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ENGEO <br /> INCORPORATED <br /> III. Site Geology <br /> Geologic Setting <br /> The Bay -O -Vista area is located at the west side of the East Bay Hills Block, a segment of the <br /> California Coast Ranges believed to have been uplifted during the Late Quaternary period. The <br /> State of California has defined an Alquist -Priolo Earthquake Fault Zone around the main active <br /> trace of the Hayward Fault, approximately 2,500 feet east of the proposed GHAD area. The <br /> local hill front in the study area is defined by other traces of the Hayward Fault that have been <br /> mapped by geologists with the United States Geologic Survey (USGS) (Robinson, 1956; <br /> • Radbruch and Case, 1967; Radbruch, 1969; Dibblee, 1980; Graymer, 1996). According to the <br /> current State map, these traces are not "sufficiently active and well - defined" to be included in the <br /> State Alquist -Priolo Earthquake Fault Zone program. However, previous workers (Radbruch <br /> and Case, 1967; Radbruch, 1969) postulated that surface fault rupture associated with the 1868 <br /> Hayward earthquake occurred along one of the traces mapped through the study area. <br /> The bedrock underlying the proposed GHAD area is mapped as Upper Jurassic -age gabbro, an <br /> igneous rock. According to the observations of Cotton, Shires.& Associates (1998) (CSA), the <br /> gabbro is highly fractured and internally sheared. The character of the gabbro is described by <br /> CSA to vary from highly weathered and crushed to moderately weathered and moderately strong. <br /> The area of the proposed GRAD has been previously graded. The existing distribution of soils is <br /> therefore a function of both the original, pre - grading soil conditions and of the pattern of <br /> excavation and filling that occurred during development of the area. Prior to development, the <br /> eastern portion of the GHAD area consisted of a relatively- steep, west - facing slope. The base of <br /> this steep slope formerly occurred in the central portion of the GHAD area; west of the former <br /> base of slope, the pre - graded ground sloped west at a relatively gentle inclination. The original <br /> ground configuration was modified to create the existing level areas now occupied by the street <br /> 4867.1.002.01 <br /> October 10, 2000 3 <br />