EARTHQUAKE. 591 EARTHQUAKE. mine*, and vice versa. When an eaitliiiuake oc- curs beneath the sea, the vertical movements of Ihe >eal)ed generate a •,'reat wave, commonly but erriinoously called a tidal wave, which is propa- g-ated outward from the centre of shock and reaches the land alter the arrival of the earth- wave. In the open sea this wave is so broad that it cannot be perceived; but when it reaches shallow water near the shore it rushes forward as an immense breaker, sometimes 60 feet or more in lieight. and overwhelms everything in its course. The sandy beach deposits and loose boulders are swept away, while inland the sur- face is strewn with debris. The velocity of these great sea-waves is much greater than the ordi- nary waves raised by the wind. A submarine earthquake near the coast of Japan in 1854 gave rise to sea-waves which traversed the whole breadth of the Pacific at a rate of about 370 miles per hour. At Simoda, .Japan, the waves were 30 feet high, while at San Diego, Cal., they measured only six inches. Tlie extent of country aflfected by an earth- quake shock depends on the violence of the shock and the geological structure of the region. Some earthquakes are local affairs, afl'ecting but a limited area. The earthquake of Lisbon in 1755, which threw down the greater part of the city in six minutes, and caused the death of about 40.000 people, disturbed an immense area, it being felt in the .lps,- 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, 1886, which threw down many buildings in the city, was observed from the Carolina coast, Georgia, and central Florida, northward to southern Xew England, across New York to Ontario, Canada, and westward to eastern Louisiana. Arkansas. Missouri, and Iowa: an area 800 miles wide by 1000 miles from north to south. By use of instruments now available, very slight tremors 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 be the ease. It is concluded, therefore, that the earthquake waves travel through the earth along chords of arcs rather than that they follow arcs located in the circumference of the earth. It is remembered, of conr.se. 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. Tbe geological effects are land slips, fissures. faults, extravasations of water and mud. vol- canic action, elevation and subsidence, the drain- ing of springs, the opening of new springs, and the formation of sea-waves (tidal waves). While elight 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 between great elevations and depressions. But earthquake shocks are not conlincd to the volcanic areas alone. One large zone particularly liable to earthquakes encircles the eartli. This bell includes the -Mediterranean lands, the Azores, the West Indies, Central America, the Sandwich Islands, Japan, China, India, Persia, and Asia Minor, Moreover, earthquakes are not confined to continents. JIany, perhaps the larger number of them, origi- nate under the sea. The places in sea-bot- toms where the water shows great variation in depth are particularly likely to experience earthquake shocks, as at the edge 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 "danger zones.' In assigning causes for earthquakes, there is, of course, wide opportunity for differences of opinion, and for the formation of a variety of liypothcses. 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 wliat was formerly land is now covered by 150 fathonis of water. Tliese shocks have caused earthquakes whose effects have been felt over many miles. Again, the lava forcing its way throiigli the crust of the earth may form fissures in the solid rocks, and undoubtedly leads to earthquake shocks. But otlicr 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 causes have been assigned, secular cooling, change of oblateness. injection of igneous matter into the crust, change of physical condition, loss of Avater 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 accumulate to such an extent as to cause violent ruptire. which would be felt as an earthquake shock. Indeed, such violent ruptures have nctually been observed on a small scale in quarries and excavations, when a superincumbent load has been taken from rocks. . The view is therefore commonly accepted by geologists that many earthquakes, perhaps even some 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 geological results are concerned, are of extremely small significance as compared with the greater slow earth move- ments which have taken place in the jiast and are now taking place without violent shock. The effects of earthquakes in the destruction of life and property and the sudden changes of surface conditions have caused undue attention to be paid to them as agents modifying the form of the earth, while the great but slow crustal
