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generally thicker beneath the Colorado Rocky Mountains than beneath the Rio Grande rift. <br /> The thinner crust of the latter is probably due to high heat flow (Wong and Chapman, 1990). <br /> Vertical separation rate estimates from published and unpublished data were converted to <br /> average down-dip slip rates, assuming a fault dip of 60°. We determined a range in slip <br /> rates for each fault based on a slip rate distribution developed for Basin and Range and Rio <br /> Grande rift faults using both long-term and short-term slip rate data. The mean of the slip <br /> rate distribution was scaled to the average slip rates estimated for the faults in this study and <br /> the loth, 50th and 9Oth percentile values extracted and weighted 0.3, 0.4 and 0.3 <br /> respectively. The weighted mean of these three values is approximately the average down- <br /> dip slip rate (Table 1). <br /> Maximum magnitudes were estimated using empirical relationships derived by Wells and <br /> Coppersmith (1994) based on surface rupture length and fault rupture area for normal <br /> faulting. Rupture areas were calculated from potential rupture lengths, a fault dip of 600 and <br /> a seismogenic depth of 15 km. Faults less than about 20 km long were assigned a maximum <br /> magnitude of M, 6.5, the threshold for surface faulting in the Basin and Range province <br /> (dePblo, 1994). <br /> Chase Gulch Fault <br /> The Chase Gulch fault is a northwest-striking fault located just west of Colorado Springs <br /> (Plate 1). This fault lies along the northeast margin of Spinney Mountain, a Laramide-age <br /> uplift formed by anticlinal folding above the Elkhorn thrust fault (Unruh et al., 1995b). The <br /> Chase Gulch fault is approximately 29 km long and dips to the northeast. This fault has <br /> approximately 2.5 m of post-Pinedale displacement, which implies a vertical slip rate of 0.07 <br /> to 0.2 mm/yr, with a preferred vertical slip rate of 0.14 mm/yr during the late Quaternary <br /> (Shaffer and Williamson, 1986). We estimate a preferred down-dip slip rate of 0.16 mm/yr. <br /> The mean maximum magnitude is MW 6.7 (Table 1). The Chase Gulch fault is considered <br /> to be active. <br /> HACONTRAMTENMILE\9 9 M0412951500 <br />