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<br /> <br />PROBABILISTIC SEISMIC HAZARD ANALYSIS <br />TROUT CREEK DAM PROJECT <br />NEAR BUENA VISTA, CHAFFEE COUNTY, COLORADO <br />1. INTRODUCTION <br />This report presents results of a probabilistic seismic hazard analysis <br />(PSHA) for the proposed Trout Creek dam located on the east side of <br />the upper Arkansas Valley graben southeast of Buena Vista, Chaffee <br />County, Colorado. The project, as presently envisioned, will include a <br />70-foot high, roller-compacted concrete dam and appurtenant facilities. <br />Existing geologic studies indicate faults on the west side of the upper <br />Arkansas Valley at the base of the Sawatch Range aze seismogenic and <br />capable of producing lazge earthquakes (Richter magnitude, M,,=7t) at <br />an epicentraUhypocentral distance of about 10 km from the dam site. <br />A deterministic seismic hazard assessment, using hypothetical <br />maximum credible earthquakes (MCEs) and widely-accepted attenua- <br />tion relationships, produces unreasonably high peak bedrock accelera- <br />tions at the site. Deterministic studies, however, do not account for the <br />probability of occurrence of hypothetical maximum credible earth- <br />quakes and, thus, aze overly conservative in azeas of low historic <br />seismicity and long recurrence intervals chazacteristic of central <br />Colorado. <br />1.1 OBJECTIVES <br />Earthquakes and related phenomena pose a credible threat to dam <br />safety and aze critical design issues. Moreover, seismic hazards are <br />commonly perceived by the public, especially project opponents, as <br />fatal flaws. Accordingly, the objectives of our probabilistic seismic <br />hazazd analysis were: <br />(1) to compile and synthesize geological, geophysical and seismol- <br />ogical literature; <br />