Seismicity and stress‐drop in the Eastern Transverse Ranges, southern California

Stress‐drops for small to moderately sized earthquakes in Southern California are found to vary systematically with source‐depth and location (tectonic environment). We determine high‐quality fault‐plane solutions, plus depth and source duration, for 17 significant (M >3.9) aftershocks associated with the June 28, 1992 Big Bear sequence, including the more recent April 4, 1994 19:04 GMT Mw 4.6 Lake Arrowhead aftershock, and a Mw 4.2 Banning Pass event which occurred on May 31, 1993 at 08:55 GMT. Given source durations and moments obtained from long‐period source estimations, and assuming a circular fault model, we estimate stress‐drop for each event. Big Bear aftershocks are moderate to high (>100 bars) stress‐drop. Events deeper than 12 km are generally high stress‐drop (>100 bars), while shallower events exhibit moderate to high stress‐drops. These results are compared with a similar analysis of Landers aftershocks in the Mojave block. For the Big Bear region, stress drops appear to correlate with depth, with the deepest events yielding the highest stress‐drops. In general, events in this region yield higher stress‐drops than events occuring in the Mojave block and those associated with the Landers and Joshua Tree sequences. Comparisons of ML to MO are consistent with the stress‐drop results: deep, high stress‐drop events show elevated ML to MO ratios.