Page:Great Neapolitan Earthquake of 1857.djvu/46



elastic or earth wave of shock, may reach a given point upon the surface, with any angle of emergence (the angle contained by the horizontal plane with the wave-path at the point of emergence), or in any azimuth. The path of the wave is a right line joining that point with the centre of impulse (or focus), the wave being assumed propagated thence in all directions outwards in spherical shells. This is strictly true only in an homogeneous elastic solid.

Every point in a coseismal line (that in which such a wave shell simultaneously reaches the earth's surface) at the moment of shock describes a closed curve in space, returning to (or almost exactly to) the point from which it started into motion. The curve is one of double curvature, the vibration taking place nearly simultaneously, in three rectangular unequal axes. For the purposes of our inquiry we may neglect the transversal vibration, and may consider the closed curve of normal vibration as confined to vertical planes passing through the centre of impulse. In fact, the movement of the wave particles may be assumed as confined to right lines, indirectum with the path of the