Seismologist Targets Fault Danger Zones : Earthquakes: UC Santa Cruz researcher unveils technique to estimate where the most severe ruptures will occur. It cannot predict when, however.

A new technique that allows more precise estimates of which parts of long-locked earthquake faults will rupture most severely when a quake does occur was unveiled Friday by a UC Santa Cruz seismologist.

Justin Revenaugh, writing in the journal Science, acknowledged that the technique cannot predict when the quake will happen, or how powerful it may be, but he said it can show where the worst slippage is likely to be found.

“There is [also] abundant evidence that fault bends and jumps play a major role in producing . . . slip variability and frequently serve as the [starting] point or termination point of major earthquakes,” Revenaugh noted.

The scientist’s article made particular reference to the 1992 Landers earthquake. There, fault displacement--the distance that one side of the fault moved in relation to the other--reached over 20 feet in some cases and varied sharply over the 47-mile length of the rupture. Had his method been applied before that quake occurred, there would have been advance knowledge where the slippage would be greatest, he said.

In an interview Friday, Revenaugh explained that he uses instrumental readings taken in Southern California of seismic waves from distant earthquakes--in Japan, the South Pacific and South America--to “light up” the complexities, the strong and weak points, of major Southern California faults.

“You can think of it as sonar used to locate fish at sea,” he said. “It shows the heterogeneity along the faults and then we make the judgments as to whether this shows strong or weak portions.”

It is the weak points where the most slippage occurs.

Revenaugh said that during the next year he plans to use the technique to project comparative slippage along locked portions--areas that have not ruptured for a long time--of two faults: the San Jacinto Fault near San Bernardino and the San Andreas Fault between Cholame in Central California and the Cajon Pass in Southern California.

Those stretches of faults are viewed by scientists as likely locales of great quakes.

By using his technique, it should be possible to say in advance where quakes will be especially intense, because presumably the greatest shaking will occur in the areas where fault displacement will be the greatest.

Earthquake scientists have tried to delineate specific segments of such faults as the San Andreas, believing that most of its quakes are confined to one or two segments, although the most powerful temblors may rip through several. Although the segments have long been observed on the surface, the new technique measures them at depths of up to 10 miles.

The San Andreas segments between Cholame and the Cajon Pass last ruptured in 1857, while segments farther east in the San Gorgonio Pass, the Coachella Valley and the Salton Sea have not ruptured since about 1480. Lengthy ruptures usually involve quakes in the magnitude 7 and 8 range.

Revenaugh said his calculations are also expected to indicate where aftershocks will be especially strong and frequent.

The multiplicity of shorter faults in the Los Angeles area, however, makes the technique considerably more difficult to apply there, the seismologist said.