Maps Identify Area’s Seismic Hazard Zones

State geologists released maps this week that for the first time identify six seismic hazard zones in Ventura County and are part of a statewide project to hold all builders accountable to the same engineering standards for earthquake safety.

The maps show areas at risk from the secondary effects of earthquakes in Fillmore, Ojai, Oxnard, Piru, Port Hueneme and the Rincon Beach area north of Ventura. They could clarify seismic conditions for city planners, developers and home buyers on the potential hazards of landslides and liquefaction.

They are part of an ongoing project to identify seismic hazards throughout California, especially in high-density and high-growth areas, said Candace Hill, a land use planner with the California Geological Survey in the state Department of Conservation.

“The bottom line is public safety,” Hill said.

The new maps identify dangers in the event of a magnitude 6.0 or greater earthquake in the six delineated areas of Ventura County. They show stretches of potential landslide zones in the coastal mountains and liquefaction zones from Fillmore to Oxnard, where a quake could turn soil saturated with water into quicksand.

“We’ve been waiting for those maps,” said Fillmore City Manager Roy Payne, whose rural town in the Santa Clara Valley was hard hit by the 1994 Northridge earthquake. “After the first of the year, we’re going to look at them and see what we need to do to respond.”

Although developers who want to build in Fillmore are required to conduct geological tests to determine the potential for liquefaction, the maps will help city officials decide if further steps need to be taken, Payne said.

The maps show that a broad liquefaction zone covers most of Fillmore and extends from east to west through the Santa Clara Valley. Landslide zones were identified north of Fillmore in the Topatopa Mountains.

In Oxnard, which has sandy soil and a high water table, the city requires builders to conduct soil studies and meet certain foundation standards to prevent quake-related damage and injuries, said city Planning Manager Marilyn Miller.

“Essentially, the entire Oxnard Plain is an area of liquefaction,” Miller said.

Liquefaction -- which makes the ground jiggle like Jell-O when water-saturated soil is disturbed by earthquake shock waves -- caused considerable damage in Ventura County during the 1994 Northridge earthquake. It triggered more than 11,000 landslides in Southern California.

Liquefaction zones also were found in rural Piru from the foot of the Topatopa Mountains south to the Santa Clara River. Most of the mountains are potential landslide hazards if an earthquake hits, the geologists said.

The soil in the Upper Ojai Valley also could liquefy if a sizable earthquake hit. There, the zone was found to run along Lion Creek from Summit School in the Upper Ojai west to Dennison Park. The same condition was found east of Ojai along Reeves Road and San Antonio Creek to Camp Comfort county park. A liquefaction zone also covers a 12-block area in northwestern Ojai. The mountain areas around the valley are potential landslide hazards.

The zone most prone to liquefaction in the low-lying Rincon Point area is restricted to the beach and includes the small residential communities of La Conchita, Seacliff and Faria Beach. The mountainous terrain just inland is a landslide hazard, geologists said.

Similar maps released by the state in 1997 for Simi Valley and Moorpark showed some locations there could experience landslides and liquefaction in an earthquake.

In areas where the state identifies a hazard, builders will be required to test the stability of the ground before getting local approval for projects. Those tests will determine whether special anchoring must be added to new homes’ foundations to resist earthquake damage.

Sales of any homes in the zones must be accompanied by full disclosure to the buyer of the seismic hazards. More than 135 cities are using the maps in their permit process.