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Sheep Mountain Fault ' Kirkham and Rogers (1981) mapped an "inferred potentially active fault" along the westem margin of Sheep Mountain. This structure is 6 to 11 km long, strikes north-northwest, and is mapped as anortheast-dipping fault (Plate 1). Unruh et al. (1992) informally refer to this as the Sheep Mountain fault. Aerial reconnaissance by Unruh et al. (1992) reve:iled that the trace of the fault is marked by a pronounced scarp, vegetation lineaments, and siprings. The vertical sepazation rate of the Sheep Mountain fault is 0.15 to 0.6 mm/yr, with a preferred ' rate of approximately 0.3 mm/yr (J. Unruh, William Lettis & Associates, personal communi- cation, 1995). This yields an estimated slip rate of 0.35 mm/yr. We estimate the mean maximum magnitude for this fault to be Mw 6.5 and the fault to be active. Williams Fork Mountain Fault The Williams Fork Mountain fault is an approximately 31-km-long, northwest-striking, northeast-dipping normal fault that borders the westem mazgin of the Williams :Fork Valley (Plate 1). Air photo analysis by Unruh et al. (1992; 1993) revealed that the trace of the fault ' is marked by aneast-facing break and vegetation and tonal lineaments, which aze best expressed along the northern part of the fault. This section of the fault shows evidence of late Quaternary surface faulting, with a preferred vertical movement of 0.1 to 0.3 mm/yr (Unruh et a1., 1992; 1993). We estimate an average slip rate of 0.24 mm/;yr for both segments of this fault. The computed mean magnitude for both segments is N[W 6.5. We do not consider amulti-segment rupture scenario because of the poor geomorphic expression of the southem segment compared to the northem segment. The southern part of the fault is less-well expressed along strike, possibly due to the fault being located in bedrock, and is probably not as active as the northem section of the fault (J. Unruh, William Lettis & Associates, personal communication, 1995). We thus assign the southern segment a probability of 10% that it is active although Quaternary fault activity cannot be precluded. The Henderson tailing dam is located adjacent to the southern segment (Plate 1). We ~' consider the northern segment to be 100% active (Table 1). L~ ' H:\WNIRACI\]34361.DUP13 13 MO109931609