VOLCANIC HAZARD ASSESSMENT AT THE PROPOSED YUCCA MOUNTAIN HIGH-LEVEL RADIOACTIVE WASTE REPOSITORY, SOUTHERN NEVADA, USA: UNCERTAINTY IN VOLCANIC RECURRENCE RATE

Volcanic hazard assessments for the proposed high-level radioactive waste repository at Yucca Mountain have been ongoing for more than 20 years. These studies are required because six small-volume alkali-basalt volcanoes have erupted within 20 km of Yucca Mountain during the Quaternary. Probabilistic estimates of the hazard to the repository depend on the recurrence rate and spatial distribution of past volcanism in the region. Most estimates of the hazard fall within the range of 10^-7 to 10^-9 disruptions per year, including the results of an expert elicitation sponsored by the U.S. Department of Energy in 1995-1996. Uncertainty in the disruption estimate is dominated by uncertainty in the volcanic recurrence rate, which generally ranges from 10^-5 to 10^-6 volcanic events per year (recurrence intervals of 10⁵ to 10⁶ years). Reasons for uncertainties in estimates of the recurrence rate include (1) differing interpretations of what constitutes a volcanic event (e.g., each individual volcano represents a volcanic event versus volcano alignments that may represent single or multiple events), (2) limited understanding of magmatic or tectonic processes that could impact recurrence rates, and  (3) uncertainty in the number of volcanoes buried in alluvial-filled basins surrounding Yucca Mountain. The potential number of buried volcanoes is primarily interpreted from air and ground magnetic surveys. Approximately 20 magnetic anomalies are currently interpreted as shallowly (<250 meters) buried basalt. Drilling indicates the presence of at least four buried basaltic bodies; the youngest has been dated at 3.8 m.y. using the ⁴⁰Ar/³⁹Ar method. Faulted Miocene tuffs with strong magnetic signals produce complex magnetic anomalies and underlie a significant proportion of alluvial-filled basins, complicating interpretation of magnetic data used to estimate the number of buried volcanoes. The density of possible buried volcanoes present in basins underlain by bedrock of relatively low magnetic intensity can be reliably determined, and if representative of the larger region, indicates that the number of buried volcanoes inferred from magnetic data has not been significantly underestimated.