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Sheep Mountain Fault <br /> Kirkham and Rogers (1981) mapped an "inferred potentially active fault" along the western <br /> margin of Sheep Mountain. This structure is 6 to 11 km long, strikes north-northwest, and <br /> is mapped as a northeast-dipping fault (Plate 1). Unruh et al. (1992) informally refer to this <br /> as the Sheep Mountain fault. Aerial reconnaissance by Unruh et al. (1992) revealed that the <br /> trace of the fault is marked by a pronounced scarp, vegetation lineaments, and springs. <br /> Vertical separation of the Sheep Mountain fault is 0.15 to 0.6 mm/yr, with a preferred rate <br /> of approximately 0.3 mm/yr (J. Unruh, William Lettis & Associates, personal communi- <br /> cation, 1995). This yields an estimated down-dip slip rate of 0.35 mm/yr. We estimate the <br /> mean maximum magnitude for this fault to be Ma, 6.5 and the fault to be active. <br /> Williams Fork Mountain Fault <br /> The Williams Fork Mountain fault is an approximately 31-km-long, northwest-striking, <br /> northeast-dipping normal fault that borders the western margin of the Williams Fork Valley <br /> (Plate 1). Air photo analysis by Unruh et al. (1992; 1993) revealed that the trace of the fault <br /> is marked by an east-facing break and vegetation and tonal lineaments, which are best <br /> expressed along the northern part of the fault. This section of the fault shows evidence of <br /> late Quaternary surface faulting, with a preferred vertical movement of 0.1 to 0.3 mm/yr <br /> (Unruh et al., 1992; 1993). We estimate an average down-dip slip rate of 0.24 mm/yr for <br /> both segments of this fault. The computed mean magnitude for both segments is MR, 6.5. <br /> We do not consider a multi-segment rupture scenario because of the poor geomorphic <br /> expression of the southern segment compared to the northern segment. The southern part <br /> of the fault is less-well expressed along strike, possibly due to the fault being located in <br /> bedrock, and is probably not as active as the northern section of the fault (J. Unruh, William <br /> Lettis & Associates, personal communication, 1995). We thus assign the southern segment <br /> a probability of 10% that it is active although Quaternary fault activity cannot be precluded. <br /> We consider the northern segment to be 100% active (Table 1). <br /> H:\CONTRACr\TENMILE\13 13 M0412951500 <br />