Properties of strong ground motion earthquakes

Abstract

The usual type of California earthquake is associated with a relative horizontal slipping of the two faces of a fault which lies essentially in a vertical plane. As a consequence of this slipping, the surface of the ground experiences a severe motion in the neighborhood of the fault during large earthquakes. This surface ground motion can be measured, and for the relatively small number of strong ground motions that have been recorded the maximum horizontal acceleration measured was 0.33 of gravity. For obvious reasons the accumulation of data on strong ground motion is a slow process, so that it will be many years before a sufficiently large body of observations is amassed to give a reasonably complete picture of what can be expected in the way of destructive seismic motions. Analysis of the problem is rendered difficult by the fact that it is not possible to make direct observations of the mechanism that generates the seismic waves, for the usual California earthquake originates at a depth of approximately ten miles, although in some large earthquakes the slipping on the fault has extended to the surface of the ground.