Age of Unstable Bedrock Landforms Southwest of Yucca Mountain, Nevada, and Implications for Past Ground Motions

Abstract

We determine minimum exposure ages for unstable outcrops at three sites in Amargosa Desert, southwestern Nevada, including a site at the southern end of Yucca Mountain. Varnish microlamination dating techniques provide minimum exposure ages of 12.5–36 k.y. for the unstable outcrops of welded tuff, including a 24 k.y. age for the south Yucca Mountain site. The youngest exposure age (12.5 k.y.) is found at the site located only 10 km 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 1 g (uncertainty bounds 0.5–2 g ) in at least 24 k.y. A PGA of 1 g is consistent with the predicted 24 k.y. return period PGAs from the near decade-old Yucca Mountain probabilistic seismic hazard (PSH) model, except for the ninety-fifth 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 0.1 g there (i.e., motions less than 0.5 g do not significantly damage the outcrops); and (2) severe damage to similar volcanic outcrops associated with PGAs of the order 0.5–1 g near a nuclear blast site from the 1960s. These observations support our suggestion that PGAs greater than 0.5–1 g have not occurred at the south Yucca Mountain site for a time period of at least 24 k.y. Significant seismic events that substantially modify the outcrops and produce associated rubble fields may therefore occur on longer time scales.