Abstract

New examples of precarious rocks are presented. These rocks provide constraints on ground motion for historical and recent large earthquakes. An approximate field calibration of the precarious-rock methodology was provided by the M7.1 Hector Mine earthquake of 16 October 1999. Previously documented rocks at Granite Pass near Kelso, California (Brune, 1996), were overturned by the earthquake, and a nearby strong-motion station indicated ground motions of about 0.2g, consistent with the toppling accelerations estimated in the published article. The time of the last earthquake for the section of the fault nearest the precarious rocks has recently been determined to be more than 10 ka, consistent with previously estimated ages for precarious rocks. This first example of an actual earthquake field test lends strong support to the precarious-rock methodology.

The other rocks discussed here give constraints on ground motions for historic earthquakes, such as the 1812 and 1857 San Andreas fault earthquakes, the 1899 and 1912 San Jacinto Fault earthquakes, the 1952 Kern County earthquake, and possibly recent but prehistoric earthquakes on the Banning and Garlock faults. The ground-motion constraints for these earthquakes are lower than predicted by some recent ground-motion attenuation curves (which are extrapolations to near-fault distances from a data set dominated by data from larger distances) but are generally consistent with peak ground accelerations observed from the recent large Turkey and Taiwan earthquakes and provide important additional information for seismic-hazard analysis.

On the other hand, there are areas where precarious rocks would be expected on the basis of previous studies but are apparently not found. This suggests possible earthquakes on previously unrecognized, or only recently recognized, faults. One such area is in northwestern San Diego and southwestern Orange County between the Elsinore and Newport–Inglewood faults. The lack of precarious rocks in this area might be attributed to recent earthquakes on the blind-thrust faults proposed in the area by Grant et al. (1999, 2002) and Rivero et al. (2000).

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