Assessment of the Site Conditions of Precariously Balanced Rocks in the Mojave Desert, Southern California

Abstract

We examine whether groups of precariously balanced rocks in the Mo- jave Desert, southern California, may have remained standing for thousands of years because anomalous site conditions prevent the rocks from toppling during large earth- quakes. Measurements of peak ground velocity at bandwidths of 0.5-1, 1-2, 2-4, 4-8, and 8-16 Hz for 56 earthquakes (ML 3.5-4.8) at two sites of precarious rocks are compared with velocities recorded for the same earthquakes by three TRI- NET* stations located on engineering rock (National Earthquake Hazard Reduction Program (NEHRP) site class B). We find that the residuals (the logarithm of the ratio of the amplitudes recorded at the precarious-rock sites to the TRINET amplitudes for the same earthquake and epicentral distance) at frequencies less than about 4 Hz are negative (i.e., deamplification of 50%-250%), whereas the residuals are slightly positive at the higher frequencies (i.e., amplification of up to 25%-50%). High- frequency ground motions (e.g., peak ground acceleration) may therefore be slightly amplified at the precarious-rock sites, which means that site conditions do not appear to explain the existence of the precarious rocks in areas where high peak ground accelerations are predicted in recent probabilistic seismic-hazard (PSH) models. This discrepancy between the precarious rocks and the PSH models should be urgently resolved.