Rocking Rocks; April 1996; Scientific American Magazine; by Schneider; 2 Page(s)

What adolescent hiker has not been tempted to knock over a boulder that is perched insecurely by the side of the trail? With one quick shove, over goes a rock that may have maintained itself in an upright but vulnerable position for centuries--perhaps thousands of years. It seems that good reason now exists to resist the impulse. Researchers have started to use such "precarious rocks" to help them determine whether a particular area may be prone to earthquakes.

The basic premise of the technique is straightforward: seismic shaking can easily topple delicately poised rocks; hence, finding such rocks undisturbed indicates that no earthquakes have occurred close by. The reasoning is elementary; however, until now, few geologists have ever attempted to quantify the relation between unstable rock formations and earthquake ground motion.