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1 Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557
2 Seismological Laboratory, University of Nevada, Reno, Nevada 89557
3 Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964
4 Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721
5 Global Change and Climate History Team, U.S. Geological Survey, Reno, Nevada 89557
Precariously balanced boulders that could be knocked down by strong earthquake ground motion are found in some seismically active areas of southern California and Nevada. In this study we used two independent surface-exposure dating techniques—rock-varnish microlamination and cosmogenic 36Cl dating methodologies—to estimate minimum- and maximum-limiting ages, respectively, of the precarious boulders and by inference the elapsed time since the sites were shaken down. The results of the exposure dating indicate that all of the precarious rocks are >10.5 ka and that some may be significantly older. At Victorville and Jacumba, California, these results show that the precarious rocks have not been knocked down for at least 10.5 k.y., a conclusion in apparent conflict with some commonly used probabilistic seismic hazard maps. At Yucca Mountain, Nevada, the ages of the precarious rocks are >10.5 to >27.0 ka, providing an independent measure of the minimum time elapsed since faulting occurred on the Solitario Canyon fault.
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