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ROU NFOUR SOIsmIC NEW Inputs
<br /> better address uncertainty, in this study we also consider an alternate scenario that extends the
<br /> fault farther south to near Breckenridge, including an apparently older fault in an alternate
<br /> rupture scenario, as mapped by Kellogg et al. (2002) and Wallace et al. (2003). This better
<br /> reflects available data and addresses remaining uncertainties. Finally, it also incorporates some
<br /> results from ongoing mapping in the region (McCalpin et al., 2012a; 2012b), including mapping
<br /> that uses recently flown LIDAR data of the upper Arkansas Valley and nearby areas (e.g.,
<br /> Bohanon and Ruleman, 2013; Shroba et al., 2014), which covers parts of the southern Mosquito
<br /> fault and other nearby Quaternary faults. These studies have implications for rupture models of
<br /> the Mosquito fault, which has a complex geometry in some locations due to the apparent
<br /> influence of pre-existing structures and multiple periods of deformation since the Precambrian in
<br /> the region (e.g., Behre, 1953; Begendahl and Koschmann, 1971; McCalpin et al, 2012a, 2012b;
<br /> Bohannon and Ruleman, 2013). Although it is not a significant contributor to the Henderson
<br /> site, it is a significant contributor to the hazard at the Climax site, and so these updates were
<br /> included here to keep the seismic source model consistent between the two sites. However,
<br /> some of the ongoing mapping was not yet available for this study (C.A. Ruleman and K.S.,
<br /> Kellogg, USGS,written communication,2015),and this update only includes what was available
<br /> online as of 16 November 2015.
<br /> We evaluated Quaternary fault sources within 100 km of the sites (Figure 2). Faults were
<br /> included that were judged to potentially contribute to the probabilistic hazard because of their
<br /> activity, length, or proximity to the site. We included all longer (> 5 km) faults that suggest or
<br /> show evidence for Quaternary activity within 50 km of the site (Figure 2). We did not include
<br /> faults less than 5 km long as separate independent sources because we consider them to be
<br /> accounted for by the areal source zones. In general, less active fault sources (slip rates < 0.1
<br /> mm/yr) beyond 50 km were judged to not contribute to the seismic hazard, due to the dominant
<br /> contribution of the nearby WFMF and Mosquito faults. However, some more distant(> 50 km)
<br /> faults that were included in previous analyses are retained here for consistency.
<br /> We included fourteen faults in our analysis: Chase Gulch, Frontal, Golden, Greenhorn
<br /> Mountain, Mosquito, and Sawatch faults, Steamboat Springs and Ute Pass fault zones,
<br /> Northeastern Boundary fault system, unnamed faults in Granby basin, unnamed faults in
<br /> Williams Fork Valley, unnamed faults near Burns, and unnamed faults near Leadville, and the
<br /> WFMF. Figure 2 shows the locations of these faults and Tables I and 2 show the parameters
<br /> used for these faults in our PSHA. Faults are listed alphabetically in Table 1. Except where
<br /> noted otherwise, fault nomenclature and numbers shown in Table 1 and Figure 2 generally
<br /> follows those used by Widmann et al. (1998) and the USGS Quaternary Fault and Fold
<br /> Database.
<br /> Of the fourteen faults, seven are considered active and seven are considered potentially active
<br /> based on available information, resulting in the probabilities of activity indicated in Table 1. In
<br /> assigning probabilities of activity for each fault source, we considered both the likelihood that
<br /> the structure is capable of independently generating earthquakes (i.e., is seismogenic), and the
<br /> likelihood that it is still active within the modern stress field. We incorporated many factors in
<br /> assessing these likelihoods, such as: orientation in the modern stress field, fault geometry
<br /> (length, continuity, depth extent, and dip), relation to other faults, age of youngest movement,
<br /> geomorphic expression, amount of cumulative offset, rates of activity, and any evidence for a
<br /> non-tectonic origin (e.g, related to evaporite deformation or dissolution). Faults with strong
<br /> evidence for repeated Quaternary activity were generally assigned probabilities of being active
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