The system could give tens of seconds of advance notice - enough time to send schoolchildren diving below their desks or to cut the flow of gas through pipelines vulnerable to rupture, scientists said.

Details appear today in the journal Science.

The system would not predict or forecast earthquakes. Instead, it would exploit the staggered way that energy travels from the underground source of quakes to the surface.

The first indication at the surface that a large earthquake has occurred is typically the jolt caused by the arrival of a fast-moving but low-energy wave called the primary, or P, wave. It is followed by the more energetic but slower-moving shear, or S, wave, which causes far more violent shaking.

Richard Allen of the University of Wisconsin-Madison and Hiroo Kanamori of the California Institute of Technology developed a way to determine the location, origin, time and - most importantly - magnitude of an earthquake from as little as four seconds of measurements of the frequency of the energy in the P wave. The system would rely on a network of seismic instruments already deployed across the region.

"If we can detect this P wave and use the information contained in it to estimate the hazard associated with an earthquake, then there is the potential to issue a warning before any significant ground motion reaches the surface," Allen said.

THE FARTHER AWAY FROM THE EPICENTER, THE MORE WARNING

The amount of forewarning would depend on the distance of the sensors from an earthquake's epicenter.

If directly above the epicenter, there would be no time for a warning, since the S wave would arrive almost immediately after the P wave. At 37 miles from the epicenter of a major quake, the system could give a magnitude estimate 20 seconds before the arrival of the S wave and the strong ground motion that accompanies it, Allen said.

A similar system is already in use in Japan, where individual sensors are used to provide early warnings.

Following the 1989 Loma Prieta earthquake that rocked the San Francisco Bay area, seismologists tracked P waves to warn rescue workers on a section of collapsed freeway about oncoming aftershocks. A Nevada company has sold siren devices triggered by P waves to several fire stations in California.

Such a system would allow a utility to quickly cut power to its grid, minimizing but not eliminating damage, said Philip Mo, a structural engineer with Southern California Edison.

With enough warning, fire officials might move vehicles out of their stations to avoid damage from building collapse, said Capt. Stephen Miller of the Orange County Fire Authority.

Other systems in Taiwan and Mexico rely on measurements of the peak ground motion associated with the S wave to relay warnings to locations far from an earthquake's epicenter but still vulnerable to shaking.

DEPLOYMENT OF SYSTEM MAY STILL BE A LONG WAY OFF

Allen and Kanamori used data from past earthquakes to simulate their system. They are now testing it on earthquakes that shake the region to determine if it can provide accurate estimates in real time. There are no immediate plans to develop an actual warning system.

If developed, it would work best if plugged into a region's infrastructure, allowing it to automatically prompt shutdowns of everything from trains to factories during major quakes, Allen said.

The study may settle the question of whether earthquakes of different magnitudes begin in different ways, said Lucy Jones, of the U.S. Geological Survey office in Pasadena. The study suggests they do.

"There's debate whether quakes start differently or if a 6 is just a 2 that doesn't stop," said Jones.

Measuring magnitude is the key to the success of any warning system relying on P waves, said professor Lisa Grant, an earthquake geologist at the University of California, Irvine, who was not involved in the study. Southern California is aswarm with tiny earthquakes that go unnoticed, except by sensors, and cause no damage.

"That's the trick," she said. "You don't want it to trip on a small earthquake P wave.

"You have to detect the P wave, analyze it, estimate magnitude and then, if it crosses a certain threshold, send out a warning. Then the warning has to be received. All of this has to happen in seconds."