Draft Environmental Impact Report - East Bay Municipal Utility District

Estates Reservoir Replacement Draft Environmental Impact ReportGeology, Soils and Seismicityseismic surveys in the dam area indicate that the shear wave velocity of the materials nearthe bedrock surface is about 1,500 feet per second. The shear wave velocity of thematerials increases rapidly with depth such that a representative average of the velocitiesbeneath the dam is at least 2,000 feet per second.Groundwater ConditionsThe groundwater level in the dam used for stability analysis was estimated based on thepiezometric data corresponding to a reservoir filled to the spillway elevation of 770 feet.The groundwater level is at approximately Elevation 746 beneath the dam crest. Fromthe dam crest, the groundwater level follows a gentle downward gradient to aboutElevation 675 feet at the downstream toe of the dam. The presence of groundwater at ahillside like the Estates Reservoir site is common and the groundwater level may besubject to large seasonal fluctuations. There are natural springs in the area that contributeto the groundwater regime at the site.SeismicitySeismic SourcesThe Hayward-Rodgers Creek fault is located about 0.3 kilometers northeast of theEstates Reservoir site. This fault was the source of an estimated magnitude (M)6.8 earthquake on October 21, 1868. The San Andreas fault, located about29 kilometers west of the dam, was the source of the 1906 Great San Franciscoearthquake. Other active faults within 50 kilometers of the dam that are considered aspotential sources of future large earthquakes include the Mount Diablo Thrust,Concord-Green Valley, North Calaveras, San Gregorio-Seal Cove, and Greenvillefaults. The locations of the main potential seismic sources in the region are shownin Figure 3.3-2.The maximum magnitudes for each identified seismic source were estimatedbased on the potential rupture length and seismogenic depth, using an empiricalrelationship that relates earthquake magnitude and rupture area as proposed by Wellsand Coppersmith (1994). Site-to-source distances were measured from the dam site tothe main trace of each fault. The estimated maximum earthquake magnitudes and siteto-sourcedistances for each of the main faults in the region are listed in Table 3.3-1.Because of its magnitude and site-to-source distance, the Hayward-Rodgers Creek fault islikely to generate the strongest ground motions at the dam site. The estimated maximummagnitude for this fault is moment magnitude (Mw) 7¼. The San Andreas fault, located about29 kilometers west of dam, is capable of generating long duration shaking due to its largemaximum magnitude (Mw 8.0). All other intermediate faults have estimated maximummagnitudes lower than the Hayward-Rodgers Creek fault.sb09_001.doc 3-3.6 7/22/2009