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4. ENVIRONMENTAL ANALYSIS as 4.7 GEOLOGY,SOILS,AND SEISMICrrY t 28 feet east of Mendota(Hotchkiss and Balding, 1971). Land subsidence caused by groundwater withdrawal was not identified in the vicinity of Tracy(Hotchkiss and Balding, 1971). Subsidence is not considered a potential adverse geologic affect and therefore is not discussed further in this chapter. Settlement Settlement is the depression of the bearing soil when a load, such as that of a building or new fill material, is placed upon it. Soils tend to settle at different rates and by varying amounts depending on the load weight, which is referred to as differential settlement. Differential settlement can be a greater hazard than total settlement if there are variations in the thickness of previous and new fills or natural variations in the thickness and compressibility of soils across an area. Settlement commonly occurs as a result of building construction or other large projects, such as the proposed RMC Pacific Vernalis Quarry, that require soil stockpiles. Although some settlement may occur within the proposed project site area due to soil stockpiling and other operations,the affect to the site would be very minimal considering that the site doesn't support development that could be adversely impacted by ground settlement. Therefore,the impact of ground settlement is not discussed further in this document. SEISMIC HAZARDS The proposed project site could experience the effects of a major earthquake from one of the active or potentially active faults located in close proximity to the project site. The four major hazards associated with earthquakes are fault surface rupture (ground displacement),ground shaking, ground failure,and settlement. This EIR considers the potential for hazards associated with ground shaking,earthquake-induced slope failure(landslides),liquefaction and settlement and does not further discuss surface fault rupture. Surface fault rupture is not discussed because no active faults capable of generating surface rupture are located beneath the proposed Vernalis Mine site. Seismic Ground Shaking Earthquakes in the San Francisco Bay and San Joaquin Valley Region could produce strong ground shaking in the project area. Ground-shaking intensity is related to the size(i.e., magnitude)of an earthquake, the distance to the site, and the response of the geologic materials that underlie the site. As a rule,the greater the earthquake magnitude and the closer the fault rupture to the site,the greater the intensity of ground shaking. Violent shaking is generally expected at and near the epicenter of a large earthquake, although studies of recent earthquakes, �r such as those conducted after the 1992 Landers earthquake,indicate that directional ground motion along a fault can cause strong ground shaking farther away from the epicenter. Seismic hazards due to ground shaking can cause the greatest amounts of damage to structure and utilities. In 1999,California Geological Survey completed the Seismic Shaking Hazard Maps for California to describe the statewide distribution of estimated ground motion through the state. r- These maps provide a conservative estimate,through probabilistic analysis, of the peak ground RMC Pacific Vernalis Quarry Muting and Reclamation Project 4.7-9 FSA/203015 Administrative Draft©R M.b 2004