PROPERTIES OF STRONG GROUND MOTION EARTHQUAKES* By

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. From a certain point of view many of the observed properties of destructive shocks can be correlated to give a consistent picture of what happens in the large, although because of the complexity many of the details cannot be accounted for precisely. The slipping that takes place along an active fault releases shear stress that existed on the fault, and the rapid release of stress sends out waves which, after reflection and refraction through various strata, produce the motion that is measured on the surface of the ground. If the initial state of stress on the fault is not uniform, and if the stress on the fault during slipping is not constallt, some relatively short-period stress waves may be sent out that would have a strong effect on the ground acceleration measured near the fault, even though their effect would be negligible on recorded ground displacements. The properties of strong groundmotion accelerograms will thus reflect the direction of slip, the size of the slip area, the strata through which the waves pass, and the details of nonuniformity of the stress that is released. The slipping along a fault may be pictured as a releasing of shear dislocations. An example of a shear dislocation, as defined in this paper, is shown in figure 1, a and b. Consider an indefinitely extended elastic material, in the interior of which there is a plane crack with boundary "a" as shown in figure 1, a. The two abutting faces of the crack have been given a lateral displacement relative to each other as shown in figure 1, b. In order to maintain this relative displacement there must exist equal and opposite shear stresses on the two faces. These shear stresses can exist only if friction forces can be developed and this requires that compressive stresses exist in the material. In a typical single dislocation the stress on the faces of the dislocation and in the adiacent material will be distributed in the manner shown in figure 1, c and d. The precise form of the shear distribution will depend upon the relative displacement of the two faces and the shape of the dislocation. If the