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<br /> <br />INTRODUCTION <br />' This report presents the results of a probabilistic seismic hazard analysis of a tailing dam <br />located at the Henderson mine in central Colorado. The purpose of this study i:l to evaluate <br />the level of ground motions associated with the probability or likelihood of being exceeded <br />in a specified time period at the damsite, which is owned and operated by the Cyprus Climax <br />Metals Company. <br />' In this study, both published and unpublished geologic and seismologic data were used to <br />evaluate potential seismic sources, the likelihood of earthquakes of various magnitudes <br />occurring on those sources, and the likelihood of the earthquakes producing ground motions <br />' over a specified level. The probabilistic seismic hazard analysis methodology used in this <br />study allows for the explicit inclusion of the range of possible interpretations in components <br />of the model, including seismic source chazacterization and ground motion estimation. <br />Uncertainties in models and parameters are incorporated into the hazazd analysis through the <br />use of logic trees. The following describes the seismotectonic setting of the clamsite, our <br />approach, the seismic source chazacterization, the attenuation relationships used in the <br />probabilistic analysis of the Henderson mine tailing dam, and the results of the study. <br />SEISMOTECTONIC SETTING <br />Colorado is located in a transition between the seismically active Basin and Range Province <br />to the west and the significantly less active, cratonic Midcontinent to the east. The majority <br />of historical seismicity is located in the westem half of the state (Wong et al., 1994), which <br />' is composed physiographically of the Colorado Plateau and Southern Rocky Mountains. <br />Based on the brief 150-year historical record, the lazgest earthquake known in Colorado is <br />the enigmatic 1882 Richter magnitude (MJ 6'/x earthquake, which may have occurred in the <br />northern Front Range (Kirkham and Rogers, 1981), although its location has beers the subject <br />of considerable debate. The most significant event known to have occurred in western <br />Colorado is the 1960 M~ 5'/: eazthquake near Ridgway which also occurred prior to adequate <br />seismographic coverage (Wong et al., 1994). <br />Significant advancements have been made in our understanding of earthquakes, their rupture <br />' processes, and associated tectonic stresses in westem Colorado because of improved, albeit <br />H:~COMRACn234561.DUPl 1 M0309951509 <br />