Balancing boulders

Earthquake- free regions may be pinpointed by studying the number of delicately poised rocks that dot them

Published: Saturday 15 June 1996

PRECARIOUSLY perched rocks, a sight common to trekkers, can actually be a repository of clues pointing to the vulnerability of a particular region to earthquakes. Geologists who until recently seem to have overlooked the fact are now trying to get to the bottom of these perched rocks. These insecurely placed rocks may have maintained their posts for centuries, or even longer. Their importance has now risen to a level where trekkers are advised not to give in to the impulse of trying to topple them over.

The connection between the delicately poised rock and earthquakes is simple. Any seismic activity would result in the toppling of these rocks, and if there are a number of such undisturbed rocks in an area, one can safely presume that no earthquakes have occurred in the surrounding area for a period of time. Recently, James N Brune and John W Bell of the University of Nevada at Reno, US, along with some colleagues, decided to infer the likelihood of an earthquake from these rocks. Working at various sites in the south- west of the US, the team is trying to estimate the amount of earthquake- induced motion such rocks can with- stand before taking the plunge. They have also tried to find out the length of time for which the rock has been in its present position, after its separation from the parent rock ( Scientific American, Vo117, No 4).

Examining the microscopic changes occurring on the rock surface is one method of checking how long the boul- der has rested. In dry climates, it is com- monto find rocks encrusted with a clay- rich coating called 'varnish', which also contains organic matter. Carbon dating of this organic matter helps determine the length of time the boulder has been perched thus. Another method involves cosmic rays -swiftly moving particles that rain down from the sky in a steady stream, and create distinctive kinds of atoms when they irradiate common minerals. Measurement of these atoms, called 'cosmogenic isotopes' helps calculate the length 9f time the rock has been exposed to the forces of nature.

In California, near Victorville and Jacumba, Brune found rocks that had been in place for the last 10,000 years, which tottered with a slight push, indicating an earthquake-free zone.

Not all geologists are, however, taken in with the new technique. According to Klaus H J acob, a seismologist at Columbia University's Lamont- Doherty Earth Observatory in the us, there are many aspects to take into consideration when trying to determine the amount of seismic shaking an area must have undergone from the shape and position of the rocks. The mathematics involved for such measurements is extremely complex. He feels before taking the results of this study seriously, it should first be conducted in areas which have experienced recent earthquakes.

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