REP33384

• Permit No: M1977342
• IBM Index Class Name: Report
• Doc Name: PROBABILISTIC SEISMIC HAZARD ANALYSIS OF HENDERSON TAILING DAM NORTH CENTRAL COLO
• Media Type: D
• Archive: No

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still spazse, regional seismographic coverage since 1983 and two aftershoc4: monitoring
studies using portable instruments (Wong et al., 1994). Consistent with the historical record,
post-1983 earthquake activity is distributed throughout most of westem Colorz~do. In part,
possibly due to the lack of dense seismographic coverage, events rarely appeaz to be
associated with specific geologic structures. Earthquakes up to about M~ 4 to :i and swarm
activity (a sequence of events clustered in space and time with no dominar~t event) are
commonly observed. Swamvs often occur in regions of relatively high heat flow which is
generally the case for the Southern Rocky Mountains. Based on the analysis of ~;everal small
but felt earthquakes and most notably, sequences near Carbondale in 1984 (toter et al., 1988)
and Crested Butte in 1986 (Wong, 1991; Bott and Wong, 1995), seismicity appears to be the
result of the reactivation of thrusdreverse faults of Laramide age. Such reactivation appears
to be extensional in nature because most earthquake focal mechanisms exhibit normal faulting
in response to anortheast-southwest oriented minimum principal stress (Wong, 1986; Wong
et al., 1994). Earthquakes in westem Colorado, as elsewhere in the western U.S., aze
confined to the upper 15 km of the crust (Wong and Chapman, 1990).
Tectonic strain rates appeaz to be low in western Colorado as reflected in the generally small
and infrequent moderate magnitudes of observed seismicity. Recurrence estimates suggest
that events of M~ 5~/x similar to the 1960 earthquake occur only once about every 60-70 years
and for 1882-like events, only once every 400 t 200 years (Wong et al., 19!)4). Recent
geologic investigations suggest that some faults in westem Colorado may bc: capable of
generating earthquakes up to M~ 7 although these events are probably rare. Such faults
probably have recurrence intervals of many tens of thousands to possibly more than 100,000
years.
SEISMIC HAZARD ANALYSIS METIIODOLOGY
The seismic hazard approach used in this study is based on the model presented by Kulkarni
et al. (1979). This approach is similaz to other available seismic hazard analysis models
(Cornell, 1968; McGuire, 1974), but includes some additional features as described below.
The occurrence of earthquakes on a fault is assumed to be a Poisson process. The Poisson
model is widely used and is a reasonable assumption in regions where data are sufficient to
provide only an estimate of average recurrence rate (Cornell, 1968). Whc:n there are
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