But it may not be, according to a study published todayin the journal Science. A number of other faults may trigger a major earthquake, but the results of cutting-edge techniques now suggest the northern Hayward Fault isn't likely to be the culprit.

"The likelihood of a large earthquake originating and centering on the northern Hayward Fault alone is rather low," says Roland Burgmann, the University of California, Berkeley, seismologist who led the study. Last year his preliminary results led the U.S. Geological Survey to downgrade the risk on the northern section of the Hayward Fault from 28 percent to 16 percent.

After considering other area faults, scientists with the USGS still assigned the area a 70 percent chance of having a major earthquake in the next 30 years. But while the overall risk remains pessimistic, scientists were excited about the potential of the new techniques to probe earthquake-causing stresses below the earth's surface.

On the surface, the Hayward Fault splits curbs and fences by about half a centimeter a year, but the hidden layers far below may not be keeping up the pace.

If the deep layers lock up under a slowly creeping surface, strain builds until the layers slip violently, producing a major earthquake.

Previously, scientists have only been able to guess at the subterranean fault movements. Burgmann captured a new view of these deeper layers using two new techniques: one to image the shifting surface, and another to locate the rumblings far below.

For five years, a satellite photographed the moving area using a new radar imaging technique called InSAR. Satellites in the Global Positioning System already could measure movement by bouncing radio waves to antennas on the ground, but the antennas were so sparse that the satellites could only map the earth's movements in chunks greater than 5 miles wide. InSAR can image patches tens of meters wide.

"We asked how can we use this method to study new problems, and (the northern Hayward Fault) was right there," Burgmann says.

By combining the GPS and InSAR images, he mapped the shifting surface with unprecedented detail. He then used a computer model to map out how creeping layers deep down might shift the surface. He found that he could most closely reproduce the measured map by moving all of the fault layers together. If all of the layers creep along together, they cannot build up strain to produce a major earthquake.

Burgmann also used another new clue to the movement of the deeper layers: micro-earthquakes. Rough patches along the layers produce tiny rumblings far below the surface, and can help listening seismologists find patches that may be building up stress for a major quake. The scientists are now able to find them more precisely than ever.

"When we take the measurements from the seismograph stations, we have to work backward to tell where the earthquakes are," explains Bill Ellsworth, a seismologist who studies micro-earthquakes for the USGS. "We've developed new ways of locating earthquakes that allow us to get much sharper pictures than we could before."

When Burgmann compares the micro-earthquakes observed along the Hayward Fault with those predicted by a computer model assuming all of the layers are moving together, "the fit ends up being astonishingly good."

But while these new measurements suggested the fault isn't building up any stresses for a sudden jolt, other scientists dug up a paradox in the El Cerritos Golf Course. For 10 days, the greens over the Hayward Fault were closed so geologists with the USGS could dig and study fissured trenches.

"Looking in trenches is a real art form," says Robert Simpson, a USGS seismologist.

By dating cracks in the trenches, geologists were able to determine that the northern Hayward Fault has violently slipped in a major earthquake several times in the past 2,000 years.

If the crack isn't building up any pressure, how can it cause a major earthquake? Some scientists speculate that it didn't cause a major earthquake, but that instead the northern or southern sections of the fault suddenly slipped, jolting the northern Hayward Fault.

Seismologists have not ruled out the possibility that the northern Hayward Fault will slip, but they are excited about the new techniques.

"The hope is that you could see changes in the ground's surface as these faults perhaps got close to an earthquake," Simpson says of the imaging technique. "The new technologies are pretty exciting in that regard."