Quake Studies Bode Well for East County : Seismology: New investigations show that Conejo Valley has no major active faults and that most of Thousand Oaks lies above bedrock.

New seismic studies in Thousand Oaks confirm previous findings that eastern Ventura County is less likely than neighboring areas to suffer grave damage in earthquakes, geologists reported last week.

The most recent seismic study was commissioned by Amgen, a biotechnology firm in Thousand Oaks, and disclosed when company officials described expansion plans at a special session with the City Council last week.

“From the studies that came out, this is a pretty stable area,” said Bruce B. Wallace, the head of Amgen’s environmental health and safety group.

However, warned Scott Simmons, an engineering geologist with the geotechnical consulting firm Gorian & Associates in Thousand Oaks, “I wouldn’t classify Thousand Oaks as being safer than another area.” Much depends on where a quake’s epicenter is and what type of building one is in during the quake, he said.

The Amgen study was conducted by Seismoview Information Services in Alhambra. Another recent study produced a similar finding--that the Conejo Valley has no major active faults, Amgen officials said.

The studies also found that most of the city lies above bedrock, not the softer, more volatile sandy soil that forms the floor of the San Fernando Valley and the flat areas of Ventura County, Wallace said.

Scientists recently announced the discovery of a potential new fault zone running alongside the Ventura Freeway through Thousand Oaks and into Camarillo. It is still not known whether the South San Fernando Fault, as it is called, is active.

The studies’ findings are backed by geologists who have studied the Ventura County area for years.

Thousand Oaks is sandwiched between the Malibu Fault to the south, which parallels the Pacific Coast Highway, and the Oak Ridge Fault to the north, which runs along the Santa Clara River between Fillmore and Santa Paula.

Both are active faults that could trigger earthquakes of magnitude 7 on the Richter scale, said Gary Hufgile, a geologist at Oregon State University, who has studied the Ventura County area. However, Hufgile added, earthquakes set off by either fault would not break the ground in Thousand Oaks or snap bridges, though they might shake the area quite a bit.

“I treat the whole area--Thousand Oaks, Simi Valley, Moorpark--as a kind of stable block in the middle” of the two active faults, he said.

Thousand Oaks is crisscrossed by faults that geologists consider inactive. Simi Valley is bisected by the Simi Santa Rosa Fault, an active fault that scientists say is probably a more serious threat at its western end in the Camarillo area than in Simi Valley.

East Ventura County is also a more stable region than the neighboring San Fernando Valley, if for no other reason than sheer distance from the San Andreas Fault. Geologists expect the fault to rupture sometime within the next 100 years, producing a strong earthquake that, if its epicenter were near Los Angeles, could wreak havoc in the San Fernando Valley. East Ventura County, however, would only feel some shaking, Simmons said.

Another advantage of the Thousand Oaks area is that most of it lies above layers of sedimentary bedrock, which absorbs seismic shock waves better than softer soils.

Portions of San Francisco and San Diego rest on soft, muddy soils that make them especially vulnerable during powerful earthquakes, said Thomas F. Blake, a geotechnical services manager with Fugro-McClelland in Ventura.

When an earthquake rumbles through bedrock, those on the ground above feel short, sharp jolts. With the softer alluvial soils, the shock waves come in rolling motions that are less severe but last longer, geologists say.

Bedrock is usually found in mountains or hilly areas, whereas alluvium covers flatlands such as the center of Thousand Oaks or the floor of the San Fernando Valley. Volcanic--or Conejo blue--rock stretches across the Santa Monica Mountains from Calabasas to the range’s western end at Point Mugu. It is the densest type of bedrock and generally the safest during an earthquake, Blake said.

Most homes and businesses south of the Ventura Freeway rest on Conejo blue rock, said Buck O’Shea, an inspector supervisor for Thousand Oaks. Though volcanic soil does extend north of the freeway at some points, he said, most of the hills in northern parts of Thousand Oaks are made of another kind of sedimentary rock called the Modelo formation.

Softer than Conejo blue, the Modelo formation was formed by ancient sediment deposits at the ocean bottom, Sessions said. Ten to 20 million years ago, Thousand Oaks was under water, he said. Through deposits over the millennia, some of the sediment may have settled into relatively unstable layers, Sessions said, and strong shaking or heavy rainfall could start landslides on the earth above. However, he added, bedrock like the Modelo formation is still safer than the alluvial soil that coats most of the flatlands in the area.