The Palos Verdes and Coronado Bank Fault Zones, Inner Continental Borderland, Southern California - Do They Connect?

The inner continental borderland offshore southern California accommodates about 7 mm/yr of slip between the North American and Pacific plates. Nearly half of this total is thought to be taken up on the Palos Verdes (PV) and Coronado Bank (CB) fault zones, which are modeled as a single, continuous fault zone in recent seismic hazard assessments for southern California. A connection between these faults increases the estimated maximum potential magnitude of an earthquake, since it would allow the entire length of both faults to rupture in a single event. Although these faults lie roughly on strike with each other, a connection between these faults has not been clearly demonstrated by previous data.

In spring 2008, a new high-resolution seismic reflection survey was conducted in the area between the PV and CB faults. North of Lasuen Knoll, the PV fault zone is well defined in seismic reflection profiles, with 1 or 2 straight strands showing seafloor offset, and new high-resolution bathymetry reveals a pronounced bathymetric scarp traceable for at least 9 km. Instead of continuing to the south to join the CB fault, however, the PV fault appears to end southwest of Lasuen Knoll, where it splits into numerous branches that become progressively buried southwards under at least 50 m of undeformed sediment. The northernmost well-defined active strands of the CB fault are about 25 km south of the southernmost active strands of the PV fault zone. Typical sedimentation rates for the inner borderland of southern California indicate that the most recent offset on buried faults in the area between the PV and CB faults occurred at least 15 to 50 thousand years ago. Thus, there appears to be no active connection between the PV and CB fault zones.