Oct. 13, 2005 — Large earthquakes have long been suspected — but rarely convicted — of triggering lesser quakes near and far, and now a theory is starting to explain how they do it.

What matters most is how "loud" the seismic trigger shock is at a distance from the epicenter, rather than which particular seismic note a main shock sings through the ground.

What's more, the lab findings have been backed up with a look at historical earthquakes that have seemed to have triggered distant smaller quakes. Reports on both studies appeared in the Oct. 6 issue of Nature.

"It's a good step," said seismologist Susan Hough of the U.S. Geological Survey.

Hough has been puzzling over remotely triggered earthquakes for some time and said she welcomed any good, scientific attempts to get to better handle on them.

Among the things that might be hard to grasp is the idea that fault-zone materials respond in rather unpredictable ways, or "nonlinearly," to seismic waves coming from other quakes, said Paul Johnson, a co-author of both Nature papers and a geoscientist at Los Alamos National Laboratory.

In Johnson and his colleagues' experiment, they used a container filled with glass beads to simulate the ripped-up, granular material inside a fault gouge — the zone where earthquake ruptures happen. They put the beads under pressure and measured how they responded to seismic-like shocks.

They discovered that the materials didn't react to the frequency of the seismic vibrations, but were weakened as a whole when hit by shocks exceeding certain seismic strengths, comparable to the "loudness" of the seismic waves.

The sudden weakening at key amplitudes, instead of gradual weakening as amplitudes steadily increased, is why they describe the triggering as nonlinear, Johnson explained.

The laboratory experiment also showed that faults likely to be most susceptible are those that are already close to slipping, said Johnson, who conducted the experiments with Xiaoping Jia in France at Université de Marne-la-Vallée. The research was funded by France's Centre National de la Recherche Scientifique and the U.S. Department of Energy.

To see if the laboratory finding could be applied to real earthquakes, Johnson teamed up with the U.S. Geological Survey's Joan Gomberg to see which sorts of historical quakes seem to be triggering other tremors.

They compared records from a number of remotely triggered quakes and found that the most important factor was amplitude, not frequency, just as in the laboratory.

"There are a number of models that have been proposed," said Hough. But few have been able to link laboratory results with what's seen in the ground. "So anything you can do to nail down part of that is really helpful."