NR 2001-38
April 17, 2001

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


Maps Show Likeliest Areas for Liquefaction, Landslides in an Earthquake

SACRAMENTO -- Significant portions of the San Jose area could be susceptible to liquefaction or landslides in the event of an earthquake magnitude 6.0 or greater, according to two Seismic Hazard Zone maps released today by the California Department of Conservation.

These preliminary maps, each of which covers an area of about 60 square miles, become official after a six-month review and comment period. Once they are official, disclosure to the buyer must be made before property in a Seismic Hazard Zone is sold.

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

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.

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.

``These maps tell local governments where they should take extra steps to make people and buildings safer," Department of Conservation Director Darryl Young said. "It is much less expensive to build in features to minimize the potential damage of liquefaction and landslides than to retrofit."

The Milpitas quadrangle map covers an area roughly bounded by Highway 680 on the east to the Alviso salt evaporator ponds, and from the Alameda County line south to the Bayshore Freeway. Most of this area is zoned for liquefaction, the exception being areas just east of Highway 680. There are landslide hazard zones in the hillsides east of Highway 680 from Jacklin Road to the county line.

The Calaveras Reservoir quadrangle map covers an area roughly bounded by Highway 680 on the west, seven miles east to the Cherry Flat Reservoir, and from Alum Rock area north to the Alameda County line.

There are liquefaction zones noted in San Jose just west of Highway 680 in Berryessa, along Penintencia Creek through Alum Rock Canyon, the low-lying area at Tujarcitos Golf Course, Calaveras Valley south of Calaveras Reservoir, and along Arroyo Hondo Creek east of the reservoir. Landslide zones occur throughout hilly areas of northern San Jose and eastern Milpitas from Hamilton Road northwest to the hills north of El Levin County Park. Landslide zones also appear from Hamilton Road north to the Alameda County line east of the Calaveras Reservoir.

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 West San Jose and Mountain View areas, among others.

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

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