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Cover Story : Accurate to a Fault : Seismology: Scientists are slowly unraveling the complicated puzzle of quake activity in the San Gabriel Valley. Their high-tech tools might show which systems to focus on.

March 03, 1994|BERKLEY HUDSON and VICKI TORRES | TIMES STAFF WRITERS

For decades, the San Gabriel Valley has been home to much of the nation's earthquake re search. And although the 6.8 Northridge temblor caused only modest damage locally, its deadly upward punch refocused attention on the theory that serious seismic activity is continuing in the Los Angeles Basin--much of it increasingly centered in or near the San Gabriel Valley.

Within the past seven years, four of six earthquakes in the basin registering 5.0 or larger were centered in the San Gabriel Valley or nearby Upland, researchers say.

"We've gone from about zero activity to a lot of activity," said Richard Morton, a geologist with the U.S. Geological Survey at UC Riverside. "That's what's really troublesome."

Geologists and seismologists sprang into action when the Jan. 17 temblor struck Northridge, killing at least 57 people causing more than $20 billion in damage. Since then, they have busied themselves in the San Fernando Valley, setting up portable seismographs, measuring ground movement and examining cracks in the earth.

Their findings about the Northridge quake could affect how scientists view the faults and earthquake activity 20 miles to the east in the San Gabriel Valley. The research "might change our picture of how these (San Gabriel Valley) faults interact together," said Lucile M. Jones, a U.S. Geological Survey seismologist. "It might tell us which are more important than others."

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Scientists have been working for years to unravel the horrendously complicated puzzle of faults in the San Gabriel Valley and much of the Los Angeles Basin. The valley's fault system is far more complicated than the notorious San Andreas fault zone just over the mountains to the east, seismologists say.

"It's not clear how the pieces fit together," said Jones, the seismologist well-known to local television viewers for her savvy translations of what all the shaking means.

Those pieces include active faults like the 60-mile-long Sierra Madre, considered the biggest threat in the valley capable of generating a 7.5-magnitude shock. Shorter, less-threatening faults such as the Raymond, Cucamonga, Whittier, Eagle Rock, San Jose and Clamshell Saw Pit also thread through the valley.

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Complicating the issue are faults such as the 50-million-year-old San Gabriel Fault. The San Andreas of its day, the only energy the San Gabriel generates now is controversy. Scientists debate whether movement 11,000 years ago on this fault means it still should be considered active, Morton said.

At the other end of the activity scale is one of the more recently discovered faults, the Elysian Park Fold and Thrust Belt. A thrust occurs when one side of a slanting fault moves closer to its parallel side.

The Elysian Park belt, skirting the San Gabriel Valley's southwestern edge, was discovered when it emerged as the prime suspect in the Whittier Narrows quake.

Between the giant lines of these faults are webs of smaller faults, not significant enough to even warrant names.

Buried and unseen faults become known, as did the Northridge fault and the Elysian Park belt, only when an earthquake shakes the earth, Morton said.

Like the Northridge fault, the Elysian Park Fold and Thrust Belt is a blind thrust, meaning the fault line cannot be seen on the surface. Small hills on the surface may give some idea of the existence of blind thrusts underneath, but because their location cannot be pinpointed, scientists may have no knowledge of these faults until an earthquake hits. With the same seismic energy as other faults, these may not be more dangerous but do carry the added punch of surprise.

"These buried systems are very important criteria," Jones said. "We can have a 6.6 anywhere in Los Angeles. . . . You can understand them, but basically you can't get the extent of them until an earthquake happens."

And earthquakes are always happening. Usually their magnitude registers below 3, the level at which seismologists begin studying quakes as major seismic events. From 1900 to 1920, seismologists saw little need to study the Los Angeles Basin, which enjoyed relative seismic quiet.

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But the following two decades--1920 to 1940--saw a cluster of temblors. Big quakes hit the southern end of the county, including a 6.4-magnitude tremor in Long Beach in 1933. Residents as far north as Monterey Park still recall pianos rolling out their back doors.

Then followed 30 more years of quiet, until a 5.2 quake rumbled through the San Gabriel Mountains in 1970. The temblor was noted mainly by scientists because of its remote location on Lytle Creek. But the following year, nearly everyone took notice when the 6.4-magnitude Sylmar quake killed 58 people and caused $511 million in damage.

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It was the only quake this century to rupture the surface of the Los Angeles Basin, a rare event because earthquake fissures typically occur one to 12 miles below the surface.

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