EARTHQUAKES. The eruption of volcanoes is frequently accompanied by a shaking of the earth; and the rising of lava into the crust and the movements of the lava before an eruption also cause earth jars. Likewise, a breaking of the rocks, or a movement of the strata along a fault plane, causes earthquakes. Indeed, any jar to the rocks, even the explosion of •gunpowder .or the falling of a cavern, will produce an earth quake shock. The jar, originating at a point, or along a plane, is transmitted through the rocks as a series of waves moving outward in spherical form from the centre, or focus. At the epicen trum, directly above the focus, the wave move ment is upward; on all sides from the epicentrum it reaches the surface at an angle, departing more and more from the vertical as distance from the epicentrum increases. The violence and the time •of appearance of the shock vary in all directions from this centre. Irregularities of rock texture and structure interfere with the regularity of these variations. The propagation of the earth
quake wave is shown in the accompanying dia gram, where E represents the epicentrum and A B coseismal curves.
Among the geological effects of earthquakes the destruction of life is best known; but the shaking of the ground sometimes changes the topography, shaking loose earth about, and opening fissures in the ground and rocks. When the earthquake originates under the sea a great water-wave is raised. This, advancing on neighboring shelving coasts, so increases in height as to wash over the lower land with highly destructive effects. These earthquake water-waves have an important in fluence on sedimentation in certain places; and the jarring of the sea floor and the ocean water sometimes causes a great destruction of life, which aids in the formation and preservation of fossils. If the jarring is too frequent, however, the tendency is toward extinction of life in the region subjected to the jarring. See EARTH QUAKE.