BIG NUMBERS FOR L.A.; 7.6; STUDIES FIND TRACES OF MAJOR QUAKES.

Byline: Jane E. Allen Associated Press 

Geologists have dug up evidence that two extremely powerful earthquakes struck only 12 miles from what is now downtown Los Angeles within the past 15,000 years, suggesting the region is far more vulnerable than previously believed.

``This is direct evidence that earthquakes in the region have been substantially larger than ever before documented - and suggests they could happen again,'' said Charles Rubin, a geologist at Central Washington University and author of the study in today's issue of the journal Science.

The quakes happened along the Sierra Madre Fault, with the more recent one taking place about 10,000 years ago. Both quakes ranged in strength from magnitude 7.2 to 7.6, the researchers said.

The scientists found traces of the quakes by digging 18 feet into the ground in Altadena, 12 miles northeast of downtown Los Angeles. Radiocarbon dating of charcoal found during the dig allowed them to estimate when the quakes took place.

``Until our recent excavation we had no clue that an earthquake of a magnitude 7.5 had ever occurred in this region,'' Rubin said.

Such quakes are far stronger than the deadly 1994 Northridge Earthquake that rocked Los Angeles, and the location on the Sierra Madre Fault is much closer to millions of people living in the nation's most populous county.

Also, while the Northridge Quake hurled much of its force northward, away from the metropolitan region, a quake of magnitude-7 or greater on the Sierra Madre Fault would rupture southward, directing energy toward the densely populated Los Angeles Basin, the scientists said.

The Northridge Quake struck with a magnitude of 6.7, killed 57 people and caused an estimated $40 billion in damage. It happened along a different and previously unknown fault about 25 miles northwest of downtown Los Angeles.

A quake of the sort detected by the geologists ``would probably be the biggest event that this country has ever experienced in a major urban area,'' said Thomas Henyey, director of the Southern California Earthquake Center at the University of Southern California.

``Confirmation of these sorts of earthquakes really requires that policy-makers, engineers, urban planners seriously take note and ask the question, Are there things we can do to help safeguard property and lives should an event like this occur, even though it may be a fairly rare event?''

James F. Dolan, an earthquake geologist at the earthquake center, called the new work ``convincing, direct proof'' of ``something we have all been thinking within the earthquake community for the past five to 10 years.''

``If anyone had any doubt that faults around metro Los Angeles were capable of producing earthquakes much larger than the Northridge Quake of 1994, this should erase that doubt,'' Dolan said.

The Sierra Madre-Cucamonga fault system runs for about 60 miles along the southern edge of the San Gabriel Mountains and northern edge of the heavily developed San Gabriel and San Fernando valleys.

Paul Somerville, an engineering seismologist in Pasadena who specializes in seismic designs for critical structures such as dams and bridges, expressed concern for the National Aeronautics and Space Administration's Jet Propulsion Laboratory and hospitals in the San Gabriel foothills, as well as freeway bridges.

But Somerville added: ``To be fair, the thing to emphasize is these big earthquakes are infrequent and so the likelihood we will actually live through one of these earthquakes is quite low.''

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