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<br /> <br />value of 0.83 for the whole of Colorado. The normalized a-value is -2.33 for the Colorado <br />Rocky Mountain region (Unruh et al., 1995a). Based on these recurrence parameters, the <br />return period for the occurrence of a Mc >_ 6.0 in the whole of the Colorado Rocky Mountains <br />region is about 400 t 200 years. <br />' For the probabilistic study we used b-values of 0.76, 0.90 and 1.04 weighted 0.3, 0.4, and 0.3 <br />' respectively, to represent the mean and one standazd deviation values. The a-value was <br />adjusted for the seismic source zone area used in this study, which was about 31,400 kmZ <br />(based on a 100 km radius circle around each dam site). <br />Earthquake Recurrence <br />The weighted combinations of all of the recurrence models, slip rates, anti maximum <br />magnitudes have been combined for each potentially significant seismic source: to produce <br />the recurrence curves presented in Figure 3. The annual mean number of events is shown <br />,~ exceeding various magnitudes for each source. Because the maximum magnitude is <br />distributed overt 0.3 magnitude range, the highest magnitude for the source is 0.3 higher <br />' than the maximum magnitude shown in Table 1. <br />The slopes of the recurrence curves for the faults were assumed to have the same slope as <br />' the areal source which is dictated by the regional b-value of 0.90. Figure 3 illustrates the <br />contrast in recurrence between the low activity faults outside the Rio Grande rift (e.g., Golden <br />fault) and the relatively active Frontal fault. The multiple bumps in the recurretlce curve of <br />the Mosquito fault are due to the treatment of its individual segments. <br />Ground Motion Attenuation <br />Because detailed site-specific information on the foundation conditions beneath the dam is <br />not available, we have used empirical attenuation relationships appropriate for stiff soil since <br />the dam is built on alluvial gravels. These relationships include those of Boore et al. (1994), <br />Sadigh et al. (1987; as described in Joyner and Boore, 1988), and Campbell and Bozorgnia <br />(1994). These empirical relationships were assigned equal weights in the analyses. <br />i <br />H:\CONIrtACn234561.DUH16 16 M0309951609 <br />