Summary
The San Francisco Bay region sits astride a dangerous “earthquake machine,” the tectonic
boundary between the Pacific and North American Plates. The region has experienced major and
destructive earthquakes in 1838, 1868, 1906, and 1989, and future large earthquakes are a certainty. The
ability to prepare for large earthquakes is critical to saving lives and reducing damage to property and
infrastructure. An increased understanding of the timing, size, location, and effects of these likely
earthquakes is a necessary component in any effective program of preparedness.
This study reports on the probabilities of occurrence of major earthquakes in the San Francisco
Bay region (SFBR) for the three decades 2000 to 2030. The SFBR extends from Healdsberg on the
northwest to Salinas on the southeast and encloses the entire metropolitan area, including its
most rapidly expanding urban and suburban areas. In this study a “major” earthquake is defined as
one with M≥6.7 (where M is moment magnitude). As experience from the Northridge, California
(M6.7, 1994) and Kobe, Japan (M6.9, 1995) earthquakes has shown us, earthquakes of this size can
have a disastrous impact on the social and economic fabric of densely urbanized areas.
To reevaluate the probability of large earthquakes striking the SFBR, the U.S. Geological Survey
solicited data, interpretations, and analyses from dozens of scientists representing a wide crosssection
of the Earth-science community (Appendix A). The primary approach of this new Working
Group (WG99) was to develop a comprehensive, regional model for the long-term occurrence of
earthquakes, founded on geologic and geophysical observations and constrained by plate tectonics.
The model considers a broad range of observations and their possible interpretations. Using this
model, we estimate the rates of occurrence of earthquakes and 30-year earthquake probabilities. Our
study considers a range of magnitudes for earthquakes on the major faults in the region—an innovation
over previous studies of the SFBR that considered only a small number of potential earthquakes of
fixed magnitude.
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First posted October 14, 1999
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