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15, 1998 • 7 • 983-2348.135 For the contribution from the Oil Creek fault, the ground motion is influenced by earthquakes in the entire magnitude range from moment magnitude Mw 5.0 to Mw 6.8 (the MCE), although about 50 percent of the PGA appeazs to be from the magnitude range of Mw 5.0-5.5 and the remaining 50 percent from Mw 5.5-6.8. For comparison, the U.S. Geological Survey (1996) would predict a PGA of 0.04 g for the same 475-yeaz return period, and Algermissen et al (1990) would predict a PGA of 0.02 g (Figure 2). The higher PGAs for given return periods derived from this investigation, as compazed to the U.S. Geological Survey (1996) and Algermissen et al (1990), aze believed to be the result of the implementation of a systematic and comprehensive PSHA that incorporates historical seismidty data as well paleoseismic data for identified seismogenic faults in the development of the ground motions. Both the U.S. Geological Survey (1996) and Algermissen et al (1990) appeaz to have primarily addressed the use of historical seismicity data for the region of the site, with limited or no use of paleoseismic data for active faults, in the development of their probabilistic PGA maps for this region. Because the area is of low historical seismidty, and the earthquake record is short, and thus incomplete (compared to the geologic or paleoseismic record), the overall seismic hazard, in terms of ground motion maybe underestimated if only the historical seismicity is considered. The use of paleoseismic data, in conjunction with historical seismidty data, more completely characterizes the seismic hazard and thus accounts for the higher PGAs. CONCLUSIONS AND RECOMMENDATIONS The CC&V Project is located in a region of Colorado that generally has a low rate of historical seismidty. In addition, although there aze 16 active and potentially active seismogenic sources within 100 km (62 mi) of the project, their rates of tectonic activity appear to also be low, and aze consistent with the appazentiy overall low rate of seismic activity of the region. Together, these suggest that, compazed to other more active regions of Colorado, and the western United States, the seismic hazard and expected ground motions (PGAs) from future seismic activity in the vidnity and region of the site should be low. The closest potential seismogenic source to the project is the Oil Creek Eault that is located about 8 km (5 mi) to the northeast. It is estimated to be capable of generating an MCE of moment magnitude Mw 6.8, and it is estimated to have a slip rate of 0.027 mm/yr. The Oil Creek fault tends to dominate the probabilistic earthquake ground motions at the site because of its close proximity, relatively high MCE, and higher slip rate compared to the other, neazby potential seismogenic sources. The results of the systematic and comprehensive PSHA of this investigation indicate that the PGAs at the CC&V site for a suite of return periods ranging from 100 yeazs to 2,500 years are: Golder Associates