CALTECH STUDY WARNS OF QUAKE RISK; DOWNTOWN FAULT LINE COULD PRODUCE NORTHRIDGE-SIZE TEMBLOR.

Byline: David R. Baker Daily News Staff Writer

A fault line beneath downtown Los Angeles could unleash a powerful earthquake rivaling the 1994 Northridge temblor, according to a study released Tuesday.

Geologists at the California Institute of Technology said they have new evidence that the fault, which runs from Hollywood to Whittier Narrows, could produce a 6.5- to 6.8-magnitude earthquake once every 1,000 to 3,000 years.

Exactly when the fault could produce its next quake, however, is impossible to predict, scientists said.

Striking some of the city's oldest buildings, such an earthquake would be devastating, producing far more damage than the deadly 6.7-magnitude Northridge temblor, researchers concluded.

``It would almost certainly be more destructive than Northridge, just because the San Fernando Valley doesn't have as much building stock,'' said Caltech professor Kerry Sieh, co-author of the study.

The Valley would likely feel just a fraction of the quake's force, Sieh said.

``You'd probably be sending your fire trucks downtown, not the other way around,'' he said.

Geologists have long known about the fault, buried 10 miles deep beneath the hills north and east of downtown. The same fault system produced the Whittier Narrows Quake of 1987, a 5.9 temblor that killed eight people.

The new study, however, pinned down just how much the fault and the fold of earth above it are moving.

By examining sedimentary rocks within the fold, Sieh and Caltech graduate student Michael Oskin found that the fold - and the neighborhoods built on it - are rising about an inch every 25 years, while the fold contracts about a half-millimeter each year.

Some of those neighborhoods are filled with older buildings, reinforced with concrete, which could collapse under the shaking of a Northridge-size quake, said Caltech professor Thomas Heaton.

``Those motions would occur where there are dozens or even hundreds of these buildings, and that could be catastrophic,'' said Heaton, who teaches geophysics and engineering.

Even steel-reinforced buildings, six- to 20-stories tall, could suffer serious damage, Heaton said. Effects of such a quake on downtown's skyscrapers are harder to predict, he said, and would depend more on how far the ground moved during the temblor than on the strength of shaking.

The next step for researchers, he said, will be to determine whether a large quake on this fault could trigger simultaneous temblors elsewhere in the Los Angeles area's vast tangle of fault lines, combining forces to create an even larger temblor.