NR 2001-75
December 27, 2001

Contact: Carol Dahmen
Mark Oldfield
Don Drysdale
Ed Wilson
(916) 323-1886

New Maps from the California Department of Conservation
Show Most Likely Areas for Liquefaction, Landslides During an Earthquake

SACRAMENTO -- Portions of several Orange County communities are on ground susceptible to landslides or have a potential for liquefaction during a strong earthquake, according to official Seismic Hazard Zone maps released today by the California Department of Conservation.

The communities affected are Alison Viejo, Irvine, Rancho Santa Margarita, San Juan Capistrano, San Clemente, Dana Point, Laguna Hills, Laguna Beach, Laguna Niguel and Mission Viejo.

During an earthquake magnitude 6.0 or greater, strong shaking causes most of the damage. However, Seismic Hazard Zone maps, produced by DOC's Division of Mines and Geology, show areas at risk from ground failure due to landsliding 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 damage 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. 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. Generally, it is more cost effective to build improvements into new structures than to retrofit existing ones.

With the official release of the maps, 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 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.

The Dana Point and San Juan Capistrano quadrangle maps each encompass 60-square mile areas. The Dana Point map extends from Three Arch Bay in Laguna Beach southeast to the City Beach in San Clemente. The San Juan Capistrano map covers an area from north Laguna Hills south to the Ortega Highway at Highway 5 in San Juan Capistrano and from Laguna Beach east to San Juan Creek in eastern San Juan Capistrano and O'Neill Regional Park in Mission Viejo. A description of each map follows:

  • Dana Point: The zones of required investigations for landslides occur over broad areas of the hills east of the San Diego Freeway in San Juan Capistrano and San Clemente, and scattered areas west of the freeway to the beach area in Dana Point and Laguna Beach. Liquefaction Hazard Zones occur along all beach areas in San Clemente, Dana Point, and Laguna Beach. A broad zone occurs along San Juan Creek east of the San Diego Freeway from the beach to Highway 74, and in the valley along Prima Deshecha Cañada Creek from the beach area to the San Juan Capistrano city boundary in San Clemente. A small zone occurs in Laguna Niguel at the El Niguel Golf Course.

  • San Juan Capistrano: The zones of required investigation for landslides occur in widely scattered areas in the hills of northern San Juan Capistrano, Laguna and Mission Viejo, in and around O'Neill Regional Park. Landslide zones are more widespread along the eastern and western border areas of the map where there are natural, undeveloped hill slopes. Liquefaction zones occur in the major drainages along the Interstate 5 corridor from Ortega Highway in San Juan Capistrano north to La Paz Road in Mission Viejo, and from I-5 at Rancho Viejo within Trabuco Canyon through O'Neill Regional Park in Las Flores. Zones also occur from the Coast Highway in Laguna Beach along Aliso Creek northwesterly all the way through Laguna Hills along Alicia Parkway.

Department of Conservation geologists use computer models as well as analyses of existing geological mapping and numerous 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 3,000 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 permitting.

A preliminary map of the San Clemente quadrangle, which covers the remaining portions of San Clemente and San Juan Capistrano, and the Santa Paula quadrangle in Ventura County were also released today for 90 days of technical review. 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 Division of Mines and Geology at (415) 904-7707 or (916) 445-5716.

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 promote beverage container recycling.