BASIX, the Bay Area Seismic Imaging eXperiment, conducted in September 1991, was designed to use a combination of verticalincidence and wide-angle seismic data to image the San Andreas and associated faults. The results of this experiment will provide, for the first time, a detailed image of the plate boundary between the Pacific and North American plates.
Basic to our understanding of the earthquake cycle in the San Francisco Bay area is detailed knowledge of the structure and evolution of the complex boundary between the Pacific and North American plates. This boundary is a broad, complex zone in which slip is distributed over the San Andreas, Hayward, Rodgers Creek, Calaveras, Green Valley, and Antioch faults. The San Andreas accounts for approximately 40–60% of the relative motion between the Pacific and North American plates—the rest is distributed across these other faults located in the East Bay.