New Insights on Active Strike-Slip Faulting in the Inner Continental Borderland, Southern California, from High-Resolution Seismic Reflection Data

J. E. Conrad, H. F. Ryan, and R. W. Sliter
U. S. Geological Survey, Menlo Park, CA, [email protected], [email protected], [email protected]

About 7 mm/yr of slip between the North American and Pacific plates is accommodated by faults in the inner Continental Borderland offshore southern California. Nearly half of this total has previously been thought to be taken up on the Palos Verdes (PV) and Coronado Bank (CB) fault zones, which have been modeled as a single, continuous fault zone in recent seismic hazard assessments for southern California. These faults lie roughly on strike with each other, but a connection between these faults has not been clearly demonstrated. High-resolution seismic reflection data collected in spring of 2008 indicate that the PV fault terminates southwest of Lasuen Knoll in a horsetail splay that becomes progressively buried to the south. The lack of an active connection between the PV and CB fault zones implies that additional slip must be taken up elsewhere in the inner Continental Borderland.

Two other offshore faults, the San Diego Trough (SDT) and San Pedro Basin (SPB) fault zones, lie about 10-15 km southwest of and subparallel to the PV and CB faults. The new seismic reflection data reveal a previously unmapped but probably active fault zone along strike and in the area between known strands of the SDT and the SPB fault zones. This fault zone links the SDT and SPB faults, forming a continuous fault zone that extends about 250 km along the inner Continental Borderland. Although there are no slip rate data available for this fault zone, its overall length, continuity, and active character suggest that a significant portion of the plate motion that occurs offshore is accommodated along the SDT-SPB fault zone, which may pose a more significant seismic hazard than previously recognized.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009