In some types of terrain, groups of precariously balanced rocks evolve naturally unless shaken down by earthquakes. The time scale for the evolution and stability of these rocks is of the order of thousands of years. Thus precarious rocks are effectively low resolution seismoscopes that have commonly been in place thousands of years. Study of their distribution, mechanical stability, and geomorphic history can provide important information about the regional variation in seismic hazard and thus make an important contribution to seismic zonation.
Road surveys were carried out in southern California to document eleven zones of precarious and semi-precarious rocks. Zones of precarious rocks do not occur near historic earthquakes, implying that zones which do have precarious rocks have not been recently exposed to strong ground shaking. Recently published probabilistic ground motion maps for southern California have been compared with the occurrence of zones of precarious and semi- precarious rocks. Although all of the maps show some general features which are consistent, for those maps which assume broadly distributed random large earthquakes there are clear discrepancies when calculations are extended to periods of thousands of years. Studies of precarious rocks may provide important constraints on the assumptions which go into producing these maps.
We have established the mechanical basis for estimating of the horizontal accelerations necessary to topple precarious rocks, using field observations and modeling. Some rocks at Victorville and Jacumba, CA, can be toppled with forces corresponding to steady state accelerations of about 0.2 g.
The precarious rock zones near Victorville and Jacumba have been studied using geomorphic and surface-exposure dating techniques. Two separate rock varnish techniques (varnish microlamination and C-14 AMS analyses) together with the Cl-36 cosmogenic isotope technique were used to establish minimum- and maximum-limiting ages, respectively, for the precarious rocks and their underlying pedestal rocks. Preliminary results indicate that the precarious rocks have been in their present positions for more than 10.5 ka. In one case, all three techniques suggest that both the precarious and pedestal rocks are about 21 ka. The oldest dates for exposure of some of the pedestal rocks range between 21 and 52 ka.
The results of our studies are very encouraging and suggest that, with further development of the various techniques involved, the study of precarious rocks has the potential for providing important quantitative constraints on seismic hazard.