SEISMICITY AND STRESS-DROP IN THE EASTERN TRANSVERSE RANGES, SOUTHERNCALIFORNIA

Stress-drops for small to moderately sized earthquakes in Southern Cal ifornia are found to vary systematically with source-depth and locatio n (tectonic environment). We determine high-quality fault-plane soluti ons, plus depth and source duration, for 17 significant (M > 3.9) afte rshocks associated with the June 28, 1992 Big Bear Sequence, including the more recent April 4, 1994 19:04 GMT M(omega) 4.6 Lake Arrowhead a ftershock, and a M(omega) 4.2 Banning Pass event which occurred on May 31, 1993 at 08:55 GMT. Given source durations and moments obtained fr om long-period source estimations, and assuming a circular fault model , we estimate stress-drop for each event. Big Bear aftershocks are mod erate to high (> 100 bars) stress-drop. Events deeper than 12 km are g enerally high stress-drop (> 100 bars), while shallower events exhibit moderate to high stress-drops. These results are compared with a simi lar analysis of Landers aftershocks in the Mojave block. For the Big B ear region, stress drops appear to correlate with depth, with the deep est events yielding the highest stress-drops. In general, events in th is region yield higher stress-drops than events occuring in the Mojave block and those associated with the Landers and Joshua Tree sequences . Comparisons of M(L) to M(o) are consistent with the stress-drop resu lts: deep, high stress-drop events show elevated M(L) to M(o) ratios.