There are many modern investigators of earthquake phenomena. The names of Milne, Omon, Dutton, Oldham, Wiechert and many others are familiar; and in Europe and Japan many earthquake observatories are in active operation. Several such observatories have been established in the United States and now that the Seismological Society of America has been founded at San Francisco, no doubt the study of earthquakes will become an important object of attention. The Seismological Society of Japan has done much for the investigation of earthquakes in the Orient and elsewhere.
The distribution of earthquakes is very similar to that of volcanoes, but there is a quite definite earthquake zone encircling the whole earth, only a part of which is distin guished by Volcanic activity. This zone in cludes the Mediterranean Sea, the Azores, the West Indies, Central America, Japan, China, India, Persia and Asia Minor. This may be regarded as a main earthquake zone from which there are many branches following the coast line of the continents and thus, in gen eral, nearly coinciding with regions of vol canic action. In many cases it is, indeed, evi dent that volcanic eruptions and earthquakes which accompany them are closely related, both being due to explosions of steam, with a con sequent readjustment of pressures and mate rial within the immediate region. A great volcanic eruption is thus usually preceded by earthquakes which are frequently felt for a great distance from the volcanic zone; upon the actual commencement of the eruption, how ever, the earthquakes diminish greatly in in tensity or wholly cease, doubtless because the interior steam pressure has been relieved by a passage through the outlet formed. Yet these disturbances are only very local compared with the effects of really great earthquakes. Nor is it true that a very large proportion of the heavier earthquakes, even in these regions, are closely associated with volcanic activity; the original centre of the disturbance is usually found to lie at a distance, where there are no volcanic rocks.
A second and more widespread. cause of earthquakes is a sudden settling, faulting or bending of the earth's crust owing to the strains which are set up within it. The slow secular contraction of the earth's interior must set up such strains in the material resting upon it; though the rocks may withstand this strain for a considerable period, yet ultimately they may suddenly give way, causing a rock fracture many miles in length, the rock on one side sinking down and causing a afault,D— or there may be a general jarring subsidence under a territory of considerable extent. These phenomena are most
frequent along the greater mountain ranges which border the oceans and are closely asso ciated with regions of present folding and upheaval or subsidence of the earth's crust. Both they and volcanoes are obviously asso ciated with lines of weakness in the crust of the earth. Similarly, earthquakes may be pro duced by continued deposition of sediment on an ocean bed, which latter being overloaded (and possibly heated and thus weakened by the superimposed material), may suddenly yield.
We have thus far no accurate data for esti mating accurately the number and situation of earthquakes which occur under the sea but dis turbances there are not unfrequently more vio lent than under the land. One of the most frequent phenomena noticed to accompany severe earthquakes along the ocean shores is the violent disturbances of the sea, giving rise to enormous waves which sometimes prove more disastrous than the earthquakes themselves. These seismic sea waves are divided into two classes: First, those in which the water is noticed slowly to withdraw from the shore some time after the earthquake and later return as a great wave carrying everything before it; and second, those in which the water rises sud denly and overflows the coast, without any pre vious recession from the shore. In the first class are to be included the waves which inun dated Lisbon after the great earthquake of 1 Nov. 1755; Callao, 28 Oct. 1746; Conception, 20 Feb. 1835; Arica, 13 Aug. 1868; and Iquique, 9 May 1877. In all these cases the water retired some little time after the earthquake, say from a quarter to half an hour and sometimes after longer intervals; and moreover the withdrawal is gradual, so that ships at anchor simply settle down upon the sea bottom laid bare by the slow withdrawal of the water. Then after an inter val of 20 minutes, or half an hour, the sea re turns as a great wave, which is said to have been 80 feet high at the overflow of Callao in 1746 and from 50 to 60 feet high at the inunda tions of Arica and Iquique. The wave is naturally made higher with a more vertical front as it approaches the shore, owing to the resistance to propagation in shallow water.