ARCHIVED REPORTS LEC APPLICATION FOR CERTIFICATION

Home Browse Search

5.4 GEOLOGIC HAZARDS AND RESOURCES Quaternary Modesto (Qm) Formation deposits —These deposits consist of undifferentiated alluvial deposits,mainly arkosic,forming a toe of Mokelumne alluvial fan, generally covered by Holocene intertidal deposits. Quaternary Modesto upper member (Qm2) Formation deposits —These deposits consist of arkosic alluvium deposits,forming Mokelumne River alluvial fan;chiefly sand. Becomes finer-grained toward fan toe. Quaternary Modesto lower member (Qm1) Formation deposits —These deposits consist of arkosic alluvium deposits,forming Mokelumne River alluvial fan;chiefly sand. 5.4.1.3 Project Site Stratigraphy The LEC site is underlain by a fairly complete stratigraphic section of Creataceous,Tertiary and Quaternary deposits. The sediments deposited prior to mid-Tertiary time were in a marine environment. Changes in sea level,valley filling, and uplift resulted in the deposition of continental sediments after Mid-Tertiary time (Kleinfelder, 1993). 5.4.1.4 Seismic Setting The project site is in central California and within the Coast Ranges-Sierran Block Basement Zone. The modern tectonic setting of central California is dominated largely by the transform plate boundary contact between the Pacific and North American plates south of the Mendocino triple junction. The Pacific plate is slipping in a north-northwest direction (N35°W to N38°W) at a rate of about 1.81 to 1.95 inches per year (46 to 47 millimeters per year) with respect to the North American plate. Right-lateral strike-slip displacement along the major branches of the San Andreas fault system accommodates most of this plate motion,with the remainder generating Holocene tectonism and seismicity at the western continental margin and to the east in the Sierra Nevada and Basin and Range Provinces. The seismicity of the project site area can be characterized as an area of potentially large- magnitude earthquakes. Active faults within 30 miles of the LEC site include the Segments 5 and 7 of the Great Valley, and Greenville. These faults are capable of generating maximum credible earthquake (MCE) moment magnitudes up to 7.3 (Blake, 2004). These fault zones represent a significant seismic hazard to the project site. No faults have been mapped crossing the project site and the site is not within an Alquist-Priolo Special Studies Zone (CGS,2007). The locations of principal faults relative to the project site are shown on Figure 5.4-2. 5.4.1.5 Potential Geologic Hazards The following subsections discuss the potential geologic hazards that might occur in the project area. 5.4.1.5.1 Ground Rupture Ground rupture is caused when an earthquake event along a fault creates rupture at the surface. Since no known active faults cross the project site,the likelihood of ground rupture to occur is considered low. 5.4-2 SAC/371322/082330011(LEC_5.4_GEOLOGIC HAZARDS AND RESOURCES.DOC)