Edition: U.S. / Global

California Quake Smaller Than First Thought

Published: November 8, 1987

The earthquake in Southern California last month was smaller than first estimated, but it also showed that faults hidden by a fold in the earth's crust may make the city more quake-prone than once believed.

''Isn't it possible we can have similar or larger earthquakes under downtown Los Angeles?'' said Clarence Allen, professor of geology and geophysics at the California Institute of Technology. ''The answer is clearly yes.'' Dr. Allen emphasized, however, that because few jolts similar to the earthquake centered near Whittier on Oct. 1 have struck the area in recorded history, he believes the likelihood of more such quakes is relatively low.

Kate Hutton, a seismologist at the university, said analysis of data from seismograph stations throughout Southern California determined the quake measured 5.9 on the Richter scale, not 6.1 as first estimated.

The largest aftershock, on Oct. 4, measured 5.3 instead of 5.5, she said.

The Richter scale is a measure of the energy released by an earthquake. Each point on the scale indicates a tenfold increase in ground motion.

The studies were outlined Wednesday as dozens of scientists met at the University of Southern California for a conference sponsored by the United States Geological Survey to debate geological findings about the recent quakes, which killed seven people and caused an estimated $213 million in damage. Earth's Crust Squeezed

The theory that a large fold in the Earth's crust conceals entirely buried faults is rapidly gaining acceptance and ''means the nearest possible source of a moderate earthquake is closer to downtown Los Angeles than we previously thought,'' said Lucy Jones, a geological survey seismologist. According to research by a private consultant, Thom Davis, the fold is three miles high and was formed three million years ago by the squeezing of two huge plates of the Earth's crust.

One, the Pacific plate, is under the Pacific Ocean and much of Southern California. It slides northwest along the San Andreas Fault past the North American plate, which is under the rest of the continent.

Mr. Davis's theory holds that the squeezing makes folds in the crust just as a rug would wrinkle when its edge was pushed. Traditional theory holds that the squeezing creates only ruptures in the ground surface as large blocks of Earth ride up over each other.

The relatively rapid rate of this squeezing - about one-half inch annually - plus the existence of hidden faults it creates ''suggest that the long-term earthquake potential of the Los Angeles basin may be underestimated,'' Mr. Davis said.

Ross Stein, a geophysicist for the geological survey, said the amount of squeezing suggests that jolts like the Oct. 1 quake will occur every 100 years on faults hidden beneath the fold. Underground Vertical Thrust

A study by Ms. Jones and her husband, Egill Hauksson of U.S.C., showed the main shock on Oct. 1 was caused by vertical thrusting along a previously unknown fault nine miles underground.

Mr. Davis's research suggests that the fault is part of an east-west line of faults that runs 12 miles farther south, well into the Los Angeles Basin, than those previously known to exist.

One previously recognized band of faults passes through Sylmar in the San Fernando Valley just north of downtown Los Angeles, where an earthquake registering 6.7 on the Richter Scale killed 65 people in 1971.

Mr. Davis suggested that a section of those faults running eastward through Beverly Hills, the Hollywood Hills and the hills in which Dodger Stadium sits, are merely the visible expression of the newly discovered fold in the Earth's crust.

The fold includes a series of smaller folds that hide buried faults, he said.

Mr. Stein said his study found the main quake lifted the fold, and thus the elevation of the Whittier Hills, by about two inches.