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MOUNTAIN HOUSE PROPOSED MACHADO TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST 4.6 GEOLOGY AND SOILS SETTING Topography and Geology The project site is relatively flat, sloping gently to the northeast at approximately one percent. Elevation ranges from approximately 130 feet above mean sea level (MSL)at the southwest — comer to 90 feet MSL at the northeast corner of the site (Associated Engineering, Inc.,2005). Regional geologic maps indicate that the geology in the vicinity of the project site is dominated by alluvial sedimentary deposits of Quaternary period. These unconsolidated — sedimentary deposits are Holocene to Recent Pleistocene in age. Alluvium consists of mixtures of sand, silt,and clay deposited by erosional processes that form soil in valleys. Seismicity The San Joaquin Valley is a seismically active region of California. The seismicity of the _ region is primarily related to the boundary between the Coast Range and the Great Valley of Central California, two diverse geographic and geologic provinces. The active and potentially active faults in the vicinity of the project site are presented below in Table 4.6-1. Table 4.6-1 Active and Potentially Active Faults in the Vicinity — of the Proposed Machado Property Site Fault Distance to Magnitude of Maximum Length Project Sites Earthquake Fault Status (miles) (miles) (Moment Magnitude)' Great Valley(Segment 6) Active 28 2 6.7 Great Valley(Segment 7) Active 28 7 6.7 Greenville-Marsh Creek Active 35 8 6.9 Calaveras(Northern) Potentially 84 21 6.8 — Active Great Valley(Segment 5) Active 17 22 6.5 Concord-Green Valley Active 44 23 6.9 — Hayward Active 50 27 7.1 Calaveras(Southern) Active 62 28 6.2 Vaca Potentially Active 17 29 6.7 San Andreas Active 800 46 7.9 Note: ' The moment magnitude is related to the physical size of fault rupture,the movement across the fault,and the strength of the material that is faulted. Earthquakes with moment magnitudes of 6 or greater are capable of causing widespread damage. Source: Kleinfelder,2002. The San Andreas fault system, located approximately 45 miles southwest of the project site, dominates the seismic activity of the Coast Ranges. The San Andreas fault system includes 4-26