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Frontal Fault <br /> The eastern side of the Gore Range is bounded by a major fault zone referred to as the <br /> Frontal fault (Plate 1). The fault is a north-northwest-striking normal fault that dips steeply <br /> to the east. It has a total length approaching 43 km and has a cumulative vertical separation <br /> approaching 1,200 in (Unruh et al., 1992). The northern section of the fault (12 km long) <br /> is characterized by anomalous geomorphic features that are permissive of late Quaternary <br /> faulting (J. Unruh, William Lettis & Associates, personal communication, 1995). If these <br /> features are tectonic, scarp heights imply a vertical slip rate of 0.2 to 0.5 mm/yr. We <br /> estimated a mean down-dip slip rate of 0.4 mm/yr and a mean maximum magnitude of Ma, <br /> 6.5 for this fault (Table 1). The southern section of the fault (31 km long) has probably not <br /> experienced any Quaternary faulting (West, 1978) and we do not consider it to be active. <br /> We assign a probability of 70% that the northern Frontal fault is active because of the lack <br /> of definitive data. <br /> Golden Fault <br /> The Golden fault is a 27-km long, northwest-striking, west-dipping reverse fault bordering <br /> the east flank of the Front Range near the town of Golden (Plate 1). This fault accounts <br /> for approximately 2,680 in of uplift along this section of the Front Range. This fault has <br /> experienced two surface-faulting events over the past 600 to 700 ka with an approximately <br /> 2.7 in vertical displacement per event (Kirkham and Rogers, 1981). This implies a vertical <br /> slip rate of 0.008 mm/yr along the Golden fault and a down-dip slip rate of 0.009 mm/yr. <br /> The mean maximum magnitude for this fault is estimated to be M,, 6.7 (Table 1). <br /> Because of its low activity rate, whether the Golden fault should be considered in seismic <br /> hazard evaluations in Colorado depends on the criteria used for defining an "active" fault. <br /> Based on the criteria adopted in this study, the Golden fault would be considered to be <br /> "potentially active". In studies for the U.S. Bureau of Reclamation (Unruh et al., 1992) <br /> using the USBR activity criteria, the Golden fault is considered to be "not active". In <br /> contrast, in a probabilistic seismic hazards analysis for Rocky Flats, Geomatrix Consultants <br /> has assigned a probability that the Golden fault (considered part of the Front Range structure) <br /> is active of 40% (K. Coppersmith, Geomatrix Consultants, personal communication, 1995). <br /> Because of its controversial nature, we have conservatively included the fault in our analysis. <br /> H-XONTRAMTENMELM10 1 10 M0412951500 <br />