The 1989 Loma Prieta earthquake, while a devastating event, did not entirely catch seismologists by surprise. The region near Loma Prieta had been the subject of numerous forecasts, indicating a long-standing awareness of seismic risk in the area. Between 1910 and 1989, a variety of predictions were made, ranging from highly specific to more general, concerning potential seismic activity. These efforts highlight the continuous scientific endeavor
to understand and anticipate earthquakes, even if the exact timing and magnitude of the 1989 event remained unpredictable.
One notable prediction came in 1983 from Allan Lindh, a seismologist with the U.S. Geological Survey (USGS). Lindh forecasted a rupture length of 40 kilometers for the San Juan Bautista segment of the San Andreas Fault, starting near Pajaro Gap and extending northwest. This prediction, which concerned a potential M w 6.5 event in the San Juan Bautista segment or an M w 7 event in the San Francisco Peninsula segment, remarkably closely matched the actual rupture length of the 1989 earthquake. By 1988, an updated forecast was presented, and Lindh took the opportunity to rename the San Juan Bautista segment as the Loma Prieta segment, further emphasizing the area's seismic significance.
Early Warnings and Probability Assessments
In early 1988, the Working Group on California Earthquake Probabilities (WGCEP) issued several statements regarding their forecasts for the northern 360-kilometer segment of the San Andreas Fault. Their assessments included the 90-kilometer San Francisco Peninsula segment and a 30- to 35-kilometer portion of that segment, which they termed the southern Santa Cruz Mountains segment. The WGCEP estimated a 30-year probability of 50% for one or more M w 7 earthquakes occurring in the study area. However, due to a lack of information and lower confidence, a 30% probability was assigned specifically to the southern Santa Cruz Mountains segment.
These probability statements were crucial in raising awareness about the potential for a major earthquake in the region. While not pinpointing the exact date, they indicated a significant risk that scientists were actively monitoring. The scientific community's ongoing work in forecasting, even with inherent uncertainties, played a role in the preparedness efforts and public advisories that would follow.
Premonitory Events and Their Significance
The period leading up to the 1989 Loma Prieta earthquake was marked by two moderate seismic events, referred to by the USGS as the Lake Elsman earthquakes. These occurred in the Santa Cruz Mountains region in June 1988 and again in August 1989. Following each of these events, the state's Office of Emergency Services issued short-term advisories for a possible large earthquake, a first for the Bay Area. These advisories indicated a slightly increased probability of an M w 6.5 event in the Santa Cruz Mountains segment of the San Andreas Fault.
The June 1988 event, with a maximum intensity of VI (strong), caused broken windows in Los Gatos and minor damage in Holy City, where an increase in water well flow was observed. Concrete pieces also fell from a parking structure in Sunnyvale. The August 1989 event, with an intensity of VII (very strong), resulted in fallen chimneys in Cupertino, Los Gatos, and Redwood Estates, along with cracked walls, foundations, and broken underground pipes. A man even died in Los Gatos after falling or jumping from a window. These premonitory quakes, occurring close to the eventual epicenter of the main shock, were considered significant by some seismologists due to their timing and location, suggesting they might have influenced the rupture process of the larger 1989 earthquake. In anticipation of a major event, University of California, Santa Cruz (UCSC) personnel deployed four accelerometers in the area after the August 1989 quake, which successfully captured valuable data from the main shock and its initial aftershocks.
