--> Abstract: Seismotectonics of the Palos Verdes-San Pedro Basin Fault Systems, Los Angeles Region, Southern California, by Bruce A. Schell; #90076 (2008)
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Seismotectonics of the Palos Verdes-San Pedro Basin Previous HitFaultNext Hit Systems, Los Angeles Region, Southern California

Bruce A. Schell
3775 Carmel Ave, Irvine, California 926606

The Palos Verdes Previous HitfaultNext Hit comprises a series of closely spaced faults trending about N40oW in a nearly straight line from the offshore area of San Pedro shelf, into the Los Angles harbor area. Within the Harbor, the Previous HitfaultNext Hit extends to the Vincent Thomas Bridge where it steps left and assumes a slightly more northerly trend to West Basin. The Previous HitfaultNext Hit continues northwesterly along the northeast margin of the Palos Verdes Hills to the Redondo Beach area where it is covered by Pleistocene sand dunes. The onshore Palos Verdes Hills segment forms a restraining bend where the Previous HitfaultNext Hit dips at a lower angle to the southwest than the southern offshore segment that is essentially vertical. The nearly pure strike-slip motion along the vertical Previous HitfaultNext Hit plane in the offshore segment adjusts to the change in trend and dip by uplifting the Palos Verdes Hills and thrusting them over the Torrance Plain, along a steeply southwest-dipping (65-70o) Previous HitfaultNext Hit plane.

Although the Palos Verdes Hills anticlinorium extends into Santa Monica Bay, but this northern segment does not appear to be associated with an active Previous HitfaultNext Hit. A few small Holocene offsets near Santa Monica Sea Canyon cannot be traced with any continuity toward the Palos Verdes Hills. The northern segment may be separated from the Palos Verdes Hills segment near Redondo submarine canyon by an east-west trending reverse Previous HitfaultNext Hit that may transfer strain onto offshore faults such as the San Pedro Basin Previous HitfaultNext Hit. The San Pedro Basin Previous HitfaultNext Hit is a nearly vertical Previous HitfaultNext Hit extending northwesterly subparallel to the Palos Verdes Previous HitfaultNext Hit for about 85 km from Avalon Knoll to the E-W trending Dume Previous HitfaultNext Hit along the southern margin of the Santa Monica Mountains uplift. The San Pedro Basin Previous HitfaultNext Hit comprises alternating normal and reverse components with flower structures suggesting it is a strike-slip Previous HitfaultNext Hit. In contrast to the northern Palos Verdes Previous HitfaultNext Hit, the San Pedro Basin Previous HitfaultNext Hit has abundant sea floor offsets and young sea floor geomorphic features indicating it has been much more active in Holocene time than the northern Palos Verdes Previous HitfaultNext Hit. A model for seismic hazards analysis employs an expected Previous HitfaultNext Hit rupture on the Palos Verdes Previous HitfaultNext Hit of 30 to 60 km, resulting in a maximum earthquake of 7.0 to 7.25. The maximum rupture would be associated with a maximum earthquake of Mw 7.25 that has an average recurrence of about 900 years. Slip on the Previous HitfaultNext Hit occurs at a rate of 3 mm/yr and the sense of motion is predominantly strike slip with about a 10 to 15 percent vertical component. Little is known about the displacement history and rate of slip on the San Pedro Basin Previous HitfaultNext Hit or the Redondo Canyon Previous HitfaultNext Hit. The San Pedro Basin Previous HitfaultNext Hit is very similar in form and activity to the onshore Newport-Inglewood structural zone that has a slip rate of about 1 mm/yr and therefore a similar rate for the San Pedro Previous HitfaultNext Hit seems reasonable. Empirical Previous HitfaultTop length/earthquake magnitude relationships suggest a maximum earthquake of about Mw 7.0 with a recurrence interval more than a thousand years.

 

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California