Abstract

A spectacular band of precariously and semiprecariously balanced rocks extends from Riverside to near Borrego Valley, California, 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 indication 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 ± 0.10 g (g = 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 motion for M7 earthquakes, but somewhat lower than +1σ 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 deviation of ground motion attenuation curves (especially for hard rock, for which few instrumental data are available), the possible existence of random background earthquakes, and the smoothing distance for historical seismicity.

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