L.A.'S SOIL LESS PRONE TO BIG QUAKE.

Byline: David R. Baker Daily News Staff Writer

Offering a measure of comfort to people living in fear of the Big One, a study of the Northridge Earthquake has found that the soil beneath Los Angeles shakes less in severe earthquakes than previously thought.

The report to be published today in the journal Nature said the Los Angeles Basin's soil amplifies the shock waves of big earthquakes less than seismologists had predicted. The degree of amplification, in fact, is smaller in major earthquakes than it is in minor ones, according to the report.

``When these waves come into the basin, it's not going to be as extreme as we thought,'' said Tom Henyey, director of the Southern California Earthquake Center at USC, which helped fund the study of the 1994 Northridge Quake and its aftershocks.

``It's good news for people living in the basin,'' he said.

Greg Butler, director of the California Earthquake Authority, said the study could lead eventually to lower insurance rates. But that will depend, he said, on the report's reception among other scientists.

``If it's put forward and accepted by the scientific community, yes, it's something we'll take into consideration,'' he said.

One conclusion is clear: Severe quakes, those measuring at least 6.5 magnitude, still will cause serious damage, regardless of how the ground reacts.

Scientists have known since the past century that sedimentary soil amplifies shock waves from the underlying bedrock during earthquakes. But the report found a dramatic difference in the way the soil reacts to big and small temblors.

In small quakes, the sedimentary soil washed into the Los Angeles Basin and the San Fernando Valley from surrounding mountains quivers like a bowl of Jell-O, shaking violently at the surface but remaining intact, Henyey said. The study found, however, that in severe quakes like the Northridge temblor, the soil acts more like a bowl of sand, with individual pieces moving but relatively little shaking at the surface.

That finding strikes at the heart of a long-running argument between seismologists and engineers, who are charged with building structures that can survive earthquakes. Seismologists, who had studied many small local quakes but few big ones, traditionally argued that the bowl-of-Jell-O model would hold true for all quakes, while engineers favored the bowl-of-sand approach.

``In fact, the engineers were probably more right than seismologists,'' Henyey said.

He doubted, however, that the results would have any influence in the near future on building codes or earthquake insurance rates, which are based in part on soil conditions under homes and businesses.