By Sid Perkins
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.