EARTHQUAKE HAZARDS AT YUCCA MOUNTAIN, NEVADA

Probabilistic seismic hazard analyses (PSHA) were performed to evaluate the ground shaking and fault displacement hazards at the potential geologic repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain. The PSHA results are being used both for developing the seismic design of preclosure facilities and for evaluating the postclosure performance of the repository. The PSHA inputs were provided by 6 teams of 3 experts each for seismic source and fault displacement characterization (SSC) and by 7 individual experts for ground motion (GM) characterization.

Approximately 20 faults within ~20 km of the site and up to 36 regional faults (out to ~100 km) were included in the SSC experts’ characterizations. The most significant local faults included the Bow Ridge, Paintbrush Canyon, Solitario Canyon, and Stagecoach Road faults, which bound major crustal blocks at Yucca Mountain. These faults are characterized by low slip rates of approximately 0.001–0.05 mm/yr and recurrence intervals of 10,000 to more than 100,000 years. The experts’ maximum magnitudes (Mmax) ranged from M 5.7–6.8. The most significant regional faults were the Death Valley and Furnace Creek faults with slip rates of 2.5–8.0 mm/yr and Mmax of M 7.0–7.5. Areal source zones were also characterized to account for background seismicity not associated with known faults and faults not specifically characterized by the experts; their parameters were derived from the historical seismicity record. Mmax in the range of M 6.0–7.5 were assigned to these zones. These Mmax were reduced to M 5.0–6.6 in the site vicinity because of the detailed characterization of faulting

The GM experts estimated ground motions and uncertainties for a matrix of magnitudes, source-to-site distances, and faulting styles based on empirical data and numerical ground motion modeling. Attenuation relationships were derived from regressing on these ground motions. Based on the experts’ input, the probabilistic hazard for ground shaking was calculated. Several approaches to characterize fault displacement potential were also developed by the SSC experts based primarily on paleoseismic observations of faulting at the site. Based on the experts’ input, the probabilistic hazard for fault displacement was calculated at several points near and within the repository footprint.