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still sparse, regional seismographic coverage since 1983 and two aftershock monitoring <br /> studies using portable instruments (Wong et al., 1994). Consistent with the historical record, <br /> post-1983 earthquake activity is distributed throughout most of western Colorado. In part, <br /> possibly due to the lack of dense seismographic coverage, events rarely appear to be <br /> associated with specific geologic structures. Earthquakes up to about ML 4 to 5 and swarm <br /> activity (a sequence of events clustered in space and time with no dominant event) are <br /> commonly observed. Swarms often occur in regions of relatively high heat flow which is <br /> generally the case for the Southern Rocky Mountains. Based on the analysis of several small <br /> but felt earthquakes and most notably, sequences near Carbondale in 1984 (Goer et al., <br /> 1988) and Crested Butte in 1986 (Wong, 1991; Bott and Wong, 1995), seismicity appears <br /> to be the result of the reactivation of thrust/reverse faults of Laramide age. Such reactivation <br /> appears to be extensional in nature because most earthquake focal mechanisms exhibit normal <br /> faulting in response to a northeast-southwest oriented minimum principal stress (Wong, 1986; <br /> Wong et al., 1994). Earthquakes in western Colorado, as elsewhere in the western U.S., <br /> are confined to the upper 15 km of the crust (Wong and Chapman, 1990). <br /> Tectonic strain rates appear to be low in western Colorado as reflected in the generally small <br /> and infrequent moderate magnitudes of observed seismicity. Recurrence estimates suggest <br /> that events of ML 51/2 similar to the 1960 earthquake occur only once about every 60-70 <br /> years and for 1882-like events, only once every 400 years (Wong et al., 1994). Recent <br /> geologic investigations suggest that some faults in western Colorado may be capable of <br /> generating earthquakes up to ML 7 although these events are probably rare. Such faults <br /> probably have recurrence intervals of many tens of thousands to possibly more than 100,000 <br /> years. <br /> SEISMIC HAZARD ANALYSIS METHODOLOGY <br /> The seismic hazard approach used in this study is based on the model presented by Kulkarni <br /> et al. (1979). This approach is similar to other available seismic hazard analysis models <br /> (Cornell, 1968; McGuire, 1974), but includes some additional features as described below. <br /> The occurrence of earthquakes on a fault is assumed to be a Poisson process. The Poisson <br /> model is widely used and is a reasonable assumption in regions where data are sufficient to <br /> provide only an estimate of average recurrence rate (Cornell, 1968). When there are <br /> H:\CONTRACT\TENMILE\2 2 M0412951500 <br />