Research overturns quake theory

By MICHAEL WOODS
Pittsburgh Post-Gazette
03/19/2004

BARCELONA, Spain -- The catastrophic San Francisco earthquake struck 98 years ago. The most devastating slip in Missouri's New Madrid fault shook Midwestern and Eastern states, including Pennsylvania, in 1812.

It's been a long time. So does that mean the next quakes on these faults are drawing nearer?

Exactly the opposite might be true, according to new research findings that challenge a widely accepted belief about earthquakes.

The late Dr. Charles F. Richter, inventor of the Richter scale for measuring earthquakes, often gets credit for the notion, "The longer it has been since the last one, the closer it is to the next one." But studies by Spanish and U.S. scientists are shaking up Richter's idea.

"The longer we've been waiting for the big one, the longer it will take for it to occur," said Dr. Alvaro Corral of the University of Barcelona. "Although it sounds counter-intuitive, that's what our findings show."

Corral's new study on the elapsed time between earthquakes appears in the current issue of Physical Review Letters, a renowned physics journal. As a physicist, he's an outsider among the community of seismologists engaged in earthquake prediction.

Dr. Yan Y. Kagan, a noted geophysicist at the University of California at Los Angeles, said he was unfamiliar with Corral's research but that he and several colleagues at UCLA have reached similar conclusions from their own studies.

Collectively, such findings challenge a leading method of forecasting earthquakes. Sometimes called the "seismic cycle" model, it assumes that the risk of big quakes follows a regular pattern.

Quakes occur because big plates of rock that underlie Earth's surface move fractions of an inch each day. The plates collide and slide past or under one another. This movement builds up stress in cracks, or "faults," in the surface. The strain eventually is released in a sudden slip, with the fault widening, moving and shaking the ground for miles around.

The seismic cycle model assumes that the risk is lowest right after a big earthquake, when the stress on a fault has been released. The stress then builds over the years in a way that allows general predictions about when the next big quake will occur.

But Kagan and his associates have found that earthquakes actually ignore the model and don't follow any regular pattern that might mirror the buildup of stress along fault lines.

Corral's latest study bolsters that idea. He analyzed data from global earthquakes since the 1970s.

Corral said his findings might help improve earthquake forecasts and ease unnecessary tension among people in earthquake zones.

(Distributed by Scripps Howard News Service, http://www.shns.com.)