Geologists Say Simi Fault May Be Active

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 last 1,200 to 8,000 years. Thus, it may rumble again.

That means it could 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,” said 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.

The designation of a fault as active is the responsibility of state geologist James F. Davis, who in many instances has been very deliberate in reaching such a conclusion.

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 somewhere between 6.5 and 7.0. He also believes the fault is “slipping” at a rate between one-half millimeter to a millimeter a 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.”

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.

“We’re talking about really old material here,” said Tom Blake, a geological consultant.

“This is the first documentation of [geologically] recent activity in Simi Valley” on this fault, said Blake, who works for Fugro West Inc. “Previously, that determination has only been made in Camarillo and the Tierra Rejada Valley. This is pretty important from that standpoint.”

Indeed, if the fault meets state activity criteria, geologists would draw up an earthquake fault zone map marking 500-foot buffers on either side of the shear. After about a year of review by the public and other geologists, that map would influence development and property sales around the fault.

Anyone wishing to build in the fault zone would have to conduct further geological studies before grading. People wishing to sell a home or land in the zone would have to notify prospective buyers of the fault.

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

Calleguas’ Bard Reservoir is a little more than three miles from the fault line--which makes some residents around it edgy.

Kendall’s good feelings had everything to do with the fault’s magnitude predictions: Scientists believe that Calleguas’ 10,000-acre-foot reservoir can withstand up to a 7.0 earthquake. And other scientists say that the fault probably can’t muster more magnitude than that. “It’s a big crack,” Kendall said of the fault. “My career’s in that crack.”