Short-term earthquake hazard assessment for the San Andreas Fault in southern California

The southernmost 200 km of the San Andreas fault in California, from Cajon Pass southeast to Bombay Beach on the Salton Sea (Figure 1), has not produced a major earthquake within the historic record. Both geodetic evidence of continuing strain accumulation (Savage et al, 1986) and the occurrence of recent prehistoric large earthquakes (Sieh, 1986; Sieh and Williams, 1990), however, lead us to conclude that this fault segment will eventually produce great earthquakes that pose one of the greatest hazards to southern California. An estimated 1.0-1.5 million people now live adjacent to the San Andreas fault within the projected zone of severe shaking for such an earthquake. A magnitude 7.5 to 8.0 earthquake on this segment would also cause widespread damage to San Bernardino, Imperial, Riverside, Orange, and Los Angeles counties, which together have over 12 million inhabitants. For these reasons, the Southern San Andreas Fault Working Group was formed in 1989 to recommend how the scientific community might best respond to anomalous geophysical activity along the fault, increase our understanding of regional seismotectonics, and offer timely scientific advice to state and local governments