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Draft Environmental Impact Report Page IV.G-8 Forward Landfill Expansion project site and these active faults would generally attenuate to minor ground shaking events at the site. Along the eastern Great Valley Margin, the less active Foothills Fault system (approximately 26 miles to the east) has a low likelihood of a major event during the project lifetime and a lower ground shaking impact to the project site. The closest known fault is the Tracy-Stockton Fault Zone passing through Stockton 6.5 miles northwest of the landfill. However, the Tracy-Stockton Fault Zone is not considered an active fault. A potential fault, referred to as the Stockton fault, is an east-west trending feature derived from oil well data projected to underlie the project site. The Stockton fault defines the northern edge of the Stockton Arch, a basement rock that divides the San Joaquin Basin on the south from the Sacramento Basin to the north. The Stockton fault is an inferred fault and considered not active. The San Andreas and associated faultsywand Ha like all known active faults in ( Calaveras etc), California (with documented historical events),fall under the State of California Alquist-Priolo Special Studies Zone Act (1972), which requires specific seismic studies to be performed prior to development, and places restrictions on certain types of development. Potential seismic impacts for the proposed expanded Forward Landfill were evaluated by GeoSyntec (JTD, 2001). The seismic analyses by GeoSyntec consisted of developing design ground motions based on the seismic hazard data in Table IV.G-1. The assessment entailed review of the impacts of potential seismic hazards on the design ground motions for the built out-landfill. The project site has no identified active faults. Available literature on the proximity of active and potentially active faults and seismic source zones within 100 miles of the project site were reviewed, and the strong ground shaking duration(D-s), and the peak horizontal ground accelerations (PHGAs) for MPE and MCE were evaluated as part of the design criteria. Table G-1 summarizes these data for the pertinent faults of the region(Abrahmson and Silva 1996, 1997;USGS/CDMG, 1996;Topposada and others, 1994)within 70 miles of the site, and presents their designated CDMG"activity" rating, distance from the project site, maximum credible earthquake magnitudes, and PHGA's (potential movement). The key determination is the identification of seismic loads that might occur at the site following regional earthquakes. The idea of such seismic analyses is to calculate the highest potential for ground shaking and ground acceleration at the landfill site from the active and .. potentially faults within the 100 miles, and then to design the landfill to mitigate such events. The fault with the highest probability to generate large earthquake loads at the site, as can be seen in Table IV.G-1,is the Great Valley fault system,which is the closest potentially active fault to the landfill(approximately 21 miles southwest of the site). A MPE moment magnitude (MW) 6.4 earthquake on the Great Valley fault with its associated 11.8 seconds of duration of strong ground shaking would produce a PHGA of approximately 0.098. A MCE Mw of 6.7 on this fault with its longer duration of 15.3 seconds of strong ground shaking would generate a PHGA of approximately 0.13. Although the Great Valley fault yields the highest PHGA, the San Andreas Fault system could produce the longest duration of ground shaking. The MPE and MCE on the San Andreas show that the same M,7.9 event at a distance of 70 miles could produce a ground j shaking duration of 45.4 seconds. Given these data, Geosyntec concluded that the M,,,of 7.9 along the San Andreas Fault would produce the most damaging seismic impact to Forward Landfill. The probability of a strong (magnitude 7.0)earthquake occurring between 1990 and 2020 in the San Francisco Bay Area region is estimated at 67 percent. The probability of a major earthquake along the segment of the San Andreas Fault closest to the site is approximately 23