Age of unstable bedrock landforms southwest of Yucca Mountain, Nevada, and implications for past ground motions

Mark W. Stirling, Janice Ledgerwood, Tanzhou Liu, & Michael Apted

Published 2010, SCEC Contribution #1399

We determine minimum exposure ages for unstable outcrops at three sites in the Amargosa Desert, southwestern Nevada, including a site at the southern end of Yucca Mountain. Varnish microlamination dating techniques provide minimum exposure ages of 12.5-36kyrs for the unstable outcrops of welded tuff, including a 24kyr age for the South Yucca Mountain site. The youngest exposure age (12.5kyrs) is found at the site located only 10km from the Death Valley Furnace Creek Fault, suggesting outcrops may be more frequently modified when close to major active earthquake sources. A simplistic stability assessment of the South Yucca Mountain outcrops suggests peak ground accelerations (PGAs) may not have exceeded about 1g (uncertainty bounds 0.5-2g) in at least 24kyrs. A PGA of 1g is consistent with the predicted 24kyr return period PGAs from the near decade-old Yucca Mountain probabilistic seismic hazard (PSH) model, except for the 95th percentile and above. We gain confidence in our interpretations by additionally observing: (1) minimal damage to the South Yucca Mountain outcrops from a recent moderate earthquake that is estimated to have produced a PGA of less than 0.1g there (i.e. motions less than 0.5g do not significantly damage the outcrops), and; (2) severe damage to similar volcanic outcrops associated with PGAs of the order 0.5-1g near a nuclear blast site from the 1960s. These observations support our suggestion that PGAs greater than 0.5-1g have not occurred at the south Yucca Mountain site for a time period of at least 24kyrs. Significant seismic events that substantially modify the outcrops and produce associated rubblefields may therefore occur on longer time scales.

Key Words
United States, bedrock, paleoseismicity, volcanic rocks, Quarternary, geologic hazards, igneous rocks, statistical analysis, Nye County Nevada, Holocene, Miocene, Cenozoic, pyroclastics, Tertiary, sampling, Neogene, sismic risk, ground motion, risk assessment, probability tuff, Yucca Mountain, earthquakes, Nevada

Stirling, M. W., Ledgerwood, J., Liu, T., & Apted, M. (2010). Age of unstable bedrock landforms southwest of Yucca Mountain, Nevada, and implications for past ground motions. Bulletin of the Seismological Society of America, 100(1), 74-86. doi: 10.1785/0120080359.