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Seismicity The regional earthquake hazard is dominated by a series of northwest -trending faults located to the west of the project site. The major active faults in the region include the San Andreas, Hayward, Calaveras, Green Valley -Concord, and the Greenville Fault. Several relatively less important faults are also present in the region, including the Tesla, Midway, and Livermore Faults. These latter faults have been active within Quaternary time (about the last 2.5 million years) but not within Holocene time (the last 11,000 years). The nearest fault to the site is the Tracy -Stockton Fault northeast of Tracy, and underlying Stockton. The shortest distances from the site to these faults are presented in Table 4.4.1. The site is not within or near an Alquist-Priolo Special Study Zone, and there are no land forms observable in the pit that would indicate the presence of active faults. No known or mapped faults or lineaments trend through or towards the site. However, the site is within an area affected by strong ground shaking resulting from seismic activity. Earthquakes are measured in two different ways; by their physical effects, and by the amount of energy being released. The scale used to measure the physical effect, or "intensity" of an earthquake is the Modified Mercalli Scale. The scale used to measure the energy released, or "magnitude', of an earthquake is the Richter Scale. Table 4.4.2 presents a comparison of "intensity" and "magnitude' (Lindeburg, 1990). TABLE 4.4.1 ACTIVE FAULT PROXIMITY FAULT DISTANCE -MILES San Andreas 96 WSW Hayward 66 W Calaveras 54 W Green Valley -Concord 72 NW Greenville 11 SW Tesla 12 SW Midway 7 SW Livermore 25 W Tracy -Stockton 6 N The significant active faults in the site region have all generated earthquakes that have produced strong vibratory ground motion in the site area in the Modified Mercalli Intensity range of VII to IX. Such intensities have caused extensive damage to poorly designed and/or constructed structures due to strong shaking. The instability of ground shaking at a given location depends on several factors, but primarily on the earthquake magnitude, the distance from the epicenter to the site of interest, and the response characteristics of the soil or bedrock units underlying the site. The term 'Maximum Credible Earthquake' (MCE) is defined as the largest earthquake that is likely to be generated along an active fault zone. The magnitude of the MCE is estimated from the geologic character and earthquake history of the fault. Most researchers, when calculating the MCE for the strike -slip faults of the California Coast Ranges, estimate the potential length of surface rupture, then use empirical ER -93-1 -57- (9-27-93)