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Shaken but Not Stirred: Rock formations reveal past quakes' size limit

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11:58am, March 15, 2006
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Dozens of precariously balanced rocks in southern California tell a story just by standing there: Earthquakes that have occurred on nearby faults in recent millennia haven't exceeded magnitude 7. Researchers developing seismic-hazard maps for that and other areas are pondering how such rocky evidence might best be incorporated into their next round of updates.

The Elsinore and San Jacinto fault zones lie about 35 kilometers apart and extend southeast from Riverside, Calif. In a 5-km-wide, 120-km-long swath centered between these faults, there are at least 60 rock formations that seem as if they'd topple with a modest jolt. Some of these stones are more than 2 meters tall, weigh a metric ton or more, and have been standing for many thousands of years, says Abdolrasool Anooshehpoor, a seismologist at the University of Nevada in Reno.

All the balancing stones are at least 14 km from the nearest fault, a hint that similar rocks closer to the faults have been toppled by past earthquakes, says Anooshehpoor. Field studies suggest that about 40 quakes of magnitude 6.5 or larger have occurred along fault zones in the region in the past 10,000 years, but the standing stones suggest that there's an upper limit to the size of those temblors.

Anooshehpoor and his colleagues estimated the stability of a dozen of the standing stones by analyzing their sizes and shapes. They found that, on average, ground motions with a side-to-side acceleration measuring about 30 percent of that of Earth's gravity would topple the rocks. A magnitude 7 or larger quake on those nearby faults would cause such vibrations, the researchers report in the March Geology.

Current estimates of seismic risk for the region suggest that ground motions large enough to topple the rocks occur, on average, every 2,500 years. However, the presence of so many precariously balanced rocks indicates that such motions are less frequent, says Anooshehpoor.

Previous estimates have included the possibility of large earthquakes occurring on previously unknown faults because other regions have been struck by such quakes, including the magnitude 6.7 quake that struck Northridge, Calif., in January 1994, says Mark D. Petersen, a seismologist at the U.S. Geological Survey in Golden, Colo. The information derived from the balanced rocks suggests that there are no active unknown faults near the San Jacinto and Elsinore faults.

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