ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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Potential seismic hazards resulting from a nearby moderate to major earthquake can generally be classified as primary and secondary. The primary effect is fault ground rupture, also called surface faulting. Surface ground rupture along faults is generally limited to a linear zone a few meters wide. Structures built over an active fault can be torn apart if the ground ruptures. The Alquist-Priolo Earthquake Fault Zoning Act is intended to prevent the construction of buildings used for human occupancy on the surface trace of active faults. There are no Earthquake Fault Zones subject to the Alquist-Priolo Act in Manteca(California Geological Survey 2005). Common secondary seismic hazards include ground shaking, liquefaction, and subsidence,which are discussed below. SEISMIC GROUND SHAKING The most important geologic hazard that could affect the project vicinity is the risk to life and property from an earthquake generated by active and potentially active faults in the Bay Area and along the western margin of the San Joaquin Valley. Seismic ground shaking is the most likely seismic hazard to affect the project area.According to the California Building Standards Code (CBC), 1998 edition,the project area is located in Seismic Zone 3. This location implies a minimum horizontal acceleration of 0.3 g(where"g"is the acceleration of gravity)for use in earthquake resistant design. Ground motions can be estimated by probability of occurrence at specified hazard levels. The intensity of ground shaking depends on the distance from the earthquake epicenter to the site,the magnitude of the earthquake, site soil conditions,and the characteristic of the source. The Probabilistic Seismic Hazard Assessment for the State of California(Petersen et al. 1996),published by the USGS and the California Geological Survey(formerly the California Department of Conservation,Division of Mines and Geology), identifies the seismic hazard based on a review of these characteristics and historical seismicity throughout California. The results of these studies suggest there is a 10%probability that the peak horizontal acceleration experienced at the site would exceed 0.2 g in 50 years. In addition,the California Geological Survey's Probabilistic Seismic Hazards Mapping Ground Motion webpage was consulted to estimate site-specific probabilistic ground acceleration for the project area. Based on the latitude and longitude for Manteca,peak horizontal ground acceleration with a 10%probability of being exceeded in 50 years was calculated for firm rock, soft rock, and alluvium. These calculations found that there is a 1-in-10 probability that an earthquake will occur within 50 years that will result in a peak level of ground shaking exceeding 0.28g on alluvium, 0.24g on soft rock, and 0.22g on firm rock(California Geological Survey 2006b). Damage to a single-family dwelling typically begins at 0.2 g(Rogers et al. 1996). The CBC specifies more stringent design guidelines where a project would be located adjacent to Class "A"or "B"faults as designated by the California Probabilistic Seismic Hazard Maps(Cao et al. 2003). Faults with an "A"classification are capable of producing large-magnitude(M) events (M greater than 7.0),have a high rate of seismic activity(e.g.,having slip rates greater than 5 millimeters per year), and have well constrained paleoseismic data(e.g., evidence of displacement within the last 700,000 years). Class`B"faults are those that lack paleoseismic data necessary to constrain the recurrence intervals of large-scale events. Faults with a"B" classification are capable of producing an event of magnitude 6.5 or greater. The Great Valley Fault System is a Type B fault.Based on preliminary segmentation of the Great Valley Fault System, a 30-kilometer-long segment with a characteristic earthquake magnitude of 6.7 (Richter scale)is indicated approximately 23 miles west of the project vicinity.A review of the available data indicates that no Class A or B faults are located within 20 miles of the project area. No Seismic Hazard Zones have been identified for Manteca(California Geological Survey 2006a). Table 4.7-2 shows the seismic source description,magnitude, and slip rate for various CBC fault classes. Manteca WQCF and Collection System Master Plans EIR EDAW City of Manteca 4-7.3 Geology,Soils,and Seismicity