Distribution of precarious rocks at the Nevada Test Site: Comparison with ground motion predictions from nuclear tests

Abstract

Precarious rocks have been proposed as ground motion seismoscopes that have been in place thousands of years, thereby providing important constraints on seismic hazard estimates. Numerous precarious rocks exist in the region of large nuclear explosions at Nevada Test Site (NTS). Ground motion recordings from nuclear tests have provided regression equations for ground acceleration and ground velocity as a function of distance and explosion yield for comparison with constraints from precarious rocks. Very near large NTS explosions, there are no precarious rocks or semiprecarious rocks. Cliff faces are shattered and numerous recent rockfalls have fresh white surfaces covered by caliche: proof that the rockfalls have been caused by the explosions. As the distance from the explosions increases and the contours for predicted ground motion decrease, fewer and fewer rockfalls are observed, some semiprecarious rocks begin to appear, and finally, numerous precarious rocks are found with no evidence of recent rockfalls. To quantify the calibration of precarious rocks and ground motion we (1) do field tests on the rocks to determine the quasi‐static and dynamic toppling accelerations, (2) carry out computer tests using estimates of waveforms from NTS explosions, and (3) compare the results with ground motions predicted by empirical regressions for nuclear explosions. We conclude that these comparisons provide an important validation of the precarious rock methodology and an approximate validation of preliminary estimates of toppling accelerations.