NR 2001-76
December 27, 2001

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

Map Shows Likeliest Areas for Liquefaction, Landslides in an Earthquake

SACRAMENTO -- Portions of the cities of Los Altos, Los Altos Hills, Mountain View, Palo Alto, San Jose, Santa Clara and Sunnyvale are on ground susceptible to liquefaction during a large earthquake, according to a preliminary Seismic Hazard Zone map released today by the California Department of Conservation.

Strong shaking causes most of the damage during an earthquake. However, Seismic Hazard Zone maps, produced by DOC's Division of Mines and Geology, show areas at risk from landslides and liquefaction during an earthquake magnitude 6.0 or greater. These secondary hazards can be dangerous when poor soil conditions exist within 40 feet of the ground surface.

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.

San Francisco's Marina District was hard hit by the effects of liquefaction during the Loma Prieta earthquake of 1989. Liquefaction occurs when water-saturated sandy soil is shaken and, much like quicksand, temporarily cannot support the weight of buildings or other heavy structures. Loma Prieta also caused landslides that blocked two lanes of Highway 17 and damaged residences in the Santa Cruz Mountains.

"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.

The Mountain View quadrangle map covers a 60-square mile area that extends from the Los Altos Hills and Cupertino in the south to the San Francisco Bay shoreline. The "zone of required investigation" for liquefaction covers most of the flat ground that lies east of the Foothill Expressway and north from Highway 280 to the bay, including the Naval Air Station at Moffett Field.

There are narrow bands of liquefaction zones along Adobe, Hale, Permanente and Stevens creeks. Landslide Hazard Zones occur along the steep stream banks in the Los Altos Hills area. For more detailed information, contact your local building department.

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 preliminary map of the Mountain View quadrangle becomes official after a six-month review period. Once the map is official, new building permits issued inside the zones will require a geologic study to determine the level of hazard. 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 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 130 cities for their use in planning and issuing construction permitting. Mapping in the Bay Area is ongoing in Berkeley, San Leandro and parts of Alameda and Santa Clara counties.

Color maps can be purchased through DOC's Division of Mines and Geology at (415) 904-7707 or (916) 445-5716.