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May 15, 1998 • 4 • 983-2345.135 Source 100-Year Return Period 475-Year Retum Period 1,000-Yeaz Retum Period 2,500-Year Retum Period U.S. Geological 0.015 g 0.04 g 0.06 g 0.10 g Survey (1996) Algermissen et - 0.02 g - 0.06 g al (1990) The different PGAs for the site area indicated by the mapping of the U.S. Geological Survey (1996), as compazed to that of Algermissen et al (1990), maybe the result of the use of a more complete historical seismicity data base, the use of active fault sources in the probabilistic analysis, and the use of different and more current attenuation relationships. Both the hazazd mapping by the U.S. Geological Survey (1996) and Algermissen et al (1990) support the fact that the seismic hazard of the project site area, in terms of PGA, is generally low. For further comparison, the 1994 United States Uniform Building Code (UBC) places the CC&V site within Seismic Zone 1. The UBC Seismic Zone 1 has an equivalent PGA of 0.075 g. DEVELOPMENT OF EARTHQUAKE STRONG GROUND MOTIONS The development of earthquake ground motions for the CC&V Project valley leach facility was based on a probabilistic approach that included an initial step involving the identification and characterization of potential seismogenic sources, within a 100 km (62 mi) radius, that could affect the seismic design at the site. Seismogenic source data and pazameters developed from this initial step were used in the probabilistic seismic hazazd assessment to develop peak ground accelerations (PGA) for use in site seismic stability analyses and site seismic design. Identification and Characterization of Potential Earthquake Sources Based on the review of the available literature, data and maps, at least 16 potential seismogenic sources were identified within about 100 km (62 mi) of the CC&V Project that may affect the development of probabilistic earthquake ground motions. These include 15 mapped faults that have been identified as being active or potentially active, and one source, termed the maximum background earthquake (MBE), that was identified to represent the historical earthquakes that have occurred, but that have yet to be spatially or kinematically assodated with mapped faults. Historically, in seismic hazard evaluations, the MBE has been termed the "random' earthquake source. The locations of the 15 faults, with respect to the CC&V site aze shown on Figure 1. The geologic and geometric characteristics of each of the potential seismogenic sources, that Golder Assaclates