Fault plane determinations for three small earthquakes along the San Jacinto Fault, California: Search for cross faults

Using rupture directivity and aftershock locations, three earthquakes ( M L 4.2 – 4.3) were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane. All three events occurred near the San Jacinto fault zone during December 1989 and February 1990 and had a large component of strike‐slip motion with nodal planes oriented in the northeast and northwest directions. Smaller earthquakes were used as empirical Green functions to deconvolve the path and site complications from the short‐period P waveforms. The resultant far‐field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data. The results of the waveform inversion showed that the earthquake near the Anza Gap (December 2, 1989) and the earthquake near San Bernardino (December 28, 1989) ruptured northwest trending planes. The earthquake near Buck Ridge (February 18, 1990) appeared to rupture a northeast trending plane, but the waveform results for this event were not considered as reliable. Available aftershock data for the three events were also examined using master event relocations. The aftershock sequence following the Buck Ridge event showed a clear lineation trending toward the northeast. The results of this study indicate that the Anza and San Bernardino earthquakes ruptured areas on or parallel to the San Jacinto fault, while the Buck Ridge event ruptured a northeast trending cross fault. In addition to determining the fault plane, the waveform analysis resolved details of the slip distribution for these relatively small earthquakes.