SACRAMENTO -- Significant areas on either side of the Ventura-Los Angeles county line could be susceptible to landslides or liquefaction in the event of a strong earthquake, according to five new Seismic Hazard Zone maps released today by the California Department of Conservation.
The communities affected include Oxnard, Camarillo, Thousand Oaks, Malibu and Westlake Village.
During an earthquake magnitude 6.0 or greater, strong shaking causes most of the damage. However, Seismic Hazard Zone maps, produced by DOC's California Geological Survey, show areas at risk from ground failure due to landslides or liquefaction triggered by shaking. The dangerous effects of these secondary hazards exist when there are poor soil or rock conditions present in the subsurface.
Liquefaction was a major cause of damage in the Kings Harbor area of Redondo Beach during the Northridge earthquake of 1994. The Northridge earthquake also caused more than 11,000 landslides, some of which; damaged structures or blocked roads.
Knowing where liquefaction and landslides are most likely to occur means that local officials can require special engineering steps on new construction to make people and buildings safer," Department of Conservation Director Darryl Young said.
City planning and building officials use the maps to identify areas that require site-specific geologic or soil investigations before new development is permitted. Generally, it is more cost effective to build improvements into new structures than to retrofit existing ones. Design changes on new development and large remodeling or restoration jobs can lessen the impact of seismic hazards and better protect life and property in future earthquakes.
With the official release of these maps following a six-month public review, new building permits issued inside the zones will require a geologic study to determine whether the hazard exists on a particular site. If a hazard is found, measures to lessen the impact must be proposed. All property sold in California requires that a natural hazards disclosure form be provided to the buyer prior to sale. Property inside a designated Seismic Hazard Zone will be part of that disclosure, as is the case for property in designated flood or wildfire zones.
Each map covers about 60 square miles. Here's a closer look at what each shows:
Camarillo quadrangle: Liquefaction zones occur west of Spanish Hills Golf Course and south to the coast. A portion of the City of Camarillo north of the Ventura Freeway between North Las Posas Road and Mobil Avenue is zoned. Areas south of Pleasant Valley Road to the Camarillo State University campus and east to the foot of the Santa Monica Mountains also are zoned. Landslide zones occur in scattered areas in the hills north of Las Posas Road and in scattered hillsides south and east of the CSU campus.
Newbury Park quadrangle: Liquefaction zones occur along Pleasant Valley and Santa Rosa Valley south of Camarillo Road, along Hill Canyon Road in Thousand Oaks, and small areas along the Ventura Freeway at Ventu Park Road and Lincoln Oaks Village area near Moorpark Road. Landslide zones occur in scattered areas in the hills north of western Thousand Oaks, in unincorporated areas south of Camarillo Springs Golf Course to Point Mugu State Park, and easterly all the way to the hills of Hidden Valley at Lake Sherwood.
Triunfo Pass quadrangle: Liquefaction zones occur only along the beach areas on this map, and also along the Mulholland Highway in Los Angeles County from Sequit Point to the Camp Bloomfield area. Another small area occurs in the upper Little Sycamore Canyon. Landslide zones occur throughout the map, particularly within three miles of the beach.
Point Mugu quadrangle: Liquefaction zones occur within Sycamore Canyon, in areas along the Roosevelt Highway beach area near the Ventura/Los Angeles County line, and all of the areas west of the foot of the Santa Monica Mountains. Landslide zones occur heavily in the Santa Monica Mountains, except in the La Jolla Valley area.
Point Dume quadrangle: Liquefaction zones occur along the beach areas and into Zuma, Ramirez, and Trancas Canyons. Zones also occur in the upper Triunfo Canyon area at Comell. Landslide zones occur frequently throughout the mountainous areas and on the slopes within three miles of the beach. There are scattered zones in the developed Malibu and Point Dume areas.
DOC/Califomia Geological Survey geologists use computer models as well as analyses of existing geological mapping and hundreds of engineering borings to produce the maps, which are drawn on a scale where one inch equals 2,000 feet.
The Department of Conservation has mapped more than 3,500 square miles in California, including most of Los Angeles, Ventura and Orange counties, and has provided this hazard information to more than 135 cities for their use in planning and issuing construction permits.
Mapping in Southern California is ongoing in Ventura County and is planned for the Lancaster and Palmdale areas of Los Angeles County.
Color maps can be purchased through DOC's California Geological Survey at (213) 239-0878. The maps also can be found on DOC's web site in PDF format.
In addition to its program to identify and map seismic hazards, the Department of Conservation manages California's earth resources through its programs that safeguard farmland and open space; oversee oil, gas and geothermal wells; ensure mined land reclamation; study earthquakes; and promotes beverage container recycling.