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May 15,1998 • 2 • 983-2348.135 at a range of PGAs from 0.1 to 0.2 g, and an average PGA of 0.14 g that was recommended for use in site seismic design. The work to develop probabilistic PGAs for this investigation was designed to build on the previous investigations, but utilize a current, systematic and comprehensive approach and methodology, as well as an updated attenuation relationship, so that the results would be technically reasonable, defensible and appropriate to the seismic hazazd of the region. The basic approach included the following steps: Identification, mapping and chazacterization of potential seismogenic sources (faults) within 100 km (62 mi) of the site; the source chazacterization included fault type, orientation and geometry, closest source-to-site distance, total fault length, rupture length, fault rupture width, rupture azea, slip rate and maximum credible earthquake (MCE); in addition, the maximum background eazthquake (MBE) and its MCE were identified, and Development of probabilistic PGAs at the site for return periods ranging from 100 yeazs to 2,500 yeazs; the development of the PGAs addressed the exposure to the site, in terms of ground motion, from all the potential seismogenic sources within a 100 km (62 mi) radius, utilizing the seismogenic source parameters from the source characterization in the previous step, and a probabilistic seismic hazard assessment approach. TECTONIC/SEISMIC SETTING The CC&V project is located in the Rocky Mountains, neaz Victor, Colorado, at elevations of about 9,000 to 10,500 feet, about 10 miles southwest of Pikes Peak. The azea within about 10 miles of the project is generally underlain by Precambrian metamorphic rock (gneiss) and intrusive rock (granite), that aze in turn overlain by Tertiary volcanics (lava flows and pyroclastic flows) (Tweto, 1976). The primary geologic structure of the area includes northwest-, north- and northeast-striking high angle faults, that are primarily normal-slip, but locally also reverse-slip (Tweto,1976; Witkind, 1976; Kirkham and Rogers, 1981). The project valley leach fadlity site is underlain by two primary bedrock units (CC&V, 1998 and 1993). The predominant unit is undifferentiated Precambrian pinkish gray, medium to coarse-grained biotite granodiorite. The granodiorite is also considered to be the basement bedrock. Locally, this basement rock may also be composed of biotite schist, augen gneiss, or quartz monzonite. The granodiorite is also often intruded by feldspar pegmatite or aplite dikes. The secondary unit underlying the heap leach fadlity site is Tertiary aphanitic phonolite that occurs solely as narrow, often discontinuous dikes that intrude the granodiorite. The aphanitic phonolite dikes are generally gray to white, with local potassium feldspaz phenocrysts, and are fissile. For purposes of site design, the valley leach facility is considered to be underlain by granodiorite bedrock (CC&V, 1993). Golder Associates