PRECARIOUS ROCK CONSTRAINTS ON GROUND MOTION FOR HISTORIC AND PRE-HISTORIC EARTHQUAKES

Previous studies of precariously balanced rocks in southern California have focused on rocks which have apparently been in place thousands of years, and thus subjected to ground motion from numerous pre-historic earthquakes. Results of these studies have indicated that the general pattern of the upper limit on ground motion for the last few thousand years is consistent in shape with maps from probabilistic seismic hazard analysis (PSHA), but some important differences in absolute values for particular fault types and overall level were found.

In addition to precarious rocks found in slow weathering types of hard granitic and volcanic materials(which give constraints over thousands of years), some are found in faster weathering materials which give constraints over only the last few to several hundred years. These rocks 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, possible recent earthquakes effecting the Santa Ynez mountains, and possible recent but pre-historic earthquakes on the Garlock fault.

The ground motion constraints for these younger earthquakes are generally consistent with ground motions observed from the recent large Turkey and Taiwan earthquakes and provide important additional information for seismic hazard analysis. The presentation will describe the recent data and give preliminary conclusions regarding ground motion from these recent earthquakes.

On the other hand there are a few 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 blind-thrust earthquakes on the recent thrusts proposed in the area by John Shaw and Lisa Grant.