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Amid Ruins, Clues to Deep Mysteries

January 19, 1994|ROBERT LEE HOTZ | TIMES SCIENCE WRITER

Amid a cacophony of distant sirens, helicopters and sputtering gas mains, Caltech earthquake geologist James F. Dolan was taking testimony from the bowed picket fences, whip-snaked foundations and buckled sidewalks north of the epicenter of Monday's 6.6 earthquake.

The evidence he collected told a story of how in a pre-dawn instant the full force of the Northridge quake surged up from a point 10 miles beneath the ground to break the surface almost two miles north, in the front yards of a few suburban households in Granada Hills. His findings also seem to indicate that a complex web of several faults--rather than a single one--were involved in Los Angeles' largest temblor in modern history.

This is urban geology on the fly, the art of reconstructing seismic mechanisms forever hidden deep in the earth.

From cracks in the asphalt, broken pipes and crumbled paving stones, Dolan identified what appeared to be a half-mile string of cracks and buckles several blocks north of the Simi Valley Freeway and one block south of the Mission Hills fault.

As far as anyone could tell so far, it was the only point where the earthquake literally surfaced.

In the widespread rubble left by the violent pre-dawn seismic spasm, the distinctive zig-zag imprint was easily overlooked. But Dolan, picking his way through debris in a battered pair of Italian hiking boots, had no trouble tracing the rupture, following the telltale signs like a trail of small crumbs dropped in the suburban wilderness.

In a gaping hole in the middle of Balboa Boulevard, it was evident to Dolan that gas mains and water pipes had been shoved together, crumpled and then jerked apart. Water bubbled from the gas main and the heady reek of gas spewed from the water main.

Two blocks away on Amestoy Avenue, the action of the rupture shoved Cindy Carter's house off its foundations and displaced one end of the hedge by her front door two feet to the west. Nearby, in front of Harold O'Hallorn's modest ranch house on Flanders Street, it buckled the sidewalk and popped the curbstones.

Dolan surveyed the site and told the two homeowners: "This is where all the energy of that earthquake came to the surface. I think you guys are at the proverbial ground zero.

"The upshot of all this," Dolan said, "is that the back of the house on that side of the street and the back of that house across the street are a foot closer."

Based on preliminary analysis, the evidence seems to suggest the quake involved the Mission Hills Fault, which runs north of the 118 Freeway. The easternmost portion of the extensive Oak Ridge fault system, which runs west to the sea, was the actual source of the vertical thrust that generated the quake, seismologists believe.

This implicates a new portion of the intricate pattern of faults underlying the Los Angeles basin.

At the Caltech Seismological Laboratory on Tuesday, scientists said the aftershock pattern also could involve portions of the San Fernando and Santa Susana fault zones. They even held out the possibility that last week's swarm of earthquakes in Santa Monica could have been precursors to the Northridge quake.

They believe the actual stretch of the Oak Ridge fault system responsible for the Northridge quake is an offshoot called the Frew Fault, which apparently has not been active for half a million years.

Caltech seismologist Kerry Sieh described the 20-million-year-old Oak Ridge Fault in the San Fernando Valley as very steep, capable of rupturing in a quake as strong as Monday's temblor about every 200 years.

Seismologists Egill Hauksson of Caltech and Ken Hudnut of the U.S. Geological Survey cautioned that all of the ideas about what faults may have been involved in Monday's quake were still only informed speculation.

It may be months before scientists and earthquake experts like Dolan can assemble their educated guesses, insights and seismic data into anything resembling a complete picture of what happened so deep beneath surface in a few seconds Monday morning.

In the end, scientists hope that the information will give them an important insight into the inscrutable mechanisms driving the Pacific Plate past the North American plate and elbowing Los Angeles on its slow journey north past San Francisco to Alaska.

Earthquakes caused by thrust faults, like the one thought to be responsible for the Northridge quake, are especially difficult to analyze because they rarely cause any rupture on the surface.

Compared to the dramatic fissures left in the aftermath of other earthquakes, the narrow rupture Dolan mapped Tuesday is no more than a snag in the earth's fabric. But to geologists trying to penetrate the veil of asphalt and concrete that normally masks clues to the dozens of faults buried deep beneath the Los Angeles basin, it may be invaluable.

"All these faults are paved," Dolan said. "In order to get access to the earthquake history normally, we have to dig up city streets."

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