Laserfiche WebLink
the Colorado Rocky Mountain region (Unruh et al., 1995a). Based on these recurrence <br /> parameters, the return period for the occurrence of a ML > 6.0 in the whole of the Colorado <br /> Rocky Mountains region is about 590 years. <br /> For the probabilistic study we used b-values of 0.76, 0.90 and 1.04 weighted 0.3, 0.4, and <br /> 0.3 respectively, to represent the mean and one standard deviation values. The a-value was <br /> adjusted for the seismic source zone area used in this study, which was about 31,400 km2 <br /> (based on a 100 km radius circle around the dam site). <br /> Earthquake Recurrence <br /> The weighted combinations of all of the recurrence models, slip rates, and 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 over ± 0.3 magnitude range, the highest magnitude for the source is 0.3 higher <br /> than the maximum magnitude shown in liable 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., <br /> Golden fault) and the relatively active Frontal fault. The multiple bumps in the recurrence <br /> curve of 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 to characterize the attenuation of ground motions from the seismic sources to <br /> the tailings dam in the probabilistic analysis, we have used empirical attenuation relationships <br /> appropriate for stiff soil since the dam is built on alluvial gravels. These relationships <br /> include those of Boore et al. (1994), Sadigh et al. (1987; as described in Joyner and Boore, <br /> 1988), and Campbell and Bozorgnia (1994). These empirical relationships were assigned <br /> equal weights in the analyses. <br /> H:\CONTRACr\TENMILE\16 16 <br /> M0412951500 <br />