EARTHQUAKE. A term applied to any tremor or shaking of the solid crust of the earth due to natural subterranean concussion. Earthquakes vary in intensity from tremors so slight it s to be observed only by the most deli cate instruments, to violent destructive shocks. While the more violent shocks are not of fre quent oceurrenee. except in certain slight tremors are shown. by the delicate ments now used. to be of surprising frequency.
The earth is indeed anything but a lcrra urine. The succession of phenomena recorded in nota ble earthquakes is first a trembling. next a severe shock or several shocks in quick succes sion, and then a trembling, gradually, hut rapid ly, becoming insensible. Noises of sundry kinds usually precede, accompany, or succeed the earthquake. These have been likened to the howling of a storm, the growling of thunder, the clanking of iron chains, the rumbling of heavy wagons along the road, the shattering and crash ing of enormous masses of etc. Such noises are conducted through the ground, through the sea, or through the air. Through the ground they travel for the greatest dis being known where they have traveled nearly 200 An earthquake begins at some point of sub terranean shock, called the centrum or focus, and travels in waves, with gradually diminish ing energy, in every direction from this point. It has been determined that these foci are com monly at depths of S or 10 miles below the sur face, and rarely as deep as 25 or :30 miles. This determination is based on the fact that at the eideentinn over the locus the shock is felt as a vertical movement, while away from this point the shock comes to the surface obliquely. The angle at wIIMh the shock emerges from the ground is determined by the results of the shocks on walls and buildings. With a.number of such observations at various situations, the point of origin of the earthquake may be located. The shock is transmitted through rock in two waves: inc. the e01111)1VS•S loll wave, V i• 'mating parallel to the direction of transmission; and the other, the distortion wave, vibrating normal to the direction of transmission. The former has the greater velocity. The waves are of small amplitude; the ground moves upward or downWard, or obliquely, or horizontally, nsnally through :t very small fraction of an inch, and seldom through a space of more than two or three inches. The destructiveness of the earth
quake is dependent more on velocity on amplitude of the Nvave-movement. With great Velocity a movement of a small fraction of an inch is sufficient to shatter brick buildings. As to the velocity of earthquake waves, Professor Milne, one of the leading investigators of earth quakes, that "different earthquakes, although they may travel across the same voila try. have very variable velocities, varying be tween several hundreds and several thousands of feet per second; that the same earthquake travels more quickly across districts near to its origin than it does across districts which are far re moved; and that the greater the intensity of the shock, the greater is the velocity." The num ber of shocks in an earthquake and the length of time-interval between them vary between wide limits. The cities of Caracas and Lisbon were destroyed in a few- minutes, while the Calabrian earthquake. beginning in 1783. eontimied for fon• years.
The structure and nature of the rocks through which the earthquake shock is transmitted have effect on the transmission of earthquake waves. eausing tumorous deflections and delays. It has been noted that localized areas are times not sensibly affected. while surrounding, areas feel the movement. Also. shocks which are not felt at the surface have been felt in mine-, and vice versa. When an earthquake oc cur- beneath the sea. the vertical usoveutcnL of the -ea-bed generate a great waNr. commonly but erroneously called a tidal wave. which is propa gated outward from the centre of shock and reaches the land after the arrival the earth nave. In the open -ea 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 no feet or more in height, and overwhelms everything in its course. The sandy beach deposits and loose boulders are swept away, while inland the sur is strewn with daris. 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 dapan iu 1S54 gave rise to sea-Nvavcs 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.