New High-Resolution Seismic Reflection Data Show An Active Fault Linking the San Diego Trough and San Pedro Basin Fault Zones, Inner Continental Borderland, Southern California

The San Diego Trough fault zone (SDTFZ) is a significant strike-slip fault in the inner continental borderland of southern California. It extends from south of the Mexican border near Punta Santo Tomas for about 150 km to the north near Crespi Knoll, which is about 40 km offshore of San Clemente, Calif. North of Crespi Knoll, the SDTFZ either 1) terminates, 2) makes a prominent left bend to join the Catalina Ridge fault (CRF), or 3) continues to the north to join the San Pedro Basin fault (SPBF). If the SDTFZ makes a major left bend, the fault could constitute a significant tsunami hazard to the Los Angeles area. If the SDTFZ instead extends to the north to join the SPBF, the combined fault zone would make this one of the longest and potentially hazardous faults in the borderland.

In spring 2008, a new high-resolution seismic reflection survey was conducted in the area northwest of the northern end of the SDTFZ. These new data show that the CRF is inactive. The CRF is clearly onlapped by sediments that are probably much older than Holocene in age. In addition, there is no evidence for active faulting on a structure that would connect the northern end of the SDTFZ to the CRF. Instead, these new data reveal a previously unknown but apparently active fault zone along strike and in the area between the known strands of the SDTFZ and the SPBF. The SPBF had been previously mapped to extend from off of Santa Catalina Island north to near Point Dume. This newly recognized fault links the SDTFZ and SPBF, forming a continuous, active fault zone that extends about 250 km along the inner continental borderland, which may pose a more significant seismic hazard than previously recognized.