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Skylark Fault <br /> Tweto et al. (1978) have mapped a series of northeast-trending faults in the Williams Fork <br /> Valley (Plate 1) that cut Pleistocene deposits and surfaces, the precise age of which is <br /> unknown (Kirkham and Rogers, 1981). Unruh et al. (1992) evaluated these faults and <br /> suggest that Quaternary surfaces have been deformed across the faults. The Skylark fault <br /> is an 8-km long, north- to northeast-striking, west to northwest-dipping curvilinear normal <br /> fault (Unruh et al., 1992). Due to the more easterly splaying of the scarp at the southern <br /> termination of the northern segment of the Williams Fork Mountains fault, the short length <br /> of the Skylark fault and the thickest accumulation of basin sediments in Williams Fork <br /> Valley being north of the Skylark fault, J. Unruh (William Lettis & Associates, personal <br /> communication, 1995) believes that the Skylark fault to be an accommodation structure <br /> associated with the northern segment of the Williams Fork Mountains fault and probably not <br /> a separate seismic source. We have conservatively assumed that this fault is a separate <br /> seismic source from the Williams Fork Mountains fault. We also assume a similar slip rate <br /> to that of the Williams Fork Mountains fault of 0.24 mm/yr down-dip rate. 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 are not associated <br /> with the known or mapped faults, an areal source around the site was incorporated into the <br /> analysis, herein defined as the Colorado Rocky Mountain source zone. It was considered <br /> to be, to the first order, uniform in terms of its tectonic, geologic, and seismogenic <br /> characteristics. In the western U.S., the maximum magnitude for the random earthquake <br /> usually ranges between ML 6 to 61/2 (dePolo, 1994). In most regions of the western U.S., <br /> events larger 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 K 6.5 t <br /> 0.3 for the MCE for the random earthquake within the Colorado Rocky Mountain source <br /> zone. <br /> Ideally, it is preferable to calculate earthquake recurrence in the immediate vicinity of the <br /> tailings impoundment dam site. However, the apparent low level of seismicity in Colorado <br /> H:\CONTRACT\TENMELE\14 14 M0412951500 <br />