Seismological investigations

Abstract

The San Fernando earthquake of 9 February 1971, now assigned a magnitude of 6.4 by the Pasadena Network, occurred in an area of low to moderate seismic activity in the years preceding 1971. There were no known reasons to suspect a major earthquake in this area more than in many other geologically and seismologically similar areas in Southern California, and there were no recognized events precursory to the main shock. It is assigned an epicenter at 34°24.7 min. N., 118°24.0 min. W., and a depth of about 8 1/2 km. The seismic and geologic field data are in good agreement in pointing to a major N-dipping thrust fault as the cause of the earthquake. The aftershock sequence has been normal and is still continuing; major aftershocks delineate a lunate-shaped area that corresponds well to the assumed edge of the broken segment of the thrust fault except near Granada Hills and Chatsworth, where several larger aftershocks that have caused additional damage are not obviously related to the thrust fault of the main shock. The principal geologic and seismologic lesson of the earthquake is that, with some exceptions, we cannot as yet delineate in detail areas of markedly different seismic risk, and that for purposes of public policy in zoning and building codes, all of coastal California must be assumed to chare a relatively high earthquake hazard.