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Stockton Unified School District Geological and Environmental Hazards Assessment CSU-Stanistaus,Stockton Campus School Site Page-24 Near-surface sedimentary deposits underlying the site consist of alluvial fan deposits belonging to the w,. Modesto Formation. These arkosic sediments are derived primarily from rocks of the Sierra Nevada Range. The distribution of surface deposits in the vicinity of the site is shown on Figure 6. 7.2.3 Seismic Setting and Fault Hazards Normal faulting uplifted the eastern edge of the Sierra Nevada Range during Cenozoic time and tilted the range westward as a relatively coherent block. While the site is located in a moderately seismic region of �• California in the north central portion of California's San Joaquin Valley (Figure 7). Catalogs of historic earthquakes indicate that nine earthquakes of estimated magnitude (M) 6.0 or greater has occurred within 100 kilometers (62 miles) of the site between the years of 1800 and 1999. An 1868 earthquake of M6.8 epicentered about 77 kilometers (48 miles) southwest probably caused the strongest seismic shaking of the site during the 199-year interval with an estimated Peak Ground Acceleration (PGA) of 0.06 gravity. The two largest of these nearby historic earthquakes are both 6.8 and occurred between 77 and 82 kilometers (48 to 51 miles) southwest of the site. Nearby mapped faults include the San Andreas Fault System (SAFS) located about 106 kilometers (km) (66 miles) west of the site. The SAFS is composed of active strike-slip faults trending northwest through the San Francisco Bay Area. Other nearby faults include the Great Valley Fault Zone, a system of northwest-trending concealed ("blind") thrust faults, about 32 km (20 miles)west of the site and beneath the eastern foothills of the Diablo Range. Since rupture of the Great Valley Fault Zone faults does not usually extend to the ground surface, this fault system has only recently been recognized as a potential source of earthquakes. The Great Valley Fault Zone was responsible for the M6.7 Coalinga earthquake of 1983 and is considered the probable source of the twin Vacaville-Winters earthquakes of 1892 of similar +- magnitude, as well as perhaps five other historic earthquakes of M6 or greater. The site is not located in a Fault-Rupture Hazard Zone as established by the Alquist-Priolo Earthquake Fault Zoning Act (Hart & Bryant, 1997). Therefore, ground rupture from faulting is not considered a significant hazard. Nevertheless, the site is near a number of major active faults capable of generating strong earthquakes. Active and potentially active faults considered capable of causing strong ground motion at the site are listed in the following table along with both their respective distances to the site and their estimated Maximum Credible Earthquake(MCE) magnitudes. REGIONAL SIGNIFICANT FAULTS Fault Distance to Site Maximum Credible Earthquake' (km) (Moment Magnitude) Great Valley Fault Zone 32 6.7 Greenville 46 6.9 LN Foothills Fault System 48 6.5 Concord—Green Valley 64 6.9 Calaveras 65 6.8 0.0 Hayward 76 6.9 San Andreas 106 7.9 ti. 2. Petersen and others, 1996. 111*4 Lo i a CONDOR