SEISMIC HAZARD STUDIES AT YUCCA MOUNTAIN, NEVADA

Potential seismic hazards at Yucca Mountain, the proposed site of a geologic repository for spent nuclear fuel and high-level radioactive waste, have been evaluated for ground shaking and fault displacement. This evaluation serves as a basis to define inputs for the seismic design of repository facilities and to assess the repository’s ability to meet regulatory performance goals following closure.

Seismic hazards at Yucca Mountain have been analyzed probabilistically using an expert elicitation process. Based on available data, experts provided interpretations of seismic sources, fault displacement, and ground motion attenuation, along with the uncertainties in their interpretations. To support these evaluations, a suite of site characterization studies has been carried out over the past 20 years. These studies included reconnaissance-level investigations of regional Quaternary faults, detailed paleoseismic analyses of local faults, development and evaluation of tectonic models, seismic monitoring, compilation of an historical earthquake catalog, evaluation of surface faulting at analogue sites in the Basin and Range Province, geophysical surveys, and studies of ground motion attenuation. These data show that regional and local faults have the potential for infrequently generating earthquakes of M 6 to 7+. Recurrence intervals typically range from about 10,000 to 100,000 years. Several active faults occur within 5 km of the repository site.

The ground motion hazard was determined for a reference rock site. Based on these results, a site response analysis was performed to estimate the ground motions at the site of the surface facilities and underground at the depth of waste emplacement drifts. Fault displacement hazard was determined for representative conditions and locations ranging from intact rock to block-bounding faults in the vicinity of the site. Both ground motion and fault displacement hazards are expressed in terms of annual exceedance frequencies. To support long-term performance assessment at Yucca Mountain, annual frequencies smaller than 10^-4 are being addressed.