The 1971 San Fernando earthquake: A double event?

abstract Evidence is presented which suggest that the 1971 San Fernando earthquake may have been a double event that occurred on two separate, subparallel thrust faults. It is postulated that the initial event took place at depth on the Sierra Madre fault zone which runs along the base of the San Gabriel Mountains. Rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km. A second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the Sierra Madre fault zone. The surface trace of this fault coincides with the San Fernando fault zone which was the principal fault associated with surface rupture. It is postulated that rupture propagated from a depth of 8 km to the free surface. The moments of the first and second events are approximately 0.7 × 1026 dyne-cm and 1.0 × 1026 dyne-cm, respectively. This model is found to explain the combined data sets of strong ground motions, teleseismic P and S waveforms, and static offsets better than previous models, which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth. Nevertheless, many features of the observed motions remain unexplained, and considerable uncertainty still exists regarding the faulting history of the San Fernando earthquake.