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SECT NFOUR Seismic Hazard inputs Unnamed Faults in Williams Fork Valley These series of northwest- to northeast-striking normal faults in Williams Fork Valley include the Battle Mountain (BMF), Lost Creek (LCF), Mule Creek (MCF), Skylark Creek (SCF), and Ute Creek(UCF) faults (Figures 2 and 3). Not much is known about the age or activity of these faults, which are included in the USGS Quaternary Fault and Fold Database as#2300. Although offsets or scarps on late Quaternary sediments have not been identified (Widmann et al., 1998; Kirkham, 2004), many of these faults generally have prominent associated lineaments and escarpments on the Troublesome Formation, especially the northernmost faults (Figure 3). A notable exception is the Ute Creek fault, which appears buried and does not offset Pinedale-age glacial deposits that bury much of its length(Olig et al.,2013). Due to their proximity to each other and their location within the WFMF hanging wall, we grouped the unnamed faults in Williams Fork Valley into a zone; however, because of their apparent difference in age of activity we included three rupture scenarios (A, B, and C) (Table 1). In Rupture Scenario A (weighted 0.7), the northernmost faults (BMF through LCF) rupture together. In Rupture Scenario B (weighted 0.25), the long, unnamed northwest-striking fault on the northeast margin of the valley also ruptures with the northern faults. In rupture Scenario C (weighted 0.05), all of the unnamed faults in Williams Fork Valley rupture (including the UCF). Thus,the weights on the three scenarios generally reflect the better geomorphic expression of the northern faults and the interpretation that the faults may be helping to accommodate differential rates of activity along the northern middle, and southern sections of the WFMF (Olig et al., 2013). Whether the valley faults rupture with the WFMF remains unknown as little is known about the timing of events, but the early Holocene paleoearthquake observed on the northern section of the WFMF appears younger than faulting on any of the valley faults (Olig et al., 2013), suggesting independent activity at least during the most recent event on the WFMF. Our model assumes independent activity. We assigned a slightly lower probability of activity (0.7) for the unnamed faults in Williams Fork Valley(Table 1)because these faults may merely be space-accommodation structures in the hanging wall of the WFMF and not seismogenic structures. A preferred characteristic magnitude of M 6.7 was based on a floating rupture length of 18 km (Table 1), which is about the total length of the SCF, the most prominent structure of the zone (Figure 4). Little is known about rates of activity; based on the apparent lack of late Quaternary scarps we assumed a slip rate distribution similar to the Mosquito fault(Table 1). 4.1.2 Background Seismicity In state-of-the-practice seismic hazard evaluations, the hazard from background (random) earthquakes is addressed. Background earthquakes are those events that do not appear to be associated with known geologic structures. They occur on crustal faults that exhibit no surficial expression (buried faults)or are unmapped due to inadequate studies. One could argue that the 1882 earthquake is an example of a background event. In this PSHA, we address the hazard from background earthquakes through(1)the use of a regional seismic source zone for the SRM, where earthquakes are assumed to occur randomly (Figure 1), and (2) the historical seismicity can be assumed to be stationary in space and hence smoothed using a Gaussian filter. Earthquake recurrence estimates in the site region are required in order to assess the hazard to the Henderson Mill Tailings Dam from background earthquakes. A SRM background zone catalog Elm W-WAR4M 15