Band of precariously balanced rocks between the Elsinore and San Jacinto, California, fault zones: Constraints on ground motion for large earthquakes

Abstract

A spectacular band of precariously and semiprecariously bal- anced rocks extends from Riverside to near Borrego Valley, Cali- fornia, about halfway between the Elsinore and San Jacinto fault zones. The rocks are distributed in a band a few kilometers wide midway between the San Jacinto and Elsinore fault zones, an in- dication that the distribution is caused by attenuation of strong ground motion from numerous large events along these two fault zones. These rocks have apparently been in place for thousands of years, and thus place important constraints on ground motions from earthquakes. On the basis of field tests and photographic analysis, the estimated quasi-static toppling accelerations for the precariously balanced rocks are ;0.32 6 0.10 g (g 5 gravitational acceleration). The dynamic toppling accelerations are within this range for earthquakes with waveforms similar to those recorded during the Izmit and Denali earthquakes. These constraints are roughly consistent with the median predicted value of ground mo- tion for M7 earthquakes, but somewhat lower than 11s ground motion curves, and much lower than the values from the 2% in 50 yr hazard maps. The evidence from these rocks has importance for some of the assumptions that go into calculating probabilistic seismic hazard assessment, including median and standard devia- tion of ground motion attenuation curves (especially for hard rock, for which few instrumental data are available), the possible exis- tence of random background earthquakes, and the smoothing dis- tance for historical seismicity.