Precariously balanced rocks and ground-motion maps for Southern California

Groups of precariously balanced rocks are effectively low-resolution strong-motion seismoscopes that have been operating on solid rock outcrops for thousands of years and, once the methodology has been developed, can provide important information about seismic risk. In one zone, near Victorville, only 30 km from the nearest point on the San Andreas fault, more than 50 precarious rocks have been documented. Widespread rock varnish suggests that many of these rocks have been in their current unstable positions for thousands of years. We have established the mechanical basis for rough estimates of the horizontal accelerations necessary to topple these rocks, using field observations and numerical and physical modeling. To verify that zones of precarious rocks do not occur near historic earthquakes, searches using binoculars were made along roads, with occasional foot surveys, near large earthquakes. Based on these reconnaissance searches, we conclude that no precarious rock zones are found within 15 km of zones of high-energy release of historic large earthquakes. To document the occurrence of precarious rocks in southern California, road surveys were carried out along major roads. Four zones of precarious rocks and seven other zones of somewhat less precarious rocks have been documented. Published probabilistic ground-motion maps for southern California are compared with the occurrence of zones of precarious and semi-precarious rocks. The results are encouraging and suggest that eventually, studies of precarious rocks will provide important constraints on the assumptions on which the maps are based. Results from studies of precarious rocks may eventually provide important information for siting and design of sensitive structures such as hospitals and power plants. Precarious rocks give a direct indication of past ground shaking, in contrast to the indirect inference provided by fault-trenching studies, which may be subject to uncertainties in the actual time history of slip due to the fault (e.g., fault creep, “slow” earthquakes, or unknown dynamic stress drop). It is concluded that precarious rocks warrant further study and quantitative analysis.