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The attenuation relationships represent median values of the peak ground motions. It is <br /> important in the probabilistic analysis to incorporate the uncertainty in the predicted <br /> acceleration value for any given earthquake magnitude and distance. This uncertainty was <br /> included by using the lognormal distribution around the median values defined by the <br /> standard deviation. <br /> SEISMIC HAZARD RESULTS <br /> The probabilistic seismic hazard was calculated for peak horizontal acceleration for the <br /> Tenmile dam at the Climax mine. The results of the hazard analysis are presented in terms <br /> of the annual number of events exceeding the peak acceleration. The annual number of <br /> events is the reciprocal of the average return period. Figure 4 presents the computed mean, <br /> 5th, 16th, 50th, 84th, and 95th percentile peak acceleration hazard curves for the site. The <br /> mean peak horizontal accelerations for return periods of 50 to 5000 years are shown in Table <br /> 2. Algermissen et al. (1990) have produced maps of probabilistic peak horizontal <br /> acceleration for the U.S. for return periods of 500 and 2500 years. Their seismic sources <br /> are typically areal source zones with the recurrence based in large part on the historical <br /> seismicity record. The maps are produced on a national scale so they do not include the <br /> level of seismic source characterization detail that would generally be included in a site- <br /> specific probabilistic seismic hazard analysis. For the study region, the national maps show <br /> a peak horizontal acceleration for most of Colorado of 0.02 g for a return period of 500 <br /> years and about 0.05 to 0.07 g for 2500 years. These values are about a factor of 4 to 5 <br /> lower than the values computed in this study. The principal reason for this difference is <br /> most likely the absence of active faults as seismic sources in the Algermissen et al. (1990) <br /> maps and a lower recurrence for the areal source zone. <br /> The contributions of the various seismic sources to the mean peak acceleration hazard are <br /> shown in Figure 5. Because of its proximity to the Tenmile dam, the Mosquito fault is the <br /> dominant contributor to hazard at return periods greater than 1000 years (Figure 5). At <br /> shorter return periods, the background seismicity of the Rocky Mountain source zone <br /> contributes most to the peak acceleration hazard at Tenmile dam because of the low slip rates <br /> of the faults in the study region (recurrence intervals exceeding the return periods of <br /> exposure). <br /> H:\CONTRACT\TENMILE\17 17 M0412951500 <br />