1. Conclusions of a Panel of Experts

In 1987, the National Earthquake Prediction Evaluation Council convened a panel of experts for the first time to evaluate the likelihood for future earthquakes in California. In a report published in 1988, the panel concluded that there was a 50 percent probability for an earthquake of magnitude 7 or larger within the San Francisco Bay Area in 30 years or less. The panel also said there was a 30 percent probability for a 6.5 to 7 magnitude earthquake in the Santa Cruz Mountains within 30 years. This was the highest probability they assigned to any single fault segment in the Bay Area. The 7.1 Loma Prieta earthquake struck in the Santa Cruz Mountains only one year later in 1989.

After the Loma Prieta earthquake, the panel of experts was again convened to determine whether they should change the estimate of the probabilities of future large earthquakes in the San Francisco Bay Area because of the earthquake and other new data. Their report, issued in July 1990, was endorsed by the National Earthquake Prediction Evaluation Council and the California Earthquake Prediction Evaluation Council.

The panel identified four fault segments in the Bay Area along which they now believe large earthquakes are most likely: the peninsula segment of the San Andreas fault between Los Gatos and Hillsborough; the Hayward fault between Fremont and San Leandro; the Hayward fault between San Leandro and San Pablo Bay; and the Rodgers Creek fault between San Pablo Bay and Santa Rosa. They estimated that the probability is about 25 percent for a large earthquake on each one of these fault segments within 30 years.

More importantly, when the probabilities of earthquakes on all of these segments are combined mathematically, there is a 67 percent chance for at least one earthquake of magnitude 7 or larger in the San Francisco Bay Area between 1990 and 2020. Such an earthquake could strike at any time, including today.

The panel also concluded that:

• There could be more than one earthquake of magnitude 7 or larger in this 30-year period.
• Major earthquakes on all four fault segments are likely within the next 100 to 150 years.
• Each earthquake is likely to be of magnitude 7. If two fault segments slip during the same earthquake, for example along the Hayward fault, then the anticipated magnitude could be as large as 7.5.
• Earthquakes of magnitude 7 are considered possible, but not as likely, on other Bay Area faults such as the Calaveras, Concord, and San Gregorio.
• A repeat of the magnitude 8.3 San Francisco earthquake of 1906, which broke several segments of the San Andreas fault from south of San Jose to Cape Mendocino (a distance of more than 270 miles), is not likely during the next few decades.
• Numerous earthquakes with magnitudes of about 6 are also likely; these smaller earthquakes could cause some damage, especially near their epicenters.