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5.ENVIRONMENTAL CHECKLIST MOUNTAIN HOUSE NEIGHBORHOODS K AND L INITIAL STUDY 6. GEOLOGY AND SOILS Regional Geology The project site, located in the upper San Joaquin Valley, is considered part of the Great Valley geomorphic province of California. The Great Valley is a relatively flat alluvial plain that is infilled with as much as 6 vertical miles of alluvial and marine sediment. The Great Valley is bounded to the west by the Coast Ranges and to the east by the Sierra Nevada, The Sacramento and San Joaquin Rivers drain the Great Valley through San Francisco Bay, Regional geologic maps indicate that the geology in the vicinity of the project site is dominated by sediments that were deposited by streams draining the eastern slopes of the Altamont Hills to the west. These unconsolidated sedimentary deposits are of Holocene age (less than 11,000 years old)and comprise the surface and near-surface soils across the Mountain House community. For the Great Valley geomorphic province, sediments within the valley were mostly derived from erosion of the Sierra Nevada mountain range to the east and Coast Range Mountains to the west(Engeo, 2007a). Seismicity The San Joaquin Valley is a seismically active region of California, subject to occasional earthquakes. The seismicity of this region is concentrated near the boundary between the Coast Ranges and the Great Valley,two diverse geographic and geologic provinces. No known active nor potentially active faults have been mapped across the project site, and the site is not located in a Fault Rupture Hazard Zone as established by the Alquist-Priolo Earthquake Fault Zoning Act. The California Geologic Survey has defined active faults as faults that have had surface displacement within Holocene time (within the last 11,000 years). Potentially active faults are faults that show evidence of surface displacement during Quaternary time(within the past 1.6 million years). The active and potentially active faults in the vicinity of the Specific Plan II area (which includes the Neighborhoods K and L project site)are listed in Table 5.6-1. The Great Valley Thrust fault is the nearest active fault to the project site. The United States Geological Survey(USGS)Working Group on California Earthquakes has mapped the surface rupture of the nearest segment of the Great Valley Thrust fault zone beneath the eastern foothills of the Diablo Range, approximately 5 miles southeast of the project site. Since rupture of the Great Valley fault zone does not usually extend to the ground surface,this fault system has only recently been recognized as a potential source of earthquakes. This fault is omitted from both the jointly published California Division of Mines and Geology(CDMG)and International Conference of Building Officials(ICBG) documents entitled "Maps of Known Active Fault Trace Near-Source Zones in California and Adjacent Properties of Nevada(1998)"and"Determining Distances from Faults Within and Bordering the State of California (1997)"for the purpose of determining near source seismic factors used in structural design (Engeo,2007a). (gnm) 5-74