NR 2001-56
August 23, 2001

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


Map Shows Likeliest Areas for Liquefaction, Landslides in an Earthquake

SACRAMENTO -- San Jose State University, Santa Clara University, San Jose International Airport, the civic center and the Compaq Center are on ground susceptible to liquefaction in the event of a large earthquake, according to the San Jose West Seismic Hazard Zone map released today by the California Department of Conservation.

Planning 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 during future earthquakes.

``Seismic Hazard Zone maps help communities prepare for earthquakes and thus help minimize damage,'' Department of Conservation Director Darryl Young said. "Now that we know where liquefaction and landslides are most likely to occur in this area, local officials can place special engineering requirements on new construction to make buildings and people safer."

Young noted that it is much less expensive to build in features to minimize the potential damage of liquefaction and landslides at the construction phase than to retrofit.

The preliminary map of the San Jose West quadrangle, which covers an area of about 60 square miles, becomes official after a six-month review period. Once this map is official, disclosure to the buyer must be made before property inside a designated Seismic Hazard Zone is sold, as is the case for property in designated flood or wildfire zones.

Shaking causes most of the damage during an earthquake. Seismic Hazard Zone maps, produced by DOC's Division of Mines and Geology, show areas at risk from landslides and liquefaction -- hazards that also can be dangerous -- during an earthquake magnitude 6.0 or greater.

Liquefaction was a major cause of damage during the Loma Prieta earthquake of 1989, especially in San Francisco's Marina District. It occurs when water-saturated sandy soil within 40 feet of the surface is shaken and temporarily acts like quicksand. Loma Prieta also caused landslides that blocked two lanes of Highway 17 and damaged residences in the Santa Cruz Mountains.

The San Jose West quadrangle map covers downtown San Jose. The southern part of Santa Clara is in the northwestern corner of the map and the entire city of Campbell is in the south-central portion. Parts of the cities of Los Gatos, Saratoga, and Sunnyvale also extend into the southern and western margins of the map.

Liquefaction zones are widespread in Santa Clara from the Bayshore Freeway south to San Jose at Stevens Creek Blvd., and from I-280 at San Carlos St. south to Blossom Hill Road, a mile or more wide along the Guadalupe River. Landslide zones occur only in the La Rinconada Hills of northern Los Gatos.

Department of Conservation 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.

Fifty-one maps covering more than 115 cities are now official. The effort to identify California's seismic hazards is ongoing. Preliminary mapping is ongoing in the Los Gatos and Mountain View areas, among others. The San Jose East map was released in February followed by maps for the Milpitas (directly north of San Jose West) and Calaveras Reservoir quadrangles in April.

Black and white copies of the completed maps are available at cost through BPS Reprographics Services in San Francisco, telephone (415) 512-6550.