County Cited as Likely Big Quake Site

By KENNETH REICH

May 11, 2001 12 AM PT

TIMES STAFF WRITER

Research by earthquake scientists suggests that parts of Ventura County, especially the area around Piru in the Santa Clara River Valley, are among the most likely sites in California for a major earthquake, possibly exceeding magnitude 7.

The two sites near Piru that are of greatest concern are the San Cayetano and Oak Ridge faults. An extension of the Oak Ridge fault into the nearby San Fernando Valley is believed by some scientists to be the origin of the magnitude 6.7 Northridge quake of Jan. 17, 1994.

A 200-foot-long, 15-foot-deep trench dug in 1999 along the Piru strand of the San Cayetano fault, two miles southwest of Piru, uncovered evidence of a quake as high as magnitude 7.5 in the vicinity sometime since 1660.

Another, secondary area of concern is along the Red Mountain fault north and west of Ventura, where the rate of movement has been high.

The slip rate, or the average annual movement, of the San Cayetano fault over thousands of years has been measured at higher levels than almost anywhere else in California, according to James F. Dolan, a USC quake scientist who oversaw the trench study with Tom Rockwell of San Diego State University.

Meanwhile, delineations of the earth’s shape show a sizable compression, or shortening of terrain, between the north and south sides of the East Ventura Basin near Piru. At a quarter of an inch a year, the so-called geodetic movement leads scientists to believe the accumulating strain will bring forth periodic major quakes in the vicinity.

Andrea Donnellan of the Jet Propulsion Laboratory, who directed the geodetic studies, said the data also show accumulating strain along the Transverse mountain ranges extending from Santa Barbara east through Ventura and Los Angeles counties to Cucamonga in San Bernardino County.

The quake risk is considerable for communities along those ranges, including the San Fernando Valley, Sierra Madre and Upland. Other areas with a high quake risk include the San Francisco Bay Area, the north coast in Humboldt County and the Imperial Valley.

Slip rates measure how fast faults have moved in the past during earthquakes. Around Piru, the slip rates and the geodetic measurements indicate future big quakes are likely. But the data give no indication on when.

In his new book, “Living With Earthquakes in California, A Survivor’s Guide,” Robert S. Yeats, a geoscientist at Oregon State University and an expert on the seismicity of the Ventura County region, says of the San Cayetano, Oak Ridge and Red Mountain faults:

“It seems likely to me that one of these three fast-moving faults is likely to sustain an earthquake in the near future, but I can’t be specific about how long the ‘near future’ is likely to be, or which one of the faults will go first.”

Yeats’ hesitance is common in earthquake science. Scientists can use research tools to identify areas of greatest strain and fastest slip as those where big quakes are ultimately likely.

But they cannot say precisely when these quakes will strike or where they will be centered.

Lucy Jones, scientist-in-charge of the U.S. Geological Survey’s Pasadena office, said it is similar to knowing a region’s general climate but not being certain what the weather will be in the immediate future.

“We know in the long run, Ventura has a pretty high long-term risk for quakes, but we don’t know when they will arrive,” Jones said.

The uncertainty extends even to where and when quakes have occurred in the past.

Yeats and Dolan, leaders of the recent quake research for Ventura County, disagree over the epicenter of a magnitude 8 quake that occurred Dec. 21, 1812.

Dolan believes slippage from the quake is found in the trench near Piru. This was likely the last big quake to hit the region, Dolan says.

Quake Center May Have Been in Channel

But Yeats believes the 1812 quake was centered farther west in the Santa Barbara Channel and would have been felt more strongly in Santa Barbara than Ventura County.

He accordingly sees a “gap” between quakes that have occurred in the past two centuries in neighboring Los Angeles and Santa Barbara counties.

The locale of the 1812 quake, which occurred 13 days after a magnitude 8 quake on the San Andreas fault between Palmdale and the Cajon Pass, could affect how soon enough strain will accumulate to cause the next big quake in Ventura County. If Dolan is right, it might take longer.

But Yeats and Dolan do agree there is a substantial Ventura County risk. And other quake scientists in Southern California agree with them.

For instance, Thomas Heaton, earthquake engineering professor at Caltech, finds the area from the San Fernando Valley north through Ventura County to Santa Barbara has “one of the fastest uplifts of coastal deposits anywhere in the world.”

“The slip rates are high. The geodetic measurements show accumulating strain,” Heaton said. “All this leads to the conclusion that there is a relatively high risk.”

But the scientists caution this does not necessarily mean Ventura County or nearby areas are next up for a big earthquake.

Destructive earthquakes, such as the one in Kobe, Japan, in 1995 that killed 6,000 people, seem frequently to occur in areas where they are not expected.

So any statement that Ventura County is a particularly risky area for large quakes does not necessarily mean the next big one in California will occur there.

It does, however, very likely mean that over the next few thousand years Ventura County will probably be the site of more big quakes than most other counties in California and the Western states.

In his book, Yeats recognizes that over the past two centuries Ventura County has not been the center of major quakes, unlike the adjoining counties of Santa Barbara, Kern and Los Angeles.

“Earthquakes are not part of the culture of Ventura,” he observes. “The great earthquake of 1812 was too far back in time to have any meaning to the people who live there now.”

But, he observes, two centuries is very little time when it comes to earthquakes, certainly too little for anyone to assume their relative absence means that no big ones will occur.

During an early April field trip that Yeats led into Ventura County in conjunction with a Los Angeles meeting of the Geological Society of America, the geoscientist declared:

“Slip rates on the San Cayetano indicate that earthquakes in modern times in this area have not been as big as they’re going to get.”

Outside Piru, as the bus carrying about 30 scientists headed up Piru Canyon Road toward the Santa Felicia Dam, Yeats said there have been proposals to build tract homes in the vicinity.

“Should they be building here?” he questioned. “I have some doubts about that.”

The land immediately surrounding this convergence zone is mainly agricultural.

Dolan, meanwhile, told the field trip participants that a slip rate of a quarter of an inch or more a year on the San Cayetano fault is “the fastest in the lower 48 states, except for possibly the Cascadia subduction zone” in the Pacific Northwest.

Trench Exposed a ‘Surface Rupture’

The trench that allowed Dolan and Rockwell to more precisely calculate the slip rates was funded by the National Earthquake Hazards Reduction Program of the U.S. Geological Survey and was dug in the summer of 1999.

It was excavated across a scarp, or vertical rise created by quake movement, along the San Cayetano fault, not far from the Oak Ridge fault.

The trench has now been filled. But Dolan offered the visiting scientists a chart showing how it exposed what he called “a young surface rupture.”

This means the various layers of rock and earth have been offset, in this case up to 13 feet, by the quake that occurred sometime after 1660. Radiocarbon dating and other techniques are used to calculate how much time has passed since various offsets occurred.

But the calculations are not so precise as to end all dispute about the dates of past earthquakes. Nor is the recurrent interval between quakes--how much time will pass before the next one strikes--clear.

Ideally, says Ken Hudnut of the Geological Survey, “if you measure the slip rate, and the amount of slip on the last earthquake, and you have the date of that quake, then you should know approximately how long it will be before the next quake occurs. Radiocarbon dating can give us approximate figures going back 50,000 years.”

But a $20-million prediction experiment--based, in part, on recurrent interval--went awry a decade ago at the Parkfield site on the San Andreas fault in Monterey County.

Launched with considerable fanfare, Geological Survey scientists said in 1985 that based on past records of quakes of approximate magnitude 6 in the Parkfield area, it was 95% likely a magnitude 6 quake would occur by the start of 1993. But no quake has occurred there yet, diminishing the faith that quake prediction is reliable.

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On Shaky Ground

Recent earthquake science suggests that faults near Piru in eastern Ventura County are among the most likely in California to cause a major earthquake. Scientists say accumulating strain along the San Cayetano and Oak Ridge faults, and compression between the two, strongly suggests that an earthquake exceeding magnitude 7 could strike in the vicinity.

But they can’t say when. Recent trench studies confirming past quakes and observation that the two faults are slowly moving closer mean only that quakes are more likely to occur there than along other major faults.

Scientists have also noted a 200-year gap in significant quakes in Ventura County compared with neighboring counties, an absence some say increases the likelihood that Ventura County is due for one.

Significant quakes near Ventura County

1. San Andreas Fault

Wrightwood, San Bernardino County, Dec. 8, 1812. Magnitude 6.9, ruptured between Palmdale and Cajon Pass.

2. Santa Barbara County

Between Gaviota and Santa Barbara, Dec. 21, 1812. Magnitude about 8.0; some scientists believe this was centered near Piru in Ventura County.

3. San Andreas Fault

Fort Tejon, Jan. 9, 1857.

Magnitude 7.8, ruptured from Cholame to Wrightwood.

4. Santa Barbara County

June 29, 1925. Magnitude 6.3, killed 20. 5. Santa Barbara County

Off Point Arguello, Nov. 4, 1927.

Magnitude 7.5, caused small tsunami, damaged Lompoc.