This is Earthquakes in the San Francisco Bay Region,the result from strain energy constantly accumulating across the region because of the northwestward motion of the Pacific Plate relative to the North American Plate (Figure 1 ). The region experienced large and destructive earthquakes in 1838, 1868, 1906, and 1989, and future large earthquakes to relieve this continually accumulating strain are a certainty.
For our study we define the SFBR as extending from Healdsburg on the northwest to Salinas on the southeast. It encloses the entire metropolitan area, including its most rapidly expanding urban and suburban areas. We have used the term "major" earthquake as one with M>=6.7 (where M is moment magnitude). As experience from recent earthquakes in Northridge, California (M6.7, 1994, 20 killed, $20B in direct losses) and Kobe, Japan (M6.9, 1995, 5500 killed, $147B in direct losses), earthquakes of this size can have a profound impact on the social and economic fabric of densely urbanized areas.
To evaluate the probability of future large earthquakes in the San Francisco Bay Region, the U.S. Geological Survey has established a series of Working Groups on California Earthquake Probabilities (hereafter referred to as WG88, WG90, WG99). Each of these Working Groups has expanded on the work of its predecessors, applying, in turn, the data and methodology available at the time and drawing on input from broad cross-sections of the earth science community.
