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4.Environmental Analysis Faults considered to be potentially active are typically not zoned under the Alquist-priolo Earthquake Fault Zoning Act(Hart, 1997).5 The project site is not located on a known active fault and is not within an Earthquake Fault Hazard Rupture Zone as defined by the Alquist-Priolo Earthquake Zoning Act. Therefore,the risk of ground rupture within the limits of the project site is considered very low. Considering this very low probability, seismic hazards related to surface ground rupture are not discussed further in this document. Ground Motion Strong ground motion is described as motion of sufficient strength to affect people and their — environment or any ground movement recorded on a strong motion instrument or seismograph. The common way to describe ground motion during an earthquake is with the motion parameters of acceleration and velocity in addition to the duration of the shaking.A common measure of ground motion is the peak ground acceleration(PGA), which is the largest value of horizontal acceleration obtained from a seismograph. PGA is expressed as the percentage of the acceleration due to gravity(g),which is approximately 980 centimeters per second squared. In terms of automobile accelerations,one"g"of acceleration is a rate of increase in speed equivalent to a car traveling 328 feet from rest in 4.5 seconds.For comparison purposes,the maximum peak acceleration value recorded during the Loma Prieta earthquake was in the vicinity of the epicenter, near Santa Cruz,at 0.64 g. The highest value measured in the East Bay was 0.29 g, recorded at the Oakland Wharf near the Naval Supply Center. Soils at the wharf are artificial fill y over Bay Mud.The lowest values recorded were 0.06 g in the bedrock on Yerba Buena Island. Geologic Hazards This following section provides a description of common geologic hazards that currently exist or that could result from quarry development of agricultural areas in the project vicinity. These _ geologic hazards are the basis for the Significance Criteria(discussed later in this chapter)which determines the level of impact significance. This section discusses only those geologic hazards that pertain to the site and vicinity and are necessary to provide adequate environmental setting information. Slope Instability and Landslides Slope failures,commonly referred to as landslides,include many phenomena that involve the downslope displacement and movement of material,either triggered by static(i.e.,gravity)or _ dynamic (i.e.,earthquake)forces.Exposed rock slopes undergo rockfalls, rockslides,or rock avalanches, while soil slopes experience shallow soil slides,rapid debris flows, and deep-seated rotational slides. Under existing conditions,the proposed project site is a flat agricultural area with no hill or slope features susceptible to landsliding. However,change in topography proposed as part of the project would require excavation of steep slopes in certain mining areas. Currently, there are existing aggregate mines near the proposed site that maintain excavations into material — 5 The Alquist-Priolo Earthquake Fault Zoning Act,signed into law in December of 1972,requires the delineation of -- zones along active,potentially active,and well-defined faults.The purpose of the Alquist-Priolo Act is to regulate development on or near active fault traces to reduce the hazard of fault rupture. RMC Pacific Vemalis Quarry Mining and Reclamation Project 4.7-8 ESA/203015 Draft Environmental Impact Report May 2[x18