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EARTHQUAKE
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EARTHQUAKE . Although the terrible effects which often accompany earthquakes have in all ages forced themselves upon. the See also:attention of See also:man, the exact investigation of seismic phenomena See also:dates only from the See also:middle of the 19th See also:century. A new See also:science has been thus established under the name of seismology (Gr. o-ecoµos, an earthquake). See also:History.—Accounts of earthquakes are to be found scattered through the writings of many See also:ancient authors, but they are, for the most See also:part, of little value to the seismologist. There is a natural tendency to exaggeration in describing such phenomena, sometimes indeed to the extent of importing a supernatural See also:element into the description. It is true that attempts were made by some ancient writers on natural See also:philosophy to offer a rational explanation of earthquake phenomena, but the hypotheses which their explanations involved are, as a See also:rule, too fanciful to be See also:worth reproducing at the See also:present See also:day. It is therefore unnecessary to dwell upon the references to seismic phenomena which have come down to us in the writings of such historians and philosophers as See also:Thucydides, See also:Aristotle and See also:Strabo, See also:Seneca, See also:Livy and See also:Pliny. Nor is much to be gleaned from the pages of See also:medieval and later writers on earthquakes, of whom the most notable are Fromondi (x527), Maggio (1571) and Travagini (1679). In See also:England, the earliest See also:work worthy of mention is See also:Robert See also:Hooke's Discourse on Earthquakes, written in 1668, and read at a later date before the Royal Society. This discourse, though containing many passages of considerable merit, tended but little to a correct See also:interpretation of the phenomena in question. Equally unsatisfactory were the attempts of See also:Joseph See also:Priestley and some other scientific writers of the 18th century to connect the cause of earthquakes with See also:electrical phenomena. The See also:great earthquake of See also:Lisbon in 1755 led the Rev. See also: The See also:principal See also:shock quake, occurred at about 10 P.m. on the 16th of See also:December lases 1857; but, as is usually the See also:case, it had been preceded by See also:minor disturbances and was followed by numerous after-shocks which continued for many months. See also:Early in 1858, aided by a See also: Mallet located his epicentre near the See also:village of Caggiano, not far from Polla, while the other seems to have been in the neighbourhood of Montemurro, about 25 M. to the south-east. The intensity, or violence, of an earthquake is greatest in or near the epicentre, whence it decreases in all directions. A line See also:drawn through points of equal intensity forms a See also:curve See also:round the epicentre known as an isoseist, an isoseismal or an isoseismic line. If the intensity declined equally in all directions the isoseismals would be circles, but as this is rarely if ever the case in nature they usually become ellipses and other closed curves. The tract which is most violently shaken was termed by Mallet the meizoseismic area, whilst the line of maximum destruction is known as the meizoseismic line. That isoseismal along which the decline of See also:energy is most rapid was called by K. von See also:Seebach a pleistoseist. In order to determine the position of the seismic centre, Mallet made much use of the cracks in damaged buildings, especially in walls of See also:masonry, holding that the direction of such fractures must generally be at right angles to that in which the normal earthquake-wave reached them. In this way he obtained the " See also:angle of emergence " of the wave. He also assumed that See also:free-falling bodies would be overthrown and projected in the direction of See also:propagation of the wave, so that the epicentre might immediately be found from the intersection of such directions. These data are, however, subject to much See also:error, especially through want of homogeneity in the rocks, but Mallet's work was still of great value. A different method of ascertaining the depth of the focus was adopted by See also:Major C. E. Dutton in his investigation of the See also:Charleston Charleston earthquake of the 31st of See also:August 1886 earth- for the U.S. See also:Geological Survey. This catastrophe quake, was heralded by shocks of greater or less severity a 1886' few days previously at Summerville, a village 22 M. north-west of Charleston. The great earthquake occurred at 9.51 P.M., See also:standard time of the 75th See also:meridian, and in about 70 seconds almost every See also:building in Charleston was more or less seriously damaged, while many lives were lost. The epicentral tract was mainly a See also:forest region with but few buildings, and the principal records of seismological value were afforded by the lines of railway which traversed the disturbed area. In many places these rails were flexured and dislocated. Numerous fissures opened in the ground, and many of these discharged See also:water, mixed sometimes with See also:sand and silt, which was thrown up in jets rising in some cases to a height of 20 ft. Two epicentres were recognized—one near See also:Woodstock station on the South Carolina railway, and the other, being the centre of a much smaller tract, about 14 M. south-west of the first and near the station of Ran towles on the Charleston and See also:Savannah line. Around these centres and far away isoseismal lines were drawn; the relative intensity at different places being roughly estimated by the effects of the catastrophe on various structures and natural See also:objects, or, where visible records were wanting, by See also:personal See also:evidence, which is often vague and variable. The See also:Rossi-Forel See also:scale was adopted. This is an arbitrary scale formulated by Professor M. S. de Rossi, of See also:Rome, and Dr F. A. Forel, of See also:Geneva, based mostly on the See also:ordinary phenomena observed during an earthquake, and consisting of ten degrees, of which the lowest is the feeblest, viz. I. Microseismic shock; II. Extremely feeble shock; III. Very feeble shock; IV. Feeble; V. Shock of moderate intensity; VI. Fairly strong shock; VII. Strong shock; VIII. Very strong shock; IX. Extremely strong shock; X. Shock of extreme intensity. Other conventional scales, some being less detailed, have been drawn up by observers in such earthquake-shaken countries as Italy and Japan. A curve, or theoretical isoseismal, drawn through certain points where the decline of intensity on receding from the epicentre seems to be greatest was called by Dutton an " See also:index-circle "; and it can be shown that the See also:radius of such a circle multiplied by the square See also:root of 3 gives the focal depth theoretically. In this way it was computed that in the Charleston earthquake the origin under Woodstock must have had a depth of about 12 M. and that near Rantowles a depth of nearly 8 m. The determination of the index-circle presents much difficulty, and the conclusions must be regarded as only approximate. It is probable, according to R. D. See also:Oldham, that See also:local earthquakes may originate in the " See also:outer skin " of the earth, whilst a large world-shaking earthquake takes its origin in the deeper part of the " crust," whence such a disturbance is termed a balhyseism. Large earthquakes may have very extended origins, with no definite centre, or with several foci. The gigantic disaster known as the "Great See also:Indian Earthquake," which occurred on the 12th of See also:June 1897, was the subject of areal careful investigation by the Geological Survey of Indian See also:India and was described in detail by the super- earth- intendent, R. D. Oldham. It is sometimes termed
quake,
1897. the See also:Assam earthquake, since it was in that See also:pro-
vince that the effects were most severe, but the shocks were felt over a large part of India, and indeed far beyond its boundaries. Much of the area which suffered most disturbancewas a See also:wild See also:country, sparsely populated, with but few buildings of See also:brick or See also: It occurred on I9os the 4th of See also:April 1905, and the first great shocks were felt in the See also:chief epifocal See also:district at about 6.9 A.M., See also:Madras time. Although the tract chiefly affected was around Kangra and See also:Dharmsala, there was a subordinate epifocal tract in See also:Dehra Dun and the neighbourhood of See also:Mussoorie, whilst the effects of the earthquake extended in slight measure to See also:Lahore and other cities of the See also:plain. It is estimated that the earthquake was felt over an area of about 1,625,000 M. Immediately after the calamity a scientific examination of its effects was made by the Geological Survey of India, and a report was drawn up by the See also:superintendent, C. S. Middlemiss. The great earthquake, which, with the subsequent See also:fire, wrought such terrible destruction in and around See also:San Francisco on the 18th of April 1906, was the most disastrous ever recorded in See also:California California. It occurred between ro and 15 minutes earth- after 5 A.m., standard time of the 120th meridian. uake, The moment at which the disaster began and the I906 duration of the shock varied at different localities in the great area over which the earthquake was felt. At San Francisco the main shock lasted rather more than one See also:minute. According to the See also:official Report, the earthquake was due to rupture and movement along the plane of the San Andreas fault, one of a series which runs for several See also:hundred See also:miles approximately in a N.W. and S.E. direction near the See also:coast line. Evidence of fresh movement along this plane of dislocation was traced for a distance of 190 m. from San Juan on the south to Point See also:Arena on the north. There the trace of the fault is lost beneath the See also:sea, but either the same fault or another appears 75 M. to the north at Point Delgada. The See also:belt of disturbed country is notoriously unstable, and part of the fault had been known as the " earthquake crack." The direction is marked by lines of straight cliffs, See also:long ponds and narrow depressions, forming a Rift, or old line of seismic disturbance. According to Dr G. K. See also: The elaborate Report of the State Investigation Committee, by the chairman, Professor A. C. See also:Lawson, was published in 1908. On the 17th of August 1906 a disastrous earthquake occurred at See also:Valparaiso, and the year 1906 was marked generally by exceptional seismic activity. The See also:Jamaica earthquake of the 14th of See also:January 1907 appears to have accompanied movement of rock along an east and west fracture or series of fractures under the sea a few miles from the See also:city of See also:Kingston. The statue of See also:Queen See also:Victoria at Kingston was turned upon its See also:pedestal the eighth of a revolution. A terrible earthquake occurred in See also:Calabria and See also:Sicily on December 28, 1908, practically destroying See also:Messina and Reggio. Messina According to the official returns the See also:total loss of See also:life earth- was 77,283. Whilst the principal centre seems to quake, have been in the Strait of Messina, whence the dis- 1908• turbance is generally known as the Messina earthquake, there were See also:independent centres in the Calabrian See also:peninsula, a country which had been visited by severe earthquakes not long previously, namely on See also:September 8, 1905, and See also:October 23, 1907. The principal shock of the great Messina earthquake of 1908 occurred at 5.21 A.M. (4.21 See also:Greenwich time), and had a duration of from 30 to 40 seconds. Neither during nor immediately before the catastrophe was there any See also:special volcanic disturbance at See also:Etna or at Stromboli, but it is believed that there must have been movement along a great plane of weakness in the neighbourhood of the Strait of Messina, which has been studied by E. Cortese. The sea-See also:floor in the strait probably suffered great disturbance, resulting in the remarkable movement of water observed on the coast. At first the sea retired, and then a great wave rolled in, followed by others generally of decreasing See also:amplitude, though at See also:Catania the second was said to have been greater than the first. At Messina the height of the great wave was 2.70 metres, whilst at See also:Ali and Giardini it reached 8.4o metres and at San Alessio as much as 11.7 metres. At See also:Malta the See also:tide-See also:gauge recorded a wave of 0.91 See also:metre. The depth of the chief earthquake-centre was estimated by Dr E. Oddone at about 9 kilometres. The earthquake and accompanying phenomena were studied also by Professor A. See also:Ricci), Dr M. Baratta and Professor G. Platania and by Dr F. Omori of Tokyo. After the great disturbance, shocks continued to affect the region intermittently for several months. In certain respects the earthquake of 1908 presented much resemblance to the great Calabrian catastrophe of 1783. It has been proposed by R. D. Oldham that the disturbance which causes the fracture and permanent displacement of the rocks during an earthquake should be called an "earthshake," leaving the See also:term earthquake especially for the vibratory motion. The movement of the earthquake is molecular, whilst that l,: the earthshake is molar. Subsequently he suggested the terms mochleusis and orchesis (µoXXfuw, I heave; 6p)6ouat, I See also:dance), to denote respectively the molar and the molecular movement, retaining the word earthquake for use in its ordinary sense. In most earthquakes the proximate cause is generally regarded as the fracture and sudden movement of underground rock-masses. Disturbances of this type are known as " tectonic " earthquakes, since they are connected with the folding and faulting of the rocks of the earth's crust. They indicate a See also:relief of the See also:strain to which the rock-masses are subjected by See also:mountain-making and other crustal movements, and they are consequently See also:apt to occur along the steep See also:face of a table-See also:land or the margin of a See also:continent with a great slope from land to sea. In many cases the immediate seat of the originating impulse is located beneath the sea, giving rise to submarine disturbances which have been called " seaquakes." Much attention has been given to these suboceanic disturbances by Professor E. See also:Rudolph. Professor J. H. Jeans has pointed out that the regions of the earth's crust most affected by earthquakes See also:lie on a great circle corresponding with the See also:equator of the slightly See also:pear-shaped figure that he assigns to the earth. This would represent a belt of weakness, subject to crushing, from the tendency of'the pear to pass into a spherical or spheroidal form under the See also:action of See also:internal stresses. According to the See also:comte de Montessus de Ballore, the regions of maximum seismic instability appear to be arranged on two great circles, inclined to each other at about 67°. These are the Circumpacific and Mediterranean zones. Maps of the world, showing the origins of large earthquakes each year, accompany the See also:Annual Reports of the Seismological Committee of the British Association, drawn up by Professor Milne. It is important to See also:note that Professor Milne has shown a relationship between earthquake-frequency and the wandering of the earth's See also:pole from its mean position. Earthquakes seem to have been most frequent when the displacement of the pole has been comparatively great, or when the See also:change in the direction of movement has been marked. Valuable earthquake catalogues have been compiled at various times by See also:Alexis Perrey, R. and J. W. Mallet, John Milne, T. Oldham, C. W. C. See also:Fuchs, F. de Montessus de Ballore and others.
Such earthquakes as are felt from time to time in Great See also:Britain may generally be traced to the formation of faults, or rather to incidents in the growth of old faults. The East Anglian earthquake of the 22nd of April 1884—the most disastrous that had occurred in the British Isles for centuries—was investigated by Prof. R. Meldola and W. See also: An See also:attempt has been made by H. See also:Darwin, for the Seismological Committee of the British Association, to detect and measure any See also:gradual movement of the strata along a fault, by observation at the Ridgeway fault, near Upway, in See also:Dorsetshire. Dr C. Davison in studying the earthquakes which have originated in Britain since 1889 finds that several have been " twins." A twin earth-quake has two See also:maxima of intensity proceeding from two foci, whereas a See also:double earthquake has its successive impulses from what is practically a single focus. The See also:Hereford earthquake of December 1896, which resulted in great structural damage, was a twin, having one epicentre near Hereford and the other near See also:Ross. Davison refers it to a slip along a fault-plane between the anticlinal areas of Woolhope and May See also: Such, it is commonly believed, were the earthquakes which disturbed the Isle of See also:Ischia in 1881 and 1883, and were studied by Professor J. See also:Johnston-Lavis and G. Mercalli. In addition to the tectonic and volcanic types, there are occasional earthquakes of minor importance which may be referred to the collapse of the roof of British earth-quakes. caverns, or other falls of rock in underground cavities at no great depth. According to Prof. T. J. J. See most earthquakes are due, directly or indirectly, to the explosive action of See also:steam, formed chiefly by the leakage of sea-water through the ocean floor. Whatever the nature of the impulse which originates the earthquake, it gives rise to a series of waves which are propagated through the earth's substance and also superficially. In Earth- one See also:kind, known as normal or condensational waves, quake or waves of elastic See also:compression, the particles vibrate waves. to and from the centre of disturbance, moving in the direction in which the wave travels, and therefore in a way analogous to the movement of air in a See also:sound-wave. Associated with this type are other waves termed transverse waves, or waves of elastic distortion, in which the particles vibrate across or around the direction in which the wave is propagated. The normal waves result from a temporary change of See also:volume in the See also:medium; the transverse from a change of shape. The distance through which an earth-particle moves from its mean position of See also:rest, whether radially or transversely, is called the amplitude of the wave; whilst the double amplitude, or total distance of movement, to and fro or up and down, like the distance from See also:crest to trough of a water wave, may be regarded as the range of the wave. The See also:period of a wave is the time required for the vibrating particle to See also:complete an oscillation. As the rocks of the earth's crust are very heterogeneous, the earthquake-waves suffer See also:refraction and reflection as they pass from one rock to another differing in See also:density and See also:elasticity. In this way the waves break up and become much modified in course of transmission, thus introducing great complexity into the phenomena. It is known that the normal waves travel more rapidly than the transverse. Measurements of the surface See also:speed at which earthquake-waves travel require very accurate time-measurers, and these are not generally available in earthquake-shaken regions. Observations during the Charleston earthquake of 1886 were at that time of exceptional value, since they were made over a large area where standard time was kept. Lines drawn through places around the epicentre at which the shock arrives at the same moment are called coseismal lines. The motion of the wave is to be distinguished from the movement of the vibrating particles. The velocity of the earth-particle is its See also:rate of movement, but this is constantly changing during the vibration, and the rate at which the velocity changes is technically called the See also:acceleration of the particle. Unfelt movements of the ground are registered in the earthquake records, or seismograms, obtained by the delicate instruments used by See also:modern seismologists. From the study of the records of a great earthquake from a distant source, some-times termed a teleseismic disturbance, some interesting inferences have been drawn with respect to the constitution of the interior of the earth. The complete See also:record shows two phases of " preliminary tremors " preceding the principal waves. It is believed that while the preliminary tremors pass through the body of the earth, the principal waves travel along or parallel to the surface. Probably the first phase represents condensational, and the second phase distortional, waves. Professor Milne concludes from the speed of the waves at different depths that materials having similar physical properties to those at the surface may extend to a depth of about 30 m., below which they pass into a fairly homogeneous See also:nucleus. From the different rates of propagation of the precursors it has been inferred by R. D. Oldham that below the outer crust, which is probably not everywhere of the same thickness, the earth is of practically See also:uniform See also:character to a depth of about six-tenths of the radius, but the remaining four-tenths may represent a core differing physically and perhaps chemically from the outer part. Oldham also suggests, from his study of oceanic and See also:continental wave-paths, that there is probably a difference in the constitution of the earth beneath oceans and beneath continents. The surface waves, which are waves of great length and long period and are propagated to great distances with practically a See also:constant velocity, have been regarded as quasi-elastic gravitationalwaves. Further, in a great earthquake the surface of the ground is sometimes visibly agitated in the epifocal district by undulations which may be responsible for severe superficial damage. (See also for elastic waves ELASTICITY, § 89.) An old See also:classification of earthquake-shocks, traces of which still linger in popular nomenclature, described them as " undulatory," when the movement of the ground was mainly in a horizontal direction; " subsultory," when the motion was vertical, like the effect of a normal wave at the epicentre; and " vorticose," when the movement was rotatory, apparently due to successive impulses in varying directions. The sounds which are associated with seismic phenomena, often described as subterranean rumbling and roaring, are not without scientific interest, and have been carefully studied by Davison. " Isacoustic lines " are curves drawn through places where the sound is heard by the same percentage of observers. The sound is always See also:low and often inaudible to many. The refined instruments which are now used by seismologists for determining the elements of earthquake motion and for recording earthquakes from distant origins are described in the See also:article See also:SEISMOMETER. These instruments were See also:developed as a consequence of the attention given in modern times to the study of earthquakes in the Far East. (F. W. R.*) See also:Strange as it may appear, the advances that have been made in the study of earthquakes and the world-wide interest shown in their phenomena were initiated in work commenced in Japan. When the See also:Japanese See also:government, seis- mology in desiring to adopt Western knowledge, invited to Japan. its shores bodies of men to See also:act as its instructors, the attention of the newcomers was naturally attracted to the frequent shakings of the ground. Interest in these phenomena increased more rapidly than their frequency, and at length it was felt that something should be done for their systematic study. At midnight on the 22nd of See also:February 188o movements more violent than usual occurred; chimneys were shattered or rotated, tiles slid down from See also:roofs, and in the See also:morning it was seen that See also:Yokohama had the See also:appearance of a city that had suffered a See also:bombardment. The excitement was intense, and before the ruins had been removed a See also:meeting was convened and the Seismological Society of Japan established. The twenty volumes of See also:original papers published by this body summarize to a large extent the results of the later study of seismology.' The attention of the students of earthquakes in Japan was at first directed almost entirely to seismometry or earthquake measurement. Forms of apparatus which then existed, as for example the seismographs, seismometers and seismoscopes of Mallet, Palmieri and others, were subjected to trial; but inasmuch as they did little more than indicate that an earthquake had taken place—the more elaborate forms recording also the time of its occurrence—they were rapidly discarded, and instruments were constructed to measure earthquake motion. Slightly modified types of the new instruments devised in Japan were adopted throughout the See also:Italian peninsula, and it is See also:fair to say that the seismometry developed in Japan revolutionized the seismometry of the world. The records obtained from the new instruments increased our knowledge of the character of earth-quake motion, and the engineer and the architect were placed in a position to construct so that the effects of known movements could be minimized. It was no doubt the marked success, both See also:practical and scientific, attending these investigations that led the Japanese government to establish a See also:chair of seismology at its university, to organize a system of nearly x000 observing stations throughout the country, and in 1893 to appoint a committee of scientific and practical men to carry out investigations which might palliate the effects of seismic disturbances. In the first year this committee received a grant of £5000, and as liberal sums for the same purpose appear from time to time in the ' The publications for 1880-1892 were termed the Transactions of the Seismological Society of Japan, and for 1893-1895 the Seismological See also:Journal of Japan. The observations are now published by the Earthquake Investigation Committee of Japan, and edited by F. Omori, professor of seismology at the university of Tokyo. See also:parliamentary estimates, it may be assumed that the work has been fraught with See also:good results. In their publications we find not only records of experiences and experiments in Japan, but descriptions and comments upon earthquake effects in other countries. In two of the volumes there are long and extremely well illustrated accounts of the earthquake which on the 12th of June 1897 devastated Assam, to which country two members of the above-mentioned committee were despatched to gather such See also:information as might be of value to the architect and builder in earthquake-shaken districts. A great impetus to seismological investigation in Europe and See also:America was no doubt given by the realization of the fact that a large earthquake originating in any one part of the Selsmo- world may be recorded in almost any other. Italy logical research. for many years past has had its observatories for recording earthquakes which can be felt, and which are of local origin, but at the present time at all its first-class stations we find instruments to record the unfelt movements due to earthquakes originating at great distances, and as much attention is now paid to the large earthquakes of the world as to the smaller ones originating within Italian territory.) The Kaiserliche Akademie der Wissenschaf ten of See also:Vienna established earthquake observatories in See also:Austria,2 and the Central Observatorium of St See also:Petersburg has carried out similar work in See also:Russia. See also:Germany attached a seismological See also:observatory to its university at See also:Strassburg, whilst See also:provision has been made for a professorship of Earth Physics (Geophysik) at See also:Gottingen.3 In accordance with the recommendation of the British Association, seismographs of a similar character have been installed at stations all over the world.' The principal objects of this extended and still extending system of stations are to determine the velocity with which motion is propagated over the surface and through the interior of the earth, to locate the positions of sub-oceanic earth-quake origins, and generally to extend our knowledge respecting the physical nature of the See also:planet on which we live. We now know that earthquakes are many times more frequent than was previously supposed. In Japan, for example, between 1885 and 1892 no fewer than 8331 were recorded—that is to say, on the See also:average there were during that time more than See also:rood disturbances per year. Although many of these did not cause a sensible shaking over areas exceeding a few hundred square miles, many of them were sufficiently intense to propagate vibrations round and through the globe. If we pick out the well-marked earthquake districts of the world, and give to each of them a seismicity or earthquake frequency per unit area one-third of that in Japan, the conclusion arrived at is that considerable areas of our planet are on the average shaken every See also:half-See also:hour. The knowledge which we now possess respecting the localities where earthquakes are frequent and the forms of the foci from which they have spread, enables us to speak definitely volcanoes respecting the originating causes of many of these and phenomena. It is found, for example, that although earth- quakes. s. in many countries there may be displays of volcanic and seismic activity taking See also:place almost See also:side by side, it is only rarely that there is See also:direct relationship between the two. Now and then, however, before a volcano breaks into eruption there may be a few ineffectual efforts to form a vent, each of which 1 The chief Italian station is at Rocca di Papa near Rome. It is equipped with delicate instruments designed by its director, Giovanni Agamennone. The records since 1895 are published in the Bollettino della Societd Sismologica Italiana, edited by See also:Luigi Palazzo, director of the Central See also:Office for See also:Meteorology and Geodynamics at Rome. 2 The chief See also:Austrian publications are :—Mittheilungen der Erdbebencommission der k. Akad. der Wissen. in Wien (since 1897) ; Die Erdbebenwarte (1901–1907) ; and the " Neueste Erdbebennachrichten, Beilage der Monatsschrift ` Die Erdbebenwarte." 3 The " See also:International Seismological Association " was founded at Strassburg in 1903, and publishes the Beitrage zur Geophysik, edited by See also:George Gerland, director of the Strassburg station; the papers are printed in several See also:languages. 3 The records of the British Association stations are published (since 1896) in the Reports. See also:Chile has a See also:national earthquake service (founded after the Valparaiso earthquake of August 1906) directed by comte de Montessus de Ballore.is accompanied by no more than a slight local shaking of the ground. This is true even for the largest and most violent eruptions, when mountains have with practically a single effort blown off their heads and shoulders. Thus the earthquake which accompanied the eruption of Bandaisan, in central Japan, in 1888 was felt only over a radius of 25 M. The analyses of the seismic registers of Japan clearly indicate that comparatively few shakings originate near to the volcanoes of the country, the See also:majority of them, like those of many other countries, coming from regions where volcanic rocks are absent. The greatest number spread inland from the Pacific seaboard, the movement becoming more and more feeble as it approaches the backbone of the country, which is drilled with numerous volcanic vents. What is true for Japan is generally true for the western coasts of North and South America. Speaking broadly, earthquakes are most frequent along the steeper flexures in the earth's surface, and in those regions where there is geological evidence to show that slow See also:secular movements in the earth's crust are possibly yet in progress. With a unit distance of 2 degrees, or 120 See also:geographical m., we find that the slopes running eastwards from the See also:highlands of Japan and westwards from the Andean ridges down into the Pacific vary from 1 in 20 to r in 30, and it is on the faces or near to the bottom of these slopes that seismic efforts are frequent. The slopes running from See also:Australia, eastern America and western Europe into the neighbouring oceans vary between r in 70 and r in 250, and in these regions earthquakes are of rare occurrence. The seismic activity met with in the Himalayas and the See also:Alps finds its best explanation in the fact that these mountains are geologically See also:recent, and there are no reasons to doubt that the forces which brought their folds into existence are yet in action. This See also:peculiar association of earthquakes with pronounced topographical configuration and certain geological conditions evidently indicates that the origin of many of them is connected with rock folding. Inasmuch as certain large earthquakes have been accompanied by rock fracture, as for example in 1891, when in central Japan a fault some 5o M. in length was created, whilst the origins of others have been distinctly traced to the line of an existing fault or its continuation, we may conclude that the majority of earthquakes are spasmodic accelerations in the secular movements which are creating (and in some instances possibly obliterating) the more prominent features of the earth's surface. These secular movements, which include upheavals, subsidences, horizontal displacements—all of which are explained on the See also:assumption of a crust seeking support on a nucleus gradually contracting by loss of See also:heat, are collectively referred to as bradyseismical (13paSus, slow) movements. To these may be added movements directly attributable to the influence of gravity. Sub-oceanic districts in a state of seismic strain may be so far loaded by the See also:accumulation of sediments that See also:gentle bending may be accompanied by sudden yieldings. This possibly accounts for the frequency of earthquakes off the mouth of the Tonegawa on the eastern side of Japan. The distortions so frequently observed, in fossils and pebbles, the varying thickness of contorted strata, and the " creep " in See also:coal-mines, together with other phenomena, indicate that rocks may flow. Observations of this nature See also:lead to the supposition that high See also:plateau-like regions may be gradually subsiding under the influence of their own See also:weight, and that the See also:process of See also:settlement may from time to time be spasmodic in its character. Whether the earthquakes which originate round the submerged basal frontiers of the continents bounding the Pacific are ever attributable to such activities, it is impossible to say. All that we know with certainty is that they are sometimes accompanied by such a vast displacement of material that the ocean has been set into a state of oscillation for periods of 24 See also:hours, that in some instances there have been marked changes in depth, and that enormous sub-oceanic landslips have occurred. These phenomena are, however, equally well explained on the assumption of sudden faulting accompanied by violent shaking, which would dislodge steeply inclined beds of material beneath the ocean as it does upon the land. Frequency of earth-quakes. Origin of earth-quakes. Although the proximate cause of earthquake motion is traced to sudden yieldings in the crust of the earth brought about Two types by some form of bradyseismical action, the exist-of earth- ence of at least two distinct types of seismic motion quake indicates that the See also:mechanical conditions accompany-'''. See also:ing the fracturing of rocks are not always identical. 90 or 95 % of the earthquakes which can be recorded consist of elastic or quasi-elastic vibrations. The See also:remainder, including the large earthquakes, not only exhibit the elastic movements, but are accompanied by surface undulations which are propagated most certainly for some hundreds of miles round their origin, and then as horizontal movements sweep over the whole surface of the globe. The former of these may accompany the formation of a new fault or the sudden renewal of movement along an old one; they are cracking or rending effects, without any great displacement. The latter are probably fracturings accompanied by vertical and horizontal displacements of masses of the earth's crust sufficiently great to set up the observed surface undulations. These shocks are so frequently followed a few minutes later by disturbances, which from their similarity to the movements which have preceded them may be called earthquake echoes, that we are led to the See also:speculation that we are here dealing with the caving-in of See also:ill-supported portions of the earth's crust, the waves from which are radiated to boundaries and then returned to their origin to coalesce and give rise to a second impulse not unlike' the See also:primary. Succeeding the first repetition of motion recorded by the seismograph there is often a rhythmical repetition of similar wave See also:groups, suggesting the existence within our earth of phenomena akin to multiple echoes. The introduction of new methods into seismometry quickly revolutionized our ideas respecting the character of earthquake character motion. Although an earthquake may be strongly of earth- felt within a distance of 50 M. from its origin, and quake although the movements in the upper storeys of motion. buildings within the shaken area may be large, the actual range of the horizontal motion of the ground is usually less than 116 of an See also:inch. With such earthquakes ordinary seismographs for recording vertical motion do not show any disturbance. When the movement reaches z in. it becomes dangerous, and a back-and-forth movement of an incl. is usually accompanied by destructive effects. In this latter case the amplitude of the vertical record which indicates the existence of surface waves will vary between z and i o of an inch. In the earthquake which devastated central japan on the 26th of October 189 1, nearly every building within the epifocal district See also:fell, the ground was fissured, forests slipped down from mountain sides to See also:dam up valleys, whilst the valleys themselves were permanently compressed. The horizontal movements seem to have reached 9 in. or r ft., and the surface undulations were visible to the eye. The rapidity with which the movements are performed varies throughout a disturbance. A typical earthquake usually com- mences with minute elastic vibrations, the periods period of which vary between g and z~ of a second. These and duration. are recorded by seismographs, and are noticed by certain of the See also:lower animals like pheasants, which before the occurrence of movement perceptible to human beings scream as if alarmed. When an earthquake is preceded by a sound we have evidence of preliminary tremors even more rapid than those recorded by seismographs. Following these precursors there is a shock or shocks, the period of which will be r or 2 seconds. From this See also:climax the movements, although irregular in character, become slower and smaller until finally they are imperceptible. The duration of a small earthquake usually varies from a few seconds to a minute, but large earth- quakes, which are accompanied by surface undulations, may be felt for 2 or 3 minutes, whilst an ordinary seismograph indicates a duration of from 6 to 12 minutes. A free horizontal pendulum tells us that with severe earthquakes the ground comes to rest by a series of more or less rhythmical surgings. continuing over r or 2 hours. Although the maximum displacement has a 3efinite direction, the successive vibrations are frequently aerformed in many different azimuths. The predominating direction at a given station in certain instances is apparently at right angles to the strike of the neighbouring strata, this being the direction of easiest yielding. Earthquake motion as recorded at stations several thousands of miles distant from its origin exhibits characteristics strikingly different from those just described. The precursors now show periods of from r to 5 seconds, whilst the velocity. largest movements corresponding to the shocks may have periods of from 20 to 40 seconds. The See also:interval of time by which the first tremors have outraced the maximum movement has also become greater. Within a few hundreds of miles from an origin this interval increases steadily, the velocity of propagation of the first movements being about 2 km. per second, whilst that of the latter may be taken at about i•6 km. per second. Beyond this distance the velocity of transmission of the first movements rapidly increases, and for great distances, as for example from Japan to England, it is higher than we should expect for waves of compression passing through See also:steel or See also:glass. This observation precludes the See also:idea that these preliminary tremors have travelled through the heterogeneous crust of the earth, and since the average velocity of their trans-See also:mission increases with the 'length of the path along which they have travelled, and we but rarely obtain certain evidence that a seismograph has been disturbed by waves which have reached it by travelling in opposite directions round the world, we are led to the conclusion that earthquake precursors pass through our earth and not round its surface. The following table See also:relating to earthquakes, which originated off the coast of See also:Borneo on the 20th and 27th of September 1897, is illustrative of the velocities here considered: Localities. Distance Velocity Average from in kms. depth of origin per sec. if chord in in degrees. on chord. kms. Nicolaieff 81° 8•r 8•o See also:Potsdam 92° 8.4 9•r Catania, Ischia, Rocca di 96° 9.0 q 5 Papa, Rome . Isle of.See also:Wight 503° 9.8 10.2 The chords referred to here are those joining the earthquake origins and distant observing stations, and it will be noted that one-See also:quarter of the square root of the average depths at which these run closely corresponds to observed average velocities if wave paths followed chords. This increase of velocity with average depth shows that the paths followed through the earth must be curved with their convexity towards the centre of the earth. These observations do not directly tell us to what ex-See also:tent a true wave path is deflected from the direction of a chord, but they suggest as an extremely plausible assumption that the square of the speed is a linear See also:function of the depth below the surface of the earth. With this assumption Dr C. G. Knott shows that the square of the speed (v2) can be expressed linearly in terms of the average depth of the chord d, thus: v2= 2.9+•026 d, the See also:units being miles and seconds. The See also:formula applies with fair accuracy to moderate and high values of d, but it gives too high a value for See also:short chords. It follows that the square of the speed increases o•9% per mile of descent in the earth. The conclusion we arrive at is that the preliminary tremors which pass through the earth do so in the vicinity of their origin at the rate of almost 2.3 km. per second. This velocity increases as the wave path plunges downwards, attaining in the central regions a velocity of 16 to 17 kms., whilst the highest average velocity which is across a See also:diameter lies between ro and 12 kms. per second. The large surface waves radiating from an origin to a distant place have velocities lying between r•6 and 4 kms. per second, and it has been observed that when the higher velocity has been noted this refers to an observation at a station very remote from the origin. One explanation of this is the assumption that only very large waves indicating a large initial disturbance are capable of travelling to great distances, and as pointed out by R. D. Oldham, large waves under the influence of gravity will travel faster than small waves. These waves (which may be gravitational or distortional) are recorded as slow tiltings of the ground measured by angles of o•5 to zo or 15 seconds of arc, or as horizontal displacements of o•5 or several millimetres. Their calculated lengths have reached 50 kms. (31 m.). In the See also:section of this article relating to the cause of earthquakes a little has been said about their frequency or the number of Frequency times these phenomena are repeated during a given interval of time. It has been shown that all countries are very often moved by earthquakes which have originated at great distances. Great Britain, for example, is crossed about zoo times a year by earthquake waves having durations of from 3 minutes to 3 hours, whilst the vibratory motions which originate in that country are not only small but of rare occurrence. In the earlier stages of the world's history, because the contraction of its nucleus was more rapid than it is at present, it is commonly inferred that phenomena accompanying bradyseismical activity must have been more pronounced and have shown themselves upon a grander scale than they do at the present time. Now, although the records of our rocks only carry us back over a certain portion of this history, they certainly represent an interval of time sufficiently long to furnish some evidence of such enfeeblement if it ever existed. So far from this being the case, however, we meet with distinct evidences in the later chapters of geological history of plutonic awakenings much more violent than those recorded at its commencement. During Palaeozoic times many mountain ranges were formed, and accompanying these orogenic processes there was marked volcanic activity. In the succeeding Secondary period plutonic forces were quiescent, but during the formation of the early See also:Tertiaries, when some of the largest mountain ranges were created, they awoke with a vigour greater than had ever been previously exhibited. At this period it is not improbable that See also:Scotland was as remarkable for its volcanoes and its earthquakes as Japan is at the present day. If the statement relating to the See also:general decrease in bradyseismical changes referred merely to their frequency, and omitted reference to their magnitude, the views of the geologist and physicist might harmonize. One explanation for this divergence of See also:opinion may rest on the fact that too little attention has been directed to all the conditions which accompany the See also:adaptation of the earth's crust to its shrinking nucleus. As the latter grows smaller the puckerings and foldings of the former should grow larger. Each succeeding geological See also:epoch should be characterized by mountain formations more stupendous than those which preceded them; whilst the fracturing, dislccation, caving-in of ill-supported regions, and creation of lines of freedom for the See also:exhibition of volcanic activity which would accompany these changes, would grow in magnitude. The written records of many countries reflect but on a smaller scale the crystallized records in their hills. In 1844, at Comrie, in See also:Perthshire, as many as twelve earthquakes were recorded in a single See also:month, whilst now there are but one or two per year. Earthquake frequency varies with time. A district under the influence of hypogenic activities reaches a See also:condition of seismic strain which usually is relieved rapidly at first, but subsequently more slowly. The small shocks which follow an initial large disturbance are known as after-shocks. The first shock which in 1891 devastated central Japan was accompanied by the formation of a large fault, and the 3364 small shocks which succeeded this during the following two years are regarded as due to intermittent settlements of disjointed material. The decreasing frequency with which after-shocks occur may be represented by a curve. Dr F. Omori points out that the continuation of such a curve gives the means of determining the length of time which will probably elapse before the region to which it refers will return to the same seismic quiescence that it had See also:prior to the initial disturbance. The See also:positive results that we have respecting the periodicity of earthquakes are but few. Generally earthquakes are some-what more frequent during See also:winter than during summer, and this applies to both the See also:northern and See also:southern hemispheres. The annual periodicity, which, however, does not show itself if onlydestructive earthquakes are considered, finds an explanation, according to Dr Knott, in the annual periodicity of long-continued stresses, as for example those due to the accumulation of See also:snow and to barometric gradients. Period• For certain earthquake regions there appears to be a iciey. distinct semi-annual period for which no satisfactory explanation has yet been adduced. Although the elaborate registers of japan, which have enabled us to See also:group earthquakes according to their respective origins and varying intensities, and to See also:separate after-shocks from initial disturbances, have been subjected by Dr Knott to most careful See also:analysis, with the See also:object of discovering periodicities connected with the ebb and flow of the tides, the lunar day or lunar months, nothing of marked character has been found. Certainly there is slight evidence of a periodicity connected with the times of See also:conjunction and opposition of the See also:sun and See also:moon, and a maximum frequency near the time of See also:perigee, but the effect of lunar stresses is comparatively insignificant. Ordinary earthquakes, and especially after-shocks, show a diurnal period, but we cannot say that there are more earth-quakes during the See also:night than during the day. Many experiments and investigations have been made to determine a possible relationship between earthquakes and electrical phenomena, but beyond See also:drawing attention to the fact that luminous appearances may accompany M8$nonettc the See also:friction of moving masses of rock, and that a he• menm temporary current may be established in a line by the disturbance of an earth-See also:plate, these inquiries have yielded but little of importance. The inquiries respecting a possible relation-See also:ship between adjustments so frequently taking place within and beneath that region called the crust of the earth and magnetic phenomena are, however, of a more promising nature. We have seen that at or near the origin of earthquakes which for several hours disturb continents, and occasionally cause oceans to oscillate for longer periods, we sometimes have direct evidence of the bodily displacement of many cubic miles of material. When this material is volcanic it is almost invariably magnetic, and we perceive in its sudden rearrangement causes which should produce magnetic effects within an epifocal district. In Japan, where attention is being directed to phenomena of this description, not only have such effects been observed, but unusual magnetic disturbances have been noted prior to the occurrence of large earthquakes. These may, of course, be regarded as See also:mere coincidences, but when we consider volcanic and seismic activities as evidences of physical and chemical changes, together with mechanical displacements of a magnetic magma, it is See also:reason-able to suppose that they should have at least a local influence upon magnetic needles. Another form of disturbance to which magnetic needles are subjected is that which accompanies the passage of large earth-waves beneath certain observatories situated at great distances from earthquake origins. At See also:Utrecht, Potsdam and Wilhelmshaven the magnetographs are frequently disturbed by seismic waves, whilst at many other See also:European observatories such effects are absent or only barely appreciable. To explain these marked See also:differences in the behaviour of magnetic needles at different stations we are at present only in a position to formulate hypotheses. They may be due to the fact that different needles have different periodic times of oscillation; it is possible that at one observatory the mechanical movements of the ground are much greater than at others; we may speculate on the existence of materials beneath and around various observatories which are different in their magnetic characters; and, lastly, we may picture a crust of varying thickness, which from time to time is caused to rise and fall upon a magnetic magma, the places nearest to this being the most disturbed. A subject to which but little attention has been directed is the effect which displays of seismic and volcanic activities have had upon the human mind. The effects are distinctly dual and opposite in character. In countries like P `k8humanO the England, where earthquakes are seldom experienced, mind. the prevailing idea is that they are associated with all that is baneful. For certain earthquakes, which fortunately are less than 1 % of those which are annually recorded, this is partially true. A disastrous shock may unnerve a whole community. Effects of this nature, however, differ in a marked manner with different nationalities. After the shock of 1891, when Japan lost 9960 of its inhabitants, amongst the wounded indications of See also:mental excitement were shown in See also:spinal and other trouble. Notwithstanding the lightheartedness of this particular nation, it is difficult to imagine that the long series of seismic effects chronicled in Japanese history, which culminated in 1896 in the loss of 29,000 lives by sea-waves, has been without some effect upon its mental and moral character. Several earthquakes are annually commemorated by special services at temples. In bygone times governments have recognized earthquakes as visitations of an angry deity, whom they have endeavoured to appease by repealing stringent laws and taxes. In other countries the sermons which have been preached to show that the tremblings of the world were visitations consequent on impiety, and the prayers which have been formulated to See also: The Kamchadales picture a subterranean deity called Tuil, who in Scandinavian See also:mythology is represented by the evil See also:genius Loki. We have only to think of the reference in the See also:Decalogue forbidding the making of graven images of that which is in the earth beneath, to see in early Biblical history evidence of a subterranean mythology; and it seems probable that the same causes which led to the creation of See also:Pluto, See also:Vulcan and See also:Poseidon gave rise to practices condemned by See also:Moses. Perhaps the greatest practical benefits derived from seismological investigations relate to important changes and new Building principles which have been introduced into the arts of to with. the engineer and builder when constructing in earth- stand quake countries. The new rules and formulae, rather earth- than being theoretical deductions from hypotheses, quakes, are the outcome of observation and experiment. True See also:measures of earthquake motion have been given to us by modern seismometers, with the result that seismic destructivity can be accurately expressed in mechanical units. From observation we now know the greatest acceleration and maximum velocity of an earth particle likely to be encountered; and these are measures of the destructivity. The engineer is therefore dealing with known forces, and he has to See also:bear in mind that these are chiefly applied in a horizontal direction. A formula connecting the acceleration requisite to overturn bodies of different dimensions has been given. The acceleration which will fracture or shatter a See also:column firmly fixed at its See also:foundation to the moving earth may be expressed as follows: i gFAB a _ 6 fw where a= the acceleration per sec. per sec. F=the force of cohesion, or force per unit surface, which when gradually applied produces fracture. A=area of base fractured. B = thickness of the column. f =height of centre of gravity of column above the fractured base. w =the weight of the portion broken off. With this formula and its derivatives we are enabled to state the height to which a See also:wall, for example, may be built capable of resisting any assumed acceleration. Experience has shownthat yielding first shows itself at the base of a See also:pier, a wall or a building, and it is therefore clear that the lower portion of such structures should be of greater dimensions or stronger than that above. Piers having these increased dimensions below, and tapering upwards in a proper manner, so that every horizontal section is sufficiently strong to resist the effects of the inertia of its superstructure, are employed to carry See also:railways in Japan. In that country See also:cast-See also:iron piers are things of the past, whilst piers of masonry, together with their See also:foundations, no longer follow the rules of ordinary See also:engineering practice. After See also:flood, fire, earthquake, or when opportunity presents itself, changes are introduced in the construction of ordinary buildings. In a so-called earthquake-See also:proof See also:house, although externally it is similar to other dwellings, we find rafters running from the See also:ridge pole to the floor sills, an exceedingly See also:light roof, iron straps and sockets replacing mortices and tenons, and many other departures from ordinary rules. Masonry See also:arches for bridges or arched openings in walls (unless protected by lintels), heavy gables, ornamental copings, tappings for chimneys, have by their repeated failure shown that they are undesirable features for construction in earthquake countries. As sites for buildings it is well to avoid soft ground, on which the movement is always greater than on hard ground. Excessive movement also takes place along the face of unsupported openings, and for this reason the edges of scarps, bluffs, cuttings and See also:river-See also:banks are localities to be avoided. In short, the rules and precautions which have to be recognized so as to avoid or mitigate the effects of earthquake movement are so numerous that students of engineering and See also:architecture in Japan receive a special course of lectures on this subject. When it is remembered that a large earthquake may See also:entail a loss of life greater than that which takes place in many See also:wars, and that for the reconstruction of ordinary buildings, factories and public See also:works an See also:expenditure of several million pounds See also:sterling is required, the importance of these studies cannot be overrated. Severe earthquakes are fortunately unknown in the British Isles, but we have simply to turn our eyes to earthquake-shaken colonies and lands in See also:close commercial See also:touch with Great Britain to realize the importance of mitigating such disasters as much as possible, and any endeavour to obviate the wholesale destruction of life should See also:appeal to the civilized communities of the world. An unexpected application of seismometry has been to record the vibration of railway trains, bridges and steamships. An See also:instrument of suitable construction will give records Applicaof the more or less violent jolting and vibratory tions of movements of a See also:train, and so localize irregularities seismodue to changes in the character of See also:ballast and sleepers, metr*, to variation in gauge, &c. An instrument placed on a See also:locomotive throws considerable light upon the effects due to the methods of balancing the wheels, and by alterations in this respect a saving of See also:fuel of from i to 5 lb of coal per mile per locomotive has sometimes been effected. By mapping the centres from which earthquakes originate off the coast of Japan, we have not only determined districts where geological activity is pronounced, but have placed before the See also:cable engineer well-defined localities which it is advisable to avoid; and in the records of unfelt earthquakes which originate far from land similar information is being collected for the deeper parts of the oceans. Occasionally these records have almost immediately made clear the cause of a cable failure. From lack of such information in 1888, when the cables connecting Australia with the outer world were simultaneously broken, the sudden See also:isolation was regarded as a possible operation of See also:war, and the colonists called out their See also:naval and military reserves. Records of earthquakes originating at great distances have also frequently enabled us to anticipate, to correct, to extend, or to disprove telegraphic accounts of the disasters. Whatever information a seismogram may give is certain, whilst the information gathered from telegrams may in the process of transit become exaggerated or minimized. Otherwise unaccountable disturbances in records from magnetographs, barographs and other instruments employed in observatories are frequently explained by reference to the traces yielded by seismometers. Perhaps the greatest See also:triumph in seismological investigation has been the determination of the varying rates at which motion is propagated through the world. These measurements have already thrown new light upon its effective rigidity, and if we assume that the density of the earth increases uniformly from its surface towards its centre, so that its mean density is 5.5, then, according to Knott, the coefficient of elasticity which governs the trans-mission of preliminary tremors of an earthquake increases at a rate of nearly 1.2 % per mile of descent. U. M1.) EARTH-See also:STAR (Geaster), in See also:botany, a kind of puff-See also:ball, with a distinct outer coat which, on separating from the inner, splits into several divisions, which be-come reflexed and spread like a star. The inner coat enveloping the spores is supported, like a ball, either with or without a stalk on the upper face of the star. The spores See also:escape generally by means of a distinct See also:aperture which appears in the See also:top of the ball. There are several See also:species in Britain found on the ground or on decaying leaves. They are rare or local, but more See also:common in the south or south-east of England than in other parts of Britain. Additional information and CommentsThere are no comments yet for this article.
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