Seismotectonics of the Palos Verdes-San Pedro Basin Fault Systems, Los Angeles Region, Southern California

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

The Palos Verdes fault 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 fault extends to the Vincent Thomas Bridge where it steps left and assumes a slightly more northerly trend to West Basin. The fault 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 fault 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 fault 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) fault 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 fault. 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 fault that may transfer strain onto offshore faults such as the San Pedro Basin fault. The San Pedro Basin fault is a nearly vertical fault extending northwesterly subparallel to the Palos Verdes fault for about 85 km from Avalon Knoll to the E-W trending Dume fault along the southern margin of the Santa Monica Mountains uplift. The San Pedro Basin fault comprises alternating normal and reverse components with flower structures suggesting it is a strike-slip fault. In contrast to the northern Palos Verdes fault, the San Pedro Basin fault 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 fault. A model for seismic hazards analysis employs an expected fault rupture on the Palos Verdes fault 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 fault 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 fault or the Redondo Canyon fault. The San Pedro Basin fault 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 fault seems reasonable. Empirical fault 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