Earthquake: The Long Road Back : Area’s Long-Term Forecast Calls for Big Temblor : Seismology: Experts say county’s numerous faults leave it prone to a major hit ‘sometime in the next 1,000 years.’

The chances of a major earthquake in Ventura County have increased slightly since Monday’s magnitude 6.6 quake that devastated parts of Ventura and Los Angeles counties, quake experts said Tuesday.

“But that chance is very slight,” said Lucille M. Jones, a scientist at the U.S. Geological Survey at Caltech. “Only about 5% or 10% of the time is an earthquake followed by another in the same general area.”

As each day passes, the chances diminish that Monday’s earthquake was merely a foreshock of a greater event to come in the near future, she added.

“In a week or so, the chances that this was a foreshock will be very, very low,” Jones said.

But in the long term--which in geology is measured in centuries or more--Ventura County remains ripe for a major quake, seismologists said, because it is riddled with more than 300 faults.

Seismologists have not pinpointed which fault was responsible for Monday’s quake, which was centered at Northridge about 10 miles east of the Ventura County line.

But they say it may well have occurred on the eastern end of the Oak Ridge Fault, a geologically fast-moving fracture that extends through Ventura County to the ocean.

The Northridge quake did nothing to relieve pressure that has built up during the last 200 years on the Ventura County portion of the fault, Jones said. That leaves what seismologists call the Ventura Basin prone to a major earthquake, she said.

“The Ventura Basin is more vulnerable to an earthquake than other parts of Southern California because it has more active faults,” Jones said. “The Oak Ridge potential (for a major earthquake) is up quite high because it’s a fast-moving fault.”

But not necessarily any time soon, she said.

“It’s geologic time, not human time,” Jones said. “On a geological time frame, that means we’re predicting an earthquake sometime in the next 1,000 years.”

In 1990, scientists reported that mountain ranges north and south of the Santa Clara River Valley are moving together, possibly along the Oak Ridge Fault or the San Cayetano Fault to the north.

In her report, Caltech geophysicist Andrea Donnelian called Ventura County the “second most hazardous area in Southern California next to the San Andreas Fault.”

The mountains are moving together at the rate of about half an inch per year--considered rapid in geological time.

“That’s a pretty high rate of movement,” said James J. Mori, a seismologist colleague of Jones at the U.S. Geological Survey. “If all that movement is being stored in terms of thrust on the fault, it certainly means there is a high potential for a damaging earthquake in the Ventura Basin.”

But Mori and Jones said they need much more information before they could say how quickly the earthquake might occur.

Because the region is laced with about 300 faults capable of producing an earthquake of 6 or greater on the Richter scale, another fault in the region also could produce a large temblor, she said.

Mori said seismologists are not yet certain that the quake occurred along the Oak Ridge Fault because more studies are needed. But when seismologists plot the quake and its aftershocks on a map, the pattern indicates that the Oak Ridge is at least partly responsible.

“It’s our best guess,” Mori said.

Robert S. Yeats, a geologist at Oregon State University who is a former Ojai resident, has studied the area and found that earthquakes relieve pressure along Ventura County faults about once every 200 years.

He said there is no evidence of a major earthquake in the area for at least that long, although there may have been one offshore in 1812.

Ventura County Fault Lines Scientists say the fast-moving Oak Ridge Fault could be the source of a major quake in Ventura County. It is suspected of causing Monday’s quake in Northridge.