This report describes the methodology and results of the use of precariously balanced rocks to study seismic hazard at Yucca Mountain, Nevada, the site of a potential geologic repository for high-level radioactive waste. Precarious rocks are effectively strong-motion seismoscopes that have been in for place thousands of years. Numerous precarious rocks exist in and near Solitario Canyon, Nevada, immediately above the site of the potential repository. Estimates of toppling accelerations using computer models, physical models, and field tests indicate these rocks would be toppled by ground acceleration of a few tenths of the acceleration of gravity (g). Rock-surface age dating using rock varnish layering and cosmogenic age dating indicate that the rocks have been in place tens of thousands of years. Thus, the precarious rock methodology indicates that Yucca Mountain ground accelerations have not exceeded about 0.3 g for the last several tens of thousands of years. These results are consistent with paleoseismic studies of the Solitario Canyon fault that demonstrate the last significant surface offset capable of producing 0.3 g at the surface took place 75,000 to 80,000 years ago. This report presents recent results of testing, calibration, and application of the precarious rock methodology both at Yucca Mountain and in southern California. In southern California, the seismicity rate is much higher than at Yucca Mountain. Thus, the comparison of results of the precarious rock methodology with Probabilistic Seismic Hazard Analysis (PSHA) results is easier and more accurate. This lends confidence to the application of the methodology to Yucca Mountain. Preliminary results from these precarious rock studies suggest: (1) Probabilistic Seismic Hazard Analysis using current procedures gives values that are too high at low probabilities (e.g.,10"4 annual probabilities). This is true both for Yucca Mountain and for southern California. This is a result of unverified statistical assumptions in PSHA procedures. (2) Current ground-motion estimates for the footwall of major normal faults gives values that are too high. This tends to make the PSHA ground-motion estimates for Yucca Mountain too high. (3) Current ground motion estimates for trans-tensional strike-slip faults are too high. This tends to make the PSHA ground-motion estimates for Yucca Mountain too high. (4) Recent preliminary surface-exposure age dates for the pedestals of precarious rocks at Yucca Mountain are considerably older than some previous estimates. This result tends to confirm the conclusion in (1) above. Although the conclusions of this report have not been formally verified by rigorous peer review, they collectively lend strong support to the conclusion that the ground-motion estimates resulting from the Yucca Mountain Probabilistic Seismic Hazard Analysis (PSHA) are too large, perhaps excessively so.
Earthquake hazard analysis; Faults (Geology); Nevada – Yucca Mountain; Paleoseismology; Rocks – Surfaces – Dating
Earth Sciences | Geophysics and Seismology
Brune, J. N.,
Seggern, D. v.,
Smith, K. D.,
Smiecinski, A. J.
Constraints on ground motion at Yucca Mountain provided by precarious rocks.
Available at: http://digitalscholarship.unlv.edu/yucca_mtn_pubs/70