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ABSTRACT

The Santa Susana fault extends along the southern edge of the Santa Susana Mountains from the San Fernando Valley in Los Angeles County 28 km west-northwest into Ventura County. It marks an older hinge line between a thick, continuous middle Miocene to Pliocene sequence on the north and a thin, discontinuous sequence of the same age on the south. The Frew reverse-fault system, south-side up, was overridden by the north-side-up Torrey and Roosa faults that formed before deposition of the Saugus Formation and are precursors to the Santa Susana fault. The Santa Susana fault overrides the Saugus and older formations together with alluvial-fan deposits that unconformably overlie the Saugus; younger alluvial-fan deposits overlie the fault trace. The fault is low dipping and lobate near the surface but steepens at depth to a uniform 55°-60° to maximum well control at -1.5 km. Comparisons with aftershocks of the 1971 San Fernando earthquake suggest that the fault maintains this dip to depths greater than 12 km. Minimum separation varies from zero west of Oak Ridge oil field, where the Santa Susana apparently becomes a bedding fault, to more than 4 km near Aliso-Canyon oil field; true displacement is probably much larger. Comparison of separation on the Santa Susana fault and on the older Torrey and Frew systems suggests that fault displacements have been accelerating since the initiation of the Frew fault during deposition of the Pico Formation. Modern seismieity on the fault is relatively low, although the April 4, 1893, Pico Canyon earthquake may have occurred on the fault. The Santa Susana fault trace steps left and steepens in dip at Gillibrand Canyon and at the west end of Sylmar basin; both steps (lateral ramps) are seismically active. The Gillibrand step is outlined by aftershocks of an M = 4.6 event on April 8, 1976, and the Sylmar (San Fernando) step is outlined by aftershocks of the February 9, 1971, earthquake. The Santa Susana fault is part of a discontinuous north-dipping thrust-fault system that extends from the Red Mountain and San Cayetano faults east to the San Fernando, Sierra Madre, and Cucamonga faults, all of which lie on the south-facing margin of the Palmdale uplift

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