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Draft Environmental p Impact Report Page IV.G-7 Forward Landfill Expansion little sandier than group A. Sediments are distinctly less oxidized than Group A showing a gray brown color. {. • Group C.: Top: 55 to 75 feet deep, bottom: approximately 110 feet deep - Sediments dominated by fine-to medium-grained sand; some coarse-grained or gravelly layers. A few layers of silt and clay material occur. Color is generally light brown to yellow brown. Grains are large enough to assess that the sediments appear to be granitic in origin. A few monitoring wells downgradient of the Austin Road Landfill, drilled to deeper levels up l to 160 feet bgs, encountered a 40-foot thick clay unit at approximately 120 to 160 feet bgs. This clay unit could represent an effective aquitard (barrier to flow of groundwater), insulating the upper Victor Formation saturated sediments with residual VOC contamination from migrating to the aquifer materials below the 160-foot depth. However, as there is a limited set of deep wells reaching this formation,the continuity of the potential aquitard unit is not known. Seismic Conditions i The proposed expanded Forward Landfill is in the geologically stable area of the Great Valley that has a history of relatively low seismic activity except on its margins. The active and potentially active faults of the region-with the exception of the Great Valley Fault and Foothills Fault system - are related to the San Andreas Fault system which reflects the geologic evolution of the convergent northwest-trending boundary between the Pacific Ocean and the North American tectonic plates (the tectonic source of earthquakes in California). The seismic y expression is dominated by major strike-slip fault systems of which the San Andreas Fault is the earliest and most dominant. An active fault is defined by the California Division of Mines and Geology as one which has had active surface displacement within Holocene time (i.e. over the past 11,000 years). Some faults are characterized as active based on surface displacements within historic time (over the last 200 years), while others are characterized as active based on surface displacements in rocks or sediments that occurred within the last 11,000 years. The Hayward and San Andreas faults are active faults based on both recorded historical activity and geologic displacement. The Roger Creek fault is active based on the geologic record. This definition of"active fault" does not mean that all faults for which there is no evidence of surface displacement during the Holocene are necessarily inactive. Some faults may have been active in this time period,but ? they did not result in changes to the surfaces that are easily identifiable. Meanwhile,other i faults may still be active although they have not been active during the Holocene period. Occasionally,earthquakes occur on blind thrust faults that are buried and show no evidence of past surface rupture, as was the case with the Northridge earthquake in 1994. fThe Division of Mines and Geology has defined potentially active faults as those for which there is evidence of surface displacement within the Quaternary period;that is,within the last 1.6 million years, approximately. Faults classified as potentially active show no evidence of surface displacements within the past 11,000 years,but this period of time is short in geologic terms and thus such faults are considered potentially active. Potentially active faults are estimated not to have shown any activity for the last 700,000 years, and have no designated maximum credible earthquake (MCE) or maximum probable earthquake (MPE) values assigned by the CDMG. Such potentially active faults are also reviewed but considered by the CDMG to be at low risk i for activity. 1 The most significant cluster of active faults are the San Andreas, Calaveras, Hayward, Antioch, and Green Valley faults located 30 to 70 miles west of the project site. The distance between the