NR 2001-01
January 16, 2001

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


Maps Indicate Liquefaction and Landslide Zones

SACRAMENTO -- Planning officials, developers and homeowners in three parts of Orange County have a new tool with which to work in preparing for future earthquakes.

The California Department of Conservation today released Seismic Hazard Zone maps covering three 60-square-mile sections of Orange County. The maps detail areas that are susceptible to the secondary earthquake hazards of landslides and liquefaction. They are used to plan for safer buildings to help protect life and property. These maps cover portions of the communities of Anaheim, Orange, Yorba Linda, Irvine, Laguna Hills, Mission Viejo and El Toro. In addition, a revised map was issued for the Tustin area.

Liquefaction occurs when water-saturated sandy soil within 40 feet of the surface is shaken and temporarily acts like quicksand. That was a major cause of damage in the King Harbor area of Redondo Beach during the Northridge earthquake of 1994. The Northridge earthquake also caused more than 11,000 landslides, some of which blocked roads.

``These maps are a tool that can help communities minimize earthquake damage by being prepared," Department of Conservation Director Darryl Young said. ``Local officials can place special engineering requirements on new construction to make buildings and people safer now that we know where these hazards are most likely to occur."

On the map of the Black Star Canyon quadrangle, liquefaction zones are shown along the Santa Ana River at Feathery Regional Park and the Green River Golf Course in East Anaheim and along Wier Canyon Road south of the Riverside Freeway. Liquefaction zones are also found along Santiago Creek, around Santiago Reservoir (Irvine Lake), and in Irvine Park and Fremont Canyon. Landslide zones are shown over much of the hilly area between Feathery Park and Santiago Reservoir.

On the El Toro quadrangle map, which shows part of Irvine and the El Toro area, liquefaction zones are shown along Aliso, Serrano, Oso, Santiago and San Diego creeks; in Borrego Canyon; and in some areas north and east of the Marine Corps Air Station. There are landslide zones north and east of El Toro.

The Prado Dam quadrangle covers part of Orange and San Bernardino counties, but the San Bernardino portion is unmapped. It shows a landslide zone east of Yorba Linda and a liquefaction zone along the Santa Ana River.

The Tustin quadrangle includes portions of Irvine, Tustin and Santa Ana. It was revised to modify the liquefaction zone in two areas. A channel of San Diego Creek east of Jeffry Road was added; in the northeast corner of the map, the upper part of Rattlesnake Canyon north of the Portola Parkway was taken out of the zone. In Irvine, there are liquefaction zones in the Bonita Canyon Drive area south of Aldrich Park, along Macarthur Boulevard. from Ford Road to San Diego Creek, and along San Diego Creek. Also affected are areas east of UC Irvine along Culver Drive, and most of Irvine north of the San Diego Freeway (Highway 405), including Woodbridge. Areas in Irvine susceptible to landslides include the hillsides south of Ford Road and east of Bonita Reservoir as well as some hillside areas east of the Turtle Rock area. Also affected are the slopes of French Hill, within the UC Irvine campus.

Areas in Tustin susceptible to liquefaction include all areas north of the Irvine boundary to approximately Chestnut Avenue, and from Irvine Boulevard at Browning Avenue west to the Santa Ana boundary. Santa Ana is affected from Flower Street east to the Tustin city boundary, from First Street south to the San Diego Freeway.

Geologists from the Department of Conservation's Division of Mines and Geology use computer models as well as analyses of existing geological mapping and hundreds of engineering borings to produce seismic hazard zone maps. Before becoming official, the maps are reviewed by local planning, public works and building permits departments.

Each map covers approximately 60 square miles and is drawn on a scale where one inch equals 2,000 feet. Building officials use the maps to identify areas that need site-specific geologic or soil investigations before permitting new development. Construction sites where the DOC Division of Mines and Geology's investigations indicate landslide or liquefaction hazards are present, can introduce design changes to minimize the impact on new development and large remodeling or restoration projects. With the release of these official maps, property sellers must disclose to buyers if the property is in a state-mapped seismic hazard zone.

Fifty-two maps have now been released, covering more than 120 cities. The effort to identify California's seismic hazards is ongoing. Mapping is ongoing in Southern California in several areas, including in and around San Juan Capistrano, Dana Point, Rancho Margarita, San Clemente, Trabuco Highlands and Coto De Caza.

Black and white copies of the completed maps are available at cost through BPS Reprographics Services of San Francisco, telephone (415) 512-6550. Color copies are available for $12 each from the DOC's Division of Mines and Geology in Los Angeles at (213) 239-0878. The charge for rolled maps shipped in a tube will be slightly higher.

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.