FORECASTS OF THE 1989 LOMA-PRIETA, CALIFORNIA, EARTHQUAKE

The magnitude (M-w) 6.9 Loma Prieta earthquake struck the San Francisc o 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 propert y damage. Scientists were not surprised by the occurrence of a destruc tive earthquake in this region and had in fact been attempting to fore cast the location of the next large earthquake in the San Francisco Ba y area for decades. This article summarizes more than 20 scientificall y based predictions made before the 1989 Loma Prieta earthquake for a large earthquake that might occur in the Loma Prieta region. The predi ctions geographically closest to the actual earthquake primarily speci fied slip on the San Andreas fault northwest of San Juan Bautista. A n umber of the predictions did encompass the magnitude of the actual ear thquake and at least one approximately encompassed the along-strike ru pture length. Post-Loma Prieta studies of the 1906 San Francisco, Cali fornia, earthquake in the Loma Prieta region of the San Andreas fault zone show the Loma Prieta and 1906 events with different senses of sli p and fault-plane dip. Therefore, some have argued that the 1989 earth quake was not foreseen, even though (1) this earthquake appears to hav e released much of the horizontal strain accumulated since 1906, and ( 2) not all of the forecasts were based on 1906 behavior.