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Fault That Triggered LA Quake Poses Danger to Other Areas

February 2, 1995

LOS ANGELES (AP) _ The fault responsible for last year’s deadly Northridge earthquake could cause an even stronger temblor a few miles away in heavily populated southern Ventura County, a study found.

Oregon State University geologists Robert S. Yeats and Gary Huftile said in a study published in Thursday’s issue of the journal Nature that the Northridge fault is a continuation of the Oak Ridge fault system.

The Northridge quake occurred at the eastern end of the 50-mile fault, which extends west into southern Ventura County, the geologists said. The magnitude-6.7 quake killed 61 people, buckled freeways and caused $20 billion in damage in January 1994.

The Northridge rupture occurred at the slowest-slipping area of the fault, ``the one less likely to have an earthquake than the Ventura Basin,″ Yeats said Wednesday from his office in Corvallis, Ore.

``Northridge basically increased the perception of hazard″ on the western end of the Oak Ridge fault, Yeats said.

The western edge of the fault includes the heavily populated areas of Oxnard, Ventura, Simi Valley and Thousand Oaks, which are about 30 to 60 miles from downtown Los Angeles.

Although there are no predictions in the study, Yeats said he expects to see a quake on Oak Ridge or the nearby San Cayetano fault in the next 20 to 30 years.

Slip rates refer to the long-term speed at which one side of a fault is sliding past the other. At Northridge it’s 1.5 millimeters a year; farther west toward the city of Ventura, it’s 5 millimeters a year.

A higher slip rate leads to greater pressure and a greater chance for an earthquake.

Yeats’ and Huftile’s contention that the Northridge fault is part of the Oak Ridge fault has been debated.

Egill Hauksson, a seismologist with the California Institute of Technology, believes the faults are separate.

``Another approach which I prefer is to say the Oak Ridge fault is a fault that’s very steeply dipping, much more vertical than the fault that caused the Northridge earthquake,″ Hauksson said. ``I think of them as two separate geologic faults.″

Although the western section of the Oak Ridge fault hasn’t had a major earthquake in 200 years of record-keeping, it could generate one bigger than Northridge’s, Yeats said.

``Based upon other settings similar to this, we can have earthquakes much larger than a 7 on this fault,″ he said.

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