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Range-Sierran Block Boundary Zone (CRSBBZ), has been the subject of recent geologic ' and seismic research; it is interpreted to represent not only a regional geologic and geomorphic boundary but a regional fault or suture zone (Wong et al., 1988). Alluvial and colluvial sediments cover the bedrock at the base of the eastern mountain front of the Diablo Range and along streams which drain the mountains (Figure 4.2-1). Sediment deposition and erosional processes have resulted in the development of the ( ' landforms which dominate the project site and surrounding areas. The most pronounced geomorphic features are the large alluvial fans deposited at the points at which the streams, draining the mountains to the east, emerge onto the San Joaquin Valley. The major alluvial fans include those formed along Corral Hollow, Hospital, and Ingram creeks. The incision (downcutting) of the creeks into the alluvial fans has resulted in the formation of terraces along those streams. The alluvial fans and terraces have been formed in the Quaternary period (two million years ago to present). The sediment'deposits, which underlie these landforms, are a heterogenous mixture of unconsolidated to weakly cemented, interbedded gravels, sands, silts, and clays. The sands and gravels of these J deposits are mined as aggregate resources. Regional Seismicity The tectonic setting of western California creates a relatively high potential for the occurrence of moderate to large earthquakes. Large earthquakes can cause damaging ground shaking throughout a large area. The'active and potentially active faults potentially affecting the project site are shown on Figure 4.2-2. The characteristics of these faults are summarized in Table 4.2-1. O In general, the regional fault zones, including the San Andreas, Hayward, Calaveras, and Rodgers Creek fault zones, are typically characterized as strike-slip faults with the major component of movement being horizontal and right-lateral.2 Moderate to large earthquakes (M 53 or greater) are considered capable of causing rupture of the ground surface. Major (M 7 or greater) right-lateral strike-slip earthquakes within the region have historically occurred on the San Andreas (1838, 1906, 1989) and on the Hayward faults (1836, 1868). These earthquakes were felt over large areas. Western San Joaquin County has experienced moderate ground shaking (up to MMI' VII) during large earthquakes generated on these major fault zones to the west. The combined probability of a major earthquake (M 7 or greater) occurring on the major strike-slip faults of the San Francisco Bay region is estimated to be 67 percent (USGS, 1990). This probability represents a minimum because not all faults capable of generating a large earthquake (including the Calaveras and San Gregorio-Seal Cove fault zones) were included in the development of the estimate. 'Right-lateral motion is a convention defined by the observation of the direction of movement across the fault D� when looking across the fault. Within the San Andreas Fault Zone,the observer would recognize that the western side of a fault has moved northward after right-lateral displacement along the fault. 3M 5 denotes Richter Magnitude 5. "MMI denotes Modified Mercalli intensity Scale. Draft Environmental Impact Report 4-6 ER-96-3