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SEO NFIVE SOISmic Hamra UNINS <br /> Several parameters may be used to characterize earthquake ground motions. The common <br /> parameters include: PGA, velocity, and displacement; response spectral accelerations or <br /> velocities; duration; and time histories in acceleration, velocity, or displacement. In this <br /> analysis, we have estimated PGA and horizontal spectral accelerations (SA) for a location at <br /> 39.8661°N. and -106.0997°W. This location is approximately the center of 1 Dam. 3 Dam is <br /> sufficiently close and the site condition is assumed to be the same; thus the hazard needs to be <br /> calculated at only a single location. <br /> 5.1 PROBABILISTIC HAZARD RESULTS <br /> The results of the PSHA are presented in terms of ground motion as a function of annual <br /> frequency of exceedance (AFE). The AFE is the reciprocal of the average return period. The <br /> hazard curves are shown on both log-linear and log-log plots. Figure 1 l shows the mean,median <br /> (50th percentile), 5th, 15th, 85th, and 95th percentile hazard curves for PGA. The fiactiles <br /> indicate the range of uncertainties about the mean hazard. For example, at a return period of <br /> 10,000 years,the difference between the 5th and 95th percentile values is more than a factor of 4, <br /> a large range of uncertainty. The 0.2 and 1.0 sec horizontal SA hazard curves, shown on Figures <br /> 12 and 13,have similar ranges of uncertainty. The PGA, 0.2 and 1.0 sec SA values for selected <br /> return periods are summarized in Table 4. <br /> The contributions of the various seismic sources to the mean PGA hazard are shown on Figure <br /> 14. The fractional contributions to the PGA hazard from the dominant sources are shown on <br /> Figure 15. The WFMF contributes more than 50 percent to the hazard at return periods beyond <br /> 2,000 years, with the Frontal fault contributing between 30 and 35 percent. At shorter return <br /> periods, the background seismicity is the major contributor. The patterns are similar for the 0.2 <br /> sec SA hazard (Figures 16 and 17) and the 1.0 sec SA hazard (Figures 18 and 19), with the <br /> background seismicity contributing relatively less as the spectral period increases. This is not <br /> surprising given the proximity of the faults to the damsites and the relatively high activity rate of <br /> both the WFMF and the Frontal fault. <br /> By deaggregating the PGA. 0.2 and 1.0 sec SA hazard by magnitude (M), distance (D), and <br /> epsilon (e)bins,the contributions by events at range of return periods can be evaluated(Figures <br /> 20 to 28). Epsilon is the difference between the logarithm of the ground motion amplitude and <br /> the mean logarithm of ground motion(for that M and R)measured in units of standard deviation <br /> (a). At a return period of 500 years, the hazard is dominated by magnitude M 5.0 to M 6.5 <br /> within 20 km (Figures 20, 23 and 26). At return periods of 3,000 years and greater, events of <br /> magnitude M 6.5 to M 7.4 within 20 km(Figures 21 to 22,24 to 25,and 27 to 28). As expected, <br /> as the return period increases, the hazard is controlled by larger epsilon ground motions (above <br /> average ground motions for a given event)(e.g.,Figures 20 to 22 for the PGA hazard). <br /> Based on the magnitude and distance bins(e.g.,Figures 20 to 28),the controlling earthquakes as <br /> defined by the modal magnitude M*, distance D*, and epsilon s* and mean magnitude M-bar <br /> and distance D-bar can be calculated. Table 5 lists the M*, D*, 0, M-bar and D-bar for a suite <br /> of return periods for PGA,0.2 and 1.0 sec horizontal SA. <br /> Figures 29 to 31 illustrate the sensitivity of the mean PGA, 0.2 and 1.0 sec SA hazard to the <br /> selection of ground motion models. Each hazard curve shown is calculated using only that <br /> model and one branch of the additional epistemic uncertainty, i.e., assigned 1.0 weight. At PGA, <br /> 20 <br />