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4. ENVIRONMENTAL ANALYSIS 4.7 GEOLOGY,SOILS,AND SEISMICITY acceleration for all regions of California. Based on estimates of this seismic hazards assessment, y peak ground acceleration in the region of the proposed project site could reach or exceed 0.3 to 0.4g(CGS, 2003x).TS Seismic ground shaking is discussed further in the Impacts and Mitigation section of this chapter. , 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. 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. 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 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. 7 A probabilistic seismic hazard map shows the hazard from earthquakes that geologists and seismologists agree could occur. It is"probabilistic"in the sense that the analysis takes into consideration the uncertainties in the size and location of earthquakes and the resulting ground motions that can affect a particular site. The maps are typically expressed in terms of probability of exceeding a certain ground motion. For example,the 10 percent probability of exceedance in 50 yews maps depict an annual probability of 1 in 475 of being exceeded each year. -� This level of ground shaking has been used for designing buildings in high seismic areas. The maps for 10 percent probability of exceedance in 50 years show ground motions that geologists and seismologists do not think will be exceeded in the next 50 years. In fact,there is a 90 percent chance that these ground motions will not be exceeded. This probability level allows engineers to design buildings for larger ground motions that geologists and --- seismologists think will occur during a 50-year interval,which makes buildings safer than if there were only designed for the ground motions that are expected to occur in the next 50 years. Seismic shaking maps are prepared using consensus information on historical earthquakes and faults. These levels of ground shaking are used primarily for formulating building codes and for designing buildings. The maps can also be used for estimating _ potential economic losses and preparing for emergency response(Peterson et al., 1999). 8 The damage level represents the estimated overall level of damage that will occur for various MMI levels. The damage,however,will not be uniform. Some buildings will experience substantially more damage than this overall level,and others will experience substantially less damage. Not all buildings perform identically in an earthquake. -� The age,material,type,method of construction,size,and shape of a building all affect its performance(ABAG, 1998). RMC Pacific Vemalis Quany Mining and Reclamation Project 4.7-10 PSA/203015 ' Adminishative Draft t]R Match 2004