EXPERTS estimate that during the past 1,200 years, at least eight major earthquakes have occurred along the southern San Andreas fault, with intervals of approximately 140 years. The last such earthquake occurred in 1857; statistics indicate that the overall probability of another major earthquake striking this region in the next 30 years is about 60 percent. At least 10 active faults, each able to produce damaging earthquakes, have been identified in the entire Bay Area. The probability of a major earthquake's occurring there within the next 30 years is about 50 percent. Many of San Francisco's prime building sites could be affected by strong ground shaking because of the relatively poor quality of soft, alluvial, and water-saturated soils in nearly all the flatlands of the region. As was seen in the Mexico City earthquake of 1985, these soft soils can extend the impact of strong ground shaking over a larger area.
Events within this decade also show that the earthquake threat is not just a bigcity problem. Seismic events in El Centro, Mono County, Palm Springs, Coalinga, and Eureka over the past 10 years show that far-reaching damage can occur anywhere in the state.