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<br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />IJ <br /> <br /> <br /> <br />Skylark Fault <br />'I\veto et al. (1978) mapped a series of northeast-trending faults in the Williams Fork Valley <br />(Plate 1) that cut Pleistocene deposits and surfaces, the precise age of which is unknown <br />(Kirkham and Rogers, 1981). Unruh et al. (1992) evaluated these faults and suggested that <br />Quaternary surfaces have been deformed across the faults. The 5kylazk fault is an 8-km long, <br />north- to northeast-striking, west- to northwest-dipping curvilineaz normal fault (Unruh et al., <br />1992) and is the closest fault to the Henderson mine. Due to the more easterly splaying of <br />the scarp at the southern termination of the northern segment of the Williams Fork Mountains <br />fault, the short length of the Skylark fault, and the thickest accumulation of basin sediments <br />in Williams Fork Valley being north of the Skylazk fault, J. Unruh (William Lettis & <br />Associates, personal communication, 1995) believes that the Skylark (fault is an <br />accommodation structure associated with the northern segment of the Williams Fork <br />Mountains fault and probably not a separate seismic source. However, due to its close <br />proximity to the Henderson tailing dam, we have conservatively assumed that this fault is a <br />separate seismic source from the Williams Fork Mountains fault. We also assume a similar <br />slip rate to that of the Williams Fork Mountains fault of 0.24 mmlyr. We assign a best <br />estimate magnitude of Mw 6.5 and consider it to be 100% active (Table 1). <br />Colorado Rocky Mountain Source Zone <br />To account for the hazard from background (random) earthquakes that aze not associated with <br />the known or mapped faults, an areal source azound the site was incorporal:ed into the <br />analysis, herein defined as the Colorado Rocky Mountain source zone. It was considered to <br />be, to the first order, uniform in terms of its tectonic, geologic, and ;>eismogenic <br />characteristics. In the western U.S., the maximum magnitude for the random earthquake <br />usually ranges between M~ 6 to 6r/i (dePolo, 1994). In most regions of the western U.S., <br />events lazger than these magnitudes are usually accompanied by surface rupture and thus <br />repeated events of this size will produce recognizable fault or fold-related features at the <br />earth's surface (e.g. Doser, 1985). We consider a mean maximum magnitude of Mq 6.5 t <br />0.3 for the MCE for the random earthquake within the Colorado Rocky Mountain source <br />zone. <br />H:\CONiRACT234561.DUH10 14 M0709951609 <br />