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Frontal Fault ' The eastern side of the Gore Range is bounded by a major fault zone referred to as the Frontal fault (Plate 1). The fault is anorth-northwest-striking normal fault that dips steeply to the east. It has a total length approaching 43 km and has a cumulative vertical separation approaching 1,200 m (Unruh et al., 1992). The northern section of the fault (12 km long) ' is characterized by anomalous geomorphic features that are permissive of late Quaternary faulting (J. Unruh, William Lettis & Associates, personal communication, 1995). If these features are tectonic, scarp heights imply a vertical separation rate of 0.2 to 0.5 mm/yr. We estimated a mean slip rate of 0.4 mm/yr and a mean maximum magnitude of M~, 6.5 for this fault (Table 1). The southern section of the fault (31 km long) has probably not experienced any Quaternary faulting (West, 1978) and we do not consider it to be active. We assign a probability of 70% that the northern Frontal fault is active because of the lack of definitive data. Golden Fault The Golden fault is a 27-km long, northwest-striking, west-dipping reverse fault bordering the east flank of the Front Range neaz the town of Golden (Plate 1). This fault accounts for ' approximately 2,680 m of uplift along this section of the Front Range. Thds fault has experienced two surface-faulting events over the past 600 to 700 ka with an approximately 2.7 m vertical displacement per event (Kirkham and Rogers, 1981). This implies a vertical separation rate of 0.008 mm/yr along the Golden fault and a slip rate of 0.009 Itun/yr. The mean maximum magnitude for this fault is estimated to be M.. 6.7 (Table 1). ' Because of its low activity rate, whether the Golden fault should be considered in seismic hazazd evaluations in Colorado depends on the criteria used for defining an "active" fault. Based on the criteria adopted in this study, the Golden fault would be considered to be ' "potentially active". In studies for the U.S. Bureau of Reclamation using the U:~BR activity criteria, the Golden fault is considered to be "not active" (Unruh et al., 1992). In contrast, in a probabilistic seismic hazards analysis of the Rocky Flats, Geomatrix Consultants has t assigned a probability that the Golden fault (considered part of the Front Range structure) is active of 40% (K. Coppersmith, Geomatrix Consultants, personal communication, 1995). ' Because of its controversial nature, we have conservatively included the fault in our analysis. H:\CONTRACT234561DUN10 10 M0309951509