Abstract

The magnitude (Mw) 6.9 Loma Prieta earthquake struck the San Francisco Bay area of central California at 5:04 p.m. local time on 17 October 1989, killing 62 people and generating billions of dollars in property damage. Scientists were not surprised by the occurrence of a destructive earthquake in this region and had in fact been attempting to forecast the location of the next large earthquake in the San Francisco Bay area for decades. This article summarizes more than 20 scientifically based predictions made before the 1989 Loma Prieta earthquake for a large earthquake that might occur in the Loma Prieta region. The predictions geographically closest to the actual earthquake primarily specified slip on the San Andreas fault northwest of San Juan Bautista. A number of the predictions did encompass the magnitude of the actual earthquake and at least one approximately encompassed the along-strike rupture length. Post-Loma Prieta studies of the 1906 San Francisco, California, earthquake in the Loma Prieta region of the San Andreas fault zone show the Loma Prieta and 1906 events with different senses of slip and fault-plane dip. Therefore, some have argued that the 1989 earthquake was not foreseen, even though (1) this earthquake appears to have released much of the horizontal strain accumulated since 1906, and (2) not all of the forecasts were based on 1906 behavior.

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