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SEPINFOUR Seismic Hazard Inputs Inputs into the seismic hazard analyses include characterization of both earthquake sources and ground motion attenuation. These inputs for the PSHA and DSHA, including the WFMF that is particularly significant to the Henderson site,are described below. 4.1 EARTHQUAKE SOURCES In this study, all known seismic sources which could potentially generate strong ground shaking at the site were characterized. Seismic source characterization is concerned with three fundamental elements: (1) the identification of significant sources of earthquakes; (2) the maximum size of these earthquakes; and (3) for probabilistic analyses, the rate at which they occur. In the PSHA, we estimated and used source parameters for the significant faults in the region, and areal source zones to represent background earthquakes, which are described in Sections 4.1.1 and 4.1.2, respectively. For the DSHAs,the WFMF is the source for the scenario event at the Henderson site and is discussed at the end of Section 4.1.1. 4.1.1 Quaternary Faults In marked contrast to the historical earthquake record, the geologic record indicates that large surface-faulting earthquakes (M > 6.25) have repeatedly occurred during the Quaternary (past 1.6 million years)on several faults in the northern Rio Grande rift(Machette, 1998; Machette et al., 1998; Widmann et al., 1998; Morgan et al., 2012; Figure 2). These faults accommodate overall east-west extension in a series of asymmetric late Cenozoic basins that generally are smaller (narrower and shallower) than basins in the central and southern Rio Grande rift. Quaternary faults in the northern rift typically strike north to northwest and often show long, complex histories, reactivating older reverse faults. Many are range-bounding normal faults (e.g., the WFMF and Mosquito fault), but some show more structural complexity, including intrabasin faults(e.g.,Williams Fork Valley faults),or discontinuous zones(e.g.,unnamed faults near Leadville) that may serve as relay ramps transferring slip between larger range-bounding faults. Despite the complexities, Quaternary faults in the rift are generally characterized by: (1) dominantly normal slip, (2) moderate to low slip rates (typically less than 1 mm/yr), and (3) relatively long recurrence intervals between surface-faulting events (typically tens of thousands to hundreds of thousands of years). These faults often lack associated seismicity and show orders-of-magnitude variations in rates of activity through time (Machette, 1998; McCalpin, 1995; McCalpin et al., 2006). The seismic source characterization of Quaternary faults used for this analysis is updated from our previous seismic hazard analysis for the Henderson site (Olig et al., 2013), which included results from mapping(Olig and Kirkham,2012)and trenching investigations of the WFMF. Our 2013 study was in turn enhanced and updated from previous studies that we conducted for other damsites in western Colorado, most importantly our seismic hazard analyses for the Climax site that included reconnaissance mapping and scarp profiling of the Mosquito fault (Wong et al., 2007). Additionally, this study addresses review comments on our 2013 study provided by Norm Abrahamson (personal comm., 2014) regarding better incorporating epistemic versus aleatory uncertainties on rupture models for the WFMF. This study also incorporates new data from the USBR regarding recent studies of the Frontal fault conducted to improve seismic hazard evaluations of Green Mountain Dam (Derouin, 2014). It also revises our previous rupture models and geometry for the southern Frontal fault, which had solely relied on mapping by the USBR (L. Anderson, personal comm., 2009) and terminated the southern end near Frisco. To