, 1876</td> - <td class="tdc"> 70 lbs. dynamite</td> - <td class="tdr"><div>1·360</div></td> - <td class="tdc">A</td> - <td class="tdc">1,240</td> - </tr> - <tr> - <td class="tdr"><div>7</div></td> - <td class="tdc">Sept. 6, 1877</td> - <td class="tdc">400 „ „ </td> - <td class="tdr"><div>1·169</div></td> - <td class="tdc">A</td> - <td class="tdc">3,428</td> - </tr> - <tr> - <td class="tdr"><div>8</div></td> - <td class="tdc">„</td> - <td class="tdc"> „ „ „ </td> - <td class="tdr"><div>1·169</div></td> - <td class="tdc">B</td> - <td class="tdc">8,814</td> - </tr> - <tr> - <td class="tdr"><div>9</div></td> - <td class="tdc">Sept. 12, 1877</td> - <td class="tdc">200 „ „ </td> - <td class="tdr"><div>1·340</div></td> - <td class="tdc">A</td> - <td class="tdc">6,730</td> - </tr> - <tr> - <td class="tdr"><div>10</div></td> - <td class="tdc">„</td> - <td class="tdc"> „ „ „ </td> - <td class="tdr"><div>1·340</div></td> - <td class="tdc">B</td> - <td class="tdc">8,730</td> - </tr> - <tr> - <td class="tdr"><div>11</div></td> - <td class="tdc">„</td> - <td class="tdc"> 70 „ „ </td> - <td class="tdr"><div>1·340</div></td> - <td class="tdc">A</td> - <td class="tdc">5,559</td> - </tr> - <tr> - <td class="tdr"><div>12</div></td> - <td class="tdc">„</td> - <td class="tdc"> „ „ „ </td> - <td class="tdr"><div>1·340</div></td> - <td class="tdc">B</td> - <td class="tdc">8,415</td> - </tr> - </table> - - <p>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.</p> - - <p>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.</p> - - <div class="figright"> - <img src="images/i_p063.jpg" width="283" height="151" alt="" /> - <div class="caption"><span class="smcap">Fig. 13.</span></div> - </div> - - <p>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 - <span class="pagenum" id="Page_64">64</span> us - that a still higher power above 12 might have detected still earlier - tremors.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Another point which was observed appears to have been that the rate - varied with the initial shock. Thus:—</p> - - <table summary="Shockwave velocity"> - <tr> - <th> </th> - <th>Feet per second</th> - </tr> - <tr> - <td class="left">400 lbs. of dynamite gave</td> - <td class="tdc"><div>8,814</div></td> - </tr> - <tr> - <td class="left">200 „ „ </td> - <td class="tdc"><div>8,730</div></td> - </tr> - <tr> - <td class="left"> 70 „ powder (deep) gave</td> - <td class="tdc"><div>8,415</div></td> - </tr> - </table> - - <p>Also it is probable that the rate of a wave diminished with its - advance. For,</p> - - <table summary="Shockwave velocity per distance"> - <tr> - <th> </th> - <th>Feet per second</th> - </tr> - <tr> - <td class="left"> 200 lbs. of dynamite gave for 1 mile</td> - <td class="tdc"><div>8,730</div></td> - </tr> - <tr> - <td class="left"> „ „ „ „ 5 miles</td> - <td class="tdc"><div>5,250</div></td> - </tr> - <tr> - <td class="left">50,000 „ „ „ 8 „</td> - <td class="tdc"><div>8,300</div></td> - </tr> - <tr> - <td class="left"> „ „ „ „ 13½ „</td> - <td class="tdc"><div>5,300</div></td> - </tr> - </table> - - <p>General Abbot’s general conclusions are:—</p> - - <p>1. A high magnifying power of telescope is essential in seismometric - observations.</p> - - <p>2. The more violent the initial shock the higher is the velocity of - transmission.</p> - - <p>3. This velocity diminishes as the general wave advances.</p> - - <p>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<span class="pagenum" id="Page_65">65</span> - wave, at any station, is shorter than with a slow burning - explosive.</p> - - <p><em>Results obtained in Japan.</em>—From some experiments made by the author - in the grounds of the Meteorological Department in Tokio, the following - results were obtained:—</p> - - <table id="JapanResults" class="collapse" summary="Japan shockwave velocity"> - <tr> - <th colspan="3" class="ball">No. of Explosion</th> - <th class="ball">Velocity in feet per second for the first 200 ft. (A to B)</th> - <th class="ball">Velocity in feet per second for the second 200 ft. (B to C)</th> - <th class="ball">Velocity in feet per second for 400 ft. (A to C)</th> - <th class="ball">Number of Cartridges of Dynamite (6 = 1 lb.)</th> - </tr> - <tr> - <td class="tdc bl" rowspan="4">Vertical vibrations</td> - <td class="tdr x400" rowspan="4"><div>{</div></td> - <td class="tdr0 br"><div>I.</div></td> - <td class="tdc br">464</td> - <td class="tdc br">186</td> - <td class="tdc br">265</td> - <td class="tdrx br"><div>8·3</div></td> - </tr> - <tr> - <td class="tdr0 br"><div>III.</div></td> - <td class="tdc br">—</td> - <td class="tdc br">211</td> - <td class="tdc br">—</td> - <td class="tdrx br"><div>10·1</div></td> - </tr> - <tr> - <td class="tdr0 br"><div>IV.</div></td> - <td class="tdc br">352</td> - <td class="tdc br">234</td> - <td class="tdc br">281</td> - <td class="tdrx br"><div>7·1</div></td> - </tr> - <tr> - <td class="tdr0 br"><div>V.</div></td> - <td class="tdc br">343</td> - <td class="tdc br">232</td> - <td class="tdc br">277</td> - <td class="tdrx br"><div>5·0</div></td> - </tr> - <tr> - <td class="tdc bl" rowspan="2">Normal vibrations</td> - <td class="tdr x200" rowspan="2"><div>{</div></td> - <td class="tdr0 br"><div>VI.</div></td> - <td class="tdc br">—</td> - <td class="tdc br">—</td> - <td class="tdc br">407</td> - <td class="tdrx br"><div>10·0</div></td> - </tr> - <tr> - <td class="tdr0 br"><div>VII.</div></td> - <td class="tdc br">—</td> - <td class="tdc br">—</td> - <td class="tdc br">516</td> - <td class="tdrx br"><div>12·5</div></td> - </tr> - <tr> - <td class="tdc bl bb">Transverse vibrations</td> - <td class="tdr x200 bb"><div>}</div></td> - <td class="tdr0 br bb"><div>VIII.</div></td> - <td class="tdc br bb">—</td> - <td class="tdc br bb">—</td> - <td class="tdc br bb">344</td> - <td class="tdrx br bb"><div>12·5</div></td> - </tr> - </table> - - <p>The general results to be deduced from the above appear to be:—</p> - - <p class="hang1">1. For vertical motion.</p> - - <p class="hang2">(<i>a</i>) For the first 200 feet. The velocity depends upon the initial - force—the greater the charge of dynamite the greater the velocity.</p> - - <p class="hang2">(<i>b</i>) 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.</p> - - <p class="hang1">The speed of the wave during the second 200 feet is always less - than during the first 200 feet.</p> - - <p class="hang1">2. For normal vibrations.</p> - - <p class="hang2">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.</p> - - <p><span class="pagenum" id="Page_66">66</span></p> - - <p class="hang1">3. For transverse vibrations.</p> - - <p class="hang2">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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_V"> - <span class="pagenum" id="Page_67">67</span> - <h2>CHAPTER V.<br /> - <span class="subhead">EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION ON EARTHQUAKES.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>Result of Feelings.</em>—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.</p> - - <p>Sometimes these motions gradually increase to a maximum and then die - out as gradually as they commenced.</p> - - <p>Sometimes the maximum comes suddenly, and at other times during an - earthquake our feelings distinctly tell us that there are several - maxima.</p> - - <p><span class="pagenum" id="Page_68">68</span></p> - - <p>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.’</p> - - <p>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.</p> - - <p><em>The Direction of Motion.</em>—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 - <span class="pagenum" id="Page_69">69</span> 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.</p> - - <p>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. (<em>See</em> chapter - on Effects in Buildings.)</p> - - <p>To determine the direction of movement during a small earthquake, the - most satisfactory method appears to be an appeal to instruments.</p> - - <p><em>Instruments as Indicators of Direction.</em>—The relative values of - different kinds of instruments, such as columns, pendulums, and the - like, as indicators of direction have already been discussed.</p> - - <p>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 - <a href="#Page_21">p. 21</a>); 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.</p> - - <p>Results similar to those indicated by the records of pendulum - seismographs have also been obtained upon moving plates with a double - bracket seismograph. Thus,<span class="pagenum" id="Page_70">70</span> - in the earthquake which shook Tokio at 6 - <span class="smcap">a.m.</span> on July 5, 1881, there were indications of the following - motions:—</p> - - <p>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.</p> - - <p>These particular directions of motion have been selected because they - were so definitely indicated.</p> - - <p>The commonest type of earthquake which is experienced in Japan, and - probably also in other earthquake-shaken districts, is the compound or - diastrophic form.</p> - - <p>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.</p> - - <p>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 <i xml:lang="es">vorticosi</i>, - 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.</p> - - <p>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<span class="pagenum" id="Page_71">71</span> - 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.</p> - - <p>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.</p> - - <p>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 <i xml:lang="es">sussultatore</i> is sometimes applied. Mallet has - given many elliptical and other closed curves to illustrate the nature - of such motions.</p> - - <p><em>Duration of an Earthquake.</em>—When reading accounts of earthquakes - it is often difficult to determine the length of time a shaking was - continuous. In Japan, in <span class="smcap">a.d.</span> 745, there was a shaking which - is said to have lasted sixty hours; and in <span class="smcap">a.d.</span> 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.</p> - - <p>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.</p> - - <p>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,<span class="pagenum" id="Page_72">72</span> - 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.</p> - - <p>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.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a></p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_73">73</span> - 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.</p> - - <p>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.</p> - - <p>The record of the duration of an ordinary earthquake as observed at a - given point is dependent upon the sensibility of our instruments.</p> - - <p>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.</p> - - <p>Seismometers having a multiplication of 6 to 12 - <span class="pagenum" id="Page_74">74</span> usually indicate that - motion continues longer than is perceptible to the senses.</p> - - <p><em>Period of Vibration.</em>—When an earthquake contains several prominent - vibrations which might be called the <em>shocks</em> of the disturbance, our - feelings tell us that these have occurred at unequal intervals.</p> - - <p>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.</p> - - <p>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.</p> - - <p>In the earthquake of March 11 (referred to on <a href="#Page_70">p. 70</a>) 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.</p> - - <p>This earthquake, as recorded at the author’s house in Tokio, lasted - about one and a half minute.</p> - - <p>The same earthquake, as recorded by Professor Ewing - <span class="pagenum" id="Page_75">75</span> 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.</p> - - <p>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.</p> - - <p><em>The Amplitude of Earth Movements.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_76">76</span> - 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:—</p> - - <table summary="Earthquake amplitude"> - <tr> - <th>Number of<br />Earthquakes</th> - <th>Maximum horizontal<br />motion of the ground</th> - </tr> - <tr> - <td class="tdr2"><div>10</div></td> - <td> ·0 to 0·15 mm.</td> - </tr> - <tr> - <td class="tdr2"><div>7</div></td> - <td> ·15 „ 0·5 „</td> - </tr> - <tr> - <td class="tdr2"><div>8</div></td> - <td> ·5 „ 2·5 „</td> - </tr> - <tr> - <td class="tdr2"><div>2</div></td> - <td>2·5 „ more „</td> - </tr> - </table> - - <p>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.</p> - - <p>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 ‘<em>dead beat</em>,’ 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.</p> - - <p>The results obtained for vertical motion were also very small. In Tokio - it is seldom that vertical motion can be - <span class="pagenum" id="Page_77">77</span> detected, and when it is - recorded it is seldom more than a millimetre.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Taking stations situated on or very nearly on the same line passing - through the seismic vertical (<em>epicentrum</em>), Mallet observed the - amplitude increased as some function of the distance, as will be seen - from the following table:—</p> - - <table id="amplitude2" summary="Earthquake amplitude per distance"> - <tr> - <th>Station</th> - <th>Polla</th> - <th>La Sala</th> - <th>Certosa</th> - <th>Tramutola</th> - <th>Sarconi</th> - </tr> - <tr> - <td class="tdl bl br">Distance from Seismic Vertical in geographical miles</td> - <td class="br">3·45</td> - <td class="br">11·60</td> - <td class="br">16·50</td> - <td class="br">20·60</td> - <td class="br">26·7</td> - </tr> - <tr> - <td class="tdl bl br bb">Amplitude in inches</td> - <td class="br bb">2·5</td> - <td class="br bb"> 3·5</td> - <td class="br bb"> 4·0</td> - <td class="br bb"> 4·5</td> - <td class="br bb"> 4·75</td> - </tr> - </table> - - <p>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.</p> - - <p>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 <em>two metres</em>. 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.</p> - - <p><em>Intensity of Earthquakes.</em>—In speaking of the strength - <span class="pagenum" id="Page_78">78</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>In writing about the Neapolitan earthquake of 1857, Mallet says that - ‘area alone affords no test of seismic energy.’</p> - - <p>The area over which a shock is felt will depend not only upon the - initial force of the disturbance, but also - <span class="pagenum" id="Page_79">79</span> 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.</p> - - <p>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.</p> - - <p><em>Velocity and Acceleration of an Earth Particle.</em>—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.</p> - - <p>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.<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a></p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_80">80</span></p> - - <p>The principles which guided him in making the calculations will be - understood from the following illustration.</p> - - <div class="figleft"> - <img src="images/i_p080.jpg" width="262" height="273" alt="" /> - <div class="caption"><span class="smcap">Fig. 14.</span></div> - </div> - - <p>If a column, <span class="smcap">a b c d</span>, receive a shock or be suddenly moved in - the direction of the arrow, the centre of gravity, <span class="smcap">g</span>, of this - column will revolve round the edge, and tend to describe the path <span class="smcap">g o</span>. - If it passes <span class="smcap">o</span>, the column will fall. The work done in - such a case as this is equal to lifting the column through the height - o <i>h</i>.</p> - - <p>If <span class="smcap">g a</span> = <i>a</i>, the angle <span class="smcap">g a</span> <i>h</i> = <i>ϕ</i>, and the - weight of the body = <span class="smcap">w</span>, then the above work equals</p> - - <p class="center"><span class="smcap">w</span><i>a</i> (1 − cos <i>ϕ</i>).</p> - - <p>This must equal the work acquired—that is to say, the kinetic energy of - rotation of the body, or</p> - - <div class="center"> - <span class="smcap">w</span><i>a</i> (1 − cos <i>ϕ</i>) = - <span class="frac"><sup><span class="smcap">w</span><i>w</i><sup>2</sup><span class="smcap">k</span><sup>2</sup></sup> - <span>/</span><sub>2 <i>g</i></sub></span>. - </div> - - <p>Where <i>w</i> is the angular velocity of the body at starting, <span class="smcap">k</span> - the radius of gyration round <span class="smcap">a</span>, and <i>g</i> the velocity acquired - by a falling body in one second. Whence</p> - - <div class="center"><i>w</i><sup>2</sup> <span class="smcap">k</span><sup>2</sup> = 2 <i>ga</i> (1 − cos <i>ϕ</i>),</div> - - <p class="noindent">but <i>w</i>, the angular velocity, is equal to the statical couple applied, - divided by the moment of inertia, or,</p> - - <div class="center"> - <i>w</i> = <span class="frac"><sup><span class="smcap">v</span><i>a</i> cos <i>ϕ</i></sup> - <span>/</span><sub><span class="smcap">k</span><sup>2</sup></sub></span>, - </div> - - <p class="noindent">squaring and substituting</p> - - <div class="center"> - <span class="smcap">v</span><sup>2</sup> = 2<i>g</i> × - <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span> × - <span class="frac"><sup>1 − cos <i>ϕ</i></sup><span>/</span><sub>cos<sup>2</sup> <i>ϕ</i></sub></span>, - </div> - - <p><span class="pagenum" id="Page_81">81</span></p> - - <p class="noindent">and since the length of the corresponding pendulum is <i>l</i> = - <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>, - </p> - - <div class="center"> - <span class="smcap">v</span><sup>2</sup> = 2<i>gl</i> × <span class="frac"><sup>1 − cos <i>ϕ</i></sup> - <span>/</span><sub>cos<sup>2</sup> <i>ϕ</i></sub></span>, - </div> - - <p>To apply this to any given case we must find the value of <i>l</i> or of - <span class="frac"><sup><span class="smcap">k</span><sup>2</sup></sup><span>/</span><sub><i>a</i></sub></span>. - </p> - - <p>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.</p> - - <div class="figright"> - <img src="images/i_p081.jpg" width="221" height="212" alt="" /> - <div class="caption"><span class="smcap">Fig. 15.</span></div> - </div> - - <p>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 - <em>through joints across</em> 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 - <span class="pagenum" id="Page_82">82</span> - emerged, or, if this be known, for determining the velocity.</p> - - <p>Thus by a shock in the direction <span class="smcap">o c</span>, a ball, <span class="smcap">a</span>, - on the top of a pedestal would describe a trajectory to the point - <span class="smcap">c</span>. Let the angle which <span class="smcap">o c</span> makes with the horizon be - <i>e</i>, the vertical height through which the ball has fallen be <i>b</i>, and - the horizontal distance of projection be <i>a</i>; then</p> - - <div class="center"> - <i>b</i> = <i>a</i> tan <i>e</i> + - <span class="frac"><sup><i>a</i><sup>2</sup></sup> - <span>/</span><sub>4<span class="smcap">h</span> cos<sup>2</sup> <i>e</i></sub></span>, - </div> - - <p class="noindent"><span class="smcap">h</span> being the height due to the velocity of projection. Whence</p> - - <div class="center"> - Tan <i>e</i> = - <span class="frac"><sup>2<span class="smcap">h</span> ± √<span class="sqrt">4<span class="smcap">h</span>(<span class="smcap">h</span> + <i>b</i>) − <i>a</i><sup>2</sup></span></sup> - <span>/</span><sub><i>a</i></sub></span> - </div> - - <div class="center mt1"> - <span class="smcap">v</span><sup>2</sup> = - <span class="frac"><sup><i>a</i><sup>2</sup> <i>g</i></sup> - <span>/</span><sub>2 cos<sup>2</sup> <i>e</i> (<i>b</i> − <i>a</i> tan <i>e</i>)</sub></span>. - </div> - - <p>For the back motion or subnormal wave in the direction <span class="smcap">c o</span>,</p> - - <div class="center"> - Tan <i>e</i> = - <span class="frac"><sup>2<span class="smcap">h</span> ± √<span class="sqrt">4<span class="smcap">h</span>(<span class="smcap">h</span> + <i>b</i>) − <i>a</i><sup>2</sup></span></sup> - <span>/</span><sub><i>a</i></sub></span> - </div> - - <div class="center mt1"> - <span class="smcap">v</span><sup>2</sup> = - <span class="frac"><sup><i>a</i><sup>2</sup> <i>g</i></sup> - <span>/</span><sub>2 cos<sup>2</sup> <i>e</i> (<i>b</i> + <i>a</i> tan <i>e</i>)</sub></span>. - </div> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_83">83</span> - 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 <em>opposite</em> to that given in the - figure—that is to say, in the direction of the - shock.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a></p> - - <p><em>The Absolute Intensity of the Force exerted by an Earthquake.</em>—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.</p> - - <p><span class="pagenum" id="Page_84">84</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>The capability of producing the earthquake impulse, however, depends - on the <em>suddenness</em> 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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_85">85</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Radiation of an Earthquake.</em>—The tremors preceding the more violent - movements of an earthquake may be due, as Mallet has suggested,<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a> - to the free surface waves reaching a distant point before the direct - vibrations.</p> - - <p>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.</p> - - <p>In the tunnel, although the distance may be small, - <span class="pagenum" id="Page_86">86</span>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.</p> - - <p>Lastly, we may refer to the experiences of miners underground.</p> - - <p>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.</p> - - <p>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).</p> - - <p>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.</p> - - <p>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.</p> - - <p>The explanation of these latter observations appears to be either - that, in consequence of a smaller amplitude - <span class="pagenum" id="Page_87">87</span> 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.</p> - - <p><em>Velocity of Propagation of an Earthquake.</em>—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.</p> - - <p>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.</p> - - <p>In Mallet’s British Association Report for 1858, he gives data compiled - by Mr. David Milne<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a> respecting the Lisbon earthquakes of 1755 and - 1761, from which data the tables of velocities <a href="#Page_89">(p. 89)</a> have been - calculated, omitting those which Mr. Mallet has marked as uncertain.</p> - - <p>The distances are marked in degrees of seventy English miles each, and - the time is reduced to Lisbon time.</p> - - <p><span class="pagenum" id="Page_88">88</span></p> - - <table id="propogation" class="collapse" summary="Propogation velocity"> - <tr> - <th class="ball">Occasion and Place</th> - <th class="ball">Approx. rate in feet per second</th> - <th class="ball">Formation constituting Range on surface as far as known or conjectured</th> - <th class="ball">Authority</th> - </tr> - <tr> - <td class="tdl bl br">Rev. John Mitchell’s guesses from the Lisbon earthquakes</td> - <td class="tdr br"><div>1,760</div></td> - <td class="tdl br">Sea bottom, probably on slates, secondary and crystalline rocks</td> - <td class="tdc br">Mitchell</td> - </tr> - <tr> - <td class="tdl bl br">Von Humboldt’s guesses from South America</td> - <td class="tdr br"><div>1,760 to 2,464</div></td> - <td class="tdl br">From observations in various South American rocks in great part volcanic</td> - <td class="tdc br">Humboldt</td> - </tr> - <tr> - <td class="tdc bl br pt2"><i>Lisbon Earthquake of 1761.</i></td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Lisbon to Corunna</td> - <td class="tdr br"><div>1,994</div></td> - <td class="tdl br">Transition, carboniferous and granitoid</td> - <td class="tdc br">‘Annual Register’</td> - </tr> - <tr> - <td class="tdl bl br">Lisbon to Cork</td> - <td class="tdr br"><div>5,228</div></td> - <td class="tdl br">Transition, carboniferous crystalline slates and granitoid, probably, under sea bottom</td> - <td class="tdc br">„</td> - </tr> - <tr> - <td class="tdl bl br">Lisbon to Santa Cruz</td> - <td class="tdr br"><div>3,261</div></td> - <td class="tdl br">The same with many alterations</td> - <td class="tdc br">„</td> - </tr> - <tr> - <td class="tdc bl br pt2"><i>Antilles.</i></td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Pointe à Pitre to Cayenne (doubtful)</td> - <td class="tdr br"><div>6,586</div></td> - <td class="tdl br">Probably volcanic rocks under sea bottom</td> - <td class="tdc br">Stier and Perrey’s memorandum, Dijon</td> - </tr> - <tr> - <td class="tdc bl br pt2"><i>India.</i></td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Cutch to Calcutta, 1819</td> - <td class="tdr br"><div>1,173</div></td> - <td class="tdl br">Alluvial, secondary, granitoid and later igneous rocks</td> - <td class="tdc br">‘Royal Asiatic Journal’</td> - </tr> - <tr> - <td class="tdl bl br">India, Nepauls, and basin of the Ganges, 1834:—</td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Rungpur to Arrah</td> - <td class="tdr1 br" rowspan="4"> - <table summary="Propogation sub-table"> - <tr> - <td class="tdr1"> - <div> - 2,314<br /> - 3,520<br /> - 990<br /> - 1,210 - </div> - </td> - <td class="x400">}</td> - </tr> - </table> - </td> - <td class="tdl br" rowspan="4">Deep alluvia, with occasional transition, carboniferous, granitoid, and later igneous rocks</td> - <td class="tdc br" rowspan="4">‘Royal Asiatic Journal’</td> - </tr> - <tr> - <td class="tdl bl br">Monghyr to Gorackpur</td> - </tr> - <tr> - <td class="tdl bl br">Rungpur to Monghyr</td> - </tr> - <tr> - <td class="tdl bl br">Rungpur to Calcutta</td> - </tr> - <tr> - <td class="tdl bl br bb">Ships ‘Rambler’ and ‘Millwood,’ at sea, 1851; between lat. 16° 30′ N.L., 54° 30′ W., and lat. 23° 30′ N.L., 58° 0′ W.</td> - <td class="tdr br bb"><div>1,056</div></td> - <td class="tdl br bb">Sea bottom resting on unknown rock</td> - <td class="tdc br bb">‘Nautical Magazine’</td> - </tr> - </table> - - <p><span class="pagenum" id="Page_89">89</span></p> - - <p class="center mt2"><span class="smcap">The Lisbon Earthquake on November 1, 1755.</span></p> - - <table class="collapse" summary="Propogation velocity Lisbon 1755"> - <tr> - <th class="ball">Localities</th> - <th class="ball" colspan="2">Moment observed of shock</th> - <th class="ball" colspan="2">Distance from presumed origin</th> - <th class="ball">Velocity in feet per second</th> - </tr> - <tr> - <td class="tdl bl br"> </td> - <td class="tdr"><div>h.</div></td> - <td class="tdrx br"><div>m.</div></td> - <td class="tdr"><div>°</div></td> - <td class="tdrx br"><div>′</div></td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Presumed focus of shock, lat. 30°, long. 11° W.</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>23</div></td> - <td class="tdc br" colspan="2">—</td> - <td class="tdc br">—</td> - </tr> - <tr> - <td class="tdl bl br">A ship at sea in lat. 38°, long. 10° 47′ W.</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>24</div></td> - <td class="tdr"><div>0</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>3,091</div></td> - </tr> - <tr> - <td class="tdl bl br">Colares</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdr"><div>1</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,325</div></td> - </tr> - <tr> - <td class="tdl bl br">Lisbon</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>32</div></td> - <td class="tdr"><div>1</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,030</div></td> - </tr> - <tr> - <td class="tdl bl br">Oporto</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>38</div></td> - <td class="tdr"><div>2</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,030</div></td> - </tr> - <tr> - <td class="tdl bl br">Ayamonte</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>50</div></td> - <td class="tdr"><div>4</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>916</div></td> - </tr> - <tr> - <td class="tdl bl br">Cadiz</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>48</div></td> - <td class="tdr"><div>5</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>1,236</div></td> - </tr> - <tr> - <td class="tdl bl br">Tangier and Tetuan</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>46</div></td> - <td class="tdr"><div>5</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,478</div></td> - </tr> - <tr> - <td class="tdl bl br">Madrid</td> - <td class="tdr"><div>9</div></td> - <td class="tdrx br"><div>43</div></td> - <td class="tdr"><div>6</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>1,855</div></td> - </tr> - <tr> - <td class="tdl bl br">Funchal</td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>1</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,382</div></td> - </tr> - <tr> - <td class="tdl bl br">Portsmouth</td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>3</div></td> - <td class="tdr"><div>12</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,431</div></td> - </tr> - <tr> - <td class="tdl bl br">Havre</td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>23</div></td> - <td class="tdr"><div>13</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>1,339</div></td> - </tr> - <tr> - <td class="tdl bl br">Reading</td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>27</div></td> - <td class="tdr"><div>13</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>1,304</div></td> - </tr> - <tr> - <td class="tdl bl br">Yarmouth</td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>42</div></td> - <td class="tdr"><div>15</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>1,174</div></td> - </tr> - <tr> - <td class="tdl bl br">Amsterdam</td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>6</div></td> - <td class="tdr"><div>17</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>2,444</div></td> - </tr> - <tr> - <td class="tdl bl br bb">Loch Ness</td> - <td class="tdr bb"><div>10</div></td> - <td class="tdrx br bb"><div>42</div></td> - <td class="tdr bb"><div>18</div></td> - <td class="tdrx br bb"><div>0</div></td> - <td class="tdrx br bb"><div>1,409</div></td> - </tr> - </table> - - <p class="center mt2"><span class="smcap">The Lisbon Earthquake of March 31, 1761.</span><a - id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a></p> - - <table class="collapse" summary="Propogation velocity Lisbon 1761"> - <tr> - <th class="ball">Locality</th> - <th class="ball" colspan="2">Moment observed of shock</th> - <th class="ball" colspan="2">Distance from presumed origin</th> - <th class="ball">Velocity in feet per second</th> - </tr> - <tr> - <td class="tdl bl br"> </td> - <td class="tdr"><div>h.</div></td> - <td class="tdrx br"><div>m.</div></td> - <td class="tdr"><div>°</div></td> - <td class="tdrx br"><div>′</div></td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Presumed focus, lat. 43°, long. 11° W.</td> - <td class="tdr"><div>11</div></td> - <td class="tdrx br"><div>51</div></td> - <td class="tdc br" colspan="2">—</td> - <td class="tdc br">—</td> - </tr> - <tr> - <td class="tdl bl br">Ship at sea in lat. 43°, many leagues from coast of Portugal</td> - <td class="tdr"><div>11</div></td> - <td class="tdrx br"><div>52</div></td> - <td class="tdr"><div>0</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>3,091</div></td> - </tr> - <tr> - <td class="tdl bl br">Ship in lat. 44° 8′ and about 80 leagues from coast</td> - <td class="tdr"><div>11</div></td> - <td class="tdrx br"><div>54</div></td> - <td class="tdr"><div>1</div></td> - <td class="tdrx br"><div>45</div></td> - <td class="tdrx br"><div>3,607</div></td> - </tr> - <tr> - <td class="tdl bl br">Corunna</td> - <td class="tdr"><div>11</div></td> - <td class="tdrx br"><div>51</div></td> - <td class="tdr"><div>2</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>2,576</div></td> - </tr> - <tr> - <td class="tdl bl br">Ship lat. 44° 8′ and 80 leagues NNW. of Cape Finisterre</td> - <td class="tdr"><div>11</div></td> - <td class="tdrx br"><div>58</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>3,091</div></td> - </tr> - <tr> - <td class="tdl bl br">Lisbon</td> - <td class="tdc br" colspan="2">noon</td> - <td class="tdr"><div>4</div></td> - <td class="tdrx br"><div>30</div></td> - <td class="tdrx br"><div>3,091</div></td> - </tr> - <tr> - <td class="tdl bl br">Madeira</td> - <td class="tdr"><div>12</div></td> - <td class="tdrx br"><div>6</div></td> - <td class="tdr"><div>10</div></td> - <td class="tdrx br"><div>0</div></td> - <td class="tdrx br"><div>4,122</div></td> - </tr> - <tr> - <td class="tdl bl br bb">Cork</td> - <td class="tdr bb"><div>12</div></td> - <td class="tdrx br bb"><div>11</div></td> - <td class="tdr bb"><div>9</div></td> - <td class="tdrx br bb"><div>30</div></td> - <td class="tdrx br bb"><div>2,937</div></td> - </tr> - </table> - - <p>These tables, owing to the nature of the materials - <span class="pagenum" id="Page_90">90</span>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.</p> - - <p>The great differences in transit velocity obtained for different - earthquakes is a point worthy of attention.</p> - - <p>Seebach’s velocity is a <em>true</em> transit velocity, and its determination - is dependent on the assumption that the shock radiated from the - <em>centrum</em> and not from the <em>epicentrum</em>, Seebach’s method is explained - when speaking about the determination of origins.</p> - - <p>Some interesting observations on the velocity with which the earthquake - of October 7, 1874, was propagated, are given by M. S. di Rossi.<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a></p> - - <p>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:—</p> - - <table class="collapse" summary="Rossi's velocity results"> - <tr> - <td colspan="2" class="tdc ball">Velocity in feet per second with direct radiation</td> - <td colspan="2" class="tdc ball">Velocity in feet per second by propagation along mountain chains</td> - </tr> - <tr> - <td class="tdl bl">Modigliana</td> - <td class="tdr br">820</td> - <td class="tdl">By the Valley of Marenzo</td> - <td class="tdr br"><div>1,080</div></td> - </tr> - <tr> - <td class="tdl bl">Bologna</td> - <td class="tdr br">656</td> - <td class="tdl"> „ „ Saveno</td> - <td class="tdr br"><div>1,080</div></td> - </tr> - <tr> - <td class="tdl bl">Forli</td> - <td class="tdr br">874</td> - <td class="tdl"> „ „ Montone</td> - <td class="tdr br"><div>1,080</div></td> - </tr> - <tr> - <td class="tdl vm bl">Modena</td> - <td class="tdr vm br">518</td> - <td class="tdl vm"> „ „ Panaro</td> - <td class="tdr0 pr0 br"> - <div> - <table summary="maradi1" class="mr1"> - <tr> - <td rowspan="2" class="tdr0 pr0"><div class="x200">{</div></td> - <td class="tdr0 pr1"><div>1,080</div></td> - </tr> - <tr> - <td class="tdr0 pr1"><div>984</div></td> - </tr> - </table> - </div> - </td> - </tr> - <tr> - <td class="tdl bl">Firenze</td> - <td class="tdr br">273</td> - <td class="tdl"> „ „ Sieve</td> - <td class="tdr br"><div>540</div></td> - </tr> - <tr> - <td class="tdl bl bb">Compiobbi</td> - <td class="tdr br bb">328</td> - <td class="tdl bb"> „ „ „</td> - <td class="tdr br bb"><div>540</div></td> - </tr> - </table> - - <p><span class="pagenum" id="Page_91">91</span></p> - <p>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 <a href="#Page_231">p. 231</a>). 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.<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a></p> - - <table summary="Serpieri's velocity results"> - <tr> - <td class="tdl">From Ragusa to Venice the velocity was</td> - <td class="tdr"><div>2,734</div></td> - <td class="tdl">feet per second</td> - </tr> - <tr> - <td class="tdl"> „ Spoleto „ „ </td> - <td class="tdr"><div>4,101</div></td> - <td class="tdl"> „ „</td> - </tr> - <tr> - <td class="tdl"> „ Perugia to Orvieto „ </td> - <td class="tdr"><div>601</div></td> - <td class="tdl"> „ „</td> - </tr> - <tr> - <td class="tdl"> „ „ „ Ancona</td> - <td class="tdr"><div>1,640</div></td> - <td class="tdl"> „ „</td> - </tr> - <tr> - <td class="tdl"> „ „ „ Rome</td> - <td class="tdr0 pr0"> - <div> - <table summary="Serpieri sub-table"> - <tr> - <td rowspan="2" class="tdr0 pr0"><span class="x200">{</span></td> - <td class="tdr0 pr0"><div>1,640</div></td> - </tr> - <tr> - <td class="tdr0 pr0"><div>or, 2,186</div></td> - </tr> - </table> - </div> - </td> - <td class="tdl"> „ „</td> - </tr> - </table> - - <p>The following are examples of approximate earthquake velocities which - have been determined in Japan.</p> - - <p><em>The Tokio Earthquake of October 25, 1881.</em>—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.</p> - - <p>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 <em>feet per second</em>. If - <span class="pagenum" id="Page_92">92</span>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.</p> - - <p>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 <em>homoseist</em>, travelled about - 218 miles to reach Tokio in 128 seconds, which gives a <em>velocity of - 10,219 feet per second</em>.</p> - - <p>The method here followed is equivalent to that of the hyperbola and one - direction (see <a href="#Page_204">p. 204</a>). 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.</p> - - <p>If we work by the method of circles, and assume the velocity to have - been constant in all directions, then this - <span class="pagenum" id="Page_93">93</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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:—</p> - - <table class="collapse" summary="Yokohama to Tokio transit interval"> - <tr> - <th><span class="smcap">From Yokohama to Tokio.</span></th> - <th><span class="smcap">From Tokio to Yokohama.</span></th> - </tr> - <tr> - <td class="tdl br">1880 December 20th 36 seconds</td> - <td>1882 October 25th 21 seconds</td> - </tr> - <tr> - <td class="tdl br">1881 January 7th 14–31 „</td> - <td class="tdl">1883 February 6th 23 „</td> - </tr> - <tr> - <td class="tdl br"> „ March 8th 60 „</td> - <td class="tdl"> „ March 1 53 „</td> - </tr> - <tr> - <td class="tdl br"> „ „ 17th. 66 „</td> - <td class="tdl"> „ „ „ 63 „</td> - </tr> - <tr> - <td class="tdl br"> „ November 15th 31 „</td> - <td class="tdl"> „ „ 8th 27 „</td> - </tr> - <tr> - <td class="tdl br">1882 February 16th 22 „</td> - <td class="tdl"> „ „ 11th 26 „</td> - </tr> - </table> - - <p>As these are observations which have been made with the assistance of a - telegraphic signal daily employed to - <span class="pagenum" id="Page_94">94</span> correct and rate the clocks from - which the observations were obtained, they may be regarded as being - tolerably, correct.</p> - - <p>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.</p> - - <p>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.</p> - - <p>The shock of March 11, which was recorded at Tokio at 7.51.22 - <span class="smcap">p.m.</span> and at Yokohama at 7.51.33 <span class="smcap">p.m.</span>, 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 <em>approximately</em> accurate, if - we take them with the records of previous observers they lead us to the - following conclusions:—</p> - - <p>1. Different earthquakes, although they may travel across the same - country, have very variable velocities, - <span class="pagenum" id="Page_95">95</span> varying between several - hundreds and several thousands of feet per second.</p> - - <p>2. The same earthquake travels more quickly across districts near to - its origin than it does across districts which are far removed.</p> - - <p>3. The greater the intensity of the shock the greater is the velocity.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_VI"> - <span class="pagenum" id="Page_96">96</span> - <h2>CHAPTER VI.<br /> - <span class="subhead">EFFECTS PRODUCED BY EARTHQUAKES UPON BUILDINGS.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p class="noindent"><span class="smcap">The</span> 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.</p> - - <p><em>The Destruction produced by Earthquakes is not irregular.</em>—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<span class="pagenum" id="Page_97">97</span> - recognise any law as to the relative position of the masses of - <i xml:lang="fr">débris</i> 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.</p> - - <p>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,<a id="FNanchor_20" - href="#Footnote_20" class="fnanchor">[20]</a> 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.</p> - - <p>In Caraccas it is said that every house has its <i xml:lang="es">laga securo</i>, or safe - side, where the inhabitants place their fragile property. This <i xml:lang="es">laga - securo</i> 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.<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a></p> - - <p><span class="pagenum" id="Page_98">98</span></p> - - <p><em>Cracks in Buildings.</em>—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.</p> - - <p>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.</p> - - <p>On March 3, 1879, at 4.43 <span class="smcap">p.m.</span>, an earthquake was - <span class="pagenum" id="Page_99">99</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_100">100</span> - 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.</p> - - <p>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.</p> - - <div class="figleft"> - <img src="images/i_p100.jpg" width="195" height="296" alt="" /> - <div class="caption"><span class="smcap">Fig. 16.</span>—Cracks in a corner house,<br />Belluno, June 29, 1873 - (Bittner).</div> - </div> - - <p>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).</p> - - <p>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<span class="pagenum" id="Page_101">101</span> - 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.</p> - - <div class="figright"> - <img src="images/i_p101.jpg" width="345" height="268" alt="" /> - <div class="caption"><span class="smcap">Fig. 17.</span>—Brick buildings<br />in Tokio, showing - fractures.</div> - </div> - - <p><em>Buildings in Tokio.</em>—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 - <em>similar</em> 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.</p> - - <p>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 - <span class="pagenum" id="Page_102">102</span> 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 <em>sharply</em> 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 <em>gently</em> from their abutments. The outside walls - have a thickness of 13½ inches.</p> - - <p>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:—</p> - - <p>1. In the upper windows nearly all the cracks ran from the springing, - which formed an angle with the abutment.</p> - - <p>2. In the lower arches, which <em>curved</em> 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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_103">103</span></p> - - <p>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.</p> - - <p>Another point of importance would be to build archways <em>curving</em> 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.</p> - - <p><em>Relation of Destruction to Earthquake Motion.</em>—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 <em>shape</em> rather than the <em>weight</em> of a body - which determined whether it should be overturned or projected by a - motion at its base.</p> - - <p>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.</p> - - <p>If masses of material are displaced or fractured, then Mallet remarks - that the maximum velocity will exceed √<span class="sqrt3">2<i>gh</i></span>, where <i>h</i> 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,<span class="pagenum" id="Page_104">104</span> - and there will be no relative displacements even if the - emergence of the wave be nearly or quite vertical.</p> - - <p>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.</p> - - <p>In considering cases of fracture produced by earthquake motion, it - must be remembered that these are due to stresses applied <em>suddenly</em>, - and that if the same amount of stress had been <em>slowly</em> applied to a - building, fractures might not have occurred.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>The accompanying figures are reduced from Mallet’s ‘Account of the - Neapolitan Earthquake of 1857.’</p> - - <div class="figcenter"> - <span class="pagenum" id="Page_105">105</span> - <img src="images/i_p105.jpg" width="700" height="297" alt="" /> - <div class="caption"><span class="smcap">Fig. 18.</span>—Cathedral Church, Potenza (Mallet).</div> - </div> - - <p><span class="pagenum" id="Page_106">106</span></p> - - <p>Taking <i>a b</i> as the general direction of the fractures in - fig. 18, then <i>c d</i> 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<span class="pagenum" id="Page_107">107</span> - 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.</p> - - <div class="figcenter"> - <img src="images/i_p106.jpg" width="533" height="600" alt="" /> - <div class="caption"><span class="smcap">Fig. 19.</span>—The Cathedral, Paterno (Mallet). - Neapolitan Earthquake of 1857.</div> - </div> - - <p>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.</p> - - <p>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.</p> - - <p>Fig. 19, of the cathedral at Paterno, shows the effect of a subnormal - shock striking a wall obliquely and projecting one of its corners.</p> - - <h3>MEASUREMENTS OF THE RELATIVE MOTION OF PARTS OF A BUILDING AT THE TIME - OF AN EARTHQUAKE.</h3> - - <p>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.</p> - - <p><span class="pagenum" id="Page_108">108</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Observations on Cracks.</em>—To determine whether the walls of a building - which have once been cracked, when<span class="pagenum" id="Page_109">109</span> - 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.</p> - - <p>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.</p> - - <p>In this building it was also observed that the cracks in many instances - increased their length.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_110">110</span></p> - - <p><em>Prevention of Fractures.</em>—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.</p> - - <p><em>Direction of Cracks.</em>—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.</p> - - <p><em>The Pitch of Roofs.</em>—From observation of the effects - <span class="pagenum" id="Page_111">111</span> 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.</p> - - <div class="figleft"> - <img src="images/i_p112.jpg" width="330" height="236" alt="" /> - <div class="caption"><span class="smcap">Fig. 20. Fig. 21.</span></div> - </div> - - <p><em>Relative Position of Openings in Walls.</em>—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<span class="pagenum" id="Page_112">112</span> - 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 <i>a b</i> or of a vertical movement <i>c d</i>, 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 <i>e f</i>, - 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.</p> - - <p><em>The last House in a Row.</em>—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<span class="pagenum" id="Page_113">113</span> - 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.</p> - - <div class="figcenter"> - <img src="images/i_p113.jpg" width="558" height="700" alt="" /> - <div class="caption"><span class="smcap">Fig. 22.</span>—Church of St. Augustin, Manilla. - Earthquakes of July 18–20, 1880.</div> - </div> - - <p><span class="pagenum" id="Page_114">114</span></p> - - <p><em>The Swing of Buildings.</em>—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.</p> - - <div class="figcenter"> - <img src="images/i_p114.jpg" width="536" height="336" alt="" /> - <div class="caption"><span class="smcap">Fig. 23.</span>—Webber House, San Francisco. Oct. 21, - 1868.</div> - </div> - - <p><span class="pagenum" id="Page_115">115</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>An octagonal chimney with a heavy granite capping, - <span class="pagenum" id="Page_116">116</span> 160 feet high, was - observed instrumentally to vibrate at the top nearly 5 inches.<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a></p> - - <p>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,<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a> apparently indicates a movement - of description.</p> - - <div class="figcenter"> - <img src="images/i_p116.jpg" width="534" height="337" alt="" /> - <div class="caption"><span class="smcap">Fig. 24.</span>—Stud Mill at Haywards, - California. Oct. 21, 1868.</div> - </div> - - <p><em>Principle of relative Vibrational Period.</em>—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 <span class="pagenum" id="Page_117">117</span> - 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.</p> - - <p>A particularly instructive example of this kind which came under my - notice is roughly sketched in fig. 25.</p> - - <div class="figright"> - <img src="images/i_p117.jpg" width="142" height="219" alt="" /> - <div class="caption"><span class="smcap">Fig. 25.</span></div> - </div> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_118">118</span> Italian churches - where the belfry tower is built into one of the quoins of the main - rectangular building.</p> - - <p>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.</p> - - <p>Some of the more important results dependent upon the principle of - ‘relative vibrational periods’ may be understood from the following - experiments:—</p> - - <div class="figleft"> - <img src="images/i_p118.jpg" width="258" height="155" alt="" /> - <div class="caption"><span class="smcap">Fig. 26.</span></div> - </div> - - <p>In fig. 26 <span class="smcap">a</span>, <span class="smcap">b</span>, and <span class="smcap">c</span> 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 <span class="smcap">d e</span>, - and the whole rests on a table <span class="smcap">f g</span>. 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 <span class="smcap">a</span> and <span class="smcap">b</span> are the same, but they are larger than - the weight on <span class="smcap">c</span>. Consequently the periods of <span class="smcap">a</span> and - <span class="smcap">b</span> are the same, but different to the period of <span class="smcap">c</span>. - The dimensions of these springs are as follows: height, 18 inches; - <span class="smcap">a</span> and <span class="smcap">b</span> each carry weights equal to 320 grammes, and - they make one vibration per second; <span class="smcap">c</span> has a weight of 199 - grammes, and makes 0·75 vibrations per second.</p> - - <p><em>First Experiment.</em>—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 - <span class="smcap">a</span> and <span class="smcap">b</span> may be - <span class="pagenum" id="Page_119">119</span> made to oscillate violently whilst - <span class="smcap">c</span> remains still; or <i xml:lang="la">vice versâ</i>, <span class="smcap">c</span> may be caused to - oscillate whilst <span class="smcap">a</span> and <span class="smcap">b</span> remain still. In the one - case the period of shaking will have been synchronous with the natural - period of <span class="smcap">a</span> and <span class="smcap">b</span>, whilst in the latter it will have - been synchronous with that of <span class="smcap">c</span>. 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.</p> - - <p><em>Second Experiment.</em>—Bind <span class="smcap">a</span> and <span class="smcap">b</span> 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, <span class="smcap">a</span> and <span class="smcap">b</span> 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.</p> - - <p><em>Third Experiment.</em>—Join <span class="smcap">a</span> and <span class="smcap">c</span>, or <span class="smcap">b</span> - and <span class="smcap">c</span> 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 <span class="smcap">a</span> and <span class="smcap">b</span>, or with that of - <span class="smcap">c</span>, the paper will be suddenly snapped.</p> - - <p>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 - <span class="pagenum" id="Page_120">120</span> - earthquake of February 1880 by the knocking over of chimneys. The - particular case of the chimneys is, however, better illustrated by the - next experiment.</p> - - <p><em>Fourth Experiment.</em>—Take a little block of wood three-quarters of - an inch square and about one inch high, and place it on the top of - <span class="smcap">a</span>, <span class="smcap">b</span>, or <span class="smcap">c</span>. 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.</p> - - <p>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.</p> - - <p><em>Fifth Experiment.</em>—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.</p> - - <p><em>Sixth Experiment.</em>—Bind <span class="smcap">a</span> and <span class="smcap">b</span> 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.</p> - - <p><em>Seventh Experiment.</em>—Bind <span class="smcap">a</span> and <span class="smcap">c</span>, or <span class="smcap">b</span> - and <span class="smcap">c</span> 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.</p> - - <p><em>Eighth Experiment.</em>—Take two pencils or pieces of glass tube and place - them under the board <span class="smcap">d e</span>. If the table <span class="smcap">F G</span> be now - shaken in the direction <span class="smcap">d e</span>, it will be found that the springs - will not vibrate.</p> - - <p>In a similar manner if a house or portion of a house were carried on - balls or rollers, as has already been suggested, - <span class="pagenum" id="Page_121">121</span> it would seem that - the house might be saved from much vibration.</p> - - <p><em>Ninth Experiment.</em>—Set any of the springs in violent vibration by - gently shaking <span class="smcap">d e</span> 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.</p> - - <p>This shows, that if a house is in a state of vibration the strain at - the foundations must be very great.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_VII"> - <span class="pagenum" id="Page_122">122</span> - <h2>CHAPTER VII.<br /> - <span class="subhead">EFFECTS PRODUCED UPON BUILDINGS (continued).</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>Types of buildings used in earthquake countries.</em>—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.</p> - - <p>The larger buildings, such as temples and pagodas, are - <span class="pagenum" id="Page_123">123</span> 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.</p> - - <p>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.</p> - - <p>Speaking of Japanese buildings, Mr. R. H. Brunton, who has devoted - especial attention to them says that,<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a> ‘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 <span class="pagenum" id="Page_124">124</span> - 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 - <span class="pagenum" id="Page_125">125</span> 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.</p> - - <p>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.</p> - - <p>The ordinary houses in Italy, though built of stone and - <span class="pagenum" id="Page_126">126</span> 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.</p> - - <p>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.</p> - - <p>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<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a>: ‘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.’</p> - - <p>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.</p> - - <p>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, - <span class="pagenum" id="Page_127">127</span> - 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.’</p> - - <p>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.<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a></p> - - <p>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.<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a></p> - - <p>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 <i xml:lang="la">per capita</i> 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.’<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a></p> - - <p><span class="pagenum" id="Page_128">128</span></p> - - <p><em>Typical houses for earthquake countries.</em>—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 - <span class="pagenum" id="Page_129">129</span> - 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.</p> - - <p>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 <i xml:lang="fr">papier mâché</i> and <i xml:lang="fr">carton-pierre</i>, the elastic yielding of - which is comparatively great.<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a> 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.</p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_130">130</span> - certainly, if applied in houses of the type described, would be - valuable.</p> - - <p>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.</p> - - <p><em>Destruction due to the nature of the underlying rocks.</em>—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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_131">131</span> - the observations which have been made - by various writers on this subject appear to point in a contrary - direction, I give the following examples:—</p> - - <p>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.’</p> - - <p>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.</p> - - <p>At Talcahuano, in 1835, the only houses which escaped were the - buildings standing on rocky ground; all those resting on sandy soil - were destroyed.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_132">132</span></p> - - <p>‘Humboldt observed that the Cordilleras, composed of gneiss and - mica-slate, and the country immediately at their foot, were more shaken - than the plains.’<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a></p> - - <p>‘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.’</p> - - <p>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.’</p> - - <p>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.</p> - - <p>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.’<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a></p> - - <p>After reading the above, we see that the probable reason why, in - several cases, beds of soft materials have not made - <span class="pagenum" id="Page_133">133</span>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.</p> - - <p>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.<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a></p> - - <p>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.<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a></p> - - <p>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.’<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a></p> - - <p>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 <span class="pagenum" id="Page_134">134</span> - likely to be shaken down, as in most of the previous examples.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_135">135</span> - 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.</p> - - <p>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.</p> - - <p><em>The swing of mountains.</em>—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 - <span class="pagenum" id="Page_136">136</span> 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.</p> - - <p><em>Want of support on the faces of hills.</em>—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,<span class="pagenum" id="Page_137">137</span> - the effect is to transmit a push - to the first boy, who, being unsupported, flies forward.</p> - - <p>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.’</p> - - <p><em>Earthquake shadows.</em>—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 <em>shadow</em> 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 - <span class="pagenum" id="Page_138">138</span> 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.</p> - - <div class="figcenter"> - <img src="images/i_p138.jpg" width="700" height="247" alt="" /> - <div class="caption"><span class="smcap">Fig. 27.</span>—Hypothetical section at Yokohama.</div> - </div> - - <p><em>Destruction due to the interference of waves.</em>—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<span class="pagenum" id="Page_139">139</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Under the settlement it is probable that all the reflections which took - place would be single. Thus wave fronts like <span class="smcap">a</span><sub>1</sub> advancing - in a direction parallel to the line <i>a</i><sub>1</sub>; would be reflected in - a direction <i>a</i><sub>2</sub> and give rise to a series of reflected waves - <span class="smcap">a</span><sub>2</sub>. These are shown by thicker lines. Similarly all the - <span class="pagenum" id="Page_140">140</span>neighbouring waves to the right and left of - <span class="smcap">a</span><sub>1</sub> 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 <span class="smcap">b</span><sub>1</sub> - (which is parallel to <span class="smcap">a</span><sub>1</sub> of the first supposition), - advancing in a direction parallel to <i>b</i><sub>1</sub> might be reflected - along the line <i>b</i><sub>2</sub> giving rise to waves like <span class="smcap">b</span><sub>2</sub>, - which in turn might be reflected along <i>b</i><sub>3</sub> giving rise to waves - like <span class="smcap">b</span><sub>3</sub>. 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.</p> - - <p>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.</p> - - <p><em>Earthquake bridges.</em>—In certain parts of South<span class="pagenum" id="Page_141">141</span> 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.’</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_142">142</span> head of a - monstrous catfish (Namadzu), which by its writhings causes the shakings - so often felt in this part of the world.<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a></p> - - <p>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.’<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a></p> - - <p>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.</p> - - <p><em>General examples of earthquake effects.</em>—The following examples of - earthquake effects are drawn from Mallet’s account of the Neapolitan - earthquake of 1857.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_143">143</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Protection of buildings.</em>—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<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a> 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. - <span class="pagenum" id="Page_144">144</span> 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.</p> - - <p>Quito is said to receive protection from the numerous cañons in the - neighbourhood, whilst Lactacunga, fifteen miles distant, has often been - destroyed.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>General conclusions.</em>—The following are a few of the more important - results which may be drawn from the preceding chapter:—</p> - - <p>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.</p> - - <p>2. A wide open plain is less likely to be disturbed than a position on - a hill.</p> - - <p>3. Avoid loose materials resting on harder strata.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_145">145</span></p> - - <p>5. Avoid the edges of scarps or bluffs, both above and below.</p> - - <p>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.</p> - - <p>7. Place lintels over flat arches of brick or stone.</p> - - <p>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.</p> - - <p>9. Let all portions of a building have their natural periods of - vibration nearly equal.</p> - - <p>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.</p> - - <p>11. Avoid heavy topped roofs and chimneys. If the foundations were free - the roof might be heavy.</p> - - <p>12. In brick or stone work use good cement.</p> - - <p>13. Let archways curve into their abutments.</p> - - <p>14. Let roofs have a low pitch, and the tiles, especially those upon - the ridges, be well secured.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_VIII"> - <span class="pagenum" id="Page_146">146</span> - <h2>CHAPTER VIII.<br /> - <span class="subhead">EFFECTS OF EARTHQUAKES ON LAND.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>Cracks and fissures formed in the ground.</em>—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.<a - id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a> - Besides these large cracks, many smaller ones of one or two feet in - breadth and of great length were formed. In - <span class="pagenum" id="Page_147">147</span>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.</p> - - <p>During an earthquake large cracks may suddenly open and shut.</p> - - <p>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.<a id="FNanchor_39" href="#Footnote_39" - class="fnanchor">[39]</a></p> - - <p>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.</p> - - <p>Subsequently the child was excavated, and its body, which was found a - short distance below the surface, was completely crushed.<a id="FNanchor_40" - href="#Footnote_40" class="fnanchor">[40]</a></p> - - <p><span class="pagenum" id="Page_148">148</span></p> - - <p>At the time of the Riobamba earthquake, not only were men engulfed, - but animals, like mules, also sank into the fissures which were formed.</p> - - <p>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.<a id="FNanchor_41" href="#Footnote_41" - class="fnanchor">[41]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Materials discharged from fissures.</em>—Together with the opening of - cracks in the earth it often has happened that water, mud, vapours, - gases, and other materials, have been ejected.</p> - - <p>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.</p> - - <p>From the fissures which were formed in 1692 at the time of the - earthquakes in Sicily, water issued which in some instances was - salt.<a id="FNanchor_42" href="#Footnote_42" class="fnanchor">[42]</a></p> - - <p>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 - <span class="pagenum" id="Page_149">149</span> - 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.<a id="FNanchor_43" - href="#Footnote_43" class="fnanchor">[43]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>At the time of the Jamaica earthquake men who had fallen into crevices - were in some cases thrown out again by issuing water.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_150">150</span> - was so powerful that it caused a general sickness - which swept away about 3,000 persons.<a id="FNanchor_44" href="#Footnote_44" - class="fnanchor">[44]</a></p> - - <p>From the fissures formed at Concepcion in 1835, water, which was black - and fœtid, issued.<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">[45]</a></p> - - <p>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.</p> - - <p>At one house the stink of sulphur accompanying the earthquake was so - great that the family could not bear to remain in doors.<a id="FNanchor_46" - href="#Footnote_46" class="fnanchor">[46]</a></p> - - <p>Emanations of gas sometimes appear to have burst out from submarine - sources.</p> - - <p>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.<a id="FNanchor_47" - href="#Footnote_47" class="fnanchor">[47]</a> With the - smell, flames have sometimes been observed, as, for instance, at the - time of the Lisbon earthquake.</p> - - <p>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.</p> - - <p>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.</p> - - <p>It has been suggested that flames seen above fissures - <span class="pagenum" id="Page_151">151</span>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.<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">[48]</a></p> - - <p>In addition to flames lights appear often to have been observed, the - origin of which cannot be easily explained.</p> - - <p>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.<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">[49]</a></p> - - <p><em>Explanation of fissure phenomena.</em>—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).</p> - - <p>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 <span class="pagenum" id="Page_152">152</span> - strata, which, before the earthquake, - by their continuity prevented the rising of subterranean water under - hydrostatic pressure.<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">[50]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_153">153</span></p> - - <p>From this we see that liquids may rise far beyond the level due to - hydrostatic pressure.<a id="FNanchor_51" href="#Footnote_51" class="fnanchor">[51]</a></p> - - <p>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.<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">[52]</a></p> - - <p>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.</p> - - <p>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, <span class="pagenum" id="Page_154">154</span> - 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.<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">[53]</a> 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.</p> - - <p><em>Disturbances in lakes.</em>—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.<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">[54]</a></p> - - <p><em>Disturbances in rivers.</em>—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 - <span class="pagenum" id="Page_155">155</span>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.</p> - - <p>After the earthquake of Belluno (June 29, 1873), the torrent Tesa, - which is ordinarily limpid, became very muddy.<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">[55]</a> 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.</p> - - <p>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.<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">[56]</a></p> - - <p><span class="pagenum" id="Page_156">156</span></p> - - <p>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.<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">[57]</a></p> - - <p>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.<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">[58]</a></p> - - <p>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.</p> - - <p>In these rivers similar phenomena have been observed in previous years.</p> - - <p>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.<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">[59]</a></p> - - <p><em>Effects produced in springs, wells, fumaroles, &c.</em>—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.</p> - - <p>Sometimes springs have been dried up, whilst at other times new springs - have been formed.</p> - - <p>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.<a id="FNanchor_60" href="#Footnote_60" class="fnanchor">[60]</a></p> - - <p>At and near Lisbon, in 1755, some fountains became - <span class="pagenum" id="Page_157">157</span>muddy, others - decreased, others increased, and others dried up. At Montreux, Aigle, - and other places, springs became turbid.</p> - - <p>The baths at Toplitz, in Bohemia, which were discovered in - <span class="smcap">a.d.</span> 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.<a id="FNanchor_61" href="#Footnote_61" class="fnanchor">[61]</a></p> - - <p>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.</p> - - <p>At the time of the Belluno earthquake (June 29, 1873), a hot spring, La - Vena d’Oro, suddenly became red.<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">[62]</a></p> - - <p>The following examples of like changes are taken from the writings of - Fuchs.<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">[63]</a></p> - - <p>In 1738 the hot springs of St. Euphema rose considerably in their - temperature.</p> - - <p>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.</p> - - <p>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.</p> - - <p>During the earthquake of 1835 in Chili, the springs - <span class="pagenum" id="Page_158">158</span>of Cauquenes fell - from 118° to 92° F. Subsequently, however, they again rose.</p> - - <p>Fumaroles are similarly disturbed. Thus, at the time of the earthquakes - of Martinique (September, 1875), the fumaroles there showed an abnormal - activity.<a id="FNanchor_64" href="#Footnote_64" class="fnanchor">[64]</a></p> - - <p>Wells often appear to be acted upon in the same manner as springs.</p> - - <p>At the time of the California earthquake (April, 1855), the level of - the water in certain wells was raised ten to twelve feet.</p> - - <p>A consequence of the earthquake at Neufchâtel, in 1749, was to fill - some of the wells with mud.<a id="FNanchor_65" href="#Footnote_65" class="fnanchor">[65]</a> At Constantinople, on September 2, - 1754, wells became dry.<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">[66]</a></p> - - <p><em>Explanation of the above phenomena.</em>—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.</p> - - <p>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 <span class="pagenum" id="Page_159">159</span> - 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.<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">[67]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>The sudden elevations, depressions, or lateral shifting of large tracts - of country at the time of destructive earthquakes - <span class="pagenum" id="Page_160">160</span> 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.</p> - - <p><em>Movements on coast lines and level tracts.</em>—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.<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">[68]</a></p> - - <p>By the earthquake of 1839, the island of Lemus, in the Chonos - Archipelago, was suddenly elevated eight feet.<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">[69]</a></p> - - <p>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.’<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">[70]</a> 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.</p> - - <p>Another remarkable example of sudden movement in - <span class="pagenum" id="Page_161">161</span>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.<a id="FNanchor_71" href="#Footnote_71" class="fnanchor">[71]</a></p> - - <p>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.<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">[72]</a></p> - - <p>Other examples of these permanent dislocations of strata are to be - found in almost every text-book on geology.</p> - - <p><em>Geological changes produced.</em>—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.<a id="FNanchor_73" href="#Footnote_73" class="fnanchor">[73]</a></p> - - <p><span class="pagenum" id="Page_162">162</span></p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Reason of these movements.</em>—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.</p> - - <p>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.</p> - - <p>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).</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_IX"> - <span class="pagenum" id="Page_163">163</span> - <h2>CHAPTER IX.<br /> - <span class="subhead">DISTURBANCES IN THE OCEAN.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>Sea vibrations.</em>—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.’</p> - - <p><span class="pagenum" id="Page_164">164</span></p> - - <p>In none of the cases here quoted was any disturbance of the water - observed.</p> - - <p>The great earthquake of Lisbon was felt by vessels on the Atlantic, - fifty miles away from shore.</p> - - <p>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.</p> - - <p><em>Cause of vibratory blows.</em>—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.</p> - - <p>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.<a id="FNanchor_74" href="#Footnote_74" class="fnanchor">[74]</a></p> - - <p>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 - <span class="pagenum" id="Page_165">165</span> 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.</p> - - <p><em>Sea waves.</em>—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.</p> - - <p>Again, at the earthquake in St. Thomas, in 1868, it is said that the - water receded shortly <em>before</em> the first shock. When it returned, after - the second shock, it was sufficient to throw the U.S. ship ‘Monagahela’ - high and dry.<a id="FNanchor_75" href="#Footnote_75" class="fnanchor">[75]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_166">166</span> - 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.</p> - - <p>At the time of the Jamaica earthquake (1692) the sea drew back for a - distance of a mile.</p> - - <p>In South America sea waves are common accompaniments of large - earthquakes, and they are regarded with more fear than the actual - earthquakes.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_76" href="#Footnote_76" class="fnanchor">[76]</a></p> - - <p>When Callao and Lima were destroyed, in 1746, the - <span class="pagenum" id="Page_167">167</span>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.<a id="FNanchor_77" href="#Footnote_77" class="fnanchor">[77]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>There are, however, cases known where the sea has returned as gradually - as it went out. Thus, on December - <span class="pagenum" id="Page_168">168</span>4, 1854, when Acapulco was - destroyed, the sea is said to have returned as gently as it went out.</p> - - <p>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.</p> - - <p>Sometimes, as in the present example, the first movement in the - waters is that of an incoming wave. In many - <span class="pagenum" id="Page_169">169</span> 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.</p> - - <p>The distance to which these sea waves have extended has usually been - exceedingly great.</p> - - <div class="figcenter"> - <img src="images/i_p169.jpg" width="692" height="517" alt="" /> - <div class="caption"><span class="smcap">Fig. 28.</span>—Record of Tide Gauge at Port Point, - San Francisco. Showing Earthquake Waves of May 1877.</div> - </div> - - <p>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 - <span class="pagenum" id="Page_170">170</span> 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.</p> - - <p>For example, the great earthquake of Lisbon propagated waves to the - coasts of America, taking on their journey nine and a half hours.</p> - - <p><em>Sea waves without earthquakes.</em>—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.</p> - - <p>Several examples of these are given by Mallet. Thus, at 10 - <span class="smcap">a.m.</span> 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<span class="pagenum" id="Page_171">171</span> - that a vessel entering the harbour was alternately afloat and aground.</p> - - <p>In 1761, on July 17, a similar phenomena was observed at the same place.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Cause of sea waves.</em>—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 - <span class="pagenum" id="Page_172">172</span> - 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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_78" href="#Footnote_78" class="fnanchor">[78]</a></p> - - <p>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.</p> - - <p>If, however, the submarine upheaval took place with great rapidity, say - by the sudden evolution of a large - <span class="pagenum" id="Page_173">173</span>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_174">174</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_79" href="#Footnote_79" class="fnanchor">[79]</a> 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.</p> - - <p>The complete phenomena which may accompany a violent submarine - disturbance are as follows:—</p> - - <p>By the initial impulse of explosion or lifting of the ground, a ‘great - sea wave’ is generated, which travels - <span class="pagenum" id="Page_175">175</span>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.</p> - - <p>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.</p> - - <p><em>Phenomena difficult of explanation.</em>—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.<a id="FNanchor_80" href="#Footnote_80" class="fnanchor">[80]</a> Out of 1,098 - earthquakes catalogued by Perrey for the west coast of South America, - only nineteen are said to have been accompanied - <span class="pagenum" id="Page_176">176</span>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.<a id="FNanchor_81" href="#Footnote_81" class="fnanchor">[81]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_177">177</span></p> - - <p>These sudden alterations in the levels of coast lines have already - been referred to.</p> - - <p>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 - <span class="smcap">a.m.</span>, 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?</p> - - <p>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.</p> - - <p><em>Velocity of propagation of sea waves, and depth of the ocean.</em>—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.</p> - - <p>If <i>v</i> is the velocity of the wave, and <i>h</i> the depth of the trough, - this relation may be expressed as follows:—</p> - - <div class="center"> - <i>h</i> = <span class="frac"><sup><i>v</i><sup>2</sup></sup><span>/</span><sub><i>g</i></sub></span> - or <i>h</i> = <span class="x200b">(</span><span class="frac"><sup><i>v</i></sup><span>/</span><sub><i>k</i></sub></span><span class="x200b">)</span><sup>2</sup> - </div> - - <p class="center"> - <span class="pagenum" id="Page_178">178</span> - Where <i>g</i> = 32·19 and <i>k</i> = 5·671. - </p> - - <p>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.</p> - - <p>The apparent difference is in the average value assigned to the - constant.</p> - - <p>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 <i>h</i> - by some small fraction of itself. We might also make allowance for - the different values of <i>g</i>, 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_179">179</span> - 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.</p> - - <h3><span class="smcap">Examples of Calculations on Sea Waves.</span></h3> - - <p>1. <em>The wave of 1854.</em>—This wave originated near Japan, and it was - recorded on tide gauges at San Francisco, San Diego, and Astoria.</p> - - <p>On December 23, at 9.15. <span class="smcap">a.m.</span>, 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 <span class="smcap">p.m.</span> this agitation decreased, and at 3 - <span class="smcap">p.m.</span> it was comparatively slow. Altogether there were five - large waves.</p> - - <p>On December 23 and 25, unusual waves were recorded upon the - self-registering tide gauges at San Francisco, San Diego, and Astoria.</p> - - <p>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.</p> - - <p>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.</p> - - <p>The San Francisco waves appear to indicate a recurrence of the same - phenomena.</p> - - <p>The record at San Diego shows what was probably the effect of a - series of impulses, the heights increasing to the - <span class="pagenum" id="Page_180">180</span> third wave, then - diminishing, then once more renewed, after which it died away.</p> - - <p>The result of calculations based on these data were:—</p> - - <table class="collapse" summary="Simoda sea waves"> - <tr> - <th class="ball"> </th> - <th class="ball">Distance<br />geographical<br />miles</th> - <th colspan="2" class="ball">Time of<br />transmission</th> - <th class="ball">Velocity in<br />feet per sec.</th> - <th class="ball">Depth of ocean<br />in fathoms</th> - </tr> - <tr> - <th class="bl"> </th> - <th class="bl"> </th> - <th class="bl">h.</th> - <th class="br">m.</th> - <th class="br"> </th> - <th class="br"> </th> - </tr> - <tr> - <td class="tdl bl">Simoda to San Diego</td> - <td class="tdc bl">4917</td> - <td class="tdc bl">12</td> - <td class="tdc br">13</td> - <td class="tdc br">545</td> - <td class="tdc br">2100</td> - </tr> - <tr class="bb"> - <td class="tdl bl">Simoda to San Francisco</td> - <td class="tdc bl">4527</td> - <td class="tdc bl">12</td> - <td class="tdc br">39</td> - <td class="tdc br">528</td> - <td class="tdc br">2500 or 2230</td> - </tr> - </table> - - <p>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.<a id="FNanchor_82" href="#Footnote_82" class="fnanchor">[82]</a></p> - - <p><em>The wave of 1868.</em>—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.</p> - - <p>In Japan, at Hakodate, it was observed by Captain T. Blakiston, R.A., - who very kindly gave me the following account:</p> - - <p>On August 15, at 10.30 <span class="smcap">a.m.</span>, a series of bores or tidal waves - commenced, and lasted until 3 <span class="smcap">p.m.</span> 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 <span class="smcap">p.m.</span> 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 <span class="pagenum" id="Page_181">181</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>The wave is assumed to have originated near Arica.</p> - - <table class="collapse" summary="Hochstetter sea waves"> - <tr> - <th class="ball"> </th> - <th class="ball">Distance<br />sea miles<br />from Arica</th> - <th colspan="2" class="ball">Time<br />taken by wave</th> - <th class="ball">Velocity<br />in feet per<br />second</th> - <th class="ball">Depth of<br />ocean in feet</th> - </tr> - <tr> - <th class="bl"> </th> - <th class="bl"> </th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="br"> </th> - <th class="tdr br"> </th> - </tr> - <tr> - <td class="tdl bl">Valdivia</td> - <td class="tdc bl">1,420</td> - <td class="tdr bl"><div>5</div></td> - <td class="tdr br"><div>0</div></td> - <td class="tdc br">479</td> - <td class="tdr br"><div>7,140</div></td> - </tr> - <tr> - <td class="tdl bl">Chatham Islands</td> - <td class="tdc bl">5,520</td> - <td class="tdr bl"><div>15</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdc br">608</td> - <td class="tdr br"><div>11,472</div></td> - </tr> - <tr> - <td class="tdl bl">Lyttleton</td> - <td class="tdc bl">6,120</td> - <td class="tdr bl"><div>19</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdc br">533</td> - <td class="tdr br"><div>8,838</div></td> - </tr> - <tr> - <td class="tdl bl">Newcastle</td> - <td class="tdc bl">7,380</td> - <td class="tdr bl"><div>22</div></td> - <td class="tdr br"><div>28</div></td> - <td class="tdc br">538</td> - <td class="tdr br"><div>9,006</div></td> - </tr> - <tr> - <td class="tdl bl">Apia (Samoa)</td> - <td class="tdc bl">5,760</td> - <td class="tdr bl"><div>16</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdc br">604</td> - <td class="tdr br"><div>11,346</div></td> - </tr> - <tr> - <td class="tdl bl">Rapa</td> - <td class="tdc bl">4,057</td> - <td class="tdr bl"><div>11</div></td> - <td class="tdr br"><div>11</div></td> - <td class="tdc br">611</td> - <td class="tdr br"><div>11,598</div></td> - </tr> - <tr> - <td class="tdl bl">Hilo</td> - <td class="tdc bl">5,400</td> - <td class="tdr bl"><div>14</div></td> - <td class="tdr br"><div>25</div></td> - <td class="tdc br">555</td> - <td class="tdr br"><div>9,568</div></td> - </tr> - <tr class="bb"> - <td class="tdl bl">Honolulu</td> - <td class="tdc bl">5,580</td> - <td class="tdr bl"><div>12</div></td> - <td class="tdr br"><div>37</div></td> - <td class="tdc br">746</td> - <td class="tdr br"><div>17,292</div></td> - </tr> - </table> - - <p>Calculations on the same disturbance are also given by J. E. - Hilgard.<a id="FNanchor_83" href="#Footnote_83" class="fnanchor">[83]</a></p> - - <p>Assuming the origin of the wave to have been at Arica, his results are - as follows:</p> - - <p><span class="pagenum" id="Page_182">182</span></p> - - <table class="collapse" summary="Hilgard sea waves"> - <tr> - <th class="ball"> </th> - <th class="ball">Distance<br />from Africa</th> - <th colspan="2" class="ball">Time of<br />transmission</th> - <th class="ball">Nautical miles<br />per hour</th> - <th class="ball">Mean depth<br />of ocean</th> - </tr> - <tr> - <th class="bl"> </th> - <th class="bl">miles</th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="br"> </th> - <th class="br">feet</th> - </tr> - <tr> - <td class="tdl bl">San Diego</td> - <td class="tdc bl">4,030</td> - <td class="tdr bl"><div>10</div></td> - <td class="tdr br"><div>55</div></td> - <td class="tdc br">369</td> - <td class="tdr br"><div>12,100</div></td> - </tr> - <tr> - <td class="tdl bl">Fort Point</td> - <td class="tdc bl">4,480</td> - <td class="tdr bl"><div>12</div></td> - <td class="tdr br"><div>56</div></td> - <td class="tdc br">348</td> - <td class="tdr br"><div>10,800</div></td> - </tr> - <tr> - <td class="tdl bl">Astoria</td> - <td class="tdc bl">5,000</td> - <td class="tdr bl"><div>18</div></td> - <td class="tdr br"><div>51</div></td> - <td class="tdc br">265</td> - <td class="tdr br"><div>6,200</div></td> - </tr> - <tr> - <td class="tdl bl">Kodiak</td> - <td class="tdc bl">6,200</td> - <td class="tdr bl"><div>22</div></td> - <td class="tdr br"><div>00</div></td> - <td class="tdc br">282</td> - <td class="tdr br"><div>7,000</div></td> - </tr> - <tr> - <td class="tdl bl">Rapa</td> - <td class="tdc bl">4,057</td> - <td class="tdr bl"><div>10</div></td> - <td class="tdr br"><div>54</div></td> - <td class="tdc br">372</td> - <td class="tdr br"><div>12,200</div></td> - </tr> - <tr> - <td class="tdl bl">Chatham Islands</td> - <td class="tdc bl">5,520</td> - <td class="tdr bl"><div>15</div></td> - <td class="tdr br"><div>01</div></td> - <td class="tdc br">368</td> - <td class="tdr br"><div>12,100</div></td> - </tr> - <tr> - <td class="tdl bl">Hawaii</td> - <td class="tdc bl">5,460</td> - <td class="tdr bl"><div>14</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdc br">385</td> - <td class="tdr br"><div>13,200</div></td> - </tr> - <tr> - <td class="tdl bl">Honolulu</td> - <td class="tdc bl">5,580</td> - <td class="tdr bl"><div>12</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdc br">454</td> - <td class="tdr br"><div>18,500</div></td> - </tr> - <tr> - <td class="tdl bl">Samoa</td> - <td class="tdc bl">5,760</td> - <td class="tdr bl"><div>15</div></td> - <td class="tdr br"><div>38</div></td> - <td class="tdc br">368</td> - <td class="tdr br"><div>12,100</div></td> - </tr> - <tr> - <td class="tdl bl">Lyttelton</td> - <td class="tdc bl">6,120</td> - <td class="tdr bl"><div>19</div></td> - <td class="tdr br"><div>01</div></td> - <td class="tdc br">322</td> - <td class="tdr br"><div>9,200</div></td> - </tr> - <tr> - <td class="tdl bl">Newcastle</td> - <td class="tdc bl">9,380</td> - <td class="tdr bl"><div>22</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdc br">332</td> - <td class="tdr br"><div>9,800</div></td> - </tr> - <tr class="bb"> - <td class="tdl bl">Sydney</td> - <td class="tdc bl">7,440</td> - <td class="tdr bl"><div>23</div></td> - <td class="tdr br"><div>41</div></td> - <td class="tdc br">314</td> - <td class="tdr br"><div>8,800</div></td> - </tr> - </table> - - <p><em>The wave of 1877.</em>—Two sets of calculations have been made upon the - wave of 1877 by Dr. E. Geinitz of Rostock.<a id="FNanchor_84" href="#Footnote_84" class="fnanchor">[84]</a></p> - - <p>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.</p> - - <table class="collapse" summary="Geinitz sea waves"> - <tr> - <th class="ball">Observation stations</th> - <th class="ball">Distance<br />from Iquique<br />geol. miles</th> - <th colspan="3" class="ball">Arrival of wave</th> - <th colspan="2" class="ball">Time taken<br />by wave</th> - <th class="ball">Velocity in<br />feet per<br />second</th> - <th class="ball">Mean depth<br />of ocean<br />in fathoms</th> - </tr> - <tr> - <th class="bl"> </th> - <th class="bl"> </th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr"><div>m.</div></th> - <th class="br"> </th> - <th class="tdr"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="br"> </th> - <th class="br"> </th> - </tr> - <tr> - <td class="tdl bl">Taiohāc (Marquesa Islands)</td> - <td class="tdc bl">4,086</td> - <td class="tdr bl"><div>8</div></td> - <td class="tdr"><div>40</div></td> - <td class="tdc br"><span class="smcap">a.m.</span></td> - <td class="tdr"><div>12</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdc br">563·8</td> - <td class="tdc br">1,647</td> - </tr> - <tr> - <td class="tdl bl">Apia (Samoa)</td> - <td class="tdc bl">5,740</td> - <td class="tdr bl"><div>12</div></td> - <td class="tdr"><div>0</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>15</div></td> - <td class="tdr br"><div>30</div></td> - <td class="tdc br">610·4</td> - <td class="tdc br">1,930</td> - </tr> - <tr> - <td class="tdl bl">Hilo (Sandwich Islands)</td> - <td class="tdc bl">5,526</td> - <td class="tdr bl"><div>10</div></td> - <td class="tdr "><div>24</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>0</div></td> - <td class="tdc br">667·9</td> - <td class="tdc br">2,310</td> - </tr> - <tr> - <td class="tdl bl">Kahuliu „</td> - <td class="tdc bl">5,628</td> - <td class="tdr bl"><div>10</div></td> - <td class="tdr"><div>30</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdc br">675·2</td> - <td class="tdc br">2,361</td> - </tr> - <tr> - <td class="tdl bl">Honolulu „</td> - <td class="tdc bl">5,712</td> - <td class="tdr bl"><div>10</div></td> - <td class="tdr"><div>50</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>25</div></td> - <td class="tdc br">669·7</td> - <td class="tdc br">2,319</td> - </tr> - <tr> - <td class="tdl bl">Wellington (New Zealand)</td> - <td class="tdc bl">5,657</td> - <td class="tdr bl"><div>2</div></td> - <td class="tdr"><div>40</div></td> - <td class="tdc br"><span class="smcap">p.m.</span></td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdc br">524·2</td> - <td class="tdc br">1,430</td> - </tr> - <tr> - <td class="tdl bl">Lyttelton „</td> - <td class="tdc bl">5,641</td> - <td class="tdr bl"><div>2</div></td> - <td class="tdr"><div>48</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>23</div></td> - <td class="tdc br">519·8</td> - <td class="tdc br">1,400</td> - </tr> - <tr> - <td class="tdl bl">Newcastle (Australia)</td> - <td class="tdc bl">6,800</td> - <td class="tdr bl"><div>2</div></td> - <td class="tdr"><div>32</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>7</div></td> - <td class="tdc br">633·0</td> - <td class="tdc br">2,075</td> - </tr> - <tr> - <td class="tdl bl">Sydney „</td> - <td class="tdc bl">6,782</td> - <td class="tdr bl"><div>2</div></td> - <td class="tdr"><div>35</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdc br">631·4</td> - <td class="tdc br">2,065</td> - </tr> - <tr> - <td class="tdl bl">Kamieshi (Japan)</td> - <td class="tdc bl">8,790</td> - <td class="tdr bl"><div>7</div></td> - <td class="tdr"><div>20</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>22</div></td> - <td class="tdr br"><div>55</div></td> - <td class="tdc br">649·0</td> - <td class="tdc br">2,182</td> - </tr> - <tr> - <td class="tdl bl">Hakodate „</td> - <td class="tdc bl">8,760</td> - <td class="tdr bl"><div>9</div></td> - <td class="tdr"><div>25</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>25</div></td> - <td class="tdr br"><div>0</div></td> - <td class="tdc br">592·5</td> - <td class="tdc br">1,818</td> - </tr> - <tr class="bb"> - <td class="tdl bl">Kadsusa „</td> - <td class="tdc bl">8,939</td> - <td class="tdr bl"><div>9</div></td> - <td class="tdr"><div>50</div></td> - <td class="tdc br">„</td> - <td class="tdr"><div>25</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdc br">604·9</td> - <td class="tdc br">1,895</td> - </tr> - </table> - - <p><span class="pagenum" id="Page_183">183</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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 <em>centrum</em> 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 <span class="smcap">p.m.</span> 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.</p> - - <p><span class="pagenum" id="Page_184">184</span></p> - - <table class="collapse" summary="Geinitz sea waves 2"> - <tr> - <th class="ball"> </th> - <th colspan="3" class="ball">Longitude</th> - <th colspan="3" class="ball">Arrival of<br />wave in<br />Greenwich<br /> mean time</th> - <th colspan="2" class="ball">Time taken<br />by wave</th> - <th class="ball">Distance<br />from the<br />origin<br />in miles<br />(calculated<br />in great<br />circles)</th> - <th class="ball">Velocity<br />in feet<br />per second</th> - <th class="ball">Depth of<br />the ocean<br />in feet</th> - <th class="ball">Height<br />of waves</th> - <th class="ball">Interval<br />between waves<br />in minutes</th> - </tr> - <tr> - <th class="bl"> </th> - <th class="tdr bl"><div>°</div></th> - <th class="tdr"><div>′</div></th> - <th class="tdr br"><div> </div></th> - <th class="tdr bl"><div>day</div></th> - <th class="tdr"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="br"> </th> - <th class="br"> </th> - <th class="br"> </th> - <th class="br"> </th> - <th class="br"> </th> - </tr> - <tr> - <td class="tdl bl">Origin of wave</td> - <td class="tdr bl"><div>71</div></td> - <td class="tdr"><div>5</div></td> - <td class="tdr br"><div>W.</div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>12</div></td> - <td class="tdr br"><div>59</div></td> - <td class="tdr"><div> </div></td> - <td class="tdr br"><div> </div></td> - <td class="br"><div> </div></td> - <td class="br"> </td> - <td class="br"><div> </div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">San Francisco</td> - <td class="tdr bl"><div>122</div></td> - <td class="tdr"><div>32</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>2</div></td> - <td class="tdr br"><div>28</div></td> - <td class="tdr"><div>13</div></td> - <td class="tdr br"><div>29</div></td> - <td class="tdr br"><div>4,578</div></td> - <td class="tdc br">498</td> - <td class="tdr br"><div>7,721</div></td> - <td class="br">9 in.</td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Callao</td> - <td class="tdr bl"><div>77</div></td> - <td class="tdr"><div>15</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>17</div></td> - <td class="tdr br"><div>9</div></td> - <td class="tdr"><div>4</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>658</div></td> - <td class="tdc br">231</td> - <td class="tdr br"><div>1,657</div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Iquique</td> - <td class="tdr bl"><div>70</div></td> - <td class="tdr"><div>14½</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>13</div></td> - <td class="tdr br"><div>21</div></td> - <td class="tdr"><div>0</div></td> - <td class="tdr br"><div>22</div></td> - <td class="tdr br"><div>87</div></td> - <td class="tdc br">348</td> - <td class="tdr br"><div>3,770</div></td> - <td class="br">20 ft.</td> - <td class="br">22</td> - </tr> - <tr> - <td class="tdl bl">Cobija</td> - <td class="tdr bl"><div>70</div></td> - <td class="tdr"><div>21</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>13</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdr"><div>0</div></td> - <td class="tdr br"><div>20</div></td> - <td class="tdr br"><div>80</div></td> - <td class="tdc br">352</td> - <td class="tdr br"><div>3,857</div></td> - <td class="br">30 „</td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Mejillones</td> - <td class="tdr bl"><div>70</div></td> - <td class="tdr"><div>35</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>13</div></td> - <td class="tdr br"><div>27</div></td> - <td class="tdr"><div>0</div></td> - <td class="tdr br"><div>28</div></td> - <td class="tdr br"><div>108</div></td> - <td class="tdc br">339</td> - <td class="tdr br"><div>3,587</div></td> - <td class="br">35 „</td> - <td class="br">15 or 45</td> - </tr> - <tr> - <td class="tdl bl">Chanaral</td> - <td class="tdr bl"><div>71</div></td> - <td class="tdr"><div>34</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>15</div></td> - <td class="tdr br"><div>26</div></td> - <td class="tdr"><div>2</div></td> - <td class="tdr br"><div>27</div></td> - <td class="tdr br"><div>455</div></td> - <td class="tdc br">272</td> - <td class="tdr br"><div>2,309</div></td> - <td class="br"> </td> - <td class="br">10</td> - </tr> - <tr> - <td class="tdl bl">Coquimbo</td> - <td class="tdr bl"><div>71</div></td> - <td class="tdr"><div>24</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>15</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdr"><div>2</div></td> - <td class="tdr br"><div>16</div></td> - <td class="tdr br"><div>508</div></td> - <td class="tdc br">328</td> - <td class="tdr br"><div>3,363</div></td> - <td class="br"> </td> - <td class="br">30</td> - </tr> - <tr> - <td class="tdl bl">Valparaiso</td> - <td class="tdr bl"><div>71</div></td> - <td class="tdr"><div>38</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>16</div></td> - <td class="tdr br"><div>16</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div>17</div></td> - <td class="tdr br"><div>695</div></td> - <td class="tdc br">310</td> - <td class="tdr br"><div>3,000</div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Concepcion</td> - <td class="tdr bl"><div>73</div></td> - <td class="tdr"><div>5</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>9</div></td> - <td class="tdr"><div>16</div></td> - <td class="tdr br"><div>52</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div>53</div></td> - <td class="tdr br"><div>928</div></td> - <td class="tdc br">350</td> - <td class="tdr br"><div>3,824</div></td> - <td class="br"> </td> - <td class="br">12 to 15</td> - </tr> - <tr> - <td class="tdl bl">Honolulu</td> - <td class="tdr bl"><div>157</div></td> - <td class="tdr"><div>55</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div>52</div></td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>53</div></td> - <td class="tdr br"><div>5,694</div></td> - <td class="tdc br">561</td> - <td class="tdr br"><div>9,807</div></td> - <td class="br">34 to 54 ft.</td> - <td class="br">25</td> - </tr> - <tr> - <td class="tdl bl">Hilo</td> - <td class="tdr bl"><div>155</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>5,506</div></td> - <td class="tdc br">563</td> - <td class="tdr br"><div>10,217</div></td> - <td class="br">30 or 8 „</td> - <td class="tdr0 pl0 br"> - <table summary="Geinitz sub-table"> - <tr> - <td class="x200 pl0">{</td> - <td class="tdr0 pr0 pl0"><div>3 or 15<br />18 or 27</div> - </td> - </tr> - </table> - </td> - </tr> - <tr> - <td class="tdl bl">Kahuliu</td> - <td class="tdr bl"><div>156</div></td> - <td class="tdr"><div>43</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div>12</div></td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>13</div></td> - <td class="tdr br"><div>5,611</div></td> - <td class="tdc br">579</td> - <td class="tdr br"><div>10,437</div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Samoa</td> - <td class="tdr bl"><div>171</div></td> - <td class="tdr"><div>41</div></td> - <td class="tdr br"><div>W.</div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>3</div></td> - <td class="tdr br"><div>57</div></td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>58</div></td> - <td class="tdr br"><div>5,773</div></td> - <td class="tdc br">566</td> - <td class="tdr br"><div>9,972</div></td> - <td class="br">12 ft.</td> - <td class="br">10</td> - </tr> - <tr> - <td class="tdl bl">Taurauga</td> - <td class="tdr bl"><div>176</div></td> - <td class="tdr"><div>11</div></td> - <td class="tdr br"><div>E.</div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdr"><div>19</div></td> - <td class="tdr br"><div>16</div></td> - <td class="tdr br"><div>5,615</div></td> - <td class="tdc br">427</td> - <td class="tdr br"><div>5,697</div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Wellington</td> - <td class="tdr bl"><div>174</div></td> - <td class="tdr"><div>30</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>7</div></td> - <td class="tdr br"><div>22</div></td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>23</div></td> - <td class="tdr br"><div>5,574</div></td> - <td class="tdc br">445</td> - <td class="tdr br"><div>6,168</div></td> - <td class="br">11 „</td> - <td class="br">10</td> - </tr> - <tr> - <td class="tdl bl">Akaroa</td> - <td class="tdr bl"><div>172</div></td> - <td class="tdr"><div>59</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>7</div></td> - <td class="tdr br"><div>28</div></td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>29</div></td> - <td class="tdr br"><div>5,542</div></td> - <td class="tdc br">440</td> - <td class="tdr br"><div>6,031</div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Lyttelton</td> - <td class="tdr bl"><div>172</div></td> - <td class="tdr"><div>45</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>7</div></td> - <td class="tdr br"><div>29</div></td> - <td class="tdr"><div>18</div></td> - <td class="tdr br"><div>30</div></td> - <td class="tdr br"><div>5,558</div></td> - <td class="tdc br">441</td> - <td class="tdr br"><div>6,055</div></td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl">Kameishi</td> - <td class="tdr bl"><div>140</div></td> - <td class="tdr"><div>50</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>12</div></td> - <td class="tdr br"><div>37</div></td> - <td class="tdr"><div>23</div></td> - <td class="tdr br"><div>38</div></td> - <td class="tdr br"><div>8,844</div></td> - <td class="tdc br">549</td> - <td class="tdr br"><div>9,378</div></td> - <td class="br">6 „</td> - <td class="br">15</td> - </tr> - <tr class="bb"> - <td class="tdl bl">Hakodate</td> - <td class="tdr bl"><div>140</div></td> - <td class="tdr"><div>50</div></td> - <td class="tdr br"><div> </div></td> - <td class="tdr"><div>10</div></td> - <td class="tdr"><div>14</div></td> - <td class="tdr br"><div>7</div></td> - <td class="tdr"><div>25</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>8,778</div></td> - <td class="tdc br">512</td> - <td class="tdr br"><div>8,169</div></td> - <td class="br">7 „</td> - <td class="br">20</td> - </tr> - </table> - - <p><span class="pagenum" id="Page_185">185</span></p> - - <p>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.<a id="FNanchor_85" href="#Footnote_85" class="fnanchor">[85]</a></p> - - <p><em>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.</em>—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.</p> - - <p>From the origin of the shock to Japan (Kameishi) the line would be as - follows:—</p> - - <table summary="Wave travel depth to Japan"> - <tr> - <td class="tdr"><div>about 7,441</div></td> - <td>miles</td> - <td class="tdr"><div> 15,000</div></td> - <td>feet deep</td> - </tr> - <tr> - <td class="tdr"><div>1,100</div></td> - <td class="tdc">„</td> - <td class="tdr"><div>18,000</div></td> - <td class="tdc">„</td> - </tr> - <tr> - <td class="tdr"><div>160</div></td> - <td class="tdc">„</td> - <td class="tdr"><div>27,000</div></td> - <td class="tdc">„</td> - </tr> - <tr> - <td class="tdr"><div>80</div></td> - <td class="tdc">„</td> - <td class="tdr"><div>12,000</div></td> - <td class="tdc">„</td> - </tr> - <tr> - <td class="tdr"><div>60</div></td> - <td class="tdc">„</td> - <td class="tdr"><div>6,000</div></td> - <td class="tdc">„</td> - </tr> - </table> - - <p>On account of the Tuscarora and Belkap Deeps this would be the most - irregular line over which the wave had to travel.</p> - - <p>From the origin to New Zealand (Wellington) the line would be</p> - - <table summary="Wave travel depth to New Zealand"> - <tr> - <td class="tdr"><div>about 5,274</div></td> - <td>miles</td> - <td class="tdr"><div> 15,000</div></td> - <td>feet deep</td> - </tr> - <tr> - <td class="tdr"><div>„ 300</div></td> - <td class="tdc">„</td> - <td class="tdr"><div>12,000</div></td> - <td class="tdc">„</td> - </tr> - </table> - - <p>From the origin to Samoa the line would be</p> - - <table summary="Wave travel depth to Samoa"> - <tr> - <td class="tdr"><div>about 5,773</div></td> - <td>miles</td> - <td class="tdr"><div> 15,000</div></td> - <td>feet deep</td> - </tr> - </table> - - <p>From the origin to the Sandwich Islands (Honolulu) the line would be</p> - - <p><span class="pagenum" id="Page_186">186</span></p> - - <table summary="Wave travel depth to Honolulu"> - <tr> - <td class="tdr"><div>almost 5,634</div></td> - <td>miles</td> - <td class="tdr"><div> 15,000</div></td> - <td>feet deep</td> - </tr> - <tr> - <td class="tdr"><div>and 60</div></td> - <td class="tdc">„</td> - <td class="tdr"><div> 12,000</div></td> - <td class="tdc">„</td> - </tr> - </table> - - <p>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.</p> - - <p>The actual times taken to travel the distances just referred to were,</p> - - <table summary="Wave travel time"> - <tr> - <td class="tdl">To Japan (Kameishi)</td> - <td class="tdl">23 hr. 38 min.</td> - </tr> - <tr> - <td class="tdl"> „ New Zealand (Wellington)</td> - <td class="tdl">18 „ 23 „</td> - </tr> - <tr> - <td class="tdl"> „ Samoa</td> - <td class="tdl">14 „ 58 „</td> - </tr> - <tr> - <td class="tdl"> „ Sandwich Islands (Honolulu)</td> - <td class="tdl">14 „ 53 „</td> - </tr> - </table> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_X"> - <span class="pagenum" id="Page_187">187</span> - <h2>CHAPTER X.<br /> - <span class="subhead">DETERMINATION OF EARTHQUAKE ORIGINS.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p class="noindent"><span class="smcap">One</span> 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 <em>approximately</em> 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<span class="pagenum" id="Page_188">188</span> - 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.</p> - - <p><em>Approximate determination of origins.</em>—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.</p> - - <p>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.</p> - - <p><em>Earthquake-hunting.</em>—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<span class="pagenum" id="Page_189">189</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <div class="figcenter"> - <span class="pagenum" id="Page_190">190</span> - <img src="images/i_p190.jpg" width="378" height="700" alt="" /> - <div class="caption"><span class="smcap">Fig. 29.</span>—Northern Japan. Mountainous districts - shaded with oblique lines.</div> - </div> - - <p><span class="pagenum" id="Page_191">191</span></p> - - <p>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 <span class="smcap">a</span> or - <span class="smcap">b</span>.</p> - - <p>III. This line indicates the boundary of a group of shocks which - are often experienced in Tokio. These may come in the directions - <span class="smcap">d</span>, <span class="smcap">e</span>, or - <span class="smcap">f</span>. It is probable that some of - them originate to the eastward of Yokohama, on or near to the opposite - peninsula.</p> - - <p>IV. V. and VI. The earthquakes bounded by these lines probably - originate in the directions <span class="smcap">c</span> or - <span class="smcap">d</span>.</p> - - <p>VII. The earthquakes bounded by this line probably come from the - direction <span class="smcap">e</span>.</p> - - <p>VIII. This line gives us the boundary of earthquakes which may come - from the direction <span class="smcap">b</span>.</p> - - <p>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<span class="pagenum" id="Page_192">192</span> - 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.</p> - - <p><em>Determination of earthquake origins from the direction of motion.</em>—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,<span class="pagenum" id="Page_193">193</span> 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<span class="pagenum" id="Page_194">194</span> - 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.</p> - - <p><em>Direction determined from destruction of buildings.</em>—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.</p> - - <p>From a critical examination of the <em>general</em> 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<span class="pagenum" id="Page_195">195</span> - 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.</p> - - <p><em>The rotation of bodies.</em>—It has often been observed that almost all - large earthquakes have caused objects like tombstones, obelisks, - chimneys, &c., to rotate.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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—<span class="pagenum" id="Page_196">196</span>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.</p> - - <p>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.</p> - - <p>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 <em>revolved in the - same direction</em>, namely in a direction opposite to that of the hands of - a watch.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_197">197</span></p> - - <div class="figright"> - <img src="images/i_p197.jpg" width="180" height="228" alt="" /> - <div class="caption"><span class="smcap">Fig. 30.</span></div> - </div> - - <p>If any columnar-like object, for example a prism which the basal - section is represented by <span class="smcap">a b c d</span> (see fig. 30), receives a - shock at right angles to <span class="smcap">b c</span>, there will be a tendency for - the inertia of the body to cause it to overturn on the edge <span class="smcap">b c</span>. - If the shock were at right angles to <span class="smcap">d c</span>, the tendency - would be to overturn on the edge <span class="smcap">d c</span>. If the shock were in - the direction of the diagonal <span class="smcap">c a</span>, the tendency would be to - overturn on the point <span class="smcap">c</span>. Let us, however, now suppose the - impulse to be in some direction like <span class="smcap">e g</span>, where <span class="smcap">g</span> - is the centre of gravity of the body. For simplicity we may imagine - the overturning effect to be an impulse given through <span class="smcap">g</span> in an - opposite direction—that is, in the direction <span class="smcap">g e</span>. This force - will tend to tip or make the body bear heavily on <span class="smcap">c</span>, and at - the same time to whirl round <span class="smcap">c</span> 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 <span class="smcap">e′ g</span>, then, although the turning - would still have been round <span class="smcap">c</span>, the direction would have been - <em>opposite</em> to that of the hands of a watch.</p> - - <p>To put these statements in another form, imagine <span class="smcap">g e′</span> to - be resolved into two components, one of them along <span class="smcap">g c</span> and - the other at right angles, <span class="smcap">g f</span>. Here the component of the - direction <span class="smcap">g c</span> tends to make the body tip on <span class="smcap">c</span>, whilst - the other component along <span class="smcap">g f</span> causes revolution. Similarly - <span class="smcap">g e</span> may be resolved into its two components <span class="smcap">g c</span> and - <span class="smcap">g f′</span>, the latter being the one tending to cause revolution.</p> - - <p>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<span class="pagenum" id="Page_198">198</span> - be any revolution. If we divide our - section <span class="smcap">a b c d</span> 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 <em>positive</em> - 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 <em>negative</em>, 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Determination of direction from instruments.</em>—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 - <span class="pagenum" id="Page_199">199</span> - 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.</p> - - <p>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 <em>inwards</em>, 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.</p> - - <p><em>Determination of earthquake origins by time observations.</em>—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:—</p> - - <p>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 - <span class="pagenum" id="Page_200">200</span> origin of the - disturbance, it is probable that errors of this description are small - and will not make material differences in the general results.</p> - - <p>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.</p> - - <p>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 <em>epicentrum</em> and not in a direct line from - the <em>centrum</em>. 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.</p> - - <p>I. <em>The method of straight lines.</em>—Given a number of pairs of points - <span class="smcap">a</span><sub>0</sub>, <span class="smcap">a</span><sub>1</sub>, <span class="smcap">b</span><sub>0</sub>, <span class="smcap">b</span><sub>1</sub>, - <span class="smcap">c</span><sub>0</sub>, <span class="smcap">c</span><sub>1</sub>, &c., at each of which the shock was - felt simultaneously, to determine the origin.</p> - - <p>Theoretically if we bisect the line which joins <span class="smcap">a</span><sub>0</sub> - and <span class="smcap">a</span><sub>1</sub> by a line at right angles to <span class="smcap">a</span><sub>0</sub>, - <span class="smcap">a</span><sub>1</sub>, and similarly bisect the lines <span class="smcap">b</span><sub>0</sub>, - <span class="smcap">b</span><sub>1</sub>, <span class="smcap">c</span><sub>0</sub>, <span class="smcap">c</span><sub>1</sub>, all these bisecting - lines <i>a</i><sub>0</sub>, <i>a</i><sub>1</sub>, <i>b</i><sub>0</sub>, <i>b</i><sub>1</sub>, <i>c</i><sub>0</sub>, <i>c</i><sub>1</sub>, &c., ought - to intersect in a point, which point will be the <em>epicentrum</em> or the - point above the origin.</p> - - <p>This method will fail, first, if <span class="smcap">a</span><sub>0</sub>, <span class="smcap">a</span><sub>1</sub>, - <span class="smcap">b</span><sub>0</sub>, <span class="smcap">b</span><sub>0</sub>, <span class="smcap">c</span><sub>0</sub>, <span class="smcap">c</span><sub>1</sub> form a - continuous straight line, or if they form a series of parallel lines.</p> - - <p><span class="pagenum" id="Page_201">201</span></p> - - <p>Hopkins gives a method based on a principle similar to the one which - is here employed—namely, given that a shock arrives simultaneously - at <em>three</em> 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.</p> - - <p>II. <em>The method of circles.</em>—Given the times <i>t</i><sub>0</sub>, <i>t</i><sub>1</sub>, <i>t</i><sub>2</sub>, - &c., at which a shock arrived at a number of places <span class="smcap">a</span><sub>0</sub>, - <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, &c., to determine the position from - which the shock originated.</p> - - <p>Suppose <span class="smcap">a</span><sub>0</sub> to be the place which the shock reached first, - and that it reached <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, <span class="smcap">a</span><sub>3</sub>, - &c., successively afterwards.</p> - - <p> - Let <i>t</i><sub>1</sub> - <i>t</i><sub>0</sub> = <i>a</i><br /> - <i>t</i><sub>2</sub> - <i>t</i><sub>0</sub> = <i>b</i><br /> - <i>t</i><sub>3</sub> - <i>t</i><sub>0</sub> = <i>c</i>, &c. - </p> - - <p>With <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, <span class="smcap">a</span><sub>3</sub>, &c. as centres, - describe circles with radii proportional to the known qualities <i>a</i>, - <i>b</i>, <i>c</i>, &c., and also a circle which passes through <span class="smcap">a</span><sub>0</sub> - and touches these circles. The centre of the last circle will be the - <em>epicentrum</em>. The radii proportional to <i>a</i>, <i>b</i>, <i>c</i>, &c. may be - represented by the quantities <i>ax</i>, <i>bx</i>, <i>cx</i>, &c., where <i>x</i> is the - velocity of propagation of the shock.</p> - - <p>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.</p> - - <p>It will be observed that this method is not a direct one, but is one - of trial. If, however, an imaginary case - <span class="pagenum" id="Page_202">202</span> be taken, and three given - points of observation, <span class="smcap">a</span><sub>0</sub>, - <span class="smcap">a</span><sub>1</sub>, <span class="smcap">a</span><sub>2</sub>, - be plotted on a piece of paper, it will be found that it is not a - difficult matter to determine two numbers proportional to <i>a</i> and <i>b</i> - which will allow you to draw two circles so that they may be touched - by a third circle drawn through <span class="smcap">a</span><sub>0</sub>. 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.</p> - - <p>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.</p> - - <table class="collapse" summary="South America sea waves"> - <tr> - <th class="ball"> </th> - <th colspan="2" class="ball">Arrival of<br /> sea wave</th> - <th class="ball">Time after<br />arrival at<br />Huanillos</th> - <th class="ball">Distance<br />at 350 feet<br />per second</th> - </tr> - <tr> - <th class="bl"> </th> - <th class="tdr bl"><div>h.</div></th> - <th class="tdr br"><div>m.</div></th> - <th class="tdr br"><div>minutes</div></th> - <th class="tdr br"><div>miles</div></th> - </tr> - <tr> - <td class="tdl bl">Huanillos</td> - <td class="tdr bl"><div>8</div></td> - <td class="tdr br"><div>30</div></td> - <td class="tdr br"><div>0</div></td> - <td class="tdr br"><div>0</div></td> - </tr> - <tr> - <td class="tdl bl">Tocopilla</td> - <td class="tdr bl"><div>8</div></td> - <td class="tdr br"><div>32</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdr br"><div>8</div></td> - </tr> - <tr> - <td class="tdl bl">Cobija</td> - <td class="tdr bl"><div>8</div></td> - <td class="tdr br"><div>38</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>32</div></td> - </tr> - <tr> - <td class="tdl bl">Iquique</td> - <td class="tdr bl"><div>8</div></td> - <td class="tdr br"><div>40</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>40</div></td> - </tr> - <tr class="bb"> - <td class="tdl bl">Mejillones</td> - <td class="tdr bl"><div>8</div></td> - <td class="tdr br"><div>46</div></td> - <td class="tdr br"><div>16</div></td> - <td class="tdr br"><div>64</div></td> - </tr> - </table> - - <p>The distances marked in the third column are used as radii of the - circles drawn round the places to which they respectively refer.</p> - - <p>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 - <span class="smcap">c</span>.</p> - - <p><span class="pagenum" id="Page_203">203</span></p> - - <p>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.</p> - - <div class="figcenter"> - <img src="images/i_p203.jpg" width="549" height="680" alt="" /> - <div class="caption"><span class="smcap">Fig. 31.</span></div> - </div> - - <p>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<span id="Page_204" class="pagenum">204</span> - 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 <em>by trial</em> 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 <span class="smcap">c</span>, could be - drawn which would practically touch the four circles, and at the same - time would pass through Huanillos.</p> - - <p>III. <em>The method of hyperbolas.</em>—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<span class="pagenum" id="Page_205">205</span> - latter place sixteen minutes or 960 seconds after - it was experienced at the former. Calling these places <span class="smcap">h</span> and - <span class="smcap">m</span> respectively, round <span class="smcap">m</span> 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 <span class="smcap">h</span> and touches the circle - drawn round <span class="smcap">m</span>. Join <span class="smcap">h m</span>, cutting the circle round - <span class="smcap">m</span> in <span class="smcap">y</span>. Bisect <span class="smcap">y h</span> in <span class="smcap">v</span>. It is - evident that <span class="smcap">v</span> is one possible origin for the disturbance. - Next, from <span class="smcap">m</span>, in the direction of <span class="smcap">h</span>, draw any line - <span class="smcap">m z p</span>; join <span class="smcap">z h</span>; bisect <span class="smcap">z h</span> at right angles - by the line <span class="smcap">o p n</span>. Because <span class="smcap">ph</span> = <span class="smcap">pz</span>, it is - evident that <span class="smcap">p</span> is a second possible origin. Proceeding in this - way a series of points lying to the right and left of <span class="smcap">v</span> on - the curve <span class="smcap">r v t</span> may be found, and we may therefore say that - the origin lies somewhere in the curve <span class="smcap">r v t</span>. By increasing - or decreasing our velocity we vary the position of the curve <span class="smcap">r v t</span>, - and, instead of a line on which our origin may be, we obtain a - band. As the assumed velocity increases, the circle round <span class="smcap">m</span> - becomes larger, and has its limit when it passes through <span class="smcap">h</span>, - where the two arms of the curve <span class="smcap">r v t</span> will close together - and form a prolongation of the line <span class="smcap">m y h</span> as the assumed - velocity diminishes. The circle round <span class="smcap">m</span> becomes smaller - until it coincides with the point <span class="smcap">m</span>. At such a moment the - curve <span class="smcap">r v t</span> opens out to form a straight line bisecting - <span class="smcap">m h</span> at right angles. The curve <span class="smcap">r v t</span> is a hyperbola - with a vertex <span class="smcap">v</span> and foci <span class="smcap">h</span> and <span class="smcap">m</span>. Inasmuch - as <span class="smcap">pm</span> - <span class="smcap">ph</span> = 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 <span class="smcap">r′ v′ t′</span> - 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. - <span class="pagenum" id="Page_206">206</span> - 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 <em>centrum</em> rather than - the point above the origin or <em>epicentrum</em>.</p> - - <p>IV. <em>The method of co-ordinates.</em>—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.</p> - - <p>Commencing with the place which was last reached by the shock, call - these places <i>p</i>, <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, and <i>p</i><sub>4</sub>, and let the - times taken to reach these places from the origin be respectively <i>t</i>, - <i>t</i><sub>1</sub>, <i>t</i><sub>2</sub>, <i>t</i><sub>3</sub>, and <i>t</i><sub>4</sub>.</p> - - <p>Through <i>p</i> draw rectangular co-ordinates, and with a scale measure the - co-ordinates of <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, and <i>p</i><sub>4</sub>, and let these - respectively be <i>a</i><sub>1</sub>, <i>b</i><sub>1</sub>; <i>a</i><sub>2</sub>, <i>b</i><sub>2</sub>; <i>a</i><sub>3</sub>, <i>b</i><sub>3</sub>; - <i>a</i><sub>4</sub>, <i>b</i><sub>4</sub>. Then if <i>x</i>, <i>y</i>, and <i>z</i> be the co-ordinates of the - origin of the shock, <i>d</i>, <i>d</i><sub>1</sub>, <i>d</i><sub>2</sub>, <i>d</i><sub>3</sub>, and <i>d</i><sub>4</sub>, the - respective distances of <i>p</i>, <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, and <i>p</i><sub>4</sub> - from this origin, and <i>v</i> the velocity of the shock, we have</p> - - <div class="list-container"> - <ol> - <li><i>x</i><sup>2</sup> + <i>y</i><sup>2</sup> + <i>z</i><sup>2</sup> = <i>d</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sup>2</sup></li> - <li>(<i>a</i><sub>1</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>1</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>1</sub><sup>2</sup></li> - <li>(<i>a</i><sub>2</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>2</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>2</sub><sup>2</sup></li> - <li>(<i>a</i><sub>3</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>3</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>3</sub><sup>2</sup></li> - <li>(<i>a</i><sub>4</sub> - <i>x</i>)<sup>2</sup> + (<i>b</i><sub>4</sub> - <i>y</i>)<sup>2</sup> + <i>z</i><sup>2</sup> = <i>v</i><sup>2</sup> <i>t</i><sub>4</sub><sup>2</sup></li> - </ol> - </div> - - <p>Because we know the actual times at which the waves arrived at the - places <i>p</i>, <i>p</i><sub>1</sub>, <i>p</i><sub>2</sub>, <i>p</i><sub>3</sub>, <i>p</i><sub>4</sub>, we know the values - <i>t</i>—<i>t</i><sub>1</sub>, <i>t</i>—<i>t</i><sub>2</sub>, <i>t</i>—<i>t</i><sub>3</sub>, <i>t</i>—<i>t</i><sub>4</sub>. Call these - respectively <i>m</i>, <i>p</i>, <i>q</i>, and <i>r</i>. Suppose <i>t</i> known, then</p> - - <p><span class="pagenum" id="Page_207">207</span></p> - - <div class="list-container"> - <ul> - <li><i>t</i><sub>1</sub> = <i>t</i> - <i>m</i></li> - <li><i>t</i><sub>2</sub> = <i>t</i> - <i>p</i></li> - <li><i>t</i><sub>3</sub> = <i>t</i> - <i>q</i></li> - <li><i>t</i><sub>4</sub> = <i>t</i> - <i>r</i>.</li> - </ul> - </div> - - <p>Subtracting equation No. 1 from each of the equations 2, 3, 4, and 5, - we obtain,</p> - - <div class="list-container"> - <ul> - <li><i>a</i><sub>1</sub><sup>2</sup> + <i>b</i><sub>1</sub><sup>2</sup> - 2<i>a</i><sub>1</sub> <i>x</i> - 2<i>b</i><sub>1</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>1</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>m</i><sup>2</sup> - 2<i>t</i> <i>m</i>)</li> - <li><i>a</i><sub>2</sub><sup>2</sup> + <i>b</i><sub>2</sub><sup>2</sup> - 2<i>a</i><sub>2</sub> <i>x</i> - 2<i>b</i><sub>2</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>2</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>p</i><sup>2</sup> - 2<i>t</i> <i>p</i>)</li> - <li><i>a</i><sub>3</sub><sup>2</sup> + <i>b</i><sub>3</sub><sup>2</sup> - 2<i>a</i><sub>3</sub> <i>x</i> - 2<i>b</i><sub>3</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>3</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>q</i><sup>2</sup> - 2<i>t</i> <i>q</i>)</li> - <li><i>a</i><sub>4</sub><sup>2</sup> + <i>b</i><sub>4</sub><sup>2</sup> - 2<i>a</i><sub>4</sub> <i>x</i> - 2<i>b</i><sub>4</sub> <i>y</i> = <i>v</i><sup>2</sup> (<i>t</i><sub>4</sub><sup>2</sup> - <i>t</i><sup>2</sup>) = <i>v</i><sup>2</sup> (<i>r</i><sup>2</sup> - 2<i>t</i> <i>r</i>)</li> - </ul> - </div> - - <p>Now let <i>v</i><sup>2</sup> = <i>u</i>, and 2<i>v</i><sup>2</sup> <i>t</i> = <i>w</i>.</p> - - <p>Then</p> - - <div class="list-container"> - <ol> - <li>2<i>a</i><sub>1</sub> <i>x</i> + 2<i>b</i><sub>1</sub> <i>y</i> + <i>u</i> <i>m</i><sup>2</sup> - <i>n</i> <i>m</i> = <i>a</i><sub>1</sub><sup>2</sup> + <i>b</i><sub>1</sub><sup>2</sup></li> - <li>2<i>a</i><sub>2</sub> <i>x</i> + 2<i>b</i><sub>2</sub> <i>y</i> + <i>u</i> <i>p</i><sup>2</sup> - <i>n</i> <i>p</i> = <i>a</i><sub>2</sub><sup>2</sup> + <i>b</i><sub>2</sub><sup>2</sup></li> - <li>2<i>a</i><sub>3</sub> <i>x</i> + 2<i>b</i><sub>3</sub> <i>y</i> + <i>u</i> <i>q</i><sup>2</sup> - <i>n</i> <i>q</i> = <i>a</i><sub>3</sub><sup>2</sup> + <i>b</i><sub>3</sub><sup>2</sup></li> - <li>2<i>a</i><sub>4</sub> <i>x</i> + 2<i>b</i><sub>4</sub> <i>y</i> + <i>u</i> <i>r</i><sup>2</sup> - <i>n</i> <i>r</i> = <i>a</i><sub>4</sub><sup>2</sup> + <i>b</i><sub>4</sub><sup>2</sup></li> - </ol> - </div> - - <p>We have here four simple equations containing the four unknown - quantities <i>x</i>, <i>y</i>, <i>u</i>, and <i>w</i>.</p> - - <p><i>x</i> and <i>y</i> determine the origin of the shock. The absolute velocity - <i>v</i> equals √ <i>u</i>. From <i>v</i> and <i>w</i> we obtain <i>t</i>. Substituting - <i>x</i>, <i>y</i>, <i>v</i>, and <i>t</i> in the first equation we obtain <i>z</i>.</p> - - <p>We have here assumed that the points of observation have all about the - same elevation above sea level.</p> - - <p>If it is thought necessary to take these elevations into account, a - sixth equation may be introduced.</p> - - <p>If the propagation of the wave is considered as a horizontal one, as - would be done when calculating the position of the <em>epicentrum</em> or - point above the origin, by means of the times of arrival of a sea wave, - the ordinate <i>z</i> of the first five equations would be omitted. Working - in this way the resulting four equations, viz.</p> - - <div class="center"> - 2<i>a</i><sub>1</sub> <i>x</i> + 2<i>b</i><sub>1</sub> <i>y</i> + <i>u</i><i>m</i><sup>2</sup> - <i>w</i><i>m</i><sup>2</sup> = <i>a</i><sub>1</sub><sup>2</sup> + <i>b</i><sub>1</sub><sup>2</sup><br /> - &c. &c. &c. - </div> - - <p class="noindent">remained unchanged.</p> - - <p>Applying this method to the same example as that - <span class="pagenum" id="Page_208">208</span> 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; <i>ox</i> and <i>oy</i> being, - drawn through Mejillones.</p> - - <table class="collapse" summary="South America sea waves 2"> - <tr> - <th class="ball"> </th> - <th colspan="2" class="ball">Co-ordinates</th> - <th colspan="3" class="ball">Time of arrival</th> - </tr> - <tr> - <th class="bl br"> </th> - <th class="br">OX</th> - <th class="br">OY</th> - <th class="tdr"><div>h.</div></th> - <th class="tdr "><div>m.</div></th> - <th class="br"> </th> - </tr> - <tr> - <td class="tdl bl br">Mejillones</td> - <td class="tdr br"><div><i>a</i> or 0</div></td> - <td class="tdr br"><div><i>b</i> or 0</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>46</div></td> - <td class="br">p. m.</td> - </tr> - <tr> - <td class="tdl bl br">Iquique</td> - <td class="tdr br"><div><i>a<sub>1</sub></i> or 150</div></td> - <td class="tdr br"><div><i>b<sub>1</sub></i> or 96</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>40</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="tdl bl br">Cobija</td> - <td class="tdr br"><div><i>a<sub>2</sub></i> or 36</div></td> - <td class="tdr br"><div><i>b<sub>2</sub></i> or 14</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>38</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="tdl bl br">Tocopilla</td> - <td class="tdr br"><div><i>a<sub>3</sub></i> or 66</div></td> - <td class="tdr br"><div><i>b<sub>3</sub></i> or 31</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>32</div></td> - <td class="br">„</td> - </tr> - <tr class="bb"> - <td class="tdl bl br">Huanillos</td> - <td class="tdr br"><div><i>a<sub>4</sub></i> or 102</div></td> - <td class="tdr br"><div><i>b<sub>4</sub></i> or 58</div></td> - <td class="tdr"><div>8</div></td> - <td class="tdr"><div>30</div></td> - <td class="br">„</td> - </tr> - </table> - - <p>From this data we find the co-ordinates <i>x</i> and <i>y</i> of this origin to - be 85·8 and 56·7; whilst the velocity of propagation = 45 feet per - second.</p> - - <p>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.</p> - - <p>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.</p> - - <p>In this example, as in the preceding ones, it will be - <span class="pagenum" id="Page_209">209</span> 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.</p> - - <p>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.</p> - - <p><em>Haughton’s method.</em>—Given, the time of an earthquake shock at three - places, to determine its horizontal velocity and coseismal line.</p> - - <p>The solution of this is contained in the formula</p> - - <div class="center"> - tan <i>ϕ</i> = <span class="frac"><sup><i>a</i> (<i>t<sub>2</sub></i> - <i>t<sub>1</sub></i>) - sin β</sup><span>/</span><sub><i>c</i> (<i>t<sub>3</sub></i> - <i>t<sub>2</sub></i>) + - <i>a</i> (<i>t<sub>2</sub></i> - <i>t<sub>1</sub></i>) cos β</sub></span>. - </div> - - <p>When <span class="smcap">a</span>, <span class="smcap">b</span>, and <span class="smcap">c</span> are three stations at - which a shock is observed at the times <i>t<sub>1</sub></i>, <i>t<sub>2</sub></i>, and <i>t<sub>3</sub></i>; - <i>a</i>, <i>b</i>, and <i>c</i> are the distances between <span class="smcap">a</span>, <span class="smcap">b</span>, - and <span class="smcap">c</span>, and <i>ϕ</i> is the angle made by the coseismal lines <i>x</i> - <span class="smcap">a</span> <i>x</i>, <i>y</i> <span class="smcap">b</span> <i>y</i>, and the line <span class="smcap">a b</span>, which - are assumed to be parallel.</p> - - <p>This I applied in the case of the Iquique earthquake, but owing to - the smallness of the angles between the three stations <span class="smcap">a</span>, - <span class="smcap">b</span>, and <span class="smcap">c</span>, 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.<span class="pagenum" id="Page_210">210</span> - Plot the three stations <span class="smcap">a</span>, <span class="smcap">b</span>, and - <span class="smcap">c</span> on a map, join the two stations between which there was the - greatest difference in the time observation. Let these, for example, - be <span class="smcap">a</span> and <span class="smcap">c</span>. Divide the line <span class="smcap">a c</span> at point - <span class="smcap">d</span>, so that <span class="smcap">a d</span> : <span class="smcap">d c</span> as the interval between - the shock felt at <span class="smcap">a</span> and <span class="smcap">b</span> is to the interval between - the shock felt at <span class="smcap">b</span> and <span class="smcap">c</span>. The line <span class="smcap">b d</span> will - be parallel to the direction in which the wave advanced.</p> - - <p><em>The difference in time of the arrival of two disturbances.</em>—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 - <span class="pagenum" id="Page_211">211</span> 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.</p> - - <p>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.<a id="FNanchor_86" href="#Footnote_86" class="fnanchor">[86]</a></p> - - <p><em>Seebach’s method.</em>—To determine the true velocity of an earthquake, - the time of the first shock, and the depth of the centre.</p> - - <div class="figright"> - <img src="images/i_p211.jpg" width="315" height="295" alt="" /> - <div class="caption"><span class="smcap">Fig. 32.</span></div> - </div> - - <p>Let the straight line <span class="smcap">m</span>, <i>m<sub>1</sub></i>, <i>m<sub>2</sub></i>, <i>m<sub>3</sub></i> 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.</p> - - <p>If an earthquake originates at <span class="smcap">c</span>, then to reach the surface at - <span class="smcap">m</span> it traverses a distance <i>h</i> in the time <i>t</i>. To reach the - surface at <span class="smcap">m</span><sub>1</sub> it traverses a distance <i>h</i> + <i>x<sub>1</sub></i> in a - time <i>t<sub>2</sub></i>. If <i>v</i> equals the velocity of propagation,</p> - - <div class="center"> - then <i>t</i> = <span class="frac"><sup><i>h</i></sup><span>/</span><sub><i>v</i></sub></span>, - <i>t</i><sub>1</sub> = <span class="frac"><sup><i>h</i> + <i>x</i><sub>1</sub></sup><span>/</span><sub><i>v</i></sub></span>, - </div> - - <div class="center"> - <i>t</i><sub>2</sub> = <span class="frac"><sup><i>h</i> + <i>x</i><sub>2</sub></sup><span>/</span><sub><i>v</i></sub></span>, &c. - </div> - - <p>Seebach now says that <em>if we have given the position of</em> <span class="smcap">m</span> - <em>or epicentrum of the shock</em>, and draw through it rectangular axes - like <span class="smcap">m</span> <i>m</i><sub>3</sub> and <span class="smcap">m</span> <span class="smcap">t</span><sub>3</sub>, and lay down - <span class="pagenum" id="Page_212">212</span>on <span class="smcap">m</span> - <i>m<sub>3</sub></i> in miles the distances from M of the various - stations which have been shaken, and in equal divisions for minutes - lay down on <span class="smcap">m</span> <span class="smcap">t</span><sub>3</sub> the differences of time at which - <span class="smcap">m</span>, <i>m</i><sub>1</sub>, <i>m</i><sub>2</sub>, &c. - were shaken, then <span class="smcap">m</span><sub>1</sub> - <span class="smcap">t</span><sub>1</sub>, <span class="smcap">m</span><sub>2</sub> - <span class="smcap">t</span><sub>2</sub>, &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 <em>epicentrum</em>. The apex - of the hyperbola is the <em>epicentrum</em>.</p> - - <p>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 - <em>centrum</em>. This intersection is shown by dotted lines. Knowing the - position of the <em>centrum</em>, we can directly read from our diagram how - far the disturbance has been propagated in a given time.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XI"> - <span class="pagenum" id="Page_213">213</span> - <h2>CHAPTER XI.<br /> - <span class="subhead">THE DEPTH OF AN EARTHQUAKE CENTRUM.</span> - </h2> - </div> - - <div class="summary"> - The depth of an earthquake centrum—Greatest possible depth of an - earthquake—Form of the focal cavity.</div> - - <p><em>Depth of centrum.</em>—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 <em>epicentrum</em> 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 <em>epicentrum</em> 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 <em>centrum</em>. Höfer followed this method when - investigating the earthquake of Belluno.</p> - - <p>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).</p> - - <p>By means of a number of lines parallel to twenty-six - <span class="pagenum" id="Page_214">214</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Possibly, perhaps, the earthquake may have originated from a fissure - the vertical dimensions of which was comprised between these depths.</p> - - <p>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.</p> - - <p>The following table of the depths at which certain earthquakes have - originated has been compiled from the writings of several observers.</p> - - <table class="collapse" summary="Earthquake depths"> - <tr> - <th colspan="2" class="ball"> </th> - <th colspan="3" class="ball">In feet</th> - </tr> - <tr> - <th class="bl br"> </th> - <th class="br"> </th> - <th class="br">Minimum</th> - <th class="br">Mean</th> - <th class="br">Maximum</th> - </tr> - <tr> - <td class="tdl bl br">Rhineland</td> - <td class="tdl br">1846 (Schmidt)</td> - <td class="tdr br"><div> </div></td> - <td class="tdr br"><div>127,309</div></td> - <td class="tdr br"><div> </div></td> - </tr> - <tr> - <td class="tdl bl br">Sillien</td> - <td class="tdl br">1858 (Schmidt)</td> - <td class="tdr br"><div> </div></td> - <td class="tdr br"><div>86,173</div></td> - <td class="tdr br"><div> </div></td> - </tr> - <tr> - <td class="tdl bl br">Middle Germany</td> - <td class="tdl br">1872 (Seebach)</td> - <td class="tdr br"><div>47,225</div></td> - <td class="tdr br"><div>58,912</div></td> - <td class="tdr br"><div>70,841</div></td> - </tr> - <tr> - <td class="tdl bl br">Herzogenrath</td> - <td class="tdl br">1873 (Lasaulx)</td> - <td class="tdr br"><div>16,553</div></td> - <td class="tdr br"><div>36,516</div></td> - <td class="tdr br"><div>56,477</div></td> - </tr> - <tr> - <td class="tdl bl br">Neapolitan</td> - <td class="tdl br">1857 (Mallet)</td> - <td class="tdr br"><div>16,705</div></td> - <td class="tdr br"><div>34,930</div></td> - <td class="tdr br"><div>49,359</div></td> - </tr> - <tr class="bb"> - <td class="tdl bl br">Yokohama</td> - <td class="tdl br">1880 (Milne)</td> - <td class="tdr br"><div>7,920</div></td> - <td class="tdr br"><div>17,260</div></td> - <td class="tdr br"><div>26,400</div></td> - </tr> - </table> - - <p><span class="pagenum" id="Page_215">215</span></p> - - <p>A table similar to this has been compiled by Lasaulx.<a id="FNanchor_87" href="#Footnote_87" class="fnanchor">[87]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 <em>epicentrum</em>, 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.</p> - - <p>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 - <span class="pagenum" id="Page_216">216</span>testified by the records - of well-constructed instruments, has no practical connection with the - depth from which the disturbance originated.</p> - - <p>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 <em>epicentrum</em> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 <em>epicentrum</em>. 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.</p> - - <p><span class="pagenum" id="Page_217">217</span></p> - - <p>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.</p> - - <p>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.</p> - - <div class="figright"> - <img src="images/i_p217.jpg" width="369" height="295" alt="" /> - <div class="caption"><span class="smcap">Fig. 33.</span></div> - </div> - - <p>Let a disturbance simultaneously originate from all points of the - fissure <i>f</i> <i>f</i>. This will spread outwards in ellipsoidal shells to - the surface of the earth <i>e</i> <i>e</i>. The major axis of these ellipsoidal - shells will be the direction of greatest effect. In the direction <i>c</i> - <i>d</i> 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 <em>earthquake shadow</em>. - The same expression has been<span class="pagenum" id="Page_218">218</span> - employed, somewhat differently, when - speaking of the effects produced on buildings.</p> - - <p>For places, like <i>s</i> and <i>p</i>, 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.</p> - - <p>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.</p> - - <p>The result is that the distance of equal effect from the seismic - vertical will be greatest in the direction of the more compressible - material.</p> - - <p>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.</p> - - <p><em>Greatest depth of an earthquake origin.</em>—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.</p> - - <p>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.<a id="FNanchor_88" href="#Footnote_88" class="fnanchor">[88]</a></p> - - <p><span class="pagenum" id="Page_219">219</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>Carrying the argument still further. Mallet says that if the depth of - origin of earthquakes were the same, then the <em>area of disturbance</em> - 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 <em>very great area</em> 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, - <span class="pagenum" id="Page_220">220</span> or, what is - the same thing, by the ratio of the altitudes of the volcanoes of the - Andes to that of Vesuvius.’</p> - - <p>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.</p> - - <p>Ingenious as this argument is, we can hardly admit it without certain - qualifications.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Another measure of the impulsive efforts which subterranean - <span class="pagenum" id="Page_221">221</span>terranean 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.</p> - - <p>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.<a id="FNanchor_89" href="#Footnote_89" class="fnanchor">[89]</a></p> - - <p><em>Form of the focal cavity.</em>—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.</p> - - <p>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 - <span class="pagenum" id="Page_222">222</span>be that one mass - of rock has been sliding across another mass, giving rise to shearing - strains, and producing waves of distortion.</p> - - <p>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 <i xml:lang="la">a priori</i> 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.</p> - - <p>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:—</p> - - <p>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 <em>epicentrum</em>, but as distorted oval or - elliptical figures, the major axes of which roughly coincided with each - other. Further, the <em>epicentrum</em>, did not lie in the centre of these - ovals, but was near to the narrow end where they converged.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_223">223</span> - 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.</p> - - <p>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.</p> - - <p>The nature of the arguments which were followed in discussing the sound - observations will be found in the chapter relating to these phenomena.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_224">224</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Thus, for example, Seebach, when determining the depth and nature of - the origin of the earthquake of Middle Germany, reasoned somewhat as - follows:—</p> - - <p><span class="pagenum" id="Page_225">225</span></p> - - <p>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 <em>epicentrum</em>. This region, however, - was found by observation to lie along a curved band about forty miles - in length, altogether on one side of the <em>epicentrum</em>.</p> - - <p>To explain this anomaly Seebach followed Mallet, and assumed that the - origin was not a spherical cavity, but a fissure.</p> - - <p>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 <em>epicentrum</em>. 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 - <em>centrum</em>. A second line at right angles to this one gave the dip of - the fissure.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XII"> - <span class="pagenum" id="Page_226">226</span> - <h2>CHAPTER XII.<br /> - <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>General distribution of earthquakes.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_227">227</span> - 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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_228">228</span> - 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.</p> - - <p>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.<a id="FNanchor_90" href="#Footnote_90" class="fnanchor">[90]</a> - 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 <span class="pagenum" id="Page_229">229</span> - 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.</p> - - <p>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.</p> - - <p><em>Disturbances in lines or zones.</em>—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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_230">230</span> - 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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_231">231</span> rising - and falling of these lips throw off transverse waves. Rossi adduces - observations which appear to meet with explanation on such suppositions.</p> - - <p>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.<a id="FNanchor_91" href="#Footnote_91" class="fnanchor">[91]</a> 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.<a id="FNanchor_92" href="#Footnote_92" class="fnanchor">[92]</a></p> - - <p><em>Examples of distribution.</em>—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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_232">232</span></p> - - <p>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.</p> - - <div class="figcenter"> - <img src="images/i_p232.jpg" width="302" height="430" alt="" /> - <div class="caption"><span class="smcap">Fig. 34.</span><br /> - <div class="list-container"> - <div class="left nowrap"> - <div> - Areas almost simultaneously struck from S.E. to N.W. - <img class="iglyph-a" src="images/i_p232a.png" alt="" width="40" height="40" /> - </div> - <div> - Subsequent radial disturbance - <img class="iglyph-b" src="images/i_p232b.png" width="36" height="60" alt="" /> - </div> - </div> - </div> - </div> - </div> - - <p><em>Extension of earthquake boundaries.</em>—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 - <span class="pagenum" id="Page_233">233</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XIII"> - <span class="pagenum" id="Page_234">234</span> - <h2>CHAPTER XIII.<br /> - <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</span> - </h2> - </div> - - <p><em>Seismic energy in relation to geological time.</em>—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.</p> - - <p>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.<a id="FNanchor_93" href="#Footnote_93" class="fnanchor">[93]</a> This increase in temperature as we descend into the earth - <span class="pagenum" id="Page_235">235</span> - as deduced from many observations appears to be about 1° F. for every - fifty or sixty feet of descent.</p> - - <p>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.</p> - - <p>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, - <span class="pagenum" id="Page_236">236</span> it must have had - a much greater influence in bygone times.</p> - - <p>We might speak similarly with regard to the internal heat of the earth.</p> - - <p>From the present heat gradient of our globe it is possible to calculate - how much heat flows from the earth every year.</p> - - <p>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.</p> - - <p>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.</p> - - <p>We might also calculate how many years ago it was since such a gradient - existed.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_237">237</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_94" href="#Footnote_94" class="fnanchor">[94]</a></p> - - <p>The earthquakes which from time to time shake those - <span class="pagenum" id="Page_238">238</span>newer mountains - apparently indicate that the process of mountain-making is hardly ended.</p> - - <p><em>Seismic energy in relation to historical time.</em>—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 <span class="smcap">b.c.</span> 1606 and <span class="smcap">a.d.</span> 1850. The - earthquake of <span class="smcap">b.c.</span> 1606 was on the occasion of the delivery of - the law at Mount Sinai. Between <span class="smcap">b.c.</span> 1604 and <span class="smcap">b.c.</span> - 1586 an earthquake probably occurred in Arabia, when Korah, Dathan, - and Abiram were swallowed up. Another biblical record is that of - <span class="smcap">b.c.</span> 1566, when the walls of Jericho were overthrown.</p> - - <p>The earliest records from China is in <span class="smcap">b.c.</span> 595; in Japan - <span class="smcap">b.c.</span> 285; in India <span class="smcap">a.d.</span> 894.</p> - - <p>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.</p> - - <p>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.</p> - - <p>These conclusions, based on the evidence at our - <span class="pagenum" id="Page_239">239</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_240">240</span></p> - - <div class="left hang2">Key:</div> - - <table class="ml5 mb2 lh1" summary="Earthquakes per country key"> - <tr><td class="tdr"><div>1</div></td> <td class="tdr">Centuries</td></tr> - <tr><td class="tdr"><div>2</div></td> <td>Japan</td></tr> - <tr><td class="tdr"><div>3</div></td> <td>Scandinavia and Iceland</td></tr> - <tr><td class="tdr"><div>4</div></td> <td>British Isles and Northern Isles</td></tr> - <tr><td class="tdr"><div>5</div></td> <td>Spanish Peninsula</td></tr> - <tr><td class="tdr"><div>6</div></td> <td>France, Belgium, Holland</td></tr> - <tr><td class="tdr"><div>7</div></td> <td>Rhine Basin</td></tr> - <tr><td class="tdr"><div>8</div></td> <td>Switzerland and Rhine Basin</td></tr> - <tr><td class="tdr"><div>9</div></td> <td>Danube Basin</td></tr> - <tr><td class="tdr"><div>10</div></td> <td>Italy, Sicily, Sardinia, and Malta</td></tr> - <tr><td class="tdr"><div>11</div></td> <td>Supplemental table for Italy, Sardinia, and Malta</td></tr> - <tr><td class="tdr"><div>12</div></td> <td>Turco-Hellenic Territory, Syria, Ægean Isles, and Levant</td></tr> - <tr><td class="tdr"><div>13</div></td> <td>United States and Canada</td></tr> - <tr><td class="tdr"><div>14</div></td> <td>Mexico and Central America</td></tr> - <tr><td class="tdr"><div>15</div></td> <td>Antilles</td></tr> - <tr><td class="tdr"><div>16</div></td> <td>Cuba</td></tr> - <tr><td class="tdr"><div>17</div></td> <td>Chili and La Plata Basin</td></tr> - <tr><td class="tdr"><div>18</div></td> <td>Northern Zone of Asia</td></tr> - <tr><td class="tdr"><div>19</div></td> <td>Approximate Intensity in the Kioto District of Japan</td></tr> - </table> - - <table class="collapse" summary="Earthquakes per country"> - <tr class="bb"> - <th class="ball">1</th> - <th class="ball">2</th> - <th class="ball">3</th> - <th class="ball">4</th> - <th class="ball">5</th> - <th class="ball">6</th> - <th class="ball">7</th> - <th class="ball">8</th> - <th class="ball">9</th> - <th class="ball">10</th> - <th class="ball">11</th> - <th class="ball">12</th> - <th class="ball">13</th> - <th class="ball">14</th> - <th class="ball">15</th> - <th class="ball">16</th> - <th class="ball">17</th> - <th class="ball">18</th> - <th class="ball">19</th> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>I.</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>II.</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>III.</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>IV.</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>23</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>V.</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>VI.</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td rowspan="10" class="tdr br"><div>19</div></td> - <td class="tdr br"><div>3</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>27</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>VII.</div></td> - <td class="tdr br"><div>12</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>15</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>VIII.</div></td> - <td class="tdr br"><div>11</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>12</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>17</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>IX.</div></td> - <td class="tdr br"><div>40</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>21</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>7</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>60</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>X.</div></td> - <td class="tdr br"><div>17</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdr br"><div>3</div></td> - <td class="tdr br"><div>3</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>24</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XI.</div></td> - <td class="tdr br"><div>20</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>3</div></td> - <td class="tdr br"><div>16</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>9</div></td> - <td class="tdr br"><div>7</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>28</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XII.</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>11</div></td> - <td class="tdr br"><div>4</div></td> - <td class="tdr br"><div>12</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdr br"><div>22</div></td> - <td class="tdr br"><div>23</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>20</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XIII.</div></td> - <td class="tdr br"><div>16</div></td> - <td rowspan="5" class="tdr br"><div>28</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdr br"><div>3</div></td> - <td class="tdr br"><div>9</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>3</div></td> - <td class="tdr br"><div>15</div></td> - <td class="tdr br"><div>26</div></td> - <td class="tdr br"><div>13</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>16</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XIV.</div></td> - <td class="tdr br"><div>19</div></td> - <td class="tdr br"><div>4</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>21</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdr br"><div>20</div></td> - <td class="tdr br"><div>51</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>25</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XV.</div></td> - <td class="tdr br"><div>36</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>4</div></td> - <td class="tdr br"><div>14</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>12</div></td> - <td class="tdr br"><div>18</div></td> - <td class="tdr br"><div>47</div></td> - <td class="tdr br"><div>11</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>29</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XVI.</div></td> - <td class="tdr br"><div>17</div></td> - <td class="tdr br"><div>8</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>61</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>52</div></td> - <td class="tdr br"><div>35</div></td> - <td class="tdr br"><div>32</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr br"><div>22</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>6</div></td> - <td class="tdr br"><div>1</div></td> - <td class="tdr br"><div>4</div></td> - <td class="tdr br"><div>5</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>17</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XVII.</div></td> - <td class="tdr br"><div>26</div></td> - <td class="tdr br"><div>14</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>91</div></td> - <td class="tdr br"><div>29</div></td> - <td class="tdr br"><div>120</div></td> - <td class="tdr br"><div>31</div></td> - <td class="tdr br"><div>121</div></td> - <td class="tdr br"><div>9</div></td> - <td class="tdr br"><div>53</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>7</div></td> - <td class="tdr br"><div>16</div></td> - <td class="tdr br"><div>4</div></td> - <td class="tdr br"><div>9</div></td> - <td class="tdr br"><div>—</div></td> - <td class="tdr br"><div>11</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XVIII.</div></td> - <td class="tdr br"><div>31</div></td> - <td class="tdr br"><div>111</div></td> - <td class="tdr br"><div>63</div></td> - <td class="tdr br"><div>93</div></td> - <td class="tdr br"><div>237</div></td> - <td class="tdr br"><div>71</div></td> - <td class="tdr br"><div>141</div></td> - <td class="tdr br"><div>88</div></td> - <td class="tdr br"><div>438</div></td> - <td class="tdr br"><div>20</div></td> - <td class="tdr br"><div>124</div></td> - <td class="tdr br"><div>88</div></td> - <td class="tdr br"><div>24</div></td> - <td class="tdr br"><div>85</div></td> - <td class="tdr br"><div>2</div></td> - <td class="tdr br"><div>10</div></td> - <td class="tdr br"><div>32</div></td> - <td class="tdr br"><div>8</div></td> - </tr> - <tr class="bb"> - <td class="tdr bl br"><div>XIX.</div></td> - <td class="tdr br"><div>27</div></td> - <td class="tdr br"><div>113</div></td> - <td class="tdr br"><div>110</div></td> - <td class="tdr br"><div>85</div></td> - <td class="tdr br"><div>211</div></td> - <td class="tdr br"><div>81</div></td> - <td class="tdr br"><div>173</div></td> - <td class="tdr br"><div>145</div></td> - <td class="tdr br"><div>390</div></td> - <td class="tdr br"><div>88</div></td> - <td class="tdr br"><div>194</div></td> - <td class="tdr br"><div>51</div></td> - <td class="tdr br"><div>30</div></td> - <td class="tdr br"><div>145</div></td> - <td class="tdr br"><div>50</div></td> - <td class="tdr br"><div>170</div></td> - <td class="tdr br"><div>57</div></td> - <td class="tdr br"><div>8</div></td> - </tr> - </table> - - <p><span class="pagenum" id="Page_241">241</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_242">242</span> 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.</p> - - <div class="figcenter"> - <img src="images/i_p242.jpg" width="700" height="209" alt="" /> - <div class="caption"><span class="smcap">Fig. 35.</span>—Curve of Seismic Intensity for Kioto.</div> - </div> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_243">243</span> 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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_95" href="#Footnote_95" class="fnanchor">[95]</a></p> - - <p><em>Relative frequency of earthquakes.</em>—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.</p> - - <p>From a general examination of this question, considering the - earthquakes of the whole world. Mallet arrived at the following - conclusions:—</p> - - <p><span class="pagenum" id="Page_244">244</span></p> - - <p>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.</p> - - <p>2. The shorter intervals are in connection with periods of fewer - earthquakes—not always with those of least intensity, but usually so.</p> - - <p>3. The alternations of paroxysm and of repose appear to follow no - absolute law deducible from these curves.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <table class="collapse" summary="Earthquake intervals"> - <tr> - <th class="ball">No. of shocks</th> - <th colspan="2" class="ball">Period</th> - <th colspan="2" class="ball">Interval</th> - </tr> - <tr> - <td class="bl br">6</td> - <td class="pr0">A.D.</td> - <td class="br">827–836</td> - <td class="tdr pr0"><div>10</div></td> - <td class="br">years</td> - </tr> - <tr> - <td class="bl br">6</td> - <td class="pr0">„</td> - <td class="br">880–890</td> - <td class="tdr pr0">10<div>10</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="bl br">4</td> - <td class="pr0">„</td> - <td class="br">1040–1043</td> - <td class="tdr pr0"><div>4</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="bl br">5</td> - <td class="pr0">„</td> - <td class="br">1493–1507</td> - <td class="tdr pr0"><div>5</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="bl br">4</td> - <td class="pr0">„</td> - <td class="br">1510–1513</td> - <td class="tdr pr0"><div>4</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="bl br">5</td> - <td class="pr0">„</td> - <td class="br">1645–1650</td> - <td class="tdr pr0"><div>6</div></td> - <td class="br">„</td> - </tr> - <tr> - <td class="bl br">5</td> - <td class="pr0">„</td> - <td class="br">1662–1664</td> - <td class="tdr pr0"><div>3</div></td> - <td class="br">„</td> - </tr> - <tr class="bb"> - <td class="bl br">4</td> - <td class="pr0">„</td> - <td class="br">1853–1856</td> - <td class="tdr pr0"><div>4</div></td> - <td class="br">„</td> - </tr> - </table> - - <p>Dr. E. Naumann, who has also written on the earthquakes - <span class="pagenum" id="Page_245">245</span>quakes 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.</p> - - <p>A. Caldcleugh, writing on the earthquake of Chili, in 1835,<a id="FNanchor_96" href="#Footnote_96" class="fnanchor">[96]</a> 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.</p> - - <p>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 - <span class="pagenum" id="Page_246">246</span>large earthquakes in - various parts of the world may be easily obtained by an inspection of - the table on page 240.</p> - - <p>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.<a id="FNanchor_97" href="#Footnote_97" class="fnanchor">[97]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>During the twenty-four hours succeeding the destruction of Lima - (October 28, 1746), 200 shocks were <span class="pagenum" id="Page_247">247</span> - counted, and up to the 24th of - February in the following year 451 shocks were felt.</p> - - <p>At St. Thomas, in 1868, 283 shocks were counted in nine and a quarter - hours.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Synchronism of earthquakes.</em>—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.<a id="FNanchor_98" href="#Footnote_98" class="fnanchor">[98]</a></p> - - <p>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, - <span class="pagenum" id="Page_248">248</span>when the town Euphemia - sank, and in the years 1770, 1776, 1780, and 1783.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Secondary earthquakes.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_249">249</span> - primary disturbances, they might have been treated in a previous chapter.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Another shock was felt at Cork.<a id="FNanchor_99" href="#Footnote_99" class="fnanchor">[99]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XIV"> - <span class="pagenum" id="Page_250">250</span> - <h2>CHAPTER XIV.<br /> - <span class="subhead">DISTRIBUTION OF EARTHQUAKES IN TIME (<i>continued</i>).</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>The position of the heavenly bodies and the occurrence of - earthquakes.</em>—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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_251">251</span> - 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.</p> - - <p><em>Earthquakes and the position of the moon.</em>—Many earthquake - investigators have attempted to show the connection between earthquakes - and the phases of the moon.</p> - - <p>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.</p> - - <p>1. They are more frequent at new or full moon (syzygies) than at half - moon (quadratures).</p> - - <p>2. They are more frequent when the moon is nearest the earth (perigee) - than when she is farthest off (apogee).</p> - - <p>3. They are more frequent when the moon is on the meridian than when - she is on the horizon.</p> - - <p>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.<a id="FNanchor_100" href="#Footnote_100" class="fnanchor">[100]</a></p> - - <p><span class="pagenum" id="Page_252">252</span></p> - - <p>Between 1761 and 1800 earthquakes occurred as follows:—</p> - - <table summary="Earthquakes per distance of moon"> - <tr> - <td class="tdl">In Perigee</td> - <td>526</td> - </tr> - <tr> - <td class="tdl">Apogee</td> - <td>465</td> - </tr> - </table> - - <p>The following table shows the results which enabled Perrey to deduce - his first law.</p> - - <p>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.</p> - - <table class="collapse" summary="Earthquakes per phase of moon"> - <tr> - <th class="ball"> </th> - <th class="ball">Totals</th> - <th class="ball">Syzygies</th> - <th class="ball">Quadratures</th> - <th class="ball">Difference<br />in favour of<br />the Syzygies</th> - </tr> - <tr> - <td class="bl br">1843–1847</td> - <td class="tdr br"><div>1,604</div></td> - <td class="tdr br"><div>850·48</div></td> - <td class="tdr br"><div>753·52</div></td> - <td class="tdr br"><div>69·96</div></td> - </tr> - <tr> - <td class="bl br">1848–1852</td> - <td class="tdr br"><div>2,049</div></td> - <td class="tdr br"><div>1,053·53</div></td> - <td class="tdr br"><div>995·47</div></td> - <td class="tdr br"><div>58·06</div></td> - </tr> - <tr> - <td class="bl br">1853–1857</td> - <td class="tdr br"><div>3,018</div></td> - <td class="tdr br"><div>1,534·13</div></td> - <td class="tdr br"><div>1,483·87</div></td> - <td class="tdr br"><div>50·26</div></td> - </tr> - <tr> - <td class="bl br">1858–1862</td> - <td class="tdr br"><div>3,140</div></td> - <td class="tdr br"><div>1,602·99 </div></td> - <td class="tdr br"><div>1,537·41</div></td> - <td class="tdr br"><div>65·98</div></td> - </tr> - <tr> - <td class="bl br">1863–1867</td> - <td class="tdr br"><div>2,845</div></td> - <td class="tdr br"><div>1,463·42</div></td> - <td class="tdr br"><div>1,381·58</div></td> - <td class="tdr br"><div>81·84</div></td> - </tr> - <tr> - <td class="bl br">1868–1872</td> - <td class="tdr br"><div>4,593</div></td> - <td class="tdr br"><div>2,333·48</div></td> - <td class="tdr br"><div>2,259·52</div></td> - <td class="tdr br"><div>73·96</div></td> - </tr> - <tr class="bb"> - <td class="bl br">1843–1872</td> - <td class="tdr br"><div>17,249</div></td> - <td class="tdr br"><div>8,838·03</div></td> - <td class="tdr br"><div>8,410·97</div></td> - <td class="tdr br"><div>427·06</div></td> - </tr> - </table> - - <p>The reported earthquakes between 1751 and 1843 are shown to conform - with the same rule.<a id="FNanchor_101" href="#Footnote_101" class="fnanchor">[101]</a> 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:—</p> - - <p><span class="pagenum" id="Page_253">253</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>3. When the moon was north of the equator these were 68, when south 82.</p> - - <p>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.<a id="FNanchor_102" href="#Footnote_102" class="fnanchor">[102]</a></p> - - <p><em>Frequency of earthquakes in relation to the position of the sun.</em>—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 <span class="smcap">a.d.</span> 306–1843, - Mallet gives the following results:—</p> - - <table class="collapse" summary="Earthquakes in relation to sun"> - <tr> - <th class="ball"> </th> - <th colspan="4" class="ball">For Nineteenth Century</th> - <th colspan="4" class="ball">For the whole period</th> - </tr> - <tr> - <td class="tdl bl br">Winter Solstice</td> - <td class="">177</td> - <td rowspan="3" class="pl0 pr0"><span class="x300a">}</span></td> - <td colspan="2" class="tdl br">Solstices</td> - <td class="">253</td> - <td rowspan="3" class="pl0 pr0"><span class="x300a">}</span></td> - <td colspan="2" class="tdl br">Solstices</td> - </tr> - <tr> - <td class="tdl bl br">Spring Equinox</td> - <td class="">151</td> - <td rowspan="3" class="pl0 pr0 bb"><span class="x300a">}</span></td> - <td class="br">306</td> - <td class="">170</td> - <td rowspan="3" class="pl0 pr0 bb"><span class="x300a">}</span></td> - <td class="br">403</td> - </tr> - <tr> - <td class="tdl bl br">Summer Solstice</td> - <td class="">129</td> - <td class="br">Equinoxes</td> - <td class="">150</td> - <td class="br">Equinoxes</td> - </tr> - <tr class="bb"> - <td class="tdl bl br">Autumnal Equinox</td> - <td class="">164</td> - <td class=""> </td> - <td class="br">315</td> - <td class="">159</td> - <td class=""> </td> - <td class="br">329</td> - </tr> - </table> - - <p><span class="pagenum" id="Page_254">254</span></p> - - <p>The above periods were called by Perrey <em>critical epochs</em>, 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:—</p> - - <p> - In the Northern Hemisphere— - </p> - <table summary="Earthquakes in relation to sun north"> - <tr> - <td class="tdl">Equinoxes</td> - <td><b>1324</b></td> - </tr> - <tr> - <td class="tdl">Solstices</td> - <td>1202</td> - </tr> - </table> - - <p> - In the Southern Hemisphere— - </p> - <table summary="Earthquakes in relation to sun south"> - <tr> - <td class="tdl">Equinoxes</td> - <td><b>301</b></td> - </tr> - <tr> - <td class="tdl">Solstices</td> - <td>261</td> - </tr> - </table> - - <p>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.</p> - - <p>Exceptions, however, are found in Central America and the West Indies, - in the Caucasus, and the Ægean Sea.</p> - - <p>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.</p> - - <p>In the Kuriles and Kamschatka, Sicily, and in parts of South America, - it is said that the equinoxes are regarded as dangerous seasons.</p> - - <p><em>Frequency of earthquakes in relation to the seasons and months.</em>—What - is here said respecting the relative<span class="pagenum" id="Page_255">255</span> - 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.</p> - - <p>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.</p> - - <p>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:—</p> - - <table class="collapse" summary="Earthquakes in relation to sun, hemispheres"> - <tr> - <th class="ball"> </th> - <th class="ball">Maxima</th> - <th class="ball">Minima</th> - </tr> - <tr> - <td class="tdl vatop bl br">Northern Hemisphere</td> - <td class="tdl vatop br">January, also a slight rise in August and October</td> - <td class="tdl vatop br">May, June, and July</td> - </tr> - <tr class="bb"> - <td class="tdl vatop bl br">Southern Hemisphere</td> - <td class="tdl vatop br">November, also May and June</td> - <td class="tdl vatop br">March, extending over one month, also August</td> - </tr> - </table> - - <p>Julius Schmidt, of Athens, who so carefully examined the earthquakes of - eastern Europe, came to the following conclusions:—</p> - - <p>For the earthquakes between 1200 and 1873, a maximum on September 26 - and January 17; a minimum on December 3 and June 13.</p> - - <p>For the earthquakes between 1873 and 1874, a maximum on March 1 and - October 1; a minimum on July 7 and December 15.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_256">256</span></p> - - <p>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.</p> - - <div class="figcenter"> - <img src="images/i_p256.jpg" width="541" height="454" alt="" /> - <div class="caption"><span class="smcap">Fig. 36.</span>—Curves of Monthly Seismic Intensity - (Mallet).</div> - </div> - - <p>In the following table the difference in the number of earthquakes felt - at different seasons is given more in detail.</p> - - <p>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<span class="pagenum" id="Page_257">257</span> those countries. - The Roman numerals indicate the centuries between which the records date.</p> - - <table class="collapse" summary="Earthquakes in relation to season"> - <tr> - <th class="ball"> </th> - <th colspan="2" class="ball"> </th> - <th class="ball">October to March</th> - <th class="ball">April to September</th> - </tr> - <tr class="bb"> - <td rowspan="14" class="bl br">Northern Regions</td> - <td class="tdrt"><div>1.</div></td> - <td class="tdl br">Scandinavia and Iceland, xii–xix</td> - <td class="tdrb br"><div><b>129</b></div></td> - <td class="tdrb br"><div>91</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>2.</div></td> - <td class="tdl br">British and Northern Isles, xi–xix</td> - <td class="tdrb br"><div><b>123</b></div></td> - <td class="tdrb br"><div>94</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>3.</div></td> - <td class="tdl br">Belgium, France, and Holland, iv–xix</td> - <td class="tdrb br"><div><b>395</b></div></td> - <td class="tdrb br"><div>272</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>4.</div></td> - <td class="tdl br">Rhone Basin, xvi–xix</td> - <td class="tdrb br"><div><b>115</b></div></td> - <td class="tdrb br"><div>69</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>5.</div></td> - <td class="tdl br">Switzerland and Rhine Basin, ix–xix</td> - <td class="tdrb br"><div><b>327</b></div></td> - <td class="tdrb br"><div>205</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>6.</div></td> - <td class="tdl br">Danube Basin, v–xix</td> - <td class="tdrb br"><div><b>147</b></div></td> - <td class="tdrb br"><div>128</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>7.</div></td> - <td class="tdl br">Spanish Peninsula, xi–xiv</td> - <td class="tdrb br"><div><b>114</b></div></td> - <td class="tdrb br"><div>87</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>8.</div></td> - <td class="tdl br">Italy, Sicily, Sardinia, and Malta, iv–xix</td> - <td class="tdrb br"><div><b>650</b></div></td> - <td class="tdrb br"><div>581</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>9.</div></td> - <td class="tdl br">Turco-Hellenic Territory, Syria, Ægean Isles, and Levant, iv–xix</td> - <td class="tdrb br"><div>214</div></td> - <td class="tdrb br"><div><b>222</b></div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>10.</div></td> - <td class="tdl br">Northern Zone of Asia, xviii–xix</td> - <td class="tdrb br"><div><b>46</b></div></td> - <td class="tdrb br"><div>36</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>11.</div></td> - <td class="tdl br">Japan (Tokio area), 1872–1880 (small earthquakes)</td> - <td class="tdrb br"><div><b>213</b></div></td> - <td class="tdrb br"><div>157</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>12.</div></td> - <td class="tdl br">Japan <span class="smcap">b.c.</span> 295-<span class="smcap">a.d.</span> 1872 (large earthquakes)</td> - <td class="tdrb br"><div>165</div></td> - <td class="tdrb br"><div><b>188</b></div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>13.</div></td> - <td class="tdl br">Algeria and Northern Africa</td> - <td class="tdrb br"><div><b>26</b></div></td> - <td class="tdrb br"><div>20</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>14.</div></td> - <td class="tdl br">United States and Canada, xvii–xix</td> - <td class="tdrb br"><div><b>86</b></div></td> - <td class="tdrb br"><div>48</div></td> - </tr> - <tr class="bb"> - <td rowspan="5" class="bl br">Central Regions</td> - <td class="tdrt"><div>15.</div></td> - <td class="tdl br">Java, Sumatra, and neighbouring Islands, 1873–4–7–8</td> - <td class="tdrb br"><div><b>194</b></div></td> - <td class="tdrb br"><div>182</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>16.</div></td> - <td class="tdl br">Mexico and Central America, xvi–xix</td> - <td class="tdrb br"><div>26</div></td> - <td class="tdrb br"><div>26</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>17.</div></td> - <td class="tdl br">West Indies (Mallet), xvi–xix</td> - <td class="tdrb br"><div>108</div></td> - <td class="tdrb br"><div><b>114</b></div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>18.</div></td> - <td class="tdl br">West Indies, xvi–xix</td> - <td class="tdrb br"><div>296</div></td> - <td class="tdrb br"><div><b>343</b></div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>19.</div></td> - <td class="tdl br">Cuba, xvi–xix</td> - <td class="tdrb br"><div><b>28</b></div></td> - <td class="tdrb br"><div>23</div></td> - </tr> - <tr class="bb"> - <td rowspan="5" class="bl br bb">Southern Regions</td> - <td class="tdrt"><div>20.</div></td> - <td class="tdl br">Chili, and La Plata Basin, xvi–xix</td> - <td class="tdrb br"><div>89</div></td> - <td class="tdrb br"><div>89</div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>21.</div></td> - <td class="tdl br">Peru, Columbia, Basin of Amazons, xvi–xix</td> - <td class="tdrb br"><div>506</div></td> - <td class="tdrb br"><div><b>541</b></div></td> - </tr> - <tr class="bb"> - <td class="tdrt"><div>22.</div></td> - <td class="tdl br">New Zealand, 1869–1879</td> - <td class="tdrb br"><div>166</div></td> - <td class="tdrb br"><div><b>176</b></div></td> - </tr> - </table> - - <p>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<span class="pagenum" id="Page_258">258</span> - where the greatest number have been recorded for the summer.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_259">259</span> 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.</p> - - <p>For this purpose the following table, showing the distribution of - earthquakes in different countries during the nineteenth century, has - been compiled.</p> - - <p>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.</p> - - - <p class="center"><span class="smcap">Earthquakes of the Nineteenth Century, chiefly from Perrey.</span></p> - - <div class="left hang2">Key:</div> - - <table class="ml5 mb2 lh1" summary="Earthquakes by country and month key"> - <tr><td>Jan</td> <td>January</td></tr> - <tr><td>Feb</td> <td>February</td></tr> - <tr><td>Mar</td> <td>March</td></tr> - <tr><td>Apr</td> <td>April</td></tr> - <tr><td>May</td> <td>May</td></tr> - <tr><td>Jun</td> <td>June</td></tr> - <tr><td>Jul</td> <td>July</td></tr> - <tr><td>Aug</td> <td>August</td></tr> - <tr><td>Sep</td> <td>September</td></tr> - <tr><td>Oct</td> <td>October</td></tr> - <tr><td>Nov</td> <td>November</td></tr> - <tr><td>Dec</td> <td>December</td></tr> - <tr><td>Ave</td> <td>Average per month</td></tr> - </table> - - <table class="collapse" summary="Earthquakes by country and month"> - <tr> - <th class="ball"> </th> - <th class="ball">Jan</th> - <th class="ball">Feb</th> - <th class="ball">Mar</th> - <th class="ball">Apr</th> - <th class="ball">May</th> - <th class="ball">Jun</th> - <th class="ball">Jul</th> - <th class="ball">Aug</th> - <th class="ball">Sep</th> - <th class="ball">Oct</th> - <th class="ball">Nov</th> - <th class="ball">Dec</th> - <th class="ball">Ave</th> - </tr> - <tr> - <td class="tdl bl br">Scandinavia and Iceland</td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div class="large">11</div></td> - <td class="tdrb br"><div class="large">11</div></td> - <td class="tdrb br"><div>7</div></td> - <td class="tdrb br"><div>7</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div class="large">10</div></td> - <td class="tdrb br"><div class="large">10</div></td> - <td class="tdrb br"><div class="large">11</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div>9·3</div></td> - </tr> - <tr> - <td class="tdl bl br">British Isles and Northern Isles</td> - <td class="tdrb br"><div class="large">9</div></td> - <td class="tdrb br"><div class="large">9</div></td> - <td class="tdrb br"><div class="large">10</div></td> - <td class="tdrb br"><div>7</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div>5</div></td> - <td class="tdrb br"><div class="large">11</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div class="large">11</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>9<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">France, Belgium, Holland</td> - <td class="tdrb br"><div class="large">27</div></td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div class="large">21</div></td> - <td class="tdrb br"><div>13</div></td> - <td class="tdrb br"><div>13</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div>15</div></td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div>15</div></td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div class="large">21</div></td> - <td class="tdrb br"><div class="large">25</div></td> - <td class="tdrb br"><div>17<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Basin of the Rhone</td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div class="large">8</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>4</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div class="large">8</div></td> - <td class="tdrb br"><div class="large">14</div></td> - <td class="tdrb br"><div>6·6</div></td> - </tr> - <tr> - <td class="tdl bl br">Basin of the Rhine and Switzerland</td> - <td class="tdrb br"><div class="large">15</div></td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div>13</div></td> - <td class="tdrb br"><div>12</div></td> - <td class="tdrb br"><div>11</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div>12</div></td> - <td class="tdrb br"><div>11</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div class="large">24</div></td> - <td class="tdrb br"><div class="large">25</div></td> - <td class="tdrb br"><div>14<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Basin of the Danube</td> - <td class="tdrb br"><div class="large">14</div></td> - <td class="tdrb br"><div class="large">15</div></td> - <td class="tdrb br"><div>9</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div class="large">16</div></td> - <td class="tdrb br"><div>11</div></td> - <td class="tdrb br"><div>11</div></td> - <td class="tdrb br"><div class="large">16</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>11·8</div></td> - </tr> - <tr> - <td class="tdl bl br">Spanish Peninsula</td> - <td class="tdrb br"><div class="large">10</div></td> - <td class="tdrb br"><div>5</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div class="large">7</div></td> - <td class="tdrb br"><div>4</div></td> - <td class="tdrb br"><div>6</div></td> - <td class="tdrb br"><div class="large">10</div></td> - <td class="tdrb br"><div>5</div></td> - <td class="tdrb br"><div class="large">9</div></td> - <td class="tdrb br"><div class="large">11</div></td> - <td class="tdrb br"><div class="large">7</div></td> - <td class="tdrb br"><div>5</div></td> - <td class="tdrb br"><div>7<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Italian Peninsula, Sicily, Sardinia, and Malta</td> - <td class="tdrb br"><div class="large">44</div></td> - <td class="tdrb br"><div class="large">44</div></td> - <td class="tdrb br"><div class="large">48</div></td> - <td class="tdrb br"><div class="large">43</div></td> - <td class="tdrb br"><div class="large">40</div></td> - <td class="tdrb br"><div>34</div></td> - <td class="tdrb br"><div class="large">41</div></td> - <td class="tdrb br"><div class="large">46</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div class="large">45</div></td> - <td class="tdrb br"><div>26</div></td> - <td class="tdrb br"><div class="large">39</div></td> - <td class="tdrb br"><div>39<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Turco-Hellenic Territory, Syria, Ægean Islands, and Levant</td> - <td class="tdrb br"><div class="large">22</div></td> - <td class="tdrb br"><div class="large">20</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div>16</div></td> - <td class="tdrb br"><div>15</div></td> - <td class="tdrb br"><div>14</div></td> - <td class="tdrb br"><div class="large">22</div></td> - <td class="tdrb br"><div>14</div></td> - <td class="tdrb br"><div class="large">17</div></td> - <td class="tdrb br"><div>12</div></td> - <td class="tdrb br"><div>14</div></td> - <td class="tdrb br"><div>16<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Northern Zone of Asia</td> - <td class="tdrb br"><div>4</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div>4</div></td> - <td class="tdrb br"><div>4</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div class="large">7</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>4</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div>4·7</div></td> - </tr> - <tr> - <td class="tdl bl br">1876–1881, Japan (Tokio area)</td> - <td class="tdrb br"><div class="large">39</div></td> - <td class="tdrb br"><div class="large">41</div></td> - <td class="tdrb br"><div class="large">41</div></td> - <td class="tdrb br"><div>30</div></td> - <td class="tdrb br"><div class="large">33</div></td> - <td class="tdrb br"><div>30</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div>21</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div>28</div></td> - <td class="tdrb br"><div class="large">34</div></td> - <td class="tdrb br"><div class="large">43</div></td> - <td class="tdrb br"><div>31·4</div></td> - </tr> - <tr> - <td class="tdl bl br">Japan (large earthquakes)</td> - <td class="tdrb br"><div>—</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div>—</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div>—</div></td> - <td class="tdrb br"><div>—</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div>2<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Algeria and North’rn Africa</td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div class="large">7</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div class="large">8</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div>3·8</div></td> - </tr> - <tr> - <td class="tdl bl br">United States and Canada</td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>—</div></td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div class="large">7</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div>3·8</div></td> - </tr> - <tr> - <td class="tdl bl br">Java, Sumatra, &c., 1873–4–5–7 and 9</td> - <td class="tdrb br"><div class="large">35</div></td> - <td class="tdrb br"><div>30</div></td> - <td class="tdrb br"><div class="large">38</div></td> - <td class="tdrb br"><div>33</div></td> - <td class="tdrb br"><div>22</div></td> - <td class="tdrb br"><div class="large">36</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div class="large">40</div></td> - <td class="tdrb br"><div>24</div></td> - <td class="tdrb br"><div class="large">35</div></td> - <td class="tdrb br"><div>30</div></td> - <td class="tdrb br"><div>26</div></td> - <td class="tdrb br"><div>31<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Mexico and Central America</td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div>2·5</div></td> - </tr> - <tr> - <td class="tdl bl br">Antilles</td> - <td class="tdrb br"><div>9</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div class="large">19</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div>9</div></td> - <td class="tdrb br"><div class="large">16</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div class="large">13</div></td> - <td class="tdrb br"><div class="large">12</div></td> - <td class="tdrb br"><div>11·8</div></td> - </tr> - <tr> - <td class="tdl bl br">Cuba</td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div>3</div></td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div>2</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div class="large">5</div></td> - <td class="tdrb br"><div class="large">6</div></td> - <td class="tdrb br"><div class="large">4</div></td> - <td class="tdrb br"><div>4<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Chili and La Plata</td> - <td class="tdrb br"><div class="large">14</div></td> - <td class="tdrb br"><div>10</div></td> - <td class="tdrb br"><div class="large">14</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div class="large">19</div></td> - <td class="tdrb br"><div>11</div></td> - <td class="tdrb br"><div class="large">16</div></td> - <td class="tdrb br"><div class="large">15</div></td> - <td class="tdrb br"><div class="large">16</div></td> - <td class="tdrb br"><div>9</div></td> - <td class="tdrb br"><div class="large">27</div></td> - <td class="tdrb br"><div>8</div></td> - <td class="tdrb br"><div>13·9</div></td> - </tr> - <tr> - <td class="tdl bl br">Peru, Columbia, Basins of Amazons, xvi–xix</td> - <td class="tdrb br"><div class="large">92</div></td> - <td class="tdrb br"><div>83</div></td> - <td class="tdrb br"><div class="large">92</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div class="large">106</div></td> - <td class="tdrb br"><div>79</div></td> - <td class="tdrb br"><div class="large">94</div></td> - <td class="tdrb br"><div class="large">93</div></td> - <td class="tdrb br"><div class="large">97</div></td> - <td class="tdrb br"><div>77</div></td> - <td class="tdrb br"><div>72</div></td> - <td class="tdrb br"><div class="large">90</div></td> - <td class="tdrb br"><div>87<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">New Zealand, 1869–79</td> - <td class="tdrb br"><div class="large">31</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div class="large">37</div></td> - <td class="tdrb br"><div>23</div></td> - <td class="tdrb br"><div>22</div></td> - <td class="tdrb br"><div class="large">31</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div class="large">36</div></td> - <td class="tdrb br"><div class="large">37</div></td> - <td class="tdrb br"><div>21</div></td> - <td class="tdrb br"><div>27</div></td> - <td class="tdrb br"><div>23</div></td> - <td class="tdrb br"><div>28·5</div></td> - </tr> - <tr> - <td class="tdc bl br">Jan. 1850, Dec. 1857</td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Northern Hemisphere</td> - <td class="tdrb br"><div class="large">153</div></td> - <td class="tdrb br"><div class="large">162</div></td> - <td class="tdrb br"><div>143</div></td> - <td class="tdrb br"><div class="large">161</div></td> - <td class="tdrb br"><div>126</div></td> - <td class="tdrb br"><div>124</div></td> - <td class="tdrb br"><div>141</div></td> - <td class="tdrb br"><div class="large">156</div></td> - <td class="tdrb br"><div class="large">154</div></td> - <td class="tdrb br"><div class="large">171</div></td> - <td class="tdrb br"><div class="large">151</div></td> - <td class="tdrb br"><div class="large">168</div></td> - <td class="tdrb br"><div>150<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdl bl br">Southern Hemisphere</td> - <td class="tdrb br"><div class="large">72</div></td> - <td class="tdrb br"><div>43</div></td> - <td class="tdrb br"><div class="large">61</div></td> - <td class="tdrb br"><div class="large">66</div></td> - <td class="tdrb br"><div>46</div></td> - <td class="tdrb br"><div>42</div></td> - <td class="tdrb br"><div class="large">53</div></td> - <td class="tdrb br"><div>39</div></td> - <td class="tdrb br"><div class="large">54</div></td> - <td class="tdrb br"><div class="large">55</div></td> - <td class="tdrb br"><div class="large">57</div></td> - <td class="tdrb br"><div>46</div></td> - <td class="tdrb br"><div>53<span class="s">·0</span></div></td> - </tr> - <tr> - <td class="tdc bl br">1821–1830</td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - <td class="br"> </td> - </tr> - <tr> - <td class="tdl bl br">Northern Hemisphere</td> - <td class="tdrb br"><div class="large">31</div></td> - <td class="tdrb br"><div class="large">36</div></td> - <td class="tdrb br"><div class="large">31</div></td> - <td class="tdrb br"><div>29</div></td> - <td class="tdrb br"><div class="large">33</div></td> - <td class="tdrb br"><div class="large">33</div></td> - <td class="tdrb br"><div>20</div></td> - <td class="tdrb br"><div class="large">31</div></td> - <td class="tdrb br"><div>24</div></td> - <td class="tdrb br"><div class="large">41</div></td> - <td class="tdrb br"><div>26</div></td> - <td class="tdrb br"><div class="large">34</div></td> - <td class="tdrb br"><div>30<span class="s">·0</span></div></td> - </tr> - <tr class="bb"> - <td class="tdl bl br">Southern Hemisphere</td> - <td class="tdrb br"><div class="large">2</div></td> - <td class="tdrb br"><div>—</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div class="large">2</div></td> - <td class="tdrb br"><div class="large">3</div></td> - <td class="tdrb br"><div class="large">2</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div>1</div></td> - <td class="tdrb br"><div>1·6</div></td> - </tr> - </table> - - <p><span class="pagenum" id="Page_260">260</span></p> - - <p>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.</p> - - <p>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,<a id="FNanchor_103" href="#Footnote_103" class="fnanchor">[103]</a> 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.</p> - - <p><em>Earthquakes and the planets and meteors.</em>—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.</p> - - <p><span class="pagenum" id="Page_261">261</span></p> - - <p>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.</p> - - <p>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°.</p> - - <p>As a consequence of this he predicts an increase of earthquakes in the - years 1886, 1891, 1898, 1900, &c.<a id="FNanchor_104" href="#Footnote_104" class="fnanchor">[104]</a></p> - - <p>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.<a id="FNanchor_105" href="#Footnote_105" class="fnanchor">[105]</a></p> - - <p>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.<a id="FNanchor_106" href="#Footnote_106" class="fnanchor">[106]</a></p> - - <p><em>The hours at which earthquakes are most frequent.</em>—From the - examination of a catalogue of over 2,000 earthquakes - <span class="pagenum" id="Page_262">262</span> 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.</p> - - <table class="collapse" summary="Day vs. night earthquakes"> - <tr> - <th class="ball"> </th> - <th colspan="2" class="ball">Number of Earthquakes</th> - </tr> - <tr> - <th class="bl br"> </th> - <th class="br">Day</th> - <th class="br">Night</th> - </tr> - <tr> - <td class="bl br">In the Northern Hemisphere</td> - <td class="br">938</td> - <td class="br">1592</td> - </tr> - <tr class="bb"> - <td class="bl br">In the Southern Hemisphere</td> - <td class="br">292</td> - <td class="br">357</td> - </tr> - </table> - - <p class="noindent">In the northern hemisphere the greatest number were observed between 10 - <span class="smcap">p.m.</span> and 12 <span class="smcap">p.m.</span> (360 shocks), and the fewest between - 12 and 2 <span class="smcap">p.m.</span> (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.<a id="FNanchor_107" href="#Footnote_107" class="fnanchor">[107]</a> - 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 - <span class="smcap">a.m.</span>, and less frequent about 1 <span class="smcap">p.m.</span> 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 <span class="smcap">p.m.</span> and 6 <span class="smcap">a.m.</span> - <span class="pagenum" id="Page_263">263</span></p> - - <p><em>Earthquakes and sun spots.</em>—Of late years considerable attention - has been drawn to a coincidence between the occurrence of sun spots, - magnetic disturbances, rainfall, and other natural phenomena.</p> - - <p>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.</p> - - <p>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.</p> - - <p>M. R. Wolf<a id="FNanchor_108" href="#Footnote_108" class="fnanchor">[108]</a> apparently considers that earthquakes, like volcanic - eruptions and the appearance of the aurora, are coincident with sun - spots.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_109" href="#Footnote_109" class="fnanchor">[109]</a></p> - - <p><span class="pagenum" id="Page_264">264</span></p> - - <p><em>Earthquakes and the aurora.</em>—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.<a id="FNanchor_110" href="#Footnote_110" class="fnanchor">[110]</a></p> - - <p>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.</p> - - <p class="mb0">Out of these:—</p> - - <div class="left ml5"> - 48 occur on the same day,<br /> - 5 occur in the same hour,<br /> - 30 approximate to the same time. - </div> - - <p>The nearer together that these phenomena have occurred the stronger - have they been.</p> - - <p>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.<a id="FNanchor_111" href="#Footnote_111" class="fnanchor">[111]</a> 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 - <span class="pagenum" id="Page_265">265</span>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.</p> - - <p>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.<a id="FNanchor_112" href="#Footnote_112" class="fnanchor">[112]</a></p> - - <p>The aurora was observed before the commencement of the Maestricht - earthquakes in 1751<a id="FNanchor_113" href="#Footnote_113" class="fnanchor">[113]</a> whilst at the time of the shock flashes of - light like lightning were observed in the sky.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XV"> - <span class="pagenum" id="Page_266">266</span> - <h2>CHAPTER XV.<br /> - <span class="subhead">BAROMETRICAL FLUCTUATIONS AND EARTHQUAKES—FLUCTUATIONS IN TEMPERATURE - AND EARTHQUAKES.</span> - </h2> - </div> - - <p><em>Changes in the barometer and earthquakes.</em>—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> - - <p>P. Merian, who examined the connection between the Swiss earthquakes - and atmospheric pressure, found that out of twenty-two earthquakes - observed in Basle between<span class="pagenum" id="Page_267">267</span> - 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.</p> - - <p>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.</p> - - <p>Frederick Hoffmann, who examined fifty-seven earthquakes which occurred - at Palermo between 1788 and 1838, came to the following result:—</p> - - <table summary="Palermo barometer readings"> - <tr> - <td class="tdl">The barometer was sinking</td> - <td class="tdl">in 20 cases</td> - </tr> - <tr> - <td class="tdl"> „ „ rising</td> - <td class="tdl">in 16 „</td> - </tr> - <tr> - <td class="tdl"> „ „ at a minimum</td> - <td class="tdl">in 7 „</td> - </tr> - <tr> - <td class="tdl"> „ „ maximum</td> - <td class="tdl">in 3 „</td> - </tr> - <tr> - <td class="tdl"> „ „ undetermined</td> - <td class="tdl">in 11 „<a id="FNanchor_114" href="#Footnote_114" class="fnanchor">[114]</a></td> - </tr> - </table> - - <p>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.</p> - - <p>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.</p> - - <p>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:—</p> - - <p><span class="pagenum" id="Page_268">268</span></p> - - <table summary="Tokio barometer readings"> - <tr> - <td class="tdl">The barometer was rising</td> - <td class="tdl">in 169 cases</td> - </tr> - <tr> - <td class="tdl"> „ „ falling</td> - <td class="tdl">in 154 „</td> - </tr> - <tr> - <td class="tdl"> „ „ steady</td> - <td class="tdl">in 73 „</td> - </tr> - <tr> - <td class="tdl"> „ „ below the monthly mean</td> - <td class="tdl">in 189 „</td> - </tr> - <tr> - <td class="tdl"> „ „ above</td> - <td class="tdl">in 192 „</td> - </tr> - </table> - - <p>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.</p> - - <p>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.</p> - - <p><em>Changes in temperature.</em>—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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Kluge has collected together a large number of - <span class="pagenum" id="Page_269">269</span> examples when there has - been a fall of temperature at the time of an earthquake.<a id="FNanchor_115" href="#Footnote_115" class="fnanchor">[115]</a></p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XVI"> - <span class="pagenum" id="Page_270">270</span> - <h2>CHAPTER XVI.<br /> - <span class="subhead">RELATION OF SEISMIC TO VOLCANIC PHENOMENA.</span> - </h2> - </div> - - <div class="summary"> - Want of synchronism between earthquakes and volcanic - eruptions—Synchronism between earthquakes and volcanic - eruptions—Conclusion.</div> - - <p><em>Connection between earthquakes and volcanic eruptions.</em>—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 - <span class="pagenum" id="Page_271">271</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Want of synchronism between earthquakes and volcanic eruptions.</em>—Many - of the great earthquakes of South America do not appear to have been - connected with volcanic eruptions.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<span class="pagenum" id="Page_272">272</span> - In the Sandwich Islands Mauna Loa seems to have - its eruptions independently of the disturbances which shake these - islands.<a id="FNanchor_116" href="#Footnote_116" class="fnanchor">[116]</a></p> - - <p><em>Synchronism of earthquakes and volcanic eruptions.</em>—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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_117" href="#Footnote_117" class="fnanchor">[117]</a></p> - - <p><span class="pagenum" id="Page_273">273</span></p> - - <p>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.</p> - - <p>At the time of the eruptions of Kilauea in 1789 the ground shook and - rocked so that persons could not stand.</p> - - <p>The first eruption of the volcano Irasu, in Costa Rica (1783), was - accompanied by violent earthquakes.<a id="FNanchor_118" href="#Footnote_118" class="fnanchor">[118]</a> 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.</p> - - <p>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.<a id="FNanchor_119" href="#Footnote_119" class="fnanchor">[119]</a></p> - - <p>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.</p> - - <p>In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a - volcano at the foot of which Mendoza is situated burst into eruption.</p> - - <p>The earthquake of 1822 at Valdivia was accompanied by eruptions of the - neighbouring mountains, which only lasted a few minutes.</p> - - <p>At the time of the Leghorn shocks (January 16–27, 1742) some fishermen - observed a part of the sea to rage - <span class="pagenum" id="Page_274">274</span>violently, to raise itself to a - great height, and then rush landwards.<a id="FNanchor_120" href="#Footnote_120" class="fnanchor">[120]</a></p> - - <p>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.</p> - - <p>On the night of December 10, 1874, a strong shock was felt in New - England, whilst at 4.45 <span class="smcap">a.m.</span> 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.<a id="FNanchor_121" href="#Footnote_121" class="fnanchor">[121]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_275">275</span></p> - - <p>They are due to the explosive action of steam bursting through the - molten lava.</p> - - <p><em>Volcanic eruption succeeding earthquakes.</em>—Sometimes it has happened - that an earthquake, or a series of earthquakes, have terminated with - the formation of volcanic vents.</p> - - <p>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.<a id="FNanchor_122" href="#Footnote_122" class="fnanchor">[122]</a></p> - - <p>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.<a id="FNanchor_123" href="#Footnote_123" class="fnanchor">[123]</a></p> - - <p>Antonio d’Ulloa, when speaking of the Andes, remarks that after a - volcanic eruption the shocks cease.<a id="FNanchor_124" href="#Footnote_124" class="fnanchor">[124]</a></p> - - <p><em>Conclusion.</em>—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.</p> - - <p>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 <span class="pagenum" id="Page_276">276</span> - of energy, or on the varying sorts and degrees of - resistance opposed to them.’<a id="FNanchor_125" href="#Footnote_125" class="fnanchor">[125]</a></p> - - <p>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.</p> - - <p>That many earthquakes are felt at Copiapo is attributed to the fact - that in the neighbouring mountains there are no volcanic vents.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XVII"> - <span class="pagenum" id="Page_277">277</span> - <h2>CHAPTER XVII.<br /> - <span class="subhead">THE CAUSE OF EARTHQUAKES.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p>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.</p> - - <p>The part which may be played by these various causes - <span class="pagenum" id="Page_278">278</span> in the production - of oscillations, pulsations, and tremors will be referred to.</p> - - <p><em>Earthquakes consequent on faulting.</em>—In the chapter on Earth - Oscillations, the causes producing the phenomena of elevation and - depression are briefly indicated.</p> - - <p>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.</p> - - <p>Lasaulx considered that the earthquake of Herzogenrath was more - or less intimately connected with the great mountain fissure—the - <em>Feldbiss</em>—which crosses the coal region of the Wurm.<a id="FNanchor_126" href="#Footnote_126" class="fnanchor">[126]</a> The sudden - elevation or sinking of large areas at the time of an earthquake may be - a consequence of these dislocations.</p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_279">279</span> - 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 <em>end on</em>, whilst to those stations lying in a line - perpendicular to the strike of the fissure, the motion would advance - <em>broadside on</em>.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Earthquakes consequent on the explosion of steam.</em>—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.</p> - - <p>Admitting that steam may accumulate by separating - <span class="pagenum" id="Page_280">280</span> 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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_281">281</span> - 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.</p> - - <p>The chief reasons for believing that the earthquakes of North-Eastern - Japan are partly due to explosive efforts are:—</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>The earthquakes which occur at volcanic foci constitute another class - of disturbances which may be accredited to the explosive efforts of - steam.</p> - - <p><em>Earthquakes due to volcanic evisceration.</em>—By the - <span class="pagenum" id="Page_282">282</span> 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.’<a id="FNanchor_127" href="#Footnote_127" class="fnanchor">[127]</a> 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.</p> - - <p>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.</p> - - <p><em>Earthquakes and evisceration by chemical degradation.</em>—A powerful - agent, which tends to the formation of subterranean hollows, is - chemical degradation. The effects of this have been often measured by - quantitative <span class="pagenum" id="Page_283">283</span> - 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.<a id="FNanchor_128" href="#Footnote_128" class="fnanchor">[128]</a> - Many other examples of subterranean chemical degradation will be found - in text-books of geology.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_129" href="#Footnote_129" class="fnanchor">[129]</a></p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_284">284</span> - 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.</p> - - <p>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.</p> - - <p><em>Earthquakes and the attractive influences of the heavenly bodies.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_285">285</span> - motions of precession and nutation would be subject to interference.</p> - - <p>M. Delauney objected to the views of Hopkins, on the supposition that - the fluid interior of the earth had a certain viscosity.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Effect of the attractive influences of the sun and moon.</em>—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 - <span class="pagenum" id="Page_286">286</span> 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).<a id="FNanchor_130" href="#Footnote_130" class="fnanchor">[130]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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:—</p> - - <p>1. The nearness and distance of the sun from the earth (January 1 and - July 1).</p> - - <p>2. The position of the moon with regard to the earth, which in every - twenty-seven days is once near and once distant.</p> - - <p>3. The phases of the moon—whether full or new moon (syzygies), or - whether first or last quarter (quadratures).</p> - - <p>4. The equinoxes, the position of the sun in the equator, and the - relative position of the earth.</p> - - <p>5. The position of the moon relative to the equator.</p> - - <p><span class="pagenum" id="Page_287">287</span></p> - - <p>6. The concurrence of the ‘centrifugal force’ of the earth with the - last quarter of the moon.</p> - - <p>7. The entrance of the moon on the ecliptic—the so-called nodes.</p> - - <p>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.</p> - - <p>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 <span class="smcap">b.c.</span> 4000, there should have - been a great flood, and for <span class="smcap">a.d.</span> 6400 he predicts a repetition - of such an occurrence.</p> - - <p>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 - <span class="pagenum" id="Page_288">288</span> - 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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_289">289</span> 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.</p> - - <p>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.</p> - - <p>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 <em>slightly</em> 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.</p> - - <p>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<span class="pagenum" id="Page_290">290</span> - 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.</p> - - <p><em>Earthquakes and the tides.</em>—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:—</p> - - <p>‘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.’<a id="FNanchor_131" href="#Footnote_131" class="fnanchor">[131]</a></p> - - <p>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 <span class="pagenum" id="Page_291">291</span> - 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.</p> - - <p>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.</p> - - <p>Prof. G. Darwin has calculated the amount of rise and fall of a shore - line due to tidal loads (see <a href="#Page_336">p. 336</a>, ‘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.</p> - - <p>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<span class="pagenum" id="Page_292">292</span> - 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.<a id="FNanchor_132" href="#Footnote_132" class="fnanchor">[132]</a></p> - - <p>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.</p> - - <p><em>Variations in atmospheric pressure.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_293">293</span> - variation and the times at which earthquakes have occurred.</p> - - <p>Three important laws of barometric variation are the following:—</p> - - <p>1. In the world generally the average barometric pressure is highest in - winter. (Exceptions occur near Iceland and in the North Pacific.)</p> - - <p>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.</p> - - <p>3. The greatest number of barometrical fluctuations usually take place - in winter.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_294">294</span> - 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.</p> - - <p>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.</p> - - <p><em>Fluctuations in temperature.</em>—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.</p> - - <p>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.</p> - - <p><em>Winds and earthquakes.</em>—Although it may be - <span class="pagenum" id="Page_295">295</span> 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.</p> - - <p>Storms are usually accompanied with a barometric depression, and the - relation of diminutions in atmospheric pressure to earthquakes has been - discussed.</p> - - <p><em>Rain and earthquakes.</em>—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.</p> - - <p><em>Conclusion.</em>—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<span class="pagenum" id="Page_296">296</span> - 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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XVIII"> - <span class="pagenum" id="Page_297">297</span> - <h2>CHAPTER XVIII.<br /> - <span class="subhead">PREDICTION OF EARTHQUAKES.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p><em>General nature of predictions.</em>—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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_298">298</span> - 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_133" href="#Footnote_133" class="fnanchor">[133]</a> No doubt many who - dwell in earthquake countries, and have been alarmed by earthquakes, - are at times subject to nervous expectancy.</p> - - <p>The author has had such sensations himself, due, perhaps, - <span class="pagenum" id="Page_299">299</span> 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.</p> - - <p>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.</p> - - <p><em>Prediction by the observation of natural phenomena.</em>—The above remarks - may perhaps help us to understand the prognostications of the ancient - philosophers about which Professor Antonio Favaro, of Padua, has - written.<a id="FNanchor_134" href="#Footnote_134" class="fnanchor">[134]</a> Cicero in the ‘De Divinatione,’ speaking on this subject, - says that ‘God has not predicted so much - <span class="pagenum" id="Page_300">300</span>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Before the earthquake of 1868, so severely felt at Iquique, the - inhabitants were terrified by loud subterranean noises.</p> - - <p>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.</p> - - <p>Farmers predicted the earthquake of St. Remo, in 1831, by underground - noises.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_301">301</span></p> - - <p>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.</p> - - <p>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.<a id="FNanchor_135" href="#Footnote_135" class="fnanchor">[135]</a></p> - - <p>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.’<a id="FNanchor_136" href="#Footnote_136" class="fnanchor">[136]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_302">302</span> - the records of those prognostications as the survival - of accidental guesses, and, as such, examples of the survival of the - useless.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_303">303</span> - supposed to be endowed with seismic foresight, whose verdicts - are much relied upon.</p> - - <p>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.<a id="FNanchor_137" href="#Footnote_137" class="fnanchor">[137]</a> 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.</p> - - <p>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.</p> - - <p><em>Warnings furnished by animals.</em>—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.</p> - - <p>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 - <span class="pagenum" id="Page_304">304</span>last shock it is also related - that all the dogs escaped from the city of Talcahuano.</p> - - <p><em>Earthquake warning.</em>—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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>What the results of the observations on earth tremors will lead to is - problematical.</p> - - <p>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.</p> - - <p>As to whether the movements of tromometers are destined to become - barometric-like warnings of increased - <span class="pagenum" id="Page_305">305</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XIX"> - <span class="pagenum" id="Page_306">306</span> - <h2>CHAPTER XIX.<br /> - <span class="subhead">EARTH TREMORS.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p class="noindent"><span class="smcap">During</span> 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 <em>earth tremors</em>. 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.</p> - - <p><em>Artificially produced tremors.</em>—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.</p> - - <p><span class="pagenum" id="Page_307">307</span></p> - - <p>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.</p> - - <p>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.<a id="FNanchor_138" href="#Footnote_138" class="fnanchor">[138]</a></p> - - <p>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.</p> - - <p>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 <span class="pagenum" id="Page_308">308</span> - 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.<a id="FNanchor_139" href="#Footnote_139" class="fnanchor">[139]</a></p> - - <p>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.<a id="FNanchor_140" href="#Footnote_140" class="fnanchor">[140]</a></p> - - <p>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.</p> - - <p><em>Natural tremors.</em>—Next let us turn to those microscopical disturbances - of our soil which are due to natural - <span class="pagenum" id="Page_309">309</span>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_310">310</span></p> - - <p>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.<a id="FNanchor_141" href="#Footnote_141" class="fnanchor">[141]</a></p> - - <p><em>Observations at Cambridge.</em>—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.</p> - - <p>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 - <span class="pagenum" id="Page_311">311</span>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.<a id="FNanchor_142" href="#Footnote_142" class="fnanchor">[142]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p><em>Experiments in Japan.</em>—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.</p> - - <p>A simple contrivance which may be used for the purpose of recording - small earthquakes can be made with a small compass needle.</p> - - <p>If a light, small sensitive compass needle be placed on - <span class="pagenum" id="Page_312">312</span>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.</p> - - <p>With crude apparatus like this a large number of small earth - disturbances have been recorded.</p> - - <p>Another form of apparatus, employed in Japan, has been a delicately - constructed <em>circuit closer</em>. 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_313">313</span></p> - - <p>In addition to these and other contrivances, experiments were made with - microphones.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_314">314</span> house so that it fell near to the - microphone pit caused a sharp creak in the telephone and a movement in - the needle.</p> - - <p>The nature of the records I received from this contrivance may be - judged of from the following extract from my papers.</p> - - <table summary="Microphone contrivance readings"> - <tr> - <th> </th> - <th class="tdl"> h. m.</th> - </tr> - <tr> - <td class="tdl">1879. Nov. 12th</td> - <td class="tdl">7 0 <span class="smcap">p.m.</span> contact of needle</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 2 „ difficult to set the needle</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 3 „ needle swings and telephone creaks</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 4 „ „ „ „</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 5 „ „ „ „</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 6 „ „ „ „</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 10 „ 3 more swings</td> - </tr> - <tr> - <td class="tdl"> </td> - <td class="tdl">7 11 „ again „</td> - </tr> - </table> - - <p class="noindent">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.</p> - - <p>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 <span class="smcap">a.m.</span>, the needle was found in contact, and - again at 5 <span class="smcap">p.m.</span>, and at 6 <span class="smcap">p.m.</span> the shock of a small - earthquake was felt <em>which caused a rattling sound in the telephone for - about one minute after the motion had appeared to cease</em>. The needle - swung considerably, but did not come in contact.</p> - - <p>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 - <span class="pagenum" id="Page_315">315</span> 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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_316">316</span> - 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.</p> - - <p><em>Work in Italy.</em>—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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_143" href="#Footnote_143" class="fnanchor">[143]</a></p> - - <p>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.</p> - - <p>Similar investigations were made at Nice by M. le Baron Prost.</p> - - <p>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<span class="pagenum" id="Page_317">317</span> - 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.</p> - - <p>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.</p> - - <p><em>Instruments employed in Italy.</em>—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.</p> - - <p>The most important of these instruments is the <em>Normal Tromometer</em> of - Bertelli and Rossi.</p> - - <p>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<span class="pagenum" id="Page_318">318</span> - wire. This instrument is shown in the accompanying drawing.</p> - - <div class="figleft"> - <img src="images/i_p318.jpg" width="303" height="545" alt="" /> - <div class="caption"> - <span class="smcap">Fig. 37.</span>—Normal Tromometer.<br /> - <span class="small"> - <span class="smcap">b</span>, bob of pendulum; <span class="smcap">p</span>, prism; - <span class="smcap">m</span>, microscope; <span class="smcap">s</span>, rim of scale. - </span> - </div> - </div> - - <p>Another apparatus is the <em>Microseismograph</em> 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<span class="pagenum" id="Page_319">319</span> - five pendulums being of different lengths allows the apparatus to - respond ‘to seismic waves of different - velocities.’<a id="FNanchor_144" href="#Footnote_144" class="fnanchor">[144]</a></p> - - <p>Lastly, we have Professor Rossi’s <em>Microphone</em>. 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 <em>vertical</em> - direction causes a fluctuation in the current passing between the stop - and the beam, and a consequent noise is heard in the telephone.</p> - - <p>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.</p> - - <p><em>Results obtained in Italy.</em>—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.</p> - - <p>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 - <span class="pagenum" id="Page_320">320</span>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 <em>vulcano seismic</em> movements. The relation of these storms - to barometric fluctuation has been observed to have been very marked - during the time of a volcanic eruption.</p> - - <p>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.</p> - - <p>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, - <span class="pagenum" id="Page_321">321</span> 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.</p> - - <div class="figcenter"> - <img src="images/i_p321.jpg" width="542" height="444" alt="" /> - <div class="caption"><span class="smcap">Fig. 38.</span></div> - </div> - - <p>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<span class="pagenum" id="Page_322">322</span> - the close accord there is between the results obtained at - different towns during successive months.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Storms of microseismical motions appear to travel from point to point.</p> - - <p>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> - - <p>P. C. Melzi says that the curves of microseismical motions, - earthquakes, lunar and solar motions, show a concordance with each - other.</p> - - <p><span class="pagenum" id="Page_323">323</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_324">324</span></p> - - <p>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.’</p> - - <p><em>Cause of microseismic movements.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_325">325</span> - 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.</p> - - <p>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.</p> - - <table summary="Typhoon barometer readings"> - <tr> - <th>Time<br />h. m.</th> - <th>Barometer<br />reading</th> - </tr> - <tr> - <td> 12 5 <span class="smcap">p.m.</span></td> - <td class="tdc">29·02</td> - </tr> - <tr> - <td> 12 10 „</td> - <td class="tdc">29·05</td> - </tr> - <tr> - <td> 12 12 „</td> - <td class="tdc">29·07</td> - </tr> - <tr> - <td> 12 13 „</td> - <td class="tdc">29·05</td> - </tr> - <tr> - <td> 12 25 „</td> - <td class="tdc">29·10</td> - </tr> - <tr> - <td> 12 50 „</td> - <td class="tdc">29·00</td> - </tr> - <tr> - <td> 1 10 „</td> - <td class="tdc">29·00</td> - </tr> - <tr> - <td> 1 20 „</td> - <td class="tdc">29·07</td> - </tr> - </table> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XX"> - <span class="pagenum" id="Page_326">326</span> - <h2>CHAPTER XX.<br /> - <span class="subhead">EARTH PULSATIONS.</span> - </h2> - </div> - - <div class="summary"> - 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.</div> - - <p class="noindent"><span class="smcap">The</span> 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.</p> - - <p>These movements, which have escaped our attention on account of their - slowness in period, for want of another term I call earth pulsations.</p> - - <p>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.</p> - - <p><em>Indication of pendulums.</em>—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.</p> - - <p>This motion has simply taken place on one side of their central - position, and is not due to a swing. The - <span class="pagenum" id="Page_327">327</span> 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.</p> - - <p>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.</p> - - <p>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<span class="pagenum" id="Page_328">328</span> - 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.</p> - - <p><em>Indications of levels.</em>—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.</p> - - <p>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 <span class="smcap">a.m.</span> and <span class="smcap">p.m.</span> 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.</p> - - <p>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.</p> - - <p>Between October 4, 1879, and January 28, 1880, the movement was 95·8″, - against 28·08″ of the previous year.</p> - - <p>These movements were not due alone to temperature. The north and south - level, which was not influenced by<span class="pagenum" id="Page_329">329</span> - the cold of the winter, moved - 4·56″. In the previous year 4·89″.<a id="FNanchor_145" href="#Footnote_145" class="fnanchor">[145]</a></p> - - <p>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.</p> - - <p>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:—</p> - - <table summary="Tokio level readings"> - <tr> - <th> </th> - <th class="left">Time<br />h. m.</th> - <th>Readings.</th> - </tr> - <tr> - <td>25th.</td> - <td class="left">4 00 <span class="smcap">p.m.</span></td> - <td class="left">104·5</td> - </tr> - <tr> - <td> </td> - <td class="left">4 5 „</td> - <td class="left">103</td> - </tr> - <tr> - <td> </td> - <td class="left">4 10 „</td> - <td class="left">102</td> - </tr> - <tr> - <td> </td> - <td class="left">4 25 „</td> - <td class="left">101</td> - </tr> - <tr> - <td> </td> - <td class="left">4 30 „</td> - <td class="left">100</td> - </tr> - <tr> - <td> </td> - <td class="left">4 40 „</td> - <td class="left"> 98</td> - </tr> - <tr> - <td> </td> - <td class="left">4 42 „</td> - <td class="left"> 99·5</td> - </tr> - <tr> - <td> </td> - <td class="left">4 45 „</td> - <td class="left">100</td> - </tr> - <tr> - <td> </td> - <td class="left">4 50 „</td> - <td class="left">101</td> - </tr> - <tr> - <td> </td> - <td class="left">4 55 „</td> - <td class="left">101</td> - </tr> - <tr> - <td> </td> - <td class="left">5 00 „</td> - <td class="left">100</td> - </tr> - <tr> - <td>26th.</td> - <td class="left">7 00 <span class="smcap">a.m.</span></td> - <td class="left">105</td> - </tr> - </table> - - <p>Usually this level moves through about three divisions per day.</p> - - <p>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 <em>comparatively</em> quiet. One division of the north south level - equals about 2″ of arc.</p> - - <p><span class="pagenum" id="Page_330">330</span></p> - - <p>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.</p> - - <p>The fact also that at the time of a barometrical depression a - <em>pulse-like surge</em> 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.<a id="FNanchor_146" href="#Footnote_146" class="fnanchor">[146]</a></p> - - <p>In addition to variation in the bubbles of levels which come on more or - less gradually, we have many recorded instances of <em>sudden</em> alterations - taking place in these instruments.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_331">331</span>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.</p> - - <p><em>Phenomena analogous to the pendulum and level observations.</em>—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 - <span class="smcap">p.m.</span> 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.<a id="FNanchor_147" href="#Footnote_147" class="fnanchor">[147]</a></p> - - <p>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.<a id="FNanchor_148" href="#Footnote_148" class="fnanchor">[148]</a></p> - - <p>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.</p> - - <p>Thus at Amsterdam and other towns, chandeliers in churches were - observed to swing. At Haarlem water was - <span class="pagenum" id="Page_332">332</span>thrown over the sides of tubs, - and it is expressly mentioned that no motion was perceived in the - ground.</p> - - <p>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.</p> - - <p><em>Unusual disturbances in bodies of water.</em>—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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_333">333</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>During these motions there were several maxima.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.<a id="FNanchor_149" href="#Footnote_149" class="fnanchor">[149]</a> On March 31, 1761, - <span class="pagenum" id="Page_334">334</span>Loch Ness rose suddenly for the period - of three-quarters of an hour.<a id="FNanchor_150" href="#Footnote_150" class="fnanchor">[150]</a></p> - - <p>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.</p> - - <p>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.</p> - - <p>In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’</p> - - <p>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.<a id="FNanchor_151" href="#Footnote_151" class="fnanchor">[151]</a></p> - - <p>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.<a id="FNanchor_152" href="#Footnote_152" class="fnanchor">[152]</a></p> - - <p>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 <span class="pagenum" id="Page_335">335</span> - 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.</p> - - <p>Sometimes they break suddenly upon the coast. ‘<em>They are annual and - constant in their periodicity.</em>’</p> - - <p>The periodical swellings are most noticeable between Tumbez 3° S.L. and - the Chincha Islands 14° S.L.</p> - - <p>These oceanic phenomena synchronise with the periodic intensity of - earthquake phenomena in that part of the globe, and these with tidal - movements.<a id="FNanchor_153" href="#Footnote_153" class="fnanchor">[153]</a></p> - - <p><em>Other phenomena possibly attributable to earth pulsations.</em>—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 - <span class="pagenum" id="Page_336">336</span>earthquake, - as for instance in Derbyshire, may have been produced by pulsations - disturbing the equilibrium of ground in a critical state.</p> - - <p>The falling in of subterranean excavations is also possibly connected - with these phenomena.</p> - - <p><em>Possible causes of earth pulsations.</em>—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.)</p> - - <p>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.</p> - - <p>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.</p> - - <p>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 - <span class="pagenum" id="Page_337">337</span> - surface must be taking place, causing variations in inclination of one - portion of the earth’s crust relatively to another.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_338">338</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Should these pressures then find relief without rupturing the - surface, it is not difficult to imagine them as the - <span class="pagenum" id="Page_339">339</span> originators of - vast pulsations which may be recorded on the surface of the earth as - wave-like motions of slow period.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_340">340</span></p> - - <p>As examples of these actions I will quote the following.</p> - - <p>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.<a id="FNanchor_154" href="#Footnote_154" class="fnanchor">[154]</a></p> - - <p>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.</p> - - <p>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, <span class="pagenum" id="Page_341">341</span> - 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.</p> - - <p>These illustrations are given as examples out of a large series of - other records, all showing like results.</p> - - <p>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.</p> - - <p>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.</p> - - <p><span class="pagenum" id="Page_342">342</span></p> - - <p>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.</p> - - <p>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.</p> - - <p>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.</p> - - <p>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. - <span class="pagenum" id="Page_343">343</span> 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.</p> - - <hr class="page" /> - <div class="chapter" id="CHAPTER_XXI"> - <span class="pagenum" id="Page_344">344</span> - <h2>CHAPTER XXI.<br /> - <span class="subhead">EARTH OSCILLATIONS.</span> - </h2> - </div> - - <div class="summary"> - Evidences of oscillation—Examples of oscillation—Temple of Jupiter - Serapis—Observations of Darwin—Causes of oscillation.</div> - - <p><em>Evidences of oscillation.</em>—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.</p> - - <p>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<span class="pagenum" id="Page_345">345</span> - 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.</p> - - <p><em>Examples of movements.</em>—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.<a id="FNanchor_155" href="#Footnote_155" class="fnanchor">[155]</a></p> - - <p>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.<a id="FNanchor_156" href="#Footnote_156" class="fnanchor">[156]</a></p> - - <p>Another remarkable example of earth movement is - <span class="pagenum" id="Page_346">346</span>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.<a id="FNanchor_157" href="#Footnote_157" class="fnanchor">[157]</a></p> - - <p>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.<a id="FNanchor_158" href="#Footnote_158" class="fnanchor">[158]</a></p> - - <p>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:—</p> - - <p><span class="pagenum" id="Page_347">347</span></p> - - <table summary="Darwin's observations"> - <tr> - <th> </th> - <th class="tdr"><div>Feet</div></th> - </tr> - <tr> - <td class="tdl">At Chiloe the recent elevation has been</td> - <td class="tdr"><div>350</div></td> - </tr> - <tr> - <td class="tdl"> „ Concepcion „ „</td> - <td class="tdr"><div>625 to 1,000</div></td> - </tr> - <tr> - <td class="tdl"> „ Valparaiso „ „</td> - <td class="tdr"><div>1,300</div></td> - </tr> - <tr> - <td class="tdl"> „ Coquimbo „ „</td> - <td class="tdr"><div>252</div></td> - </tr> - <tr> - <td class="tdl"> „ Lima „ „</td> - <td class="tdr"><div>85</div></td> - </tr> - </table> - - <p>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.</p> - - <p>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.<a id="FNanchor_159" href="#Footnote_159" class="fnanchor">[159]</a> - 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.</p> - - <p>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, <span class="pagenum" id="Page_348">348</span> - inasmuch as the rock in which they are found is soft and easily - weathered, indicate an exceedingly rapid elevation, earthquakes are of - common occurrence.</p> - - <p>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.</p> - - <p>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.</p> - - <p>Sudden displacements which occasionally accompany earthquakes might, it - was said, sometimes be regarded as the <em>cause</em> of an earthquake, and - sometimes as the <em>effect</em>.</p> - - <p>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.</p> - - <p>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.</p> - - <hr class="page" /> - <div class="chapter" id="APPENDIX"> - <span class="pagenum" id="Page_349">349</span> - <h2>APPENDIX.</h2> - </div> - - <div class="figcenter"> - <img src="images/divider.png" width="80" height="9" alt="" /> - </div> - - <h3><span class="smcap">List of the Principal Books, Papers, Periodicals, which are - referred to in the Preceding Pages.</span></h3> - - <hr class="r15" /> - <p class="hang1"><i>For a more complete bibliography of earthquakes refer to Mallet’s - catalogue of works given in his report to the British Association - in 1858.</i></p> - - <hr class="r15" /> - <p class="hang1">A True and Particular Relation of the Dreadful Earthquake which - happened at Lima, &c. (1746). 1768.</p> - - <p class="hang1">Abbot, Gen. H. L. On the Velocity of Transmission of Earth Waves. - <cite>Am. Jour. Sci.</cite> XV., March 1878.</p> - - <p class="hang1">— Shock of the Explosion at Hallet’s Point, Nov. 14, 1876. - <i>Battalion Press.</i></p> - - <p class="hang1">Alexander, Prof. T. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">American Journal of Science.</p> - - <p class="hang1">Annali del reale osservatorio meteorologico Vesuviano.</p> - - <p class="hang1">Annual Register, The.</p> - - <p class="hang1">Anonymous, A Chronological and Historical Account of the most - Memorable Earthquakes in the World, &c. 1750.</p> - - <p class="hang1">— A Vindication of the Bishop of London’s Letter occasioned by the - Late Earthquake. 1750.</p> - - <p class="hang1">— Phenomena of the Great Earthquake of Nov. 1, 1755.</p> - - <p class="hang1">— Serious Thoughts occasioned by the Late Earthquake at Lisbon. - 1755.</p> - - <p class="hang1">Asiatic Society of Japan, Transactions of.</p> - - <p class="hang1">Ayrton, Prof. W. E. <i>See</i> Perry, J.</p> - - <p> </p> - - <p class="hang1">Bárceno, M. Estudio del Terremoto (May 17, 1879) Mexico. 1879.</p> - - <p class="hang1">Beke, Dr. C. T. Mount Sinai a Volcano.</p> - - <p><span class="pagenum" id="Page_350">350</span></p> - - <p class="hang1">Bissett, Rev. J. A Sermon (on account of the Earthquake at Lisbon, - Nov. 1, 1755). 1757.</p> - - <p class="hang1">Bittner, A. Beiträge zur Kenntniss des Erdbebens von Belluno vom - 29. Juni 1873.</p> - - <p class="hang1">— Sitzungsb. der K. Akad. d. Wissensch., lxix. II. Abth., 1874.</p> - - <p class="hang1">Bollettino del Vulcanismo Italiano.</p> - - <p class="hang1">Boué, Dr. A. Ueber das Erdbeben welches Mittel-Albanien im - October d. J. so schrecklich getroffen hat. <cite>Die K. Akad. d. - Wissenschaften</cite>, Nov. 1851.</p> - - <p class="hang1">— Parallele der Erdbeben, des Nordlichtes und des Erdmagnetismus.</p> - - <p class="hang1">— Ueber die Nothwendigkeit die Erdbeben und vulkanischen - Erscheinungen genauer als bis jetzt beobachten zu lassen. <cite>Die K. - Akad. d. Wissenschaften</cite>, 1851 and 1857.</p> - - <p class="hang1">Bouguer, M. Of the Volcanoes and Earthquakes in Peru.</p> - - <p class="hang1">British Association, Reports of.</p> - - <p class="hang1">Brunton, R. H. Constructive Art in Japan. <cite>Trans. Asiatic Soc. of - Japan</cite>, II. and III., Pt. 2.</p> - - <p class="hang1">Bryce, J. Report to British Association, 1841.</p> - - <p class="hang1">Buffour, M. The Natural History of Earthquakes and Volcanoes.</p> - - <p> </p> - - <p class="hang1">C. H. A Physical Discussion of Earthquakes, &c. 1693.</p> - - <p class="hang1">Canterbury, Thomas, Lord Archbishop of, The Theory and History of - Earthquakes.</p> - - <p class="hang1">Casariego, E. A. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Cawley, G. Some Remarks on Construction in Brick and Wood, &c. - <cite>Trans. Asiatic Soc. of Japan</cite>, VI. Plate ii.</p> - - <p class="hang1">Chaplin, Prof. W. S. An Examination of the Earthquakes recorded - at the Meteorological Observatory, Tokio. <cite>Trans. Asiatic Soc. of - Japan</cite>, VI. Part ii.</p> - - <p class="hang1">Comptes Rendus.</p> - - <p class="hang1">Credner, H. Das Dippoldiswalder Erdbeben vom Oktober 1877.</p> - - <p class="hang1">— Zeitschr. f. d. Naturwiss. f. Sachsen u. Thüringen.</p> - - <p class="hang1">— Das Vogtländisch-erzgebirgische Erdbeben, 23. Nov. 1875.</p> - - <p class="hang1">— Zeitschr. f. d. gesammt. Naturwissenschaften, xlviii., Oktober.</p> - - <p> </p> - - <p class="hang1">Dan, T. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Darwin, Charles. Researches on Geology and Natural History.</p> - - <p class="hang1">— Geological observations.</p> - - <p class="hang1">Darwin, G. H. Reports on Lunar Disturbance of Gravity to British - Association, 1881. 1882.</p> - - <p><span class="pagenum" id="Page_351">351</span></p> - - <p class="hang1">Diffenbach, F. Plutonismus und Vulkanismus in der Periode von - 1868–1872, und ihre Beziehungen zu den Erdbeben im Rheingebiet.</p> - - <p class="hang1">Doelter, C. von. Ueber die Eruptivgebilde von Fleims, nebst einigen - Bemerkungen über den Bau älterer Vulcane.</p> - - <p class="hang1">— lxxiv. Band d. Sitzungsb. d. K. Akad. d. Wissensch., I. Abth., - Dec. Heft, Jahrg. 1876.</p> - - <p class="hang1">Doolittle, Rev. T. Earthquakes Explained and Practically Improved, - &c. 1693.</p> - - <p class="hang1">Doyle, P. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p> </p> - - <p class="hang1">Emerson, Prof., B.A. Review of Von Seebachs’ Earthquake of March 6, - 1872. <cite>Am. Jour. Sci.</cite>, Series III.</p> - - <p class="hang1">Ewing, Prof. J. A. Earthquake Measurement. A memoir published by - the Tokio University. 1883.</p> - - <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p> </p> - - <p class="hang1">Falb, R. Gedanken und Studien über den Vulkanismus, &c. 1875.</p> - - <p class="hang1">— Grundzüge zu einer Theorie der Erdbeben und Vulkanausbrüche.</p> - - <p class="hang1">— Das Erdbeben von Belluno. ‘Sirius,’ Bd. VI., Heft ii.</p> - - <p class="hang1">Flamstead, J. A Letter concerning Earthquakes. 1693.</p> - - <p class="hang1">Forel, F. A. Les Tremblements de Terre (Suisse). <cite>Arch. des - Sciences Physiques et Naturelles</cite>, VI. p. 461.</p> - - <p class="hang1">— Tremblement de Terre du 30 Décembre 1879.</p> - - <p class="hang1">Fuchs, Karl. Vulkane und Erdbeben.</p> - - <p class="hang1">— <cite>Die Vulkanischen Erscheinungen der Erde.</cite></p> - - <p> </p> - - <p class="hang1">Garcia, J. C. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Geinitz, Dr. E. Das Erdbeben von Iquique am 9. Mai 1877, &c. <cite>Die - K. Leop.-Carol.-Deutschen Akademie der Naturforscher</cite>, Band xl., Nr. 9.</p> - - <p class="hang1">Gentleman’s Magazine, The.</p> - - <p class="hang1">Geographical Society, Proceedings of.</p> - - <p class="hang1">Geological Society, Proceedings of.</p> - - <p class="hang1">Girard, Dr. H. Ueber Erdbeben und Vulkane. 1845.</p> - - <p class="hang1">Gray, T. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">— On Instruments for Measuring and Recording Earthquake Motions. - Phil. Mag. Sept. 1881.</p> - - <p class="hang1">— On Recent Earthquake Investigation. <cite>The Chrysanthemum</cite>, 1881. - Guiscardi, Prof. G. Notizie del Vesuvio. 1857.</p> - - <p class="hang1">— Il terremoto di Casamicciola del 4 Marzo. 1881.</p> - - <p> </p> - - <p><span class="pagenum" id="Page_352">352</span></p> - - <p class="hang1">Hales, S., D.D., F.R.S. Some Considerations on the Causes of - Earthquakes. 1750.</p> - - <p class="hang1">Hamilton, Sir W. Observations on Mount Vesuvius, Mount Etna, &c. - 1774.</p> - - <p class="hang1">Hattori, I. Destructive Earthquakes in Japan. <cite>Trans. Asiatic Soc. - of Japan</cite>, V. Plate i.</p> - - <p class="hang1">Heim, Prof. A. Les Tremblements de Terre et leur Etude - Scientifique. 1880.</p> - - <p class="hang1">— Prof. A. Die Schweizerischen Erdbeben in 1881–1882.</p> - - <p class="hang1">Hoeffer, Prof. H. Die Erdbeben Kärntens und deren Stosslinien. <cite>Die - Kais. Akademie d. Wissenschaften</cite>, Band xlii.</p> - - <p class="hang1">Höfer, Prof. H. Das Erdbeben von Belluno, am 29. Juni 1873. - <cite>Sitzungsb. der K. Akad. d. Wissensch.</cite>, I. Abth., Band lxxiv.</p> - - <p class="hang1">Hoff, K. E. A. von. Geschichte der durch Ueberlieferung - nachgewiesenen natürlichen Veränderungen der Erdoberfläche. 1822.</p> - - <p class="hang1">Hooke, R., M.D., F.R.S. Discourses concerning Earthquakes.</p> - - <p class="hang1">Hopkins, William. Report to the British Association on the - Geological Theories of Elevation and Earthquakes. 1847.</p> - - <p class="hang1">Horton, Rev. Mr. An Account of the Earthquake which happened at - Leghorn in Italy (Jan. 1742). 1750.</p> - - <p class="hang1">Humboldt, Alexander von. Cosmos.</p> - - <p class="hang1">— Travels.</p> - - <p> </p> - - <p class="hang1">Jeitteles, L. A. Bericht über das Erdbeben am 15. Januar 1858.</p> - - <p class="hang1">— Sitzungsberichte der mathem.-naturw. Classe d. K. Akad. d. - Wissensch., xxxv. S. 511.</p> - - <p class="hang1">Judd, J. W., Prof. Volcanoes, What they Are and What they Teach.</p> - - <p> </p> - - <p class="hang1">Knipping, E. Verzeichniss von Erdbeben wahrgenommen in Tokio, - &c. <cite>Mitt. d. Deutsch. Gesellsch. für Natur- und Völkerkunde - Ostasiens</cite>, Heft 14.</p> - - <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p> </p> - - <p class="hang1">Lasaulx, A. von. Das Erdbeben von Herzogenrath am 22. October 1873.</p> - - <p class="hang1">Lemery, M. A Physico-Chemical Explanation of Subterranean Fires, - Earthquakes, &c.</p> - - <p class="hang1">Lescasse, M. J. Etude sur les Constructions Japonaises, &c. - <cite>Mémoires de la Société des Ingénieurs Civils</cite>.</p> - - <p class="hang1">Lister, M., M.D., F.R.S. Of the Nature of Earthquakes.</p> - - <p class="hang1">Little, Rev. J. Conjectures on the Physical Causes of Earthquakes - and Volcanoes. 1820.</p> - - <p> </p> - - <p><span class="pagenum" id="Page_353">353</span></p> - - <p class="hang1">Mallet, R. The Neapolitan Earthquake, Vol. II. <cite>Reports to the - British Association</cite>, 1850, 1851, 1852, 1854, 1858, 1861.</p> - - <p class="hang1">— Secondary Effects of the Earthquake of Cachar. <cite>Proc. Geolog. - Soc.</cite>, 1872.</p> - - <p class="hang1">— Dynamics of Earthquakes. <cite>Trans. Royal Irish Acad.</cite> 1846.</p> - - <p class="hang1">Milne, David. Reports to British Association, 1841, 1843, 1844.</p> - - <p class="hang1">Milne, John. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">— On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) <cite>Trans. - Royal Soc.</cite> 1882.</p> - - <p class="hang1">— On Seismic Experiments (with T. Gray, B.Sc., F.R.S.E.) <cite>Proc. - Royal Soc.</cite> No. 217, 1881.</p> - - <p class="hang1">— Earthquake Observations and Experiments in Japan (with T. Gray, - B.Sc., F.R.S.E.) <cite>Phil. Mag.</cite>, Nov. 1881.</p> - - <p class="hang1">— On the Elasticity and Strength Constants of certain Rocks (with - T. Gray, B.Sc., F.R.S.E.) <cite>Jour. Geolog. Soc.</cite>, 1882.</p> - - <p class="hang1">— A Visit to the Volcano of Oshima. <cite>Geolog. Mag.</cite>, Dec. 2, Vol. - IV., pp. 193–197, 255.</p> - - <p class="hang1">— On the Form of Volcanoes. <cite>Geolog. Mag.</cite>, Dec. 2, Vol. V., and - Dec. 2, Vol. VI.</p> - - <p class="hang1">— Note upon the Cooling of the Earth, &c. <cite>Geolog. Mag.</cite>, Dec. 2., - Vol. VII., p. 99.</p> - - <p class="hang1">— Investigation of the Earthquake Phenomena of Japan. <cite>Rep. Brit. - Assoc.</cite>, 1881 and 1882.</p> - - <p class="hang1">— A Large Crater. <cite>Popular Science Review.</cite></p> - - <p class="hang1">— The Volcanoes of Japan (a series of Articles). <cite>Japan Gazette.</cite></p> - - <p class="hang1">— Earthquake Literature of Japan (a series of Articles). <cite>Japan Gazette.</cite></p> - - <p class="hang1">— The Earthquake of Dec. 23, 1880. <cite>The Crysanthemum</cite>, 1881.</p> - - <p class="hang1">— Earthquake Motion. <cite>The Crysanthemum</cite>, 1882.</p> - - <p class="hang1">— Seismology in Japan. <cite>Nature</cite>, Oct. 1882.</p> - - <p class="hang1">— Earth Movements. <cite>The Times</cite>, Oct. 12, 1882.</p> - - <p class="hang1">Mitchell, Rev. J. Conjectures Concerning the Cause and Observations - upon the Phenomena of Earthquakes. 1760.</p> - - <p class="hang1">Mohr, Dr. F. Geschichte der Erde. 1875.</p> - - <p> </p> - - <p class="hang1">Naturkundig Tijdschrift voor Nederlandsch Indie. 1875–1880.</p> - - <p class="hang1">Naumann, Dr. E. Ueber Erdbeben und Vulkanausbrüche in Japan. <cite>Mitt. - d. Deutsch. Gesellsch. für Natur- und Völkerkunde Ostasiens.</cite> Heft 15.</p> - - <p><span class="pagenum" id="Page_354">354</span></p> - - <p class="hang1">Noggerath, Dr. J. Die Erdbeben vom 29. Juli 1846 im Rheingebiet, &c.</p> - - <p class="hang1">— Die Erdbeben im Vispthale (1855).</p> - - <p class="hang1">— Die Erdbeben im Rheingebiet in den Jahren 1868, 1869, 1870.</p> - - <p class="hang1">— Jahrgänge d. Verbandlungen d. Natur. Vereins für 1870. <cite>Rheinland - u. Westphalen</cite>, xxvii.</p> - - <p> </p> - - <p class="hang1">Oldham, Dr. Secondary Effects of the Earthquake of Cachas. <cite>Proc. - Geolog. Soc.</cite> 1872.</p> - - <p class="hang1">— Thermal Springs of India. <cite>Memoirs of Geolog. Survey of India</cite>, - XIX. Plate 2.</p> - - <p class="hang1">— A Catalogue of Indian Earthquakes. <cite>Memoirs of Geolog. Survey of - India</cite>, XIX. Plate 3.</p> - - <p class="hang1">— The Cachas Earthquake. <cite>Memoirs of Geolog. Survey of India</cite>, XIX. - Plate 1.</p> - - <p> </p> - - <p class="hang1">Palmer, Col. H. S. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Palmieri, Prof. L., e Scacchi, A. Della Regione Volcanica del Monte - Vulture, e del Tremuoto ivi avvenuto nel dì 14 Agosto 1851, 1852.</p> - - <p class="hang1">— Annali del reale Osservatorio Meteorologico Vesuviano.</p> - - <p class="hang1">— Il Vesuvio, il Terremoto d’ Isernia e l’eruzione sottomarina di - Santorino. <cite>R. Accad. d. Sci. Fis. e Mat. di Napoli</cite>, iv. 1866.</p> - - <p class="hang1">— Sul recente Terremoto di Corleone. <cite>R. Accad. d. Sci. Fis. e - Mat.</cite>, v. 1876.</p> - - <p class="hang1">— Il Terremoto di Scio del dì 4 Aprile. <cite>R. Accad. d. Sci. Fis. e - Mat. di Napoli</cite>, v. 1881.</p> - - <p class="hang1">— Sul Terremoto di Casamicciola del 4 Marzo 1881. <cite>R. Accad. d. - Sci. Fis. e. Mat. di Napoli</cite>. 1881.</p> - - <p class="hang1">Paul, Prof. H. M. See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Perrey, Prof. A. Earthquake Catalogue and Memoirs. (For list see - Mallet’s Report to British Association. 1858.)</p> - - <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Perry, J., and W. E. Ayrton. On a Neglected Principle that may be - Employed in Earthquake Measurement.</p> - - <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Philosophical Magazine.</p> - - <p class="hang1">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.</p> - - <p> </p> - - <p class="hang1">Ray, J., F.R.S. A Summary of the Causes of the Alterations which - have happened to the Face of the Earth.</p> - - <p><span class="pagenum" id="Page_355">355</span></p> - - <p class="hang1">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.</p> - - <p class="hang1">Rockwood, Prof. C. G. Notes on Earthquakes. Annually in the <cite>Am. - Jour. Sci.</cite></p> - - <p class="hang1">— Japanese Seismology. <cite>Am. Jour. Sci.</cite>, XXII. Dec. 1881.</p> - - <p class="hang1">Romaine, W. A Discourse occasioned by the Late Earthquake. 1755.</p> - - <p class="hang1">Rossi, Prof. M. S. di. Intorno all’ odierna fase dei Terremoti in - Italia, e segnatamente sul Terremoto in Casamicciola del 4 Marzo - 1881. <cite>Società Geografica Italiana.</cite> 1881.</p> - - <p class="hang1">— La Meteorologia Endogena, 2 vols.</p> - - <p class="hang1">Royal Society, Transactions of.</p> - - <p> </p> - - <p class="hang1">Scacchi, A. <i>See</i> Palmieri.</p> - - <p class="hang1">Schmidt, Dr. J. F. Untersuchungen über das Erdbeben am 15. Januar - 1858.</p> - - <p class="hang1">— Studien über Erdbeben. 1879.</p> - - <p class="hang1">— Die Eruption des Vesuv (1855). 1856.</p> - - <p class="hang1">Scrope, G. P. Volcanoes.</p> - - <p class="hang1">Seebach. Das mittle Deutsche Erdbeben (1872). <cite>Mitt. der K.K. - geograph. Gesellsch.</cite>, II. Jahrg., 2. Heft, 1873.</p> - - <p class="hang1">Serpieri, Prof. A. C. S. Nuove Osservazioni sul Terremoto avvenuto - in Italia il 12 Marzo 1873. <cite>Istituto Lombardo.</cite> 1873.</p> - - <p class="hang1">— Il Terremoto di Rimini della notte 17–18 Marzo 1875.</p> - - <p class="hang1">— Documenti nuove e Riflessioni sul Terremoto della notte 17–18 - Marzo 1875. <cite>Meteorologia Italiana</cite>, iv. 1875.</p> - - <p class="hang1">— Determinazione delle fasi e delle leggi del grande Terremoto - avvenuto in Italia nella notte 17–18 Marzo 1875. <cite>Istituto - Lombardo.</cite> 1875.</p> - - <p class="hang1">— Dell’ influenza del Lume Solare sui Terremoti. <cite>Istituto - Lombardo.</cite> 1882.</p> - - <p class="hang1">Sherlock, T., D.D. (Lord Bishop of London). A Letter on the - occasion of the late Earthquakes. 1750.</p> - - <p class="hang1">Shower, Rev. J., D.D. Practical Reflections on the Earthquakes that - have happened in Europe and America, &c. 1750.</p> - - <p class="hang1">Stübel, A. (see Reiss, W.)</p> - - <p class="hang1">Stukeley, Rev. W., M.D., F.R.S. The Philosophy of Earthquakes, - Natural and Religious, &c. Plates 1, 2, and 3. 1756.</p> - - <p class="hang1">Sturmius, J. C. A Methodical Account of Earthquakes.</p> - - <p><span class="pagenum" id="Page_356">356</span></p> - - <p class="hang1">Suess, E. Die Erdbeben Niederösterreiches. <cite>Die Kais. Akademie der - Wissenschaften</cite>, Bd. xxxiii.</p> - - <p class="hang1">— Die Erdbeben des südlichen Italiens. <cite>Die Kais. Akademie der - Wissenschaften</cite>, Bd. xxxiv.</p> - - <p> </p> - - <p class="hang1">Volger, Dr. G. H. Untersuchungen über das Phänomen der Erdbeben. - 1857.</p> - - <p> </p> - - <p class="hang1">Wagener, Dr. G. Bemerkungen über Erdbebenmesser und Vorschläge zu - einem neuen Instrumente dieser Art. <cite>Mitt. d. Deutsch. Gesellsch. - für Natur- und Völkerkunde Ostasiens</cite>, Heft 15.</p> - - <p class="hang1">— See <cite>Trans. Seis. Soc. of Japan</cite>.</p> - - <p class="hang1">Winchilsea, The Earl of. A True and Exact Relation of the late - Prodigious Earthquake and Eruption of Mount Etna. 1669.</p> - - <p class="hang1">Woodward, J., M.D., F.R.S. Earthquake caused by some Accidental - Obstruction of a Continual Subterranean Heat.</p> - - - <h3><span class="smcap">Seismological Society Of Japan.</span></h3> - - <p class="center">The following are a list of the papers published by this Society:—</p> - - <h4>VOL. I.</h4> - - <p class="hang1">Milne, J. Seismic Science in Japan. 35 pages.</p> - - <p class="hang1">Ewing, J. A. New Form of Pendulum Seismograph. 6 pages, 3 plates.</p> - - <p class="hang1">Gray, T. Seismometer and Torsion Pendulum Seismograph. 8 pages, 2 - plates.</p> - - <p class="hang1">Mendenhall, T. C. Acceleration of Gravity at Tokio (abstract). 2 - pages.</p> - - <p class="hang1">Wagener, G., and E. Knipping. New Seismometer and Observations with - same. 18 pages, 1 plate.</p> - - <p class="hang1">Milne, J. Earthquake in Japan of Feb. 22, 1880. 116 pages, 5 - plates, 8 woodcuts.</p> - - - <h4>VOL. II.</h4> - - <p class="hang1">Milne, J. Recent Earthquakes of Yeddo, Effects on Buildings, &c. 38 - pages, 2 plates, and many tables.</p> - - <p class="hang1">Mendenhall, T. C. Gravity on Summit of Fujiyama (abstract). 2 pages.</p> - - <p class="hang1">Paul, H. M. Earth Vibrations from Railroad Trains (abstract). 4 - pages.</p> - - <p class="hang1">Ewing, J. A. Astatic Horizontal Lever Seismograph (abstract). 5 - pages, 1 plate.</p> - - <p><span class="pagenum" id="Page_357">357</span></p> - - <p class="hang1">Milne, J. Peruvian Earthquake of May, 9, 1877. 47 pages, 2 plates, - tables. Constitution, Rules, Officers and Members of the Society, - Dec., 1881.</p> - - <h4>VOL. III.</h4> - - <p class="hang1">Gray, T. Steady Points for Earthquake Measurements. 11 pages, 3 - plates.</p> - - <p class="hang1">Milne, J. Experiments in Observational Seismology. 53 pages, 1 - plate, tables.</p> - - <p class="hang1">— The Great Earthquakes of Japan. 38 pages, 1 plate, many tables.</p> - - <p class="hang1">Perry, J. Theory of a Rocking Column. 4 pages.</p> - - <p class="hang1">Knipping, E. Earthquake of July 25, 1880, with Dr. Wagener’s - Seismometer. 4 pages.</p> - - <p class="hang1">Ewing, J. A. Earthquake Observation at three or more Stations, &c. - 4 pages.</p> - - <p class="hang1">— Records of three recent Earthquakes. 6 pages, 3 plates.</p> - - <p class="hang1">— Earthquake of March 8, 1881. 8 pages, 1 plate.</p> - - <p class="hang1">Milne, J. Horizontal and Vertical Motion in Earthquake of March 8, - 1881. 8 pages, 3 plates.</p> - - <p class="hang1">Gray, T. Seismograph for Registering Vertical Motion. 3 pages, 1 - plate.</p> - - <p class="hang1">Ewing, J. A. Seismometer for Vertical Motion. 3 pages, 1 plate.</p> - - <p class="hang1">Gray, T. Seismograph for Large Motions. 2 pages.</p> - - <p class="hang1">— Compensating a Pendulum to make it Astatic. 3 pages.</p> - - <p class="hang1">Palmer, H. S. Note on Earth Vibrations. 3 pages.</p> - - <p class="hang1">Kuwabara, M. The Hot Springs of Atami. 2 pages.</p> - - <h4>VOL. IV.</h4> - - <p class="hang1">Milne, J. Distribution of Seismic Activity in Japan. 30 pages, 1 - plate.</p> - - <p class="hang1">Wada, T. Notes on Fujiyama. 7 pages.</p> - - <p class="hang1">Casariego, E. Abella y. Earthquakes of Nueva Vizcaya in 1881. 23 - pages, 2 maps.</p> - - <p class="hang1">Milne, J. Utilisation of Earth’s Internal Heat. 12 pages.</p> - - <p class="hang1">Ewing, J. A. Earthquake of March 11, 1882. 5 pages.</p> - - <p class="hang1">Doyle, P. Note on an Indian Earthquake. 6 pages.</p> - - <p class="hang1">Milne, J. Systematic Observation of Earthquakes. 31 pages, 5 plates.</p> - - <h4>VOL. V.</h4> - - <p class="hang1">Naumann, Dr. E. Notes on Secular Changes of Magnetic Declination in - Japan, p. 1–18.</p> - - <p class="hang1">Casariego, Don E. Abella y. Monografía Geológica del Volcan de - Albay ó El Máyon. p. 19–43.</p> - - <p class="hang1"><span class="pagenum" id="Page_358">358</span></p> - - <p class="hang1">Garcia, Don J. Centeno y. Abstract of a Memoir on the Earthquakes - on the Island of Luzon in 1880. p. 43–89.</p> - - <p class="hang1">Ewing, Prof. J. A. Seismological Notes.</p> - - <p class="hang1">— A Duplex Pendulum Seismometer.</p> - - <p class="hang1">— The Suspension of a Horizontal Pendulum.</p> - - <p class="hang1">— A Speed Governor for Seismograph Clocks, p. 89–95.</p> - - <p class="hang1">Dan, T., S.B. Notes on the Earthquake at Atami, in the Province of - Idzu, on September 29, 1882. p. 95–105.</p> - - <h4>VOL. VI.</h4> - - <p class="hang1">Alexander, Prof. T. The Development of the Record given by a - Bracket Machine.</p> - - <p class="hang1">Milne, J. Earth Pulsations.</p> - - <p class="hang1">Ewing, J. A. Note on a Duplex Pendulum with a Single Bob.</p> - - <p class="hang1">Gergens, F. Note on an Iron Casting, Supposed to have been - Disturbed whilst Cooling by an Earthquake.</p> - - <p class="hang1">West, C. D. On a Parallel Motion Seismograph.</p> - - <p class="hang1">Ewing, J. A. Certain Methods of Astatic Suspension.</p> - - <p class="hang1">Alexander, T. Ball and Cup Seismometer.</p> - - <p class="hang1">Knipping, Messrs. Paul and. Report on a System for Earthquake - Observation.</p> - - <p class="hang1">Catalogues of Earthquakes.</p> - - <hr class="page" /> - <div class="chapter" id="INDEX"> - <span class="pagenum" id="Page_359">359</span> - <h2>INDEX.</h2> - </div> - - <div class="figcenter"> - <img src="images/divider.png" width="80" height="9" alt="" /> - </div> - - <ul> - <li>Abbadie, M. d’, on earth tremors, <a href="#Page_309">309</a></li> - <li>Abbot, General H. L., on the transmission of vibrations, <a href="#Page_62">62</a></li> - <li>Abella, M., on the earthquake in the Philippines in 1881, <a href="#Page_77">77</a></li> - <li>Activity, on seismic, <a href="#Page_6">6</a></li> - <li>Aristotle, on the classification of earthquakes, <a href="#Page_41">41</a></li> - <li>Artificial earthquakes, experiments on, <a href="#Page_57">57</a></li> - <li>— — intensity of, <a href="#Page_61">61</a></li> - <li>Aurora, on the occurrence of, with earthquakes, <a href="#Page_264">264</a></li> - <li>Ayrton and Perry, on the effect of soft foundations, <a href="#Page_130">130</a></li> - <li>— — on the period of vibration of buildings, <a href="#Page_115">115</a></li> - <li>— — on the principle of, <a href="#Page_31">31</a></li> - <li> </li> - <li>Barometer, effect of changes of, on earthquakes, <a href="#Page_266">266</a></li> - <li>Bertelli, on aurora and earthquakes, <a href="#Page_265">265</a></li> - <li>— on earth tremors, <a href="#Page_316">316</a>, <a href="#Page_320">320</a></li> - <li>— on the normal tromometer of, <a href="#Page_317">317</a></li> - <li>Bittner, A., on the buildings of Belluno, <a href="#Page_100">100</a></li> - <li>Bridges, on earthquake, <a href="#Page_140">140</a></li> - <li>Brunton, R. H., on buildings in earthquake countries, <a href="#Page_123">123</a></li> - <li>Buckle, on the history of civilisation, <a href="#Page_1">1</a></li> - <li>Builders, interest of the study of earthquakes to, <a href="#Page_3">3</a></li> - <li>Buildings, on cracks in, <a href="#Page_98">98</a>, <a href="#Page_108">108</a></li> - <li>— the effect of earthquakes on, <a href="#Page_96">96</a></li> - <li>— on the irregular destruction of, <a href="#Page_96">96</a></li> - <li>— — effect on the end house in a row, <a href="#Page_112">112</a></li> - <li>— — church of St. Augustin at Manilla, <a href="#Page_113">113</a></li> - <li>— — relation of destruction of, to earthquake motion, <a href="#Page_103">103</a></li> - <li>— — protection of, <a href="#Page_143">143</a></li> - <li>— — pitch of the roof of, <a href="#Page_110">110</a></li> - <li>— — position of openings in the walls of, <a href="#Page_111">111</a></li> - <li>— — swing of, <a href="#Page_115">115</a></li> - <li>— — period of vibration of, <a href="#Page_115">115</a></li> - <li>— — principle of relative periods in, <a href="#Page_116">116</a></li> - <li>— — types of, for earthquake countries, <a href="#Page_121">121</a></li> - <li>— — effect of underlying rocks on, <a href="#Page_130">130</a></li> - <li>— general conclusions regarding, <a href="#Page_144">144</a></li> - <li> </li> - <li>Cacciatore, definition of the, <a href="#Page_18">18</a></li> - <li>Caldcleugh, A., on earthquake frequency, <a href="#Page_245">245</a></li> - <li>— on barometric height and earthquakes, <a href="#Page_266">266</a></li> - <li>Carruthers, J., on earthquakes and tides, <a href="#Page_291">291</a></li> - <li>Centrum, definition of, <a href="#Page_9">9</a></li> - <li>— on the depth of, <a href="#Page_213">213</a></li> - <li><span class="pagenum" id="Page_360">360</span>— on the maximum depth of, <a href="#Page_218">218</a></li> - <li>Centrum, determination of position of, <i>see</i> origins</li> - <li>Chaplin, W. S., on the bracket seismometer of, <a href="#Page_27">27</a></li> - <li>— on earthquakes and the position of the moon, <a href="#Page_252">252</a></li> - <li>Coast line, on the movement of, <a href="#Page_160">160</a></li> - <li>Cocks, R., on earthquakes and tides, <a href="#Page_290">290</a></li> - <li>Coseismic lines, definition of, <a href="#Page_10">10</a></li> - <li>Curves, on microseismic, <a href="#Page_321">321</a></li> - <li> </li> - <li>Darwin, Charles, on the movement of coast lines, <a href="#Page_160">160</a></li> - <li>— George H., on earth tides, <a href="#Page_285">285</a></li> - <li>— on tidal loads, <a href="#Page_291">291</a></li> - <li>— on earth pulsation, <a href="#Page_330">330</a></li> - <li>— on the effect of fluctuations of barometric pressure, <a href="#Page_336">336</a></li> - <li>— experiments at Cambridge, <a href="#Page_310">310</a></li> - <li>Delauney, M. J., on the influence of the planets on earthquakes, <a href="#Page_261">261</a></li> - <li>Diagonic, definition of, <a href="#Page_11">11</a></li> - <li>Diastrophic, definition of, <a href="#Page_11">11</a></li> - <li>Direction of motion, from instrumental records, <a href="#Page_198">198</a></li> - <li>Distribution, on earthquake, <a href="#Page_226">226</a></li> - <li>— examples of, <a href="#Page_231">231</a></li> - <li>Disturbance, on the propagation of, <a href="#Page_50">50</a></li> - <li>Douglas, J., on South American houses, <a href="#Page_126">126</a></li> - <li> </li> - <li>Earth particle, on the velocity and acceleration of, <a href="#Page_79">79</a></li> - <li>Earthquake motion, nature of, as deduced from the feelings, <a href="#Page_67">67</a></li> - <li>— direction of, derived from instrumental records, <a href="#Page_69">69</a></li> - <li>— duration of, <a href="#Page_71">71</a></li> - <li>— period of vibration in, <a href="#Page_74">74</a></li> - <li>— examples of extent of, <a href="#Page_75">75</a>–<a href="#Page_77">77</a></li> - <li>— absolute intensity of force in, <a href="#Page_83">83</a></li> - <li>— radiation of, <a href="#Page_85">85</a></li> - <li>— velocity of propagation of, <a href="#Page_87">87</a></li> - <li>Earthquake at Lisbon, velocity of propagation of, <a href="#Page_88">88</a></li> - <li>Earthquakes, general examples of effects of, <a href="#Page_142">142</a></li> - <li>— geological changes produced by, <a href="#Page_161">161</a></li> - <li>— hunting, <a href="#Page_187">187</a></li> - <li>— distribution of, <a href="#Page_226">226</a></li> - <li>— maps, <a href="#Page_189">189</a></li> - <li>— secondary, <a href="#Page_248">248</a></li> - <li>— table of, for nineteenth century, <a href="#Page_259">259</a></li> - <li>— on the course of, <a href="#Page_277">277</a>–<a href="#Page_281">281</a></li> - <li>— and tides, <a href="#Page_290">290</a></li> - <li>— prediction of, <a href="#Page_297">297</a>–<a href="#Page_304">304</a></li> - <li>Elastic waves, nature of, <a href="#Page_44">44</a></li> - <li>Emergence, angle of, <a href="#Page_9">9</a></li> - <li>Energy, dissipation of, in earthquakes, <a href="#Page_52">52</a></li> - <li>— seismic, in relation to geological time, <a href="#Page_234">234</a></li> - <li>— — table of, <a href="#Page_240">240</a></li> - <li>Epicentrum, definition of, <a href="#Page_9">9</a></li> - <li>Euthutropic, definition of, <a href="#Page_11">11</a></li> - <li>Ewing, J. A., pendulum seismograph of, <a href="#Page_25">25</a></li> - <li>— astatic pendulum of, <a href="#Page_26">26</a></li> - <li>— bracket seismograph of, <a href="#Page_26">26</a></li> - <li> </li> - <li>Falb, R., on the influence of the sun and moon on earthquakes, <a href="#Page_286">286</a></li> - <li>Fissures, on the material discharged from, <a href="#Page_148">148</a></li> - <li>— on the explanation of, <a href="#Page_151">151</a></li> - <li>Focal cavity, definition of, <a href="#Page_9">9</a></li> - <li>— on the form of, <a href="#Page_221">221</a></li> - <li>Forbes, D., on an earthquake in Mendoza, <a href="#Page_151">151</a></li> - <li>Frequency of earthquakes, <a href="#Page_243">243</a></li> - <li>Frere, Sir H. Bartle, on geological changes produced by earthquakes, <a href="#Page_161">161</a></li> - <li>Fuchs, on sea waves, <a href="#Page_176">176</a></li> - <li>— on the movement of the seismic centre, <a href="#Page_233">233</a></li> - <li>— on earthquakes and volcanic outbursts, <a href="#Page_271">271</a></li> - <li><span class="pagenum" id="Page_361">361</span>— on hot springs, <a href="#Page_157">157</a></li> - <li>Fumaroles, the effect of earthquakes on, <a href="#Page_156">156</a></li> - <li> </li> - <li>Geinitz, Dr., on sea waves, <a href="#Page_182">182</a></li> - <li>Geologists, on the interest of seismology to, <a href="#Page_2">2</a></li> - <li>Gray, T., astatic pendulum of, <a href="#Page_26">26</a></li> - <li>— bracket seismometer of, <a href="#Page_27">27</a></li> - <li>— conical pendulum of, <a href="#Page_29">29</a></li> - <li>— dead heat pendulum of, <a href="#Page_22">22</a></li> - <li>— on the rotation of bodies, <a href="#Page_196">196</a></li> - <li>— rolling spheres and cylinders of, <a href="#Page_29">29</a></li> - <li>— torsion pendulum seismometer of, <a href="#Page_25">25</a></li> - <li>— vertical motion seismometers of, <a href="#Page_32">32</a>, <a href="#Page_33">33</a></li> - <li>— and Milne, seismograph of, <a href="#Page_38">38</a></li> - <li> </li> - <li>Hattori, I., on the large earthquakes of Japan, <a href="#Page_244">244</a></li> - <li>Haughton, Prof., list of active volcanoes of, <a href="#Page_227">227</a></li> - <li>— method of finding earthquake origins of, <a href="#Page_209">209</a></li> - <li>Hills, on the want of support on the face of, <a href="#Page_136">136</a></li> - <li>Höfer, on an earthquake at Belluno, <a href="#Page_225">225</a></li> - <li>Hoffmann, F., on the barometer and earthquakes, <a href="#Page_267">267</a></li> - <li>Hooke, on earthquake motion, <a href="#Page_42">42</a></li> - <li>Hopkins, on the thickness of the earth’s crust, <a href="#Page_284">284</a></li> - <li>Humboldt, on meteors and earthquakes, <a href="#Page_261">261</a></li> - <li>— on the barometer and earthquakes, <a href="#Page_267">267</a></li> - <li>— on volcanoes and earthquakes, <a href="#Page_279">279</a></li> - <li> </li> - <li>Imagination, effect of earthquakes on the, <a href="#Page_2">2</a></li> - <li>Instruments, direction of motion derived from, <a href="#Page_198">198</a></li> - <li>Intensity, on earthquake, <a href="#Page_51">51</a>, <a href="#Page_71">71</a></li> - <li>— seismic curve of, for Kioto, <a href="#Page_242">242</a></li> - <li>Isoseismic circles, definition of, <a href="#Page_10">10</a></li> - <li>— areas, definition of, <a href="#Page_10">10</a></li> - <li> </li> - <li>Kluge, on sea waves, <a href="#Page_175">175</a></li> - <li>— on earthquake frequency, <a href="#Page_246">246</a></li> - <li>— on simultaneous earthquakes, <a href="#Page_248">248</a></li> - <li>— on earthquakes and sun spots, <a href="#Page_263">263</a></li> - <li>— on earth pulsations, <a href="#Page_339">339</a></li> - <li>Kreil, pendulum seismometer of, <a href="#Page_25">25</a></li> - <li> </li> - <li>Lakes, on disturbances in, <a href="#Page_154">154</a></li> - <li>Land, effect of earthquakes on, <a href="#Page_146">146</a>–<a href="#Page_162">162</a></li> - <li>— on the reason of movements of, <a href="#Page_162">162</a></li> - <li>— on cracks and fissures formed in, <a href="#Page_146">146</a></li> - <li>Level, on the use of for earth pulsations, <a href="#Page_328">328</a></li> - <li>Literature, on seismic, <a href="#Page_6">6</a></li> - <li>— on Japanese earthquake, <a href="#Page_7">7</a></li> - <li> </li> - <li>Mallet, R., on area of disturbance as a test of seismic energy, <a href="#Page_78">78</a></li> - <li>— on clock stopping, <a href="#Page_36">36</a></li> - <li>— list of works on earthquakes of, <a href="#Page_5">5</a></li> - <li>— curve of seismic energy of, <a href="#Page_238">238</a></li> - <li>— definition of earthquake of, <a href="#Page_43">43</a></li> - <li>— on earthquake frequency, <a href="#Page_243">243</a></li> - <li>— on the influence of the heavenly bodies on earthquakes, <a href="#Page_253">253</a></li> - <li>— on maximum depth of origin, <a href="#Page_218">218</a></li> - <li>— on pendulum seismometers, <a href="#Page_20">20</a></li> - <li>— projection seismometer of, <a href="#Page_17">17</a></li> - <li>— on the Neapolitan earthquake, <a href="#Page_69">69</a>, <a href="#Page_77">77</a>, <a href="#Page_83">83</a>, <a href="#Page_97">97</a>, <a href="#Page_103">103</a>, <a href="#Page_132">132</a>, <a href="#Page_142">142</a>, <a href="#Page_218">218</a>, <a href="#Page_280">280</a></li> - <li>— on sea waves, <a href="#Page_170">170</a></li> - <li>— on the swing of mountains, <a href="#Page_135">135</a></li> - <li><span class="pagenum" id="Page_362">362</span>— on propagation from a fissure, <a href="#Page_217">217</a></li> - <li>Mallet on the temperature of focal cavity, <a href="#Page_84">84</a></li> - <li>Malvasia, M. le Conte, on earth tremors, <a href="#Page_316">316</a></li> - <li>Martin, D. S., on the New England earthquake of 1874, <a href="#Page_142">142</a></li> - <li>Meizoseismic area, definition of, <a href="#Page_10">10</a></li> - <li>Melzi, on curves of microseismic motion, <a href="#Page_322">322</a></li> - <li>Meteors, on earthquakes and, <a href="#Page_260">260</a></li> - <li>Microseismic movements, on cause of, <a href="#Page_324">324</a></li> - <li>Milne, D., on the Lisbon earthquake, <a href="#Page_87">87</a></li> - <li>— on earthquake synchronism, <a href="#Page_247">247</a></li> - <li>Mitchell, on earthquake motion, <a href="#Page_42">42</a></li> - <li>Moon, effect of, on earthquakes, <a href="#Page_251">251</a>, <a href="#Page_285">285</a></li> - <li>Mountains, on the swing of, <a href="#Page_135">135</a></li> - <li> </li> - <li>Naumann, E., on meteors and earthquakes, <a href="#Page_261">261</a></li> - <li>— on sun spots and earthquakes, <a href="#Page_263">263</a></li> - <li> </li> - <li>Ocean, on disturbances in, <a href="#Page_163">163</a>–<a href="#Page_186">186</a></li> - <li>Origin, definition of, <a href="#Page_9">9</a></li> - <li>— on the determination of, <a href="#Page_187">187</a></li> - <li>— position of, deduced from direction of motion, <a href="#Page_192">192</a></li> - <li>— — from destruction of buildings, <a href="#Page_194">194</a></li> - <li>— — from rotation of bodies, <a href="#Page_195">195</a></li> - <li>— — from time of occurrence, <a href="#Page_199">199</a></li> - <li>— — examples of methods of calculating, <a href="#Page_200">200</a>–<a href="#Page_212">212</a></li> - <li>Oscillations, on earth, <a href="#Page_344">344</a></li> - <li>Overturning moment, on the area of greatest, <a href="#Page_53">53</a></li> - <li> </li> - <li>Palmer, Col. H. S., on earth tremors, <a href="#Page_307">307</a></li> - <li>Palmieri, on clock stopping, <a href="#Page_36">36</a>, <a href="#Page_62">62</a></li> - <li>Paul, H. M., on earth tremors, <a href="#Page_308">308</a></li> - <li>Perrey, A., on the influence of the moon on earthquakes, <a href="#Page_251">251</a></li> - <li>Perrey on the periodicity of earthquakes, <a href="#Page_8">8</a></li> - <li>Perry, J., on position of openings in walls, <a href="#Page_111">111</a></li> - <li>Physicists, on the interest of earthquakes to, <a href="#Page_2">2</a></li> - <li>Planets, influence of, on earthquakes, <a href="#Page_260">260</a></li> - <li>Plantamour, M., on earth pulsations, <a href="#Page_328">328</a></li> - <li>Pleistoseists, definition of, <a href="#Page_10">10</a></li> - <li>Poly, M. A., on earthquakes and sun spots, <a href="#Page_263">263</a></li> - <li>— on earthquakes and revolving storms, <a href="#Page_294">294</a></li> - <li>Prost, M. le Baron, on earth tremors, <a href="#Page_316">316</a></li> - <li>Pulsation, on earth, <a href="#Page_4">4</a>, <a href="#Page_326">326</a>–<a href="#Page_343">343</a></li> - <li> </li> - <li>Records, on receivers of, <a href="#Page_33">33</a></li> - <li>Rivers, on disturbances in, <a href="#Page_154">154</a></li> - <li>Rockwood, Prof., on American earthquakes, <a href="#Page_6">6</a></li> - <li>Ronaldson, T., on San Francisco houses, <a href="#Page_129">129</a></li> - <li>Rossi, M. S. di, on an eruption of gas in the Tiber, <a href="#Page_153">153</a></li> - <li>— aurora and earthquakes, <a href="#Page_264">264</a></li> - <li>— earth tremors, <a href="#Page_317">317</a>, <a href="#Page_320">320</a></li> - <li>— earth oscillations, <a href="#Page_346">346</a></li> - <li>— earth pulsations, <a href="#Page_327">327</a></li> - <li>— microseismograph of, <a href="#Page_318">318</a></li> - <li>— microphonic observations of, <a href="#Page_319">319</a>, <a href="#Page_323">323</a></li> - <li>— normal tromometer of, <a href="#Page_317">317</a></li> - <li> </li> - <li>Schmidt, on the influence of barometric pressure on earthquakes, <a href="#Page_267">267</a></li> - <li>Sea waves, on nature of, <a href="#Page_165">165</a></li> - <li>— on cause of, <a href="#Page_171">171</a></li> - <li>— seldom produced by earthquakes which originate inland, <a href="#Page_175">175</a></li> - <li>— on velocity of propagation of, <a href="#Page_177">177</a></li> - <li>— examples of, <a href="#Page_179">179</a></li> - <li><span class="pagenum" id="Page_363">363</span>Seasons, frequency of earthquakes at different, <a href="#Page_254">254</a></li> - <li>Seebach, on the determination of origins, <a href="#Page_211">211</a></li> - <li>— on the focal cavity, <a href="#Page_224">224</a></li> - <li>Seismic vertical, definition of, <a href="#Page_9">9</a></li> - <li>Seismic and volcanic phenomena, relation of, <a href="#Page_270">270</a></li> - <li>— — conclusions regarding, <a href="#Page_275">275</a>, <a href="#Page_295">295</a></li> - <li>Seismology, definition of, <a href="#Page_9">9</a></li> - <li>Seismometers, on various forms of, <a href="#Page_17">17</a>–<a href="#Page_40">40</a></li> - <li>Seismoscopes, on various forms of, <a href="#Page_13">13</a>–<a href="#Page_20">20</a></li> - <li>Serpieri, P. A., on distribution of seismic movement, <a href="#Page_231">231</a></li> - <li>Shadows, on earthquake, <a href="#Page_137">137</a></li> - <li>Spring, on frequency of earthquakes during, <a href="#Page_156">156</a></li> - <li>Stukeley, on earthquake motion, <a href="#Page_42">42</a></li> - <li>— earthquakes and aurora, <a href="#Page_265">265</a></li> - <li>Succussatore, definition of, <a href="#Page_10">10</a></li> - <li>Sun, on the effect of, on earthquakes, <a href="#Page_253">253</a>, <a href="#Page_285">285</a></li> - <li> </li> - <li>Temperature, effect of changes of, on earthquakes, <a href="#Page_268">268</a>, <a href="#Page_294">294</a></li> - <li>Terremoto, definition of, <a href="#Page_10">10</a></li> - <li>Thomson, Sir W., on the rigidity of the earth, <a href="#Page_285">285</a></li> - <li>Time, on recording apparatus for, <a href="#Page_35">35</a></li> - <li>Travagini, F., on earthquake motion, <a href="#Page_42">42</a></li> - <li>Trembelores, definition of, <a href="#Page_10">10</a></li> - <li>Tremors, on earth, <a href="#Page_3">3</a>, <a href="#Page_306">306</a>–<a href="#Page_325">325</a></li> - <li> </li> - <li>Understanding, effects of earthquakes on, <a href="#Page_2">2</a></li> - <li> </li> - <li>Verbeck, on the ball and plate seismometer of, <a href="#Page_31">31</a></li> - <li>Vibration, on the nature of earthquake, <a href="#Page_12">12</a></li> - <li>Vorticose motion, on, <a href="#Page_70">70</a></li> - <li>— definition of, <a href="#Page_10">10</a></li> - <li> </li> - <li>Wagener, on the pendulum seismometer of, <a href="#Page_25">25</a></li> - <li>— vertical motion seismometer of, <a href="#Page_33">33</a></li> - <li>— list of earthquakes, <a href="#Page_76">76</a></li> - <li>Wave paths, definition of, <a href="#Page_9">9</a></li> - <li>Waves, on the nature of earthquake, <a href="#Page_55">55</a></li> - <li>— on the interference of, <a href="#Page_138">138</a></li> - <li>Wells, on the effect of earthquakes on, <a href="#Page_156">156</a></li> - <li>Wenthrop, on the New Zealand earthquake of 1855, <a href="#Page_79">79</a></li> - <li>West, on the parallel motion seismometer of, <a href="#Page_28">28</a></li> - <li>Winslow, on pulsations of the ocean, <a href="#Page_334">334</a></li> - <li>Wolf, R., on earthquakes and sun spots, <a href="#Page_263">263</a></li> - <li>Woodward, on earthquake motion, <a href="#Page_42">42</a></li> - <li> </li> - <li>Young, Dr. T., on earthquake motion, <a href="#Page_43">43</a></li> - <li> </li> - <li>Zantedeschi, M. F., on the influence of the sun and moon on earthquakes, <a href="#Page_285">285</a></li> - <li>Zöllner, on the bracket seismometer of, <a href="#Page_27">27</a></li> - <li>— on earth tremors, <a href="#Page_309">309</a></li> - </ul> - - <hr class="page" /> - - <div class="footnotes"> - <div class="footheader"><b>Footnotes:</b></div> - <div class="footnote"> - <a id="Footnote_1" href="#FNanchor_1"><span class="label">[1]</span></a> - <i>Mémoires de l’Académie Imp. de Dijon</i>, vols. xiv. and xv., 2nd Series, 1855–56. - </div> - - <div class="footnote"> - <a id="Footnote_2" href="#FNanchor_2"><span class="label">[2]</span></a> - <i>Trans. Seis. Soc. of Japan</i>, vol. iii. p. 65. - </div> - - <div class="footnote"> - <a id="Footnote_3" href="#FNanchor_3"><span class="label">[3]</span></a> - <i>Gentleman’s Magazine</i>, 1753. - </div> - - <div class="footnote"> - <a id="Footnote_4" href="#FNanchor_4"><span class="label">[4]</span></a> - 1 Kings xix. 11, 12. - </div> - - <div class="footnote"> - <a id="Footnote_5" href="#FNanchor_5"><span class="label">[5]</span></a> - ‘Notes on the Great Earthquake of Japan.’ J. Milne, <i>Trans. Seis. Soc. of Japan</i>, vol. iii. - </div> - - <div class="footnote"> - <a id="Footnote_6" href="#FNanchor_6"><span class="label">[6]</span></a> - See Mallet’s List of Works on Earthquakes, <i>Report of the British Association</i>, 1858, p. 107. - </div> - - <div class="footnote"> - <a id="Footnote_7" href="#FNanchor_7"><span class="label">[7]</span></a> - <i>Quarterly Review</i>, vol. lxiii. p. 61. - </div> - - <div class="footnote"> - <a id="Footnote_8" href="#FNanchor_8"><span class="label">[8]</span></a> - <i>De Mundo</i>, c. iv. - </div> - - <div class="footnote"> - <a id="Footnote_9" href="#FNanchor_9"><span class="label">[9]</span></a> - See <i>Phil. Trans. R. S.</i>, Part III. 1882. - </div> - - <div class="footnote"> - <a id="Footnote_10" href="#FNanchor_10"><span class="label">[10]</span></a> - <i>Report of the British Association</i>, 1851. - </div> - - <div class="footnote"> - <a id="Footnote_11" href="#FNanchor_11"><span class="label">[11]</span></a> - ‘On the Velocity of Transmission of Earth Waves,’ by General H. L, Abbot, <i>American, Journal of Science - and Arts</i>, 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. - </div> - - <div class="footnote"> - <a id="Footnote_12" href="#FNanchor_12"><span class="label">[12]</span></a> - <i>West. Rev.</i>, July 1849. - </div> - - <div class="footnote"> - <a id="Footnote_13" href="#FNanchor_13"><span class="label">[13]</span></a> - <i>Phil. Trans.</i>, L., 1755. - </div> - - <div class="footnote"> - <a id="Footnote_14" href="#FNanchor_14"><span class="label">[14]</span></a> - The solution is taken from Mallet’s <i>Account of the Neapolitan Earthquake</i>, vol. i. p. 155. - </div> - - <div class="footnote"> - <a id="Footnote_15" href="#FNanchor_15"><span class="label">[15]</span></a> - <i>Neapolitan Earthquake</i>, ii. p. 300. - </div> - - <div class="footnote"> - <a id="Footnote_16" href="#FNanchor_16"><span class="label">[16]</span></a> - See <i>Edinburgh Phil. Trans.</i>, vol. xxxi. - </div> - - <div class="footnote"> - <a id="Footnote_17" href="#FNanchor_17"><span class="label">[17]</span></a> - See <i>Report of British Association</i>, 1858, p. 10. - </div> - - <div class="footnote"> - <a id="Footnote_18" href="#FNanchor_18"><span class="label">[18]</span></a> - <i>Meteorologia Endogena</i>, i. p. 306. - </div> - - <div class="footnote"> - <a id="Footnote_19" href="#FNanchor_19"><span class="label">[19]</span></a> - See remarks on the Earthquake ‘Push,’ p. 162. - </div> - - <div class="footnote"> - <a id="Footnote_20" href="#FNanchor_20"><span class="label">[20]</span></a> - See <i>Researches in Geology and Natural History</i>, p. 374. - </div> - - <div class="footnote"> - <a id="Footnote_21" href="#FNanchor_21"><span class="label">[21]</span></a> - ‘The City of Earthquakes,’ H. D. Warner, <i>Atlantic Monthly</i>, March, 1883. - </div> - - <div class="footnote"> - <a id="Footnote_22" href="#FNanchor_22"><span class="label">[22]</span></a> - Mallet, <i>Dynamics of Earthquakes</i>. - </div> - - <div class="footnote"> - <a id="Footnote_23" href="#FNanchor_23"><span class="label">[23]</span></a> - Stud Mill at Haywards. - </div> - - <div class="footnote"> - <a id="Footnote_24" href="#FNanchor_24"><span class="label">[24]</span></a> - See ‘Constructive Art in Japan,’ by R. H. Brunton, C.E., F.R.G.S., F.G.S., - <i>Transactions of Asiatic Society of Japan</i>, December 22, 1873, and January 13, 1875. - </div> - - <div class="footnote"> - <a id="Footnote_25" href="#FNanchor_25"><span class="label">[25]</span></a> - <i>Journal of the American Geographical Society</i>, vol. x. - </div> - - <div class="footnote"> - <a id="Footnote_26" href="#FNanchor_26"><span class="label">[26]</span></a> - <i>Phil. Trans.</i>, li. 1760. - </div> - - <div class="footnote"> - <a id="Footnote_27" href="#FNanchor_27"><span class="label">[27]</span></a> - <i>Ibid.</i>, xviii. - </div> - - <div class="footnote"> - <a id="Footnote_28" href="#FNanchor_28"><span class="label">[28]</span></a> - ‘The City of Earthquakes,’ H. D. Warner, <i>Atlantic Monthly</i>, March 1883. - </div> - - <div class="footnote"> - <a id="Footnote_29" href="#FNanchor_29"><span class="label">[29]</span></a> - T. Ronaldson, <i>A Treatise on Earthquake Dangers &c.</i> - </div> - - <div class="footnote"> - <a id="Footnote_30" href="#FNanchor_30"><span class="label">[30]</span></a> - <i>Principles of Geology</i>, Lyell, vol. ii. p. 106. - </div> - - <div class="footnote"> - <a id="Footnote_31" href="#FNanchor_31"><span class="label">[31]</span></a> - <i>The Neapolitan Earthquake of 1857</i>, R. Mallet, vol. ii. p. 359. - </div> - - <div class="footnote"> - <a id="Footnote_32" href="#FNanchor_32"><span class="label">[32]</span></a> - <i>Am. J. Sci.</i> x. 191. - </div> - - <div class="footnote"> - <a id="Footnote_33" href="#FNanchor_33"><span class="label">[33]</span></a> - <i>Am. J. Sci.</i> x. 191. - </div> - - <div class="footnote"> - <a id="Footnote_34" href="#FNanchor_34"><span class="label">[34]</span></a> - <i>Reports of British Association</i>, 1858, p. 106. - </div> - - <div class="footnote"> - <a id="Footnote_35" href="#FNanchor_35"><span class="label">[35]</span></a> - See chapter ‘Causes of Earthquakes’ for details of this myth. - </div> - - <div class="footnote"> - <a id="Footnote_36" href="#FNanchor_36"><span class="label">[36]</span></a> - <i>Am. Jour. Sci.</i> vol. x. p. 191. - </div> - - <div class="footnote"> - <a id="Footnote_37" href="#FNanchor_37"><span class="label">[37]</span></a> - <i>The Earth</i>, p. 599. - </div> - - <div class="footnote"> - <a id="Footnote_38" href="#FNanchor_38"><span class="label">[38]</span></a> - Lyell, <i>Principles of Geology</i>, vol. ii. chap. xxix. - </div> - - <div class="footnote"> - <a id="Footnote_39" href="#FNanchor_39"><span class="label">[39]</span></a> - <i>Gent. Mag.</i> vol. xx. p. 212. - </div> - - <div class="footnote"> - <a id="Footnote_40" href="#FNanchor_40"><span class="label">[40]</span></a> - <i>Trans. Seis. Soc.</i> vol. v. p. 67–68. - </div> - - <div class="footnote"> - <a id="Footnote_41" href="#FNanchor_41"><span class="label">[41]</span></a> - <i>Am. Jour. Sci.</i> vol. iv. - </div> - - <div class="footnote"> - <a id="Footnote_42" href="#FNanchor_42"><span class="label">[42]</span></a> - <i>Phil. Trans.</i> vol. xviii. - </div> - - <div class="footnote"> - <a id="Footnote_43" href="#FNanchor_43"><span class="label">[43]</span></a> - Oldham and Mallet, ‘Cachar Earthquake,’ <i>Proc. Geolog. Soc.</i> 1872. - </div> - - <div class="footnote"> - <a id="Footnote_44" href="#FNanchor_44"><span class="label">[44]</span></a> - <i>Phil. Trans.</i> vols. li. and xviii.; <i>Gent. Mag.</i> vol. - xx. 212. - </div> - - <div class="footnote"> - <a id="Footnote_45" href="#FNanchor_45"><span class="label">[45]</span></a> - <i>Trans. Royal Geog. Soc.</i> vol. vi. - </div> - - <div class="footnote"> - <a id="Footnote_46" href="#FNanchor_46"><span class="label">[46]</span></a> - <i>Phil. Trans.</i> vols. xxxvi. and xxxix. - </div> - - <div class="footnote"> - <a id="Footnote_47" href="#FNanchor_47"><span class="label">[47]</span></a> - <i>Am. Jour. of Sci.</i> 1865, vol. xl. p. 365. - </div> - - <div class="footnote"> - <a id="Footnote_48" href="#FNanchor_48"><span class="label">[48]</span></a> - <i>Proc. Geolog. Soc. Ap.</i> 1875, p. 270. - </div> - - <div class="footnote"> - <a id="Footnote_49" href="#FNanchor_49"><span class="label">[49]</span></a> - <i>Gent. Mag.</i> vol. xxi. p. 569. - </div> - - <div class="footnote"> - <a id="Footnote_50" href="#FNanchor_50"><span class="label">[50]</span></a> - <i>Jahrb. f. Min.</i> 1840, p. 173. - </div> - - <div class="footnote"> - <a id="Footnote_51" href="#FNanchor_51"><span class="label">[51]</span></a> - Oldham and Mallet, ‘Cachar Earthquake,’ <i>Trans. Geolog. - Soc. Ap.</i> 1872. - </div> - - <div class="footnote"> - <a id="Footnote_52" href="#FNanchor_52"><span class="label">[52]</span></a> - O. Volger, <i>Unters üb. d. Phän. d. Erdb.</i> vol. iii. p. - 414. - </div> - - <div class="footnote"> - <a id="Footnote_53" href="#FNanchor_53"><span class="label">[53]</span></a> - <i>Meteorologia Endogena</i>, vol. i. p. 166. - </div> - - <div class="footnote"> - <a id="Footnote_54" href="#FNanchor_54"><span class="label">[54]</span></a> - <i>Gent. Mag.</i> vol. xxvi. p. 91. - </div> - - <div class="footnote"> - <a id="Footnote_55" href="#FNanchor_55"><span class="label">[55]</span></a> - <i>Compte Rendu</i>, 1873, p. 66. - </div> - - <div class="footnote"> - <a id="Footnote_56" href="#FNanchor_56"><span class="label">[56]</span></a> - <i>An Historical Account of Earthquakes</i>, p. 46. - </div> - - <div class="footnote"> - <a id="Footnote_57" href="#FNanchor_57"><span class="label">[57]</span></a> - <i>Phil. Trans.</i> vol. xlix. p. 436. - </div> - - <div class="footnote"> - <a id="Footnote_58" href="#FNanchor_58"><span class="label">[58]</span></a> - <i>Am. Jour. Sci.</i> vol. xlv. p. 129. - </div> - - <div class="footnote"> - <a id="Footnote_59" href="#FNanchor_59"><span class="label">[59]</span></a> - <i>Phil. Trans.</i> vol. xlix. p. 547. - </div> - - <div class="footnote"> - <a id="Footnote_60" href="#FNanchor_60"><span class="label">[60]</span></a> - <i>Ibid.</i> vols. xlii. and xxxix. - </div> - - <div class="footnote"> - <a id="Footnote_61" href="#FNanchor_61"><span class="label">[61]</span></a> - <i>Phil. Trans.</i> vol. xlix, part i. - </div> - - <div class="footnote"> - <a id="Footnote_62" href="#FNanchor_62"><span class="label">[62]</span></a> - <i>Compte Rendu</i>, 1873, part ii. p. 66. - </div> - - <div class="footnote"> - <a id="Footnote_63" href="#FNanchor_63"><span class="label">[63]</span></a> - <i>Die Vulcan. Ers. d. Erde</i>, C. W. C. Fuchs. - </div> - - <div class="footnote"> - <a id="Footnote_64" href="#FNanchor_64"><span class="label">[64]</span></a> - <i>Comte Rendu</i>, 1875, p. 693. - </div> - - <div class="footnote"> - <a id="Footnote_65" href="#FNanchor_65"><span class="label">[65]</span></a> - <i>Gent. Mag.</i> vol. xix. p. 190. - </div> - - <div class="footnote"> - <a id="Footnote_66" href="#FNanchor_66"><span class="label">[66]</span></a> - <i>Phil. Trans.</i> vol. xlix. p. 115. - </div> - - <div class="footnote"> - <a id="Footnote_67" href="#FNanchor_67"><span class="label">[67]</span></a> - <i>Gent. Mag.</i> vol. xxi. 1751. - </div> - - <div class="footnote"> - <a id="Footnote_68" href="#FNanchor_68"><span class="label">[68]</span></a> - <i>Jour. Royal Geo. Soc.</i> vol. vi. p. 319. - </div> - - <div class="footnote"> - <a id="Footnote_69" href="#FNanchor_69"><span class="label">[69]</span></a> - Darwin, <i>Geolog. Observations</i>, p. 232. - </div> - - <div class="footnote"> - <a id="Footnote_70" href="#FNanchor_70"><span class="label">[70]</span></a> - <i>Ibid.</i> p. 245. - </div> - - <div class="footnote"> - <a id="Footnote_71" href="#FNanchor_71"><span class="label">[71]</span></a> - Lyell, <i>Principles of Geology</i>, vol. ii. pp. 107–8. - </div> - - <div class="footnote"> - <a id="Footnote_72" href="#FNanchor_72"><span class="label">[72]</span></a> - <i>Gent. Mag.</i> 1733, vol. iii. p. 217. - </div> - - <div class="footnote"> - <a id="Footnote_73" href="#FNanchor_73"><span class="label">[73]</span></a> - ‘Earthquakes of Cutch,’ <i>Jour. Royal Geo. Soc.</i> vol. xl. - </div> - - <div class="footnote"> - <a id="Footnote_74" href="#FNanchor_74"><span class="label">[74]</span></a> - M. Daussy, ‘Sur l’existence probable d’un volcan sousmarin situé ar environ 0° 20′ de lat. S., - et 22° 0′ de long, ouest,’ <i>Comptes Rendus</i>, vol. vi. p. 512. - </div> - - <div class="footnote"> - <a id="Footnote_75" href="#FNanchor_75"><span class="label">[75]</span></a> - <i>Am. Jour. Sci.</i> vol. xlv. p. 133. - </div> - - <div class="footnote"> - <a id="Footnote_76" href="#FNanchor_76"><span class="label">[76]</span></a> - <i>Am. Jour. Sci.</i> vol. xiv. p. 209. - </div> - - <div class="footnote"> - <a id="Footnote_77" href="#FNanchor_77"><span class="label">[77]</span></a> - D. C. F. Winslow, ‘Tides at Tahiti,’ <i>Am. Jour. Sci.</i> 1865, p. 45; also Mallet’s <i>Catalogue of Earthquakes</i>. - </div> - - <div class="footnote"> - <a id="Footnote_78" href="#FNanchor_78"><span class="label">[78]</span></a> - <i>Am. Jour. Sci.</i> vol. i. p. 469. - </div> - - <div class="footnote"> - <a id="Footnote_79" href="#FNanchor_79"><span class="label">[79]</span></a> - Darwin, <i>Researches in Geology, &c.</i>, p. 378. - </div> - - <div class="footnote"> - <a id="Footnote_80" href="#FNanchor_80"><span class="label">[80]</span></a> - Kluge, <i>Jahrb. f. Min.</i> 1861, p. 977. - </div> - - <div class="footnote"> - <a id="Footnote_81" href="#FNanchor_81"><span class="label">[81]</span></a> - Darwin, <i>Voyage of a Naturalist</i>, p. 309. - </div> - - <div class="footnote"> - <a id="Footnote_82" href="#FNanchor_82"><span class="label">[82]</span></a> - Prof. A. D. Bache, <i>United States Coast Survey Report</i>, 1855, p. 342. - </div> - - <div class="footnote"> - <a id="Footnote_83" href="#FNanchor_83"><span class="label">[83]</span></a> - <i>United States Coast Surrey Report</i>, or <i>Am. Jour. Sci.</i> vi. p. 77. - </div> - - <div class="footnote"> - <a id="Footnote_84" href="#FNanchor_84"><span class="label">[84]</span></a> - <i>Petermann’s Mittheilungen</i>, 1877, Heft xii. S. 454, and <i>Nova Acta der - Ksl. Leop. Carol. Deutschen Acad. d Naturforscher</i>, Band xl. No. 9. - </div> - - <div class="footnote"> - <a id="Footnote_85" href="#FNanchor_85"><span class="label">[85]</span></a> - J. Milne: ‘Peruvian Earthquake of May 9, 1877.’ See <i>Trans. Seis. Soc. of Japan</i>, vol. ii. - </div> - - <div class="footnote"> - <a id="Footnote_86" href="#FNanchor_86"><span class="label">[86]</span></a> - <i>Report of British Association</i>, 1847, p. 84. - </div> - - <div class="footnote"> - <a id="Footnote_87" href="#FNanchor_87"><span class="label">[87]</span></a> - <i>Das Erdbeben von Herzogenrath, &c.</i>, p. 134. - </div> - - <div class="footnote"> - <a id="Footnote_88" href="#FNanchor_88"><span class="label">[88]</span></a> - <i>Phil. Trans.</i> vol. li. - </div> - - <div class="footnote"> - <a id="Footnote_89" href="#FNanchor_89"><span class="label">[89]</span></a> - See <i>Am. Jour. Sci.</i> 1872. - </div> - - <div class="footnote"> - <a id="Footnote_90" href="#FNanchor_90"><span class="label">[90]</span></a> - 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.’ - </div> - - <div class="footnote"> - <a id="Footnote_91" href="#FNanchor_91"><span class="label">[91]</span></a> - E. Suess, <i>Die Erdbeben Niederösterreiches</i>. - </div> - - <div class="footnote"> - <a id="Footnote_92" href="#FNanchor_92"><span class="label">[92]</span></a> - H. Hoeffer, <i>Die Erdbeben Kärntens</i>. - </div> - - <div class="footnote"> - <a id="Footnote_93" href="#FNanchor_93"><span class="label">[93]</span></a> - <i>Six Lectures on Physical Geography</i>, by Rev. S. Haughton, F.R.S., chap. i. - </div> - - <div class="footnote"> - <a id="Footnote_94" href="#FNanchor_94"><span class="label">[94]</span></a> - Ramsay, ‘Geological History of Mountain Chains,’ <i>Mining Journal</i>. - </div> - - <div class="footnote"> - <a id="Footnote_95" href="#FNanchor_95"><span class="label">[95]</span></a> - 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. - </div> - - <div class="footnote"> - <a id="Footnote_96" href="#FNanchor_96"><span class="label">[96]</span></a> - <i>Phil. Trans.</i> vol. i. 1836. - </div> - - <div class="footnote"> - <a id="Footnote_97" href="#FNanchor_97"><span class="label">[97]</span></a> - <i>Am. Jour. of Sci.</i> vol. xxxvii. p. 1. - </div> - - <div class="footnote"> - <a id="Footnote_98" href="#FNanchor_98"><span class="label">[98]</span></a> - Milne, ‘British Earthquakes,’ <i>Edin. Phil. Jour.</i> vol. - xxxi. - </div> - - <div class="footnote"> - <a id="Footnote_99" href="#FNanchor_99"><span class="label">[99]</span></a> - <i>Phil. Trans.</i> vol. xlix. pt. i. - </div> - - <div class="footnote"> - <a id="Footnote_100" href="#FNanchor_100"><span class="label">[100]</span></a> - <i>Compte Rendus</i>, 1875, p. 690. - </div> - - <div class="footnote"> - <a id="Footnote_101" href="#FNanchor_101"><span class="label">[101]</span></a> - <i>Am. Jour. Sci.</i> vol. xi. p. 233. - </div> - - <div class="footnote"> - <a id="Footnote_102" href="#FNanchor_102"><span class="label">[102]</span></a> - <i>Transactions of the Asiatic Society of Japan</i>, vol. vi. pt. i. p. 353. - </div> - - <div class="footnote"> - <a id="Footnote_103" href="#FNanchor_103"><span class="label">[103]</span></a> - Kluge, <i>Ueber die Ursachen</i>, &c., p. 74. - </div> - - <div class="footnote"> - <a id="Footnote_104" href="#FNanchor_104"><span class="label">[104]</span></a> - <i>Am. Jour. Sci.</i> vol. xix. p. 162. - </div> - - <div class="footnote"> - <a id="Footnote_105" href="#FNanchor_105"><span class="label">[105]</span></a> - <i>Mitt. d. Deutsch.</i> Ges., Aug. 1878. - </div> - - <div class="footnote"> - <a id="Footnote_106" href="#FNanchor_106"><span class="label">[106]</span></a> - <i>Report to British Association</i>, 1850, p. 74. - </div> - - <div class="footnote"> - <a id="Footnote_107" href="#FNanchor_107"><span class="label">[107]</span></a> - Fuchs, <i>Die Vulkanischen Erscheinungen der Erde</i>, p. 424. - </div> - - <div class="footnote"> - <a id="Footnote_108" href="#FNanchor_108"><span class="label">[108]</span></a> - <i>Bern. Naturf. Gesellschaft</i>, 1852. - </div> - - <div class="footnote"> - <a id="Footnote_109" href="#FNanchor_109"><span class="label">[109]</span></a> - <i>Comptes Rendus</i>, 1874, Jan. to June, p. 51. - </div> - - <div class="footnote"> - <a id="Footnote_110" href="#FNanchor_110"><span class="label">[110]</span></a> - Boué, <i>Parallele der Erdbeben, Nordlichter und Erdmagnetismus, in Sitz. der K. A. d. Wissensch</i>. 1856, vol. iv. p. 395. - </div> - - <div class="footnote"> - <a id="Footnote_111" href="#FNanchor_111"><span class="label">[111]</span></a> - <i>Meteorologia Endogena</i>, vol. i. p. 107, &c. - </div> - - <div class="footnote"> - <a id="Footnote_112" href="#FNanchor_112"><span class="label">[112]</span></a> - <i>Phil. Trans.</i> vol. lxviii. p. 221. - </div> - - <div class="footnote"> - <a id="Footnote_113" href="#FNanchor_113"><span class="label">[113]</span></a> - <i>Gent. Mag.</i> vol. xxvii. p. 508. - </div> - - <div class="footnote"> - <a id="Footnote_114" href="#FNanchor_114"><span class="label">[114]</span></a> - <i>Die Vulkanischen Erscheinungen der Erde</i>, p. 419. - </div> - - <div class="footnote"> - <a id="Footnote_115" href="#FNanchor_115"><span class="label">[115]</span></a> - Petermann’s <i>Geogr. Mitth.</i> 1858, sec. 246. - </div> - - <div class="footnote"> - <a id="Footnote_116" href="#FNanchor_116"><span class="label">[116]</span></a> - <i>Notes on volcanoes of the Hawaiian Islands</i>, W. T. Brigham, Mem. Boston Soc. of Nat. Hist., 1868. - </div> - - <div class="footnote"> - <a id="Footnote_117" href="#FNanchor_117"><span class="label">[117]</span></a> - <i>Gent. Mag.</i> vol. xxiii., 1753. - </div> - - <div class="footnote"> - <a id="Footnote_118" href="#FNanchor_118"><span class="label">[118]</span></a> - <i>Jour. Royal Geog. Soc.</i> vol. vi. - </div> - - <div class="footnote"> - <a id="Footnote_119" href="#FNanchor_119"><span class="label">[119]</span></a> - <i>Ibid.</i> vol. vi. - </div> - - <div class="footnote"> - <a id="Footnote_120" href="#FNanchor_120"><span class="label">[120]</span></a> - <i>Phil. Trans.</i> vol. xlii. - </div> - - <div class="footnote"> - <a id="Footnote_121" href="#FNanchor_121"><span class="label">[121]</span></a> - <i>Am. Jour. Sci.</i> vol. x. p. 191. - </div> - - <div class="footnote"> - <a id="Footnote_122" href="#FNanchor_122"><span class="label">[122]</span></a> - ‘Earthquakes of San Salvador, December 21–30, 1879.’ <i>Am. Jour. Sci.</i> vol. xix. p. 415. - </div> - - <div class="footnote"> - <a id="Footnote_123" href="#FNanchor_123"><span class="label">[123]</span></a> - <i>Gent. Mag.</i> 1757, p. 323. - </div> - - <div class="footnote"> - <a id="Footnote_124" href="#FNanchor_124"><span class="label">[124]</span></a> - <i>Phil. Trans.</i> vol. li., 1760. - </div> - - <div class="footnote"> - <a id="Footnote_125" href="#FNanchor_125"><span class="label">[125]</span></a> - Mallet, <i>Report to Brit. Ass.</i>, 1858, p. 67. - </div> - - <div class="footnote"> - <a id="Footnote_126" href="#FNanchor_126"><span class="label">[126]</span></a> - Von Lasaulx, <i>Earthquakes of Herzogenrath</i>. - </div> - - <div class="footnote"> - <a id="Footnote_127" href="#FNanchor_127"><span class="label">[127]</span></a> - Lyell, <i>Principles</i>, vol. ii. p. 51. - </div> - - <div class="footnote"> - <a id="Footnote_128" href="#FNanchor_128"><span class="label">[128]</span></a> - Lyell, <i>Principles</i>, vol. i. p. 402. - </div> - - <div class="footnote"> - <a id="Footnote_129" href="#FNanchor_129"><span class="label">[129]</span></a> - Fuchs, p. 464. - </div> - - <div class="footnote"> - <a id="Footnote_130" href="#FNanchor_130"><span class="label">[130]</span></a> - <i>Comptes Rendus</i>, August 1854. - </div> - - <div class="footnote"> - <a id="Footnote_131" href="#FNanchor_131"><span class="label">[131]</span></a> - <i>Nature</i>, April 26, 1883. - </div> - - <div class="footnote"> - <a id="Footnote_132" href="#FNanchor_132"><span class="label">[132]</span></a> - <i>Phil. Soc.</i>, Wellington, New Zealand, 1875. - </div> - - <div class="footnote"> - <a id="Footnote_133" href="#FNanchor_133"><span class="label">[133]</span></a> - <i>Phil. Trans.</i>, vol. xlii. - </div> - - <div class="footnote"> - <a id="Footnote_134" href="#FNanchor_134"><span class="label">[134]</span></a> - M. S. di Rossi, <i>Earthquakes of Casamicciola</i>. - </div> - - <div class="footnote"> - <a id="Footnote_135" href="#FNanchor_135"><span class="label">[135]</span></a> - <i>Phil. Trans.</i>, vol. xviii. 1683–5. - </div> - - <div class="footnote"> - <a id="Footnote_136" href="#FNanchor_136"><span class="label">[136]</span></a> - <i>Ibid.</i> vol. xlix. - </div> - - <div class="footnote"> - <a id="Footnote_137" href="#FNanchor_137"><span class="label">[137]</span></a> - H. D. Warner, ‘The City of Earthquakes,’ <i>Atlantic Monthly</i>, March 1833. - </div> - - <div class="footnote"> - <a id="Footnote_138" href="#FNanchor_138"><span class="label">[138]</span></a> - Palmer, <i>Trans. Seis. Soc. of Japan</i>, vol. iii. p. 148. - </div> - - <div class="footnote"> - <a id="Footnote_139" href="#FNanchor_139"><span class="label">[139]</span></a> - Palmer, <i>Trans. Seis. Soc. of Japan</i>, vol. iii. p. 148. - </div> - - <div class="footnote"> - <a id="Footnote_140" href="#FNanchor_140"><span class="label">[140]</span></a> - Paul, <i>Trans. Seis. Soc. of Japan</i>, vol. ii. p. 41. - </div> - - <div class="footnote"> - <a id="Footnote_141" href="#FNanchor_141"><span class="label">[141]</span></a> - G. H. and H. Darwin, <i>Reports of British Association</i>, 1881. - </div> - - <div class="footnote"> - <a id="Footnote_142" href="#FNanchor_142"><span class="label">[142]</span></a> - <i>Reports of British Association</i>, 1881. - </div> - - <div class="footnote"> - <a id="Footnote_143" href="#FNanchor_143"><span class="label">[143]</span></a> - <i>Comptes Rendus</i>, 1875, January to June, p. 685. - </div> - - <div class="footnote"> - <a id="Footnote_144" href="#FNanchor_144"><span class="label">[144]</span></a> - <i>Tel. Jour.</i>, November 15, 1881. - </div> - - <div class="footnote"> - <a id="Footnote_145" href="#FNanchor_145"><span class="label">[145]</span></a> - <i>Minutes and proceedings of the Institute of Civil Engineers</i>, vol. lx. p. 412, and vol. lxiv. p. 343. - </div> - - <div class="footnote"> - <a id="Footnote_146" href="#FNanchor_146"><span class="label">[146]</span></a> - <i>See</i> ‘Earth Tremors,’ p. 309, experiments of M. d’Abbadie, &c. - </div> - - <div class="footnote"> - <a id="Footnote_147" href="#FNanchor_147"><span class="label">[147]</span></a> <i>Meteorologia Endogena.</i> - </div> - - <div class="footnote"> - <a id="Footnote_148" href="#FNanchor_148"><span class="label">[148]</span></a> <i>Ibid.</i> - </div> - - <div class="footnote"> - <a id="Footnote_149" href="#FNanchor_149"><span class="label">[149]</span></a> - <i>Phil. Trans.</i> vol. xlix. p. 544. - </div> - - <div class="footnote"> - <a id="Footnote_150" href="#FNanchor_150"><span class="label">[150]</span></a> - <i>Annual Register</i>, vol. iv. 1761, p. 92. - </div> - - <div class="footnote"> - <a id="Footnote_151" href="#FNanchor_151"><span class="label">[151]</span></a> - <i>Phil. Mag.</i>, May 1876, p. 447. - </div> - - <div class="footnote"> - <a id="Footnote_152" href="#FNanchor_152"><span class="label">[152]</span></a> - <i>Boston Soc. Nat. Hist.</i>, 1868. - </div> - - <div class="footnote"> - <a id="Footnote_153" href="#FNanchor_153"><span class="label">[153]</span></a> - ‘Notes on Tides at Tahiti,’ &c., <i>Am. Jour. Sci.</i> 1866, vol. xlii. p. 45. - </div> - - <div class="footnote"> - <a id="Footnote_154" href="#FNanchor_154"><span class="label">[154]</span></a> - <i>Trans. Seis. Soc. of Japan</i>, vol. iv. Milne, <i>Systematic Observation of Earthquakes</i>. - </div> - - <div class="footnote"> - <a id="Footnote_155" href="#FNanchor_155"><span class="label">[155]</span></a> - <i>Principles of Geology</i>, vol. ii. 177. - </div> - - <div class="footnote"> - <a id="Footnote_156" href="#FNanchor_156"><span class="label">[156]</span></a> - <i>Gent. Mag.</i>, vol. xxvii. p. 448. - </div> - - <div class="footnote"> - <a id="Footnote_157" href="#FNanchor_157"><span class="label">[157]</span></a> - <i>Phil. Trans.</i>, vol. xli. p. 805. - </div> - - <div class="footnote"> - <a id="Footnote_158" href="#FNanchor_158"><span class="label">[158]</span></a> - <i>Meteorologia Endogena</i>, vol. i. pp. 186, 187. - </div> - - <div class="footnote"> - <a id="Footnote_159" href="#FNanchor_159"><span class="label">[159]</span></a> - Darwin, <i>Geological Observations</i>, p. 275 <i>et seq.</i> - </div> - </div> - - <hr class="page" /> - - <div class="center mb2">THE<br /><span class="xlarge">INTERNATIONAL SCIENTIFIC SERIES.</span></div> - <hr class="r15" /> - <p class="center"><span class="smcap">Each book complete in One Volume, 12mo, and bound in Cloth.</span></p> - <hr class="r15" /> - - <ol> - <li class="biblio">FORMS OF WATER: A Familiar Exposition of the Origin and - Phenomena of Glaciers. By <span class="smcap">J. Tyndall</span>, LL. D., F. R. 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