The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis

Abstract

The (ML 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M > 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.

Pseudo green's functions and waveform tomography

“…The pGfs seem to do well in this respect for all four stations, and the source mechanism result agrees with the focal plot (Fig. 2) determined from first-motion P-wave picks from the Southern California Seismic Network (SCSN) (Hauksson, 1994). Station SBC is usually problematic when modeling events from the east, due to the complex propagation effect associated with the basins along the path.…”

“…The most straightforward application that can address this issue is to invert for the source mechanism of a closely located small event with known mechanism. To this end, we choose the small aftershock (15:37) that is located within 2 km of the big one (Hauksson, 1994). Figure 8 shows the source inversion result for this small aftershock (15:37) with the method of Zhao and Helmberger (1994).…”

“…These events are chosen because they occurred in the middle of the TERRAscope array and occurred within 2 km of each other (Hanksson, 1994). Previous studies (e.g., Dreger and Hehnberger, 1991;Zhao and Helmberger, 1994;Hauksson, 1994) provide essential information needed in our experiments, for example, source location, depth, mechanism, and source-time function. Application of the pGf technique in waveform tomography is also discussed.…”