THE QUAKE THAT WASN'T

By Jeffrey Kluger

You'd think that if the San Andreas Fault went to the trouble of having a perfectly good earthquake, the folks on the West Coast might at least notice. A new study reveals, however, that in 1992, what should have been a china-smashing 4.8 Richter-scale quake hit central California, and yet nobody felt a thing. The explanation for the odd shadow-quake was published last week in the journal Nature and may help improve science's understanding of earthquakes in general.

According to Alan T. Linde, a geophysicist at the Carnegie Institution of Washington and the study's leader, what makes seismic events so destructive is not just that the earth moves but the speed with which it does so. In many quakes the crustal movement that leads to shaking takes only seconds to unfold, sending energy exploding in all directions. But recent analysis of data from strain gauges along the San Andreas Fault reveals that four years ago, a slip occurred that took a week to play out. Such slow sliding all but eliminates an earthquake's quaking.

Exactly what determines the speed with which the earth's plates move is unclear, but scientists have some ideas. "The fault material may play a role," Linde says. "Porous rock containing water can move more smoothly than other rock. The pressure the plates are under can make a difference too; the higher the stress, the likelier the fault will fail suddenly."

Linde's work may never help seismologists determine which type of temblor is likely to strike which region, but he still believes the research has value. There may be no better way of understanding destructive quakes, he feels, than to learn what makes them less destructive.

--By Jeffrey Kluger