An Active Past

To the untrained eye, the site behind the Simi Valley Drive-In looks like nothing special: a spot along the Arroyo Simi where muck meets rock.

To a geologist, however, the shear along the stream embankment is something quite significant--a fault line where very old bedrock slides against young clay.

Even more important, earthen evidence recently found in this segment of the Simi-Santa Rosa fault indicates something that earthquake watchers have long suspected but lacked the data to prove: The Simi segment of the fault has almost certainly been active sometime in the past 1,200 to 8,000 years. Thus, it is likely to rumble again in the future.

That means it could well fall under state criteria that have a profound effect on people seeking to develop land or sell houses near the fault line.

“This fault will probably be zoned by the state as an active fault,” explained William R. Lettis, one of three geologists whose study of the Simi fault is being funded by $30,000 from the Southern California Earthquake Center at USC.

Lettis and fellow geologists Chris Hitchcock and Jerome A. Treiman unveiled their preliminary findings to other scientists, consultants and local and state government officials Wednesday.

So far, Lettis said, it looks as though the Simi fault could generate an earthquake with a maximum magnitude between 6.5 and 7. He also believes the fault is “slipping” at a rate between 0.5 and 1 millimeter per year.

But there is much more to learn.

“We don’t know how many events have happened,” Hitchcock added. “We don’t know if there have been multiple earthquakes. We don’t know when the most recent earthquake was. Those are very important answers we need to find out.”

The geologists estimate they will need another year of study before they can pinpoint those answers.

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To show off what they have already found, they led observers down a narrow, metal ladder and through the reeds, rocks and sand of the Arroyo Simi to peer into the soul of the fault.

Standing in front of a vertical earthen grid, state geologist Treiman pointed out various blue plastic markers near carbon deposits used to date the sediment on either side of the fault.

Tom Blake, a geological consultant who has worked for Ventura County and the cities of Camarillo and Simi Valley, knew exactly what he was seeing.

“Look at this!” he said. “We’re talking about really old material here--millions of years old--juxtaposed with materials that are very young--only thousands of years old.”

He leaned over and pointed to a slick spot a few feet above the arroyo’s muddy waters. “It got here by this up-down separation.”

Blake, who works for Fugro West Inc., quickly assessed the findings.

“This is the first documentation of recent activity in Simi Valley” on this fault line, he said. “Previously, that determination has only been made in Camarillo and the Tierra Rejada Valley. This is pretty important from that standpoint.”

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Indeed, if the fault meets state criteria for activity, geologists would draw up an earthquake fault zone map marking 500-foot buffers on either size of the shear. After review by the public and geologists, that map would influence development and property sales around the fault. The process, however, is well behind schedule.

If an official map is drawn up, anyone wishing to build in the fault zone would have to conduct further geological studies before grading. People who want to sell a home or land in the zone would have to notify prospective buyers of the existence of the fault.

While the prospect of labeling the fault active might make developers shudder, the presentation actually relieved Don Kendall, general manager of Calleguas Municipal Water District.

Calleguas’ massive Bard Reservoir is more than 3 miles from the fault line--which makes some neighbors of the ritzy neighborhoods around it edgy.

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Kendall’s good feelings had everything to do with the fault’s magnitude predictions. Scientists believe Calleguas’ 10,000-acre-foot reservoir can withstand up to a magnitude 7 quake.

Now other scientists are saying that the fault probably can’t muster anything bigger than that.

“It’s a big crack,” Kendall said of the fault. “My career’s in that crack.”

The Simi fault is just one segment of the Simi-Santa Rosa fault, a system that starts west of Camarillo’s Spanish Hills neighborhood. It snakes eastward through Las Posas Hills, across the Tierra Rejada Valley and skirts the northwest corner of Simi Valley. At its eastern end, the fault parallels the town’s northern edge and the Santa Susana Mountains toward the Marr Ranch neighborhood.

Some scientists believe it may be tied to the bigger Oak Ridge fault system, one of Ventura County’s largest faults.