Abstract

We have studied the seismograms recorded at the historic Berkeley (BRK) and Pasadena (PAS) stations for 20 aftershocks of the 21 July 1952 Kern County earthquake sequence. These events, in the magnitude range of MW 4.5 to 5.6, are too small to be studied teleseismically, yet they are important for better understanding the tectonics of the southern Sierra Nevada and the Tehachapi Mountains. On-scale recordings of moderate-sized events from this important earthquake sequence were first scanned, digitized, and then subjected to waveform modeling using a seismic moment tensor inverse procedure. In particular, the long-period, three-component Galitzen instrument at BRK and the 6-sec Wood-Anderson at PAS provided very high quality seismograms that could be analyzed in this manner. These two sites have been continuously operated from 1887 and 1927, respectively, and both are current sites of state-of-the-art broadband, high dynamic range instrumentation. First-motion polarities reported by Bath and Richter (1958) were used as additional constraints in the estimation of source parameters. There is considerable variability in the three-component seismograms of the 1952 aftershocks, which in turn result in a diversity of focal mechanisms. The majority of the solutions are northwest-striking reverse mechanisms that likely occurred on various mapped thrust faults in the hanging block of the mainshock. There are several events with northeast-striking, left-lateral mechanisms that are consistent with the strike of the White Wolf fault, as well as several normal slip events. The results of this study indicate that there are a variety of active fault structures adjacent to the White Wolf, Garlock and San Andreas faults in this region.

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