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SE010THREE Selsmotectonic Setting and Historical Selsmim <br /> Strong ground shaking due to earthquakes is a significant consideration for dam design and dam <br /> safety in many areas of the western U.S. Based largely on the historical earthquake record, <br /> which dates back to the 1870's, western and central Colorado east of the Front Range is <br /> characterized by a low to moderate level of seismicity (Figure 1). Also because there are few <br /> prominent late Quaternary faults in the entire state (Figure 2), Colorado has generally been <br /> considered to possess a low level of seismic hazard. The following section describes the <br /> seismotectonic setting and historical seismicity of north-central Colorado and the significant <br /> earthquake sources. <br /> 3.1 SEISMOTECTONIC SETTING <br /> The Henderson Mill lies near the northern extent of the northern Rio Grande rift, which is <br /> bordered on the east,north and west by the Southern Rocky Mountains. During the Neogene and <br /> Quaternary,the locus of extension in Colorado was the Rio Grande rift. The northern end of the <br /> rift is not easily defined and although many researchers place the northern end of the rift near <br /> Leadville (e.g., Chapin, 1971), others extend it north beyond Steamboat Springs (Tweto, 1978; <br /> Figure 2). The rift consists of a series of north-south-trending basins that form a nearly <br /> continuous, interconnected tectonic depression from central Colorado south to the Mexican <br /> border and beyond. In general, the most active late Quaternary faults in Colorado are associated <br /> with the portion of the Rio Grande Rift extending through Colorado(Figure 2). <br /> The Southern Rocky Mountains Province extends from northern New Mexico to south-central <br /> Wyoming (Fenneman, 1946 in Smith et al. 1989). Except for the San Juan volcanic field, the <br /> Southern Rocky Mountains Province is characterized by large, asymmetrical uplifts of <br /> Precambrian rock and adjacent deep basins (Unruh et al., 1996a; 1996b). Most of this crustal <br /> deformation likely occurred during the Laramide orogeny (late Cretaceous to Eocene), and has <br /> since been modified by regional extension,regional uplift, fluvial incision,and denudation. <br /> The eastern margin of the Southern Rocky Mountains Province is defined by the northern Front <br /> Range at the latitude of the project area;the Great Plains Province lies to the east(Figure 2). The <br /> Front Range has several known or suspected north- to northwest-striking late Cenozoic normal <br /> faults that are typically found within the hanging walls of Laramide thrust faults (Unruh et al., <br /> 1996b). Defining the late Quaternary activity of such faults is difficult due to the scarcity of <br /> deposits of that age, so very few, if any, late Quaternary faults have been documented in the <br /> Front Range(Unruh et al., 1996a; 1996b; Widmann et al., 1998)(Figure 2). <br /> According to focal mechanisms and geodetic data, the Colorado portion of the Southern Rocky <br /> Mountains (SRM), Rio Grande rift, and Great Plains Provinces are generally characterized by <br /> ENE-WSW extensional crustal stresses, normal faulting, and low to moderate levels of <br /> seismicity(Zoback and Zoback, 1989;Berglund et al.,2012). <br /> 3.2 HISTORICAL SEISMICITY <br /> A historical catalog was compiled for the site region for the period from 1870 to 2015 (Figure 1). <br /> Primary data sources used in the compilation include: the Decade of North American Geology <br /> (DNAG) catalog; the Stover, Reagor, and Algermission U.S. historical catalog; Kirkham and <br /> Rogers' (1985) catalog; the National Earthquake Information Center's Preliminary <br /> Determination of Epicenters; the Denver Water Department's Front Range Network catalogs <br /> umm.nwim 6 <br />