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Draft Environmental Impact Report Page IV.G-11 Forward Landfill Expansion An evaluation was performed of the potential maximum ground acceleration during the maximum credible earthquake along faults that are most likely to produce ground shaking at the site. The peak horizontal bedrock acceleration that would result at the site is estimated to be 0.13g, where "g" is the acceleration due to gravity. This peak acceleration is based on a maximum credible earthquake of 6.7 occurring along the active segment of the Great Valley Fault located approximately 21 miles from the site. The peak acceleration drops to 0.09g when the MPE is used;the difference being the duration of t assumed strong ground motion. The landfill design uses the more conservative MCE to estimate maximum ground shaking and resultant acceleration. Ground Failure Typical seismically induced ground failures are liquefaction, lateral spreading, and ground lurching. Liquefaction,where the subsurface granular material (such as sand)is transformed into a temporary liquid state and loses cohesion,tends to occur where the sediments or fill material is moist (Seed and ldriss, 1982). Lateral spreading is the horizontal movement of loose, unconsolidated sedimentary ` imported fill material. Lurching is the horizontal movement of soil, sediments,or fill deposits and p g found on steer slopes and embankments. The risk of surface rupture at the site occurs only when an r r y actual fault trace traverses the site. Surface rupture, if it were to occur,would be strike-slip in movement, given that it is the dominant movement of the fault systems in Northern California. SIope Stability - Slope stability impacts are limited to the slopes of the landfill area and the banks of the forks of Littlejohn Creek. Slope stability of the landfilled material was evaluated by the applicant's geotechnical consultant, Geologic Associates, based on the proposed landfill expansion plans and available information on the liner material, likely refuse, and soil cover UTD, Appendix E,2001). Per the CCR Title 27 requirements, stability of the cover soil and slopes of the planned expanded landfill are evaluated. The stability analyses includes both static and pseudo-static loading conditions, as well as potential seismic displacement that could occur under ground shaking conditions triggered by t earthquake loads. Based on a review of available seismic data, two conservative potential earthquake events, using the more conservative model assumptions of the MCE versus MPE,were considered for the landfill design. One scenario anticipates a magnitude 7.9 earthquake along the San Andreas Fault (70 miles away from the site) that generates peak horizontal accelerations of 0.06 g. and occurs over a duration of 45.4 seconds. The other event is designed along the Great Valley fault 21 miles away for a magnitude 6.7 l earthquake, peak horizontal bedrock acceleration of 0.13, and ground shaking duration of 15.3 seconds. I Land Subsidence I � Areas susceptible to earthquake-induced settlement include those areas underlain by thick layers of colluvial material or unengineered fill. This engineered nature of the landfill is designed to control subsidence as part of the landfilling process. Earthquake-induced subsidence is not expected to be significant. Structural Damage or Collapse The earthquake performance of structures varies considerably due to a number of factors. These include building construction(wood frame, unreinforced masonry, and nonductile concrete frame), magnitude and intensity of the earthquake and duration of strong ground shaking, distance from the causative faults, and similar factors. There are no planned building in the landfill expansion areas proposed. J