M5.3 normal? faulting event adjacent to the San Andreas fault on the San Francisco Peninsula

Richard Marsden, Dr. Mary Lou Zoback, Dr. Bruce Julian, Dr. Doug Dreger, Dr. Tom Parsons, Jean Olson

A M5.3 earthquake occurred near Daly City on March 22, 1957, the largest event on the northern San Francisco peninsula since 1906. It was located within a kilometer of the "locked" section of the San Andreas fault and roughly 20 km southeast of the presumed epicentral area of the 1906 earthquake, offshore from the Golden Gate (Boore, 1981, BSSA). Available local and regional first motion data were originally interpreted to suggest primarily dip-slip movement on a near vertical and a near horizontal plane (Tocher, 1959, CDMG Report 57, Uhrhammer, 1981, BSSA) indicating that, despite its location, this event was not a typical right--lateral strike slip San Andreas earthquake. New first motion data and 3--D ray tracing to improve local take--off angles yielded a normal faulting mechanism (338, 66, -50). To supplement the first motions which only weakly constrain the mechanism, we did both forward and inverse waveform analysis of numerous intermediate and long period seismograms from California. Unfortunately, no full 3 component set of onscale recordings are available for the mainshock, so data inversion was not feasible and only forward models could be tested. Synthetic seismograms were generated using Green's functions for a suite of possible mechanisms similar to those common for microearthquakes on the Peninsula today. The results of the forward tests are non--unique, although thrust faults resulting from fault--normal compression (common earthquakes on the southern Peninsula) can be eliminated. The first motion data also rule out a simple right lateral strike--slip event on a NW--trending vertical plane. Mechanisms for two aftershocks were determined by waveform inversion using seismograms from Berkeley, both indicated dominantly normal faulting. The mainshock and aftershock solutions are quite similar to normal faulting micro-earthquake mechanisms presently occurring in the 1957 region (and just offshore to the NW) suggesting that the seismc deformation within 20 km of the epicentral region of the 1906 earthquake is characterized by ongoing, dominantly normal faulting.

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