The extent of country affected by an earth quake shock depends on the violence of the shock and the geological structure of the region. Some earthquakes are local affairs. affecting but a limited area. The earthquake of Lisbon in 1755, which threw down the greater part of the city in -ix minutes. and caused the death of about 40.000 people. disturbed an immense area, it being felt in the Alps; Great Britain. in the Baltic, and in northern Germany; and at Algiers and Fez as severely as in Spain and Portugal; while the effects of the sea-waves were noticeable at much greater distances. The Charleston. S. C., earth quake of August 31. ISM which threw down many buildings in the city, was observed from the Carolina coast. Georgia. and central Florida, northward to southern New England, across New York to Ontario, Canada. and westward to eastern Louisiana, Arkansas. Missouri, and Iowa ; an area S00 miles wide by 1000 miles from north to south. By use of instruments now available. very slight may be de tected at great distances from their origin. The question arises whether the shocks are trans mitted along the crust of the earth or through the earth. i.e. along the circumference or along a chord. If the earthquake wave is transmitted through the crust. the wave coming around the earth one way ought to reach a distant point (not the antipode) earlier or later than the wave going around the earth the other direction. In other words. the instruments at any distant point ought to record two periods of disturbance from the same initial shock. This is found not to he the ease. It is concluded, therefore, that the earthquake wave; travel through the earth along chords of rather than that they follow are located in the circumference of the earth. It is remembered, of course. that waves of other kinds are simultaneously transmitted about the circumference through air and water.
The effect of earthquakes in modifying fea tures of the earth's surface is considerable. The destruction of life and property is well known. The geological effects are land slips, fissures. faults. extravasations of water and mud, vol canic action. elevation and subsidenee, the drain ing of springs, the opening of new springs. and the formation of sea-waves (tidal waves). \\idle Blight tremors occur in almost any part of the world, violent shocks are ordinarily confined to limited areas. As a rule, earthquake shocks are more frequent in volcanic districts. and particu larly along the boundary bet m great and depression-. But earthquake shocks are not confined to the volcanic areas alone. thie large zone particularly liable to earthquakes encircle; the earth. This belt includes the Mediterranean lands. the Azores. the West Central America. the Sandwich Islands, .la pall. China. India, Persia. and Asia Minor. Moreover, earthquakes are not confined to contineitt;. Many, perhaps the larger number of them, origi nate under the sea. The places in sealmt tcnis where the water shows great variation in depth are particularly likely to experience earthquake shocks. as at the (Age of the Tusca rora Deep. The earthquakes at these points have been found to be so destructive of cables that they are called by the cable authorities •da.nger zones.' In assigning causes for earthquakes, there is, of cour-e, wide opportunity for differences of opinion, and for the formation of a variety of hypotheses. It is well established that certain earthquakes are the direct concomitants of vol canic action. Lava in eruption contains a large quantity of steam, which under certain con ditions may cause violent explosion. hurling vast masses of lava into the air. Such violent explosions may even blow off the entire top of a volcano, as in the great explosion of Krakatoa in the Sunda straits, where what was formerly land is now covered by 150 fathoms of water. These -hocks have caused earthquakes whose effects have been felt over many miles. Again. the lava forcing its way through the crust of the earth may form fissures in the solid rocks, and undoubtedly leads to earthquake shocks. But other earthquakes are apparently not associated with volcanic action. and for these another ex planation is necessary. A considerable amount of corrugation of the earth's crust has occurred, for which a number of enu-es have been assigned, secular cooling, change of oblateness. inject ton
of igneous matter into the crust, change of physieal condition. loss of water and gas. etc. Whatever the causes. it is certain that stresses have accumulated sufficiently to raise great mountain systems and bring about much de formation of the rocks of the earth's crust. It is reasonable to suppose that these stresses may at certain times and places aceumulate to such an extent as to enlist. violent impture. which would be felt as an earthquake shock. Indeed. such violent ruptures have actually been observed on a small scale in quarries and excavations, when a superineumbent load has been taken from rocks.. The view is therefore commonly accepted by geologists that many earthquakes. perhaps even sonic of those associated with volcanic eruptions. are but incidents in the readjustment of the earth's crust to changing conditions of pressure. and, so far as oologieal results are concerned, are of extremely small signifiranee as compared with the greater slow earth move ments wide]] have taken place in the past and are Low taking place without violent shriek. The effects of earthquakes in the destruction of life and property and the sudden changes of surface conditions have caused undue to be paid to them as agents modifying the form of tl e earth, while the great lit slow crustal movements of which are but inci dents have been given hut passing notice.
The instruments used in the study of earth quake, are of much interest. The seismograph (q.v.) automatically records the time, violence, and duration of exceedingly slight tremors. In some localities slight tremors have been found to be of daily occurrence, while in no part of the world are they altogether lacking. So delicate are these instruments that the pressure of a hand against a stone column on which they rest will in,tantly cause a variation of the pointer; the sag in the heart of a city during the heavy traffic hours of the day and the sag of valleys between hills during heavy rainfall are indicated. The centre of disturbance of any earthquake shock may be quickly located by comparing the records of time and violence of the shock in different observatories passed by the wave in its course, and then with a globe and pair of com passes determining the point from which a wave would reach all of these observatories at the times recorded. Also by connecting points of simultaneous arrival of the wave, a •oseismal curve may lie drawn; perpendiculars erected upon chords of this curve will pass through the epi eentrum. This determination of the locus of an earthquake is of much economic importance.
Among memorable earthquakes of recent times may be noted that of Lisbon, November 1, 1755, which left the city a heap of ruins, destroyed 00,000 lives, and was felt from the Aladeiras to Britain; that in Calabria in 1753: that which destroyed Caracas in 1812; that which destroyed Aleppo in that at _Mount Ararat in 1840; those at Brusa, Asia Minor, in 1855; Naples, 1857: at Quito, 1859; Mendoza, Argentina, South America, 1860; Manila, 1563; in Peru, 1868; 18,50: Valparaiso. 1880: Ischia, 1883: the earthquake phenomena accompanying the vol canic eruption of Krakatoa, 1883; Colchester, and the eastern counties of England, 1884; Ma laga and Granada, 1884 and 1885; Charleston, 1880; Japan. 1891; Chilpan•ingo. Mexico, 1892; and Quezaltenango. Guatemala. 1902.
ltmmounArny. Reports of British Association Committee on Earthquakes (London, 1850, 1852. 1854, 1858, 1861): Transact ions of the Seismological Society of Japan (Yoko hama, 1880-90) : SIsIlct. "Dynamics of Earth quakes." in Transactions of the Royal Irish Academy (Dublin, 1846); Mallet, The Great iVeapolitan Earthquake of 1,1':;"1 (Loudon, 1862) Ewing, "Earthquake Measurement," in Memoirs of the Science Department of the rnirersity of Tokio, No. 9 (Tokio, 1883) ; Whitney, "The Owens; I ley Ea rthquake," in Overland lily, vol. ix. (San Franeisco, 1872) ; Dutton, "The Charleston Earthquake of August 31, 1886," in Ninth Annual Report, United States Geological Survey (Washington. 1889) ; Earthquakes and Gther Earth Movements (New York, 188(1) ; Humboldt, Cosmos (London. 18115), and Travels (London, 1840); Fompuc, Les tremblements de ferre (Paris, 188S).