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{ Draft Environmental p Impact Report Page IV.G-9 �rForward Landfill Expansion percent(USGS, 1996). The USGS estimates a repeat interval of 150 years for an earthquake of k magnitude 7.0 or greater along the San Andreas Fault. An attenuation model by Abrahmson and Silva(1997)was used to target the acceleration- response envelope for each of the MCE/MPE potential events of concern using the median and { 84 percentile. This is a method acceptable to the CDMG (Geosyntec 2001). Based on the target nd shaking and acceleration response for each event, significant duration of strong grou Geosyntec developed four model events (potential earthquakes) to represent potential ground motions at a hypothetical bedrock outcrop (of which there are none) at the site. The landfill YP p 1�;.. uses the maximumg p otential round accelerations as design criteria. ' Potential hazards to the project area that can be generated by major earthquake events include fill ground shaking and related secondary ground failures (Idriss, 1991;Seed and Idriss, 1982). No I surface rupture is anticipated, given that there are no active traces crossing the site. I ' j Liquefaction Liquefaction generally occurs as a result of strong ground shaking during earthquakes in areas where granular sediment or fill material occur with high moisture content in or immediately below the material. The ground shaking transforms the material from a solid state to a temporarily liquid state. Liquefaction is a serious hazard because buildings in areas that experience liquefaction may sink or suffer major structural damage. Liquefaction is most often triggered by seismic shaking,but can also be due to improper grading,landslides or other factors. In dry soils, seismic shaking may cause soil to settle rather than flow, a process known as densification. Since groundwater occurs below a depth of approximately 60 feet, at the site �y the potential for liquefaction of site soils appears remote. Liquefaction at the project site is possible, in the swale areas proposed for filling with granular material,but the risk of liquefaction is low if these areas are properly engineered. Liquefaction and dynamic settlement analyses at a landfill also are examined in the context of how the vibratory motion due to earthquakes might affect landfill cells in the project expansion areas. The geotechnical data collected and modeled indicated that under the loading design of the maximum credible earthquake horizontal site acceleration, the material is too dense to liquefy. The calculated factor of safety against liquefaction ranged from 1.4 to 8.0 compared to the 1.1 to 1.3 required factor of safety (GeoLogic, 2008a,2008b). } Ground Shaking A Ground shaking is a complex function of the distance to the earthquake source (hypocenter);the magnitude of the earthquake;its duration;and the type,thickness, and condition of the subsurface materials. A major earthquake along the Great Valley, San Andreas Fault or other faults would cause strong ground shaking at the project site. Ground shaking impacts from an earthquake is the most likely seismic impact to occur at the Forward Landfill site over the course of its proposed life. a �F f