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4.7 Geology,Soils,and Seismicity r Earthquake-Induced Landslides ` Earthquake motions can induce significant horizontal and vertical dynamic stresses in slopes that produce dynamic normal and shear stresses along potential failure surfaces within a slope.The ` susceptibility for native and engineered slopes to fail depends on the gradient and localized geology as well as the amount of rainfall, excavation,or seismic activities. During a slope failure, a mass of rock, soil, and debris is displaced down slope by sliding, flowing, or falling. Steep slopes and down slope creep of surface materials characterize areas most susceptible to failure. Engineered slopes have a tendency to fail during an earthquake if not properly designed, constructed or compacted. Unsupported slopes are part of the proposed active quarry operations. r Within the mining areas, saturated, improperly supported, or unengineered slopes, however temporary, could fail in localized areas if not properly engineered. Earthquake-induced slope failure is discussed further in the Impacts and Mitigation section of this document. r Liquefaction Liquefaction is the sudden temporary loss of shear strength in saturated, loose to medium dense, granular sediments subjected to ground shaking. Liquefaction generally occurs when seismically induced ground shaking causes pore water pressure to increase to a point equal to the overburden r pressure.Liquefaction can cause foundation failure of buildings and other facilities due to the reduction of foundation bearing strength. The potential for liquefaction depends on the duration and intensity of earthquake shaking, particle size distribution of the soil,density of the soil,and elevation of the groundwater. Areas at risk due to the effects of liquefaction are typified by a high groundwater table and underlying loose to medium-dense,granular sediments, particularly younger alluvium and artificial fill. Liquefaction has been responsible for ground failures during almost all of California's large earthquakes. In the Central Valley of California, ground shaking can liquefy saturated sand and r gravel deposits up to 50 feet below ground surface. When deep deposits of liquefiable soils fail, localized surface settlement can occur over time. Liquefaction is not considered an adverse geologic effect at the proposed project site because groundwater is over 100 feet deep and alluvial deposits underlying the site are unsaturated and consist of well distributed grain sizes varying from sand to gravel. Because the geologic conditions at the proposed project site are not _ conducive to liquefaction or the associated secondary ground failures,liquefaction is not discussed further in this chapter. .- Earthquake-Induced Settlement Settlement of the ground surface can be accelerated and accentuated by earthquakes. During an earthquake, settlement can occur as a result of the relatively rapid compaction and settling of ` subsurface materials (particularly loose, non-compacted, and variable sandy sediments)due to the rearrangement of soil particles during prolonged ground shaking. Settlement can occur both ` uniformly and differentially(i.e., where adjoining areas settle at different rates). Typically, areas underlain by artificial fills, unconsolidated alluvial sediments, and slope wash, and areas with improperly engineered construction fills are susceptible to this type of settlement. During an earthquake, some settlement of onsite soil materials may occur but because this is an active quarry and because engineered earthen structures (i.e.,berms, permanent slopes) would be RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.7-11 ESA/203015 Draft Environmental Impact Report May 2006