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jian and Shaw and published in Shaw (1982). In this approach, a poisson distri- bution model was used to calculate the probability of occurrence of various mag- nitude earthquakes along the Bear Mountains and Melones faults in a 100 -year period. The results of this analysis confirmed that the probability of having the MCE along these faults in a 100 -year period is low. Furthermore, the analysis indicates that the preferred MPE for these faults is a magnitude 5.7 earthquake. This MPE estimate is consistent with the maximum historic earthquake (magnitude 5.7, Oroville Earthquake) associated with the northern segment of Sierran Foothills faults. Ground accelerations resulting from earthquakes may be estimated from widely accepted, published relationships that relate ground shaking to earthquake mag- nitude and distance from the causative fault or earthquake epicenter. For the North County Sanitary Landfill site, relationships developed by Seed and Schn- abel and published in Seed and Idriss (1982) were used to estimate the level of mean peak horizontal ground accelerations that may occur at the site due to MPEs occurring in the area. Because the MPE for the Bear Mountains fault zone and the Melones fault zone are the same, and the Bear Mountains zone is near- est the site, ground accelerations at the site for the MPE on the Bear Mountains fault zone will be greater than for the Melones fault zone. Design mean peak horizontal ground accelerations (0.10g), therefore, will result from the MPE (magnitude 5.7) on the Bear Mountains fault zone, which has been mapped 28 km east of the site. 4.4 HYDROGEOLOGY 4.4.1 Regional HydrogeologX Most of the ground water used in eastern San Joaquin County is pumped from the Arroyo Seco and underlying Laguna Formations. Although Quaternary units, such as the Riverbank and Turlock Lake Formations, generally contain less ground water, some wells in the region draw large yields from discontinuous sand and gravel layers within these units. Ground water is primarily recharged by per- colation from surface channels that originate in the foothills of the adjacent Sierra 9 Nevada Mountains. Nal Rev. 0 July 20, 1989