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Seismicity Compared with the coastal regions of California, the project is within an area of low to moderate seismicity. Plots of historic earthquake epicenters for the region, as shown in Figure 3.1-1, indicate that no earthquakes with Richter magnitudes equal to or greater than 5.0 have been recorded within at least a 50 mile radius of the site. Seismically active or potentially active faults that may affect the site include fault zones in the San Francisco Bay Area, the eastern Sierra frontal fault system, and the Sierra Foothills fault system, the Midland fault, and the Tracy-Stockton fault. These fault systems and their relationship to the project area are shown in Figure 3.1-2. The faults in the San Francisco Bay Area that have the greatest potential to impact the site are the San Andreas, Calaveras, Hayward, and Green Valley-Concord faults. These faults have the potential to generate maximum credible earthquakes of Richter magnitude 8.3, 7.5, 7.5, and 7.0 respectively (Sedivay Cooke Associates, 1989). The eastern Sierra frontal fault system is a system of high angle faults with large magnitude offsets that extend along the eastern margin of the Sierra Nevada Mountains. This fault system has the potential to generate a maximum credible earthquake of 7.5. The Sierra Foothills fault system, about 15 miles to the east, is the closest of these systems to the project site, and includes the Melones and Bear Mountain fault zones. Some previous investigators have considered this system of faults to be inactive (Department of (Vater Resources, 1965); however, Borchardt (1980) presents evidence that the Foothills fault system should be considered active. According to the California Division of Mines and Geology (CDNIG, 1979), this fault system is capable of a 6.5 Richter magnitude earthquake. Estimated recurrence intervals for earthquakes of this magnitude range from 33,000 to 67,000 years along this fault (CDMG, 1979). The Midland fault is buried beneath the San Joaquin Delta and extends from Bethel Island to east of Lake Berryessa. Evidence exists to indicate that movement has occurred along this fault during Quaternary time. This fault could generate a maximum credible earthquake of Richter magnitude 7.0 (Sedtivay Cooke Associates, 1989). The Tracy-Stockton faults extend through the central portion of San Joaquin County from Tracy to about 5 miles east of Linden. Although subsurface data does not 20