Dating precariously balanced rocks in seismically active parts of California and Nevada

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 (CI)-C-36 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 do,vn for at least 10.5 ky., 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.