Archive of Spotlight Feature Articles

Submarine Faults of the San Andreas:
Southern California's Hidden Hazards

by Jennifer Reynolds

The San Andreas fault system forms the boundary where the Pacific and North American plates grind past each other at 5.5 cm/year. Earthquake hazards related to the San Andreas fault system are widely recognized. However, studies of the San Andreas system have been almost completely confined to the faults on land. Geologists have only a general idea of where the offshore faults are located and little information about how active they are. The hazard potential of the submarine faults has not been assessed due to lack of data, and even generalizations about the offshore part of the system are controversial.

Geologists led by Chris Goldfinger (Oregon State University) have begun investigations of the offshore structures and earthquake history near San Diego and Los Angeles (Figure 1). These studies are funded by the West Coast & Polar Regions Undersea Research Center, NOAA/NURP.

Chris Goldfinger, Marta Torres, and their collaborators started with the San Clemente strike-slip fault near San Diego. Their new bathymetric map established the basic framework of the fault (Figure 2). To study its recent history, they conducted ALVIN submersible dives on scarps associated with this fault, and found clear evidence for recent fault activity (Figure 1). They saw abundant fluid venting along the younger-looking scarps. A particularly fresh-looking scarp 0.3 to 1.5 meters high (Figure 3) has uniform weathering and bioturbation, and lacks multiple slope breaks. It probably represents a single earthquake with magnitude > 6. Thus this section of the San Clemente fault, with no significant earthquakes recorded by modern seismometers, nevertheless is active and potentially hazardous.

Figure 2. Bathymetric map of the seafloor, centered on the San Clemente Fault. The map is compiled from new surveys by Goldfinger and colleagues and from archived data of the Scripps Insitution of Oceanography. The colored arrows point to matching features that have been separated by fault motion. The average offset on the fault is 58 kilometers. Click on image for larger view. Figure 3. Seafloor photos of a fresh fault scarp on the San Clemente Fault, from ALVIN Dive 3534. The scarp is approximately 60 cm high in these photos. The scalloping is a result of slumping on the steep face of the scarp.

Another team headed by Goldfinger is studying large submarine faults near Los Angeles. These have both NW and EW orientiations (Figure 1). Many of the faults along the southern California coast formed with the San Andreas; others are structures inherited from earlier deformation of the continental margin. The stresses that created those older faults are long gone, but some of those faults have been reactivated by the San Andreas, forming a complex family of structures. The field program in 2000 included side-scan sonar mapping with an AMS-60 sonar, and DELTA submersible dives (Figure 1). A second set of DELTA dives funded by NURP is scheduled for October, 2001.

Ultimately, these results will enable geologists to incorporate the offshore faults into models of the San Andreas system, leading to a realistic assessment of earthquake and tsunami hazards along the southern California coast.

[7/30/01]

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