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0' Ground Motions for the Foothills Landfill Site Introduction The Foothills Landfill site is located in Northern California on the east edge of the central valley. It is located in the Sierran Block which is a region a relatively low seismic activity in California. The Sierran Block is bounded by the Foothills faults on the east and by the Coast Range faults on the west. For a class III landfill, the California regulations require considering the Maximum Probable Earthquake WE). The California Division of Mines and Geology (CDMG) defines the WE as the earthquake that is likely to occur in 100 years but it is not to be smaller than the largest historical earthquake. In this study, we have treated "likely to occur in 100 years" as the event with about a 1/3 chance of occurrence in 100 years. This is a reasonable approach to use to determine the MPE magnitude. To estimate the ground motion at the site, the MPE for each fault is located at the closest point on the fault to the site and the median ground motion computed from empirical attenuation relations is used. Description of the faults that have a potential to impact the MPE ground motions at the site are given below. There are several additional faults which are located within 100 km of the site location, however, these fault will not control the MPE ground motions and are not discussed below. Descriptions of Faults Foothills Fault System The western edge of the Foothills fault system is located about 19 km east of the Foothills Landfill site. In the Jennings (1994) California State Fault Map, the Foothills faults are mostly labeled as Mesozoic age with some labeled as late Quaternary age. Analysis of the Foothills fault system (Lettis Assoc, 1994) has found that the Foothills faults are active but with very low activity rates. Slip rates estimated for the Foothills fault system range from 0.001 mm/yr (Lettis Assoc., 1994) to 0.05 mm/yr (CDMG, 1996; WGNCEP,1996). Using the Youngs and Coppersmith (1985) characteristic model with a mean characteristic magnitude of 6.5 (CDMG, 1996), these low slip rates result in probabilities of 0.1% to 3% for a magnitude 5 or larger earthquake within 25 km of the