Draft Environmental Impact Report - East Bay Municipal Utility District

Estates Reservoir Replacement Draft Environmental Impact ReportGeology, Soils and SeismicityTABLE 3.3-3Calculated Horizontal Peak Ground AccelerationCalculated 84th-% Horizontal Peak GroundAcceleration, gMCE M W Distance, km AS97 SD97 BR97 MeanHayward-Rodgers Creek 7 1/4 0.3 1.25 1.10 0.83 1.06San Andreas 8.0 29 0.33 0.37 0.34 0.35Mt. Diablo Thrust 6 3/4 17 0.34 0.35 0.26 0.32Concord-Green Valley 6 3/4 22 0.26 0.27 0.21 0.25Northern Calaveras 7 19 0.32 0.34 0.27 0.31Greenville 7 36 0.17 0.17 0.17 0.17San Gregorio-Seal Cove 7 1/2 36 0.22 0.23 0.22 0.22Note: AS97= Abrahamson and Silva (1997). SD97 = Sadigh et al.(19997). BR 97 = Boore et al (1997).Seismic HazardsPrimary HazardsSurface Fault Rupture - Seismically induced ground rupture is defined as the physicaldisplacement of surface deposits in response to an earthquake’s seismic waves. Themagnitude and nature of fault rupture can vary for different faults or even along differentstrands of the same fault. Ground rupture is considered more likely along active faults,which are referenced in Table 3.3-1.The Estates Reservoir site is not within an Alquist-Priolo Earthquake Fault Zone, asmapped and designated through the Alquist Priolo Earthquake Fault Zoning Act, and nomapped active faults are known to pass through the immediate Project site (Lienkaemper,1992). Therefore the risk of ground rupture at the Estates site is considered low.Groundshaking - Earthquakes in the Bay Area could produce strong groundshaking in theProject region. Groundshaking intensity is partly related to the size of an earthquake, thedistance to the site, and the response of the geologic materials that underlie a site. As arule, the greater the earthquake magnitude and the closer the fault rupture to a site, thegreater the intensity of groundshaking. Violent groundshaking is generally expected atand near the epicenter of a large earthquake; however, different types of geologicmaterials respond differently to earthquake waves. For instance, deep unconsolidatedmaterials can amplify earthquake waves and cause longer periods of groundshaking.While the magnitude is a measure of the energy released in an earthquake, intensity is ameasure of the observed groundshaking effects at a particular location. The ModifiedMercalli (MM) scale is commonly used to measure earthquake intensity due togroundshaking. Table 3.3-4 presents a description of the MM scale. The MM values forintensity range from I (earthquake not felt) to XII (damage nearly total). MM intensitiessb09_001.doc 3-3.9 7/22/2009