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1 <br />1 <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />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 azound 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 />Henderson tailing dam. The results of the hazard analysis are presented in terms of the <br />annual number of events exceeding the peak acceleration. The annual number of events is <br />the reciprocal of the average return period. Figure 4 presents the computed mean, 5th, 16th, <br />50th, 84th, and 95th percentile peak acceleration hazard curves for the site. Th.; mean peak <br />horizontal accelerations for return periods of 50 to 5000 years are shown in Table 2. <br />Algermissen et al. (1990) have produced maps of probabilistic peak horizontal acceleration <br />for the U.S. for return periods of 500 and 2500 years. Their seismic sources ;tre typically <br />areal source zones with the recurrence based in large part on the historical seismicity record. <br />The maps are produced on a national scale so they do not include the level of seismic source <br />characterization detail that would generally be included in asite-specific probabilistic seismic <br />hazazd analysis. For the study region, the national maps show a peak horizontal acceleration <br />for most of Colorado of 0.02 g for a return period of 500 years and about 0.05 to 0.07 g for <br />2500 years. These values aze about a factor of 4 to 5 lower than the values comiputed in this <br />study. The principal reason for this difference is most likely the absence of active faults as <br />seismic sources in the Algermissen et al. (1990) maps and a lower recurrence For the azeal <br />source zone. <br />The contributions of the various seismic sources to the mean peak acceleration hazazd is <br />shown in Figure 5. The Golden, Chase Gulch, Leadville and Sawatch faults an: not shown <br />' because they do not contribute significantly to the hazard at the damsite. At thl; Henderson <br />tailing dam, the Colorado Rocky Mountain Bounce zone is the dominant contributor to peak <br />acceleration hazard at short return periods, less than 600 years (Figure 5). This is because <br />of the low slip rates of the faults in the study region (recurrence intervals exceeding the <br />return periods of exposure). At longer return periods of 1,000 years or more, the northern <br />' H:\CON7RACT234561DUP17 17 MO109951609 <br />