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Ground Motions for the North County Site <br /> Introduction <br /> The North County site is located in Northern California on the east edge of the central <br /> _valley. It is located in the Sierran Block which is a region a relatively low seismic <br /> activity in California. The Sierran Block is bounded by the Foothills faults on the east <br /> and by the Coast Range faults on the west. <br /> For a class III landfill, the California regulations require considering the Maximum <br /> Probable Earthquake (MPE). The California Division of Mines and Geology (CDMG) <br /> defines the MPE as the earthquake that is likely to occur in 100 years but it is not to be <br /> smaller than the largest historical earthquake. In this study,we have treated "likely to <br /> occur in 100 years" as the event with about a 1/3 chance of occurrence in 100 years. <br /> This is a reasonable approach to use to determine the MPE magnitude. To estimate the <br /> ground motion at the site, the MPE for each fault is located at the closest point on the <br /> fault to the site and the median ground motion computed from empirical attenuation <br /> relations is used. <br /> Description of the faults that have a potential to impact the MPE ground motions at the <br /> site are given below. <br /> Descriptions of Faults <br /> Foothills Fault System <br /> The western edge of the Foothills fault system is located about 34 km east of the North <br /> County site. In the Jennings (1992) Preliminary Fault Map, the Foothills faults are <br /> mostly labeled as Mesozoic age with some labeled as late Quaternary age. Recent <br /> analyses of the Foothills fault system (Lettis Assoc, 1994) has found that the Foothills <br /> faults are active but with very low activity rates. The Foothills faults along the east <br /> edge of the Sierran Block (closest to the site) have slip-rates of about 0.001 mm/yr, <br /> resulting in return periods of greater than 20,000 years for earthquakes with <br /> i <br /> magnitudes greater than 6. Since the activity rate of the Foothills faults is so low, they <br /> are not considered in developing the MPE ground motions for the North County site. <br /> R <br /> I <br />