The 1991 Sierra Madre Earthquake Sequence in Southern California: Seismological and Tectonic Analysis
Egill HaukssonPublished August 1994, SCEC Contribution #55
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 degrees on a plane striking N 62 degrees fE and dipping 500 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.
Citation
Hauksson, E. (1994). The 1991 Sierra Madre Earthquake Sequence in Southern California: Seismological and Tectonic Analysis. Bulletin of the Seismological Society of America, 84(4), 1058-1074.
