ADVANCES MADE IN EVALUATING SEISMIC HAZARDS AT THE YUCCA MOUNTAIN SITE



Seismic hazards were recognized at the onset of the Yucca Mountain site characterization as a significant issue that needed to be addressed. The resulting study conducted in the 1990’s was and still is the largest and most comprehensive seismic hazard evaluation ever performed for a site. Because of the complex seismotectonic and geologic setting of Yucca Mountain and the requirement that the underground facilities needed to safely contain radioactive waste for tens of thousands of years, numerous advances in evaluating both ground shaking and fault displacement hazards were needed and developed. The most significant advances were: (1) Probabilistic hazard analyses were performed following the Senior Seismic Hazard Analysis Committee guidelines using individual and teams of experts. A strong emphasis was placed on the quantification of epistemic uncertainty. The Yucca Mountain analyses provided not only a template for future studies but also motivated further refinements in the use of expert judgment. (2) Several short, closely-spaced faults around Yucca Mountain have recurrence intervals of 10 to >100 ky. Characterizing such low activity faults led to several innovations in paleoseismic techniques and age dating. (3) These local faults also posed a fault displacement hazard that required the first-of-its-kind probabilistic fault displacement hazard analysis (PFDHA) . Methodologies and empirical models for characterizing both primary and secondary faulting were developed specifically for Yucca Mountain. Most PFDHAs today have their roots in the Yucca Mountain methodology. (4) To develop ground motion inputs for preclosure design and postclosure performance evaluation, an innovative site response analysis approach was performed to accommodate the effects of the depth of the underground facilities and the subsurface geology. (5) Because of the very low annual exceedance probabilities (down to 10-8) that needed to be considered for postclosure, the estimated hazard needed to be conditioned for extreme ground motions (> 3 g peak ground acceleration). This was accomplished by utilitizing innovative geologic observations of the underground, laboratory rock testing, and numerical modeling of rock mass deformation. The Yucca Mountain experience motivated subsequent studies of extreme ground motions.