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Abstract: Earthquakes and Style of Faulting in the Transverse Ranges, Southern California

Egill Hauksson

Systems of seismically active, west to west-northwest-striking thrust faults and numerous west to southwest-striking left-lateral faults have been recognized in the Transverse Ranges. In general, the thrust systems cause crustal thickening, while the left-lateral strike-slip faults allow crustal blocks to move out of the way to the west, around the big bend of the San Andreas fault. Numerous thrust faulting moderate-sized earthquakes such as the 1987 ML5.9 Whittier Narrows, the 1930 ML5.2, 1979 ML5.2 and 1989 ML5.O Santa Monica Bay, and the 1973 MW5.3 Point Mugu earthquakes have occurred along the southern margin of the Transverse Ranges. Some of these earthquakes suggest that the thrust faulting extends a considerable distance to the south of the range bounding reverse faults. The MW6.7 1971 San Fernando and the 1991 ML5.8 Sierra Madre earthquakes also contributed to our understanding of the compressional tectonics of the Transverse Ranges. In contrast, the 1988 ML5.O Pasadena and 1990 ML5.2 Upland earthquakes both showed left-lateral faulting consistent with lateral block movements. The most recent seismicity in the eastern Transverse Ranges was aftershock activity following the 1992 MW7.3 Landers mainshock. This activity included the MW6.2 Big Bear aftershock that exhibited left-lateral strike-slip faulting on a southwest-striking and steeply dipping plane. Throughout the Transverse Ranges significant sections of active late Quaternary faults hav not ruptured in large earthquakes during historic times.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994