Ventura County a Likely Site for a Big Quake, Scientists Say

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 the subject of greatest concern are the San Cayetano and the Oak Ridge faults. An eastern extension of the Oak Ridge into the 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 great as magnitude 7.5 in the vicinity sometime after 1660.

Another, secondary area of concern is along the Red Mountain fault just north and west of the city 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 those 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.

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 what are known as 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 all along those ranges, including the San Fernando Valley, Sierra Madre and Upland. Other areas in California with a high quake risk include the San Francisco Bay Area, the North Coast in Humboldt County and the Imperial Valley.

Slip rates are measurements of how fast specific faults have moved during periodic earthquakes. Around Piru, both the slip rates and the geodetic measurements indicate that future big quakes are likely. But the data give no indication of 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, writes 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. The scientists can use existing research tools to identify the areas of greatest strain and fastest slip as those where big quakes are ultimately likely.

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

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

For instance, Yeats and Dolan, the leaders of the recent quake research for Ventura County, disagree over the location of the epicenter of a magnitude 8 quake known to have occurred Dec. 21, 1812.

Dolan said he believes slippage from that quake is found in the trench near Piru. But Yeats said he believes the 1812 quake was centered farther west, in the Santa Barbara Channel, and would have been felt more strongly in Santa Barbara than in Ventura County.

The locale of the 1812 quake could affect how soon enough strain accumulates 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 a wide range of other quake scientists in Southern California agree with them.

For instance, Thomas Heaton, earthquake engineering professor at Caltech, says the area from the San Fernando Valley 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, often seem 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 mean, however, 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 last two centuries, Ventura County has not been the center of major quakes.

“Earthquakes are not part of the culture of Ventura,” he writes. “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 no big ones will occur.

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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.