Читать книгу A Study of Recent Earthquakes - Charles Davison - Страница 15

Fig. 3.—Diagram to illustrate wave-path and angle of emergence.ToList

When the impulse causing the earthquake takes place at the focus, two elastic waves spread outwards from it in all directions through the earth's crust. The first wave which reaches a point P consists of longitudinal vibrations, that is, the particle of rock at P moves in a closed curve with its longer axis in the direction FP. Mallet supposes this curve to be so elongated that it is practically a straight line coincident in direction with FP. In the second or transversal wave, the vibration of the particle at P takes place in a plane at right angles to FP. These vibrations Mallet, for his main purpose, neglects.

Returning to the longitudinal wave, Mallet calls the line FP the wave-path at P. The direction EP gives the azimuth of the wave-path, or its direction along the surface of the earth. The angle LPA, or EPF, he defines as the angle of emergence at the point P. If Q be farther from E than P, the angle EQF is less than the angle EPF, or the angle of emergence diminishes as the distance from the epicentre increases. At the epicentre, the angle of emergence is a right-angle; at a great distance from the epicentre, it is nearly zero.

Mallet argued that the direction of the wave-path FPA, or its equivalents, the horizontal direction EPL and the angle of emergence EPF, should be discoverable from the effects of the shock at P. The cracks in damaged buildings, he urged, would be at right angles to the wave-path FPA; overturned monuments or gate-pillars should fall along the line EPL, either towards or from the epicentre according to their conditions of support; loose or slightly attached bodies, such as the stone balls surmounting gate-pillars, should be projected nearly in the direction of the wave-path FPA, and their subsequent positions, supposing the balls not to have rolled, should give the horizontal direction EPL of the wave-path, and might, in some circumstances, determine the angle of emergence and the velocity with which they were projected. I shall return to details later on. For the present, it is clear that, in the destruction wrought by the earthquake, Mallet expected to find the materials most valuable for his purpose. Indeed, so obvious did this mode of examination appear to him, that he could not conceal his surprise at the blindness of his predecessors. They seem, he says, "to have been perfectly unconscious that in the fractured walls and overthrown objects scattered in all directions beneath their eyes, they had the most precious data for determining the velocities and directions of the shocks that produced them."