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U period between characteristic earthquakes) within the CRSBBZ is estimated to be 360 to 440 years (Wakabayashi and Smith, 1994). (,J The estimated position of the CRSBBZ in the area of the project site generally corresponds to the location of the San Joaquin fault (Noller and others, 1993). This fault is considered potentially active, with the most fault displacement probably occurring in late Quaternary time (during the last 700,000 years) (Jennings, 1994). The Black Butte fault, located approximately 2.2 miles southwest of the San Joaquin fault is also considered a potentially active fault. Interpretation of the historic records for earthquakes affecting central California indicates that possibly eleven earthquakes of M 6 or greater have been generated along the CRSBBZ (Wakabayashi and Smith, 1994). An evaluation of more recent earthquakes (recorded by seismographs) indicate that the earthquakes of the western Sacramento-San Joaquin Valley region are characteristically caused by reverse or thrust faulting. Wong and others (1988) suggest that clusters of seismicity recorded within the western Sacramento Valley near Williams in Colusa County during the period 1980 to 1985 are representative of earthquakes on reverse faults. Their evaluation of recorded earthquakes for the period 1969 to 1985 also indicates similar clusters of seismicity in other areas along the CRSBBZ, €J including one centered at Patterson in western Stanislaus County. In addition to the potential for earthquakes within the San Andreas faultsystem and the CRSBBZ, earthquakes along the Sierran Nevada Frontal fault system could occur. This fault system, developed along the western flank of the Sierra Nevada, includes the Foothills and Melones fault zones; that system has an estimated maximum credible earthquake of M 6.5 (Mualchin and Jones, 1992). Mapping of expected seismic shaking within the area of the project site has been prepared by the National Earthquake Hazards Reduction Program (FEMA, 1992). The maximum ground acceleration with a 90 percent probability of nonexceedence (10 percent chance of being exceeded) is approximately 0.308. This probabilistic seismic shaking event is the design criteria required by the Uniform Building Code for structures. Local Geology The project site is located on an alluvial fan surface north of Hospital Creek as the creek emerges into the San Joaquin Valley. The topography slopes gently northeastward from an elevation of 230 feet above the National Geodetic Vertical Datum of 1929 (NGVD) at the southwestern corner of the project site to 190 feet above NGVD in the northeastern corner. This topography reflects the location of the project site on the northern portion of the large Hospital alluvial fan. The majority of the project site is inactive agricultural fields (orchards), most of which have undergone agricultural grading. Subsurface exploration (Kleinfelder, 1997) provides information on the'stratigraphy of the alluvial sediments which underlie the project site. Eleven borings were drilled to depths ranging from 80 to 102 feet below ground surface (bgs). The elevations of the bottom of Draft Environmental Impact Report 4-10 ER-96-3