Earth motions in the vicinity of a fault slip
Existing lack of knowledge regarding the mechanism of fault slipping and resultant earth motions is in part met by new and important displacement data now available for the Imperial Valley earthquake of 1940 and the Long Beach earthquake of 1933. The ground displacements were not recorded directly but obtained by mathematical treatment of seismograph acceleration records. The accuracy of the original and deduced records has been, proved adequate. In the case of the El Centro record of the Imperial Valley earthquake maximum accelerations of 0.3 g, velocity of 24 inches a second, and displacement of 15 inches are all large and indicate greater values in stronger earthquakes. The record was obtained 7 miles from the epicenter and 5 miles from the nearest point of the fault on which slipping occurred. All major wave types appear to be present, and they appear to have their genesis at a point or in a small area. The wave motions are not dominated by the direction of fault slip but are related to the direction from epicenter to recording instrument. The results show how improved records would make it possible to measure permanent displacement when it occurs. Doubt is thrown on the presence of really dominant wave periods during an earthquake in California.