NR 2003-22
August 14, 2003

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

NEW SANTA CLARA COUNTY SEISMIC HAZARD ZONE MAP OFFICIAL Liquefaction, Landslide Potential Affects New Construction

SACRAMENTO – A new Seismic Hazard Zone map covering part of the city of San Jose and unincorporated hilly land at the southern end of the Santa Clara Valley became official today. The map, issued by the Department of Conservation’s California Geological Survey, impacts planners, developers, property sellers and real estate agents.

If property is located in a Zone of Required Investigation, where liquefaction or earthquake-induced landslides could occur during a large earthquake, the local building department must require geologic studies before projects are issued permits. Also, property sellers and real estate agents must inform buyers if property they're selling is in a Seismic Hazard Zone, as is the case when property is in a designated flood zone.

“This map will help improve public safety by ensuring these earthquake hazards are taken into account during new construction,” DOC Director Darryl Young said.

The Santa Teresa Hills Quadrangle covered in this map includes some significantly developed land between Coyote Creek on the east and Alamitos Creek on the west in the Santa Clara Valley. Highway 101 and the Almaden Expressway run through the quadrangle. The Santa Cruz Mountains lie in the southern half of the map.

The liquefaction zone covers the Santa Clara Valley floor, the lowlands along Coyote Creek, Alamitos Creek and its tributary in the Arroyo Calaro, and the bottoms of several other creek canyons. About 56 percent of the quadrangle lies in an earthquake-induced landslide hazard zone, but virtually all of the zoned land is in the hills and mountains. None is within the Santa Clara Valley and only a few zoned areas are within the Almaden Valley.

Shaking causes most of the damage during earthquakes, and in many cases, it is cost effective to retrofit houses and buildings to minimize damage caused by severe shaking. Local public libraries have a number of publications by the Governor’s Office of Emergency Services, American Red Cross and the Federal Emergency Management Agency that can be used as guides to making homes more earthquake-ready.

Seismic Hazard Zone maps show areas at risk from the secondary earthquake hazards of landslides and liquefaction. It is generally not as cost effective to retrofit an existing building for the impacts of liquefaction or landslides as it is to build in safety features at the design stage. Therefore, design changes to better protect life and property during future earthquakes are required before new developments are approved and built. “It’s easier and less expensive – not to mention better for public safety -- to institute design changes as a precaution in the construction phase than to rebuild after liquefaction or landslide damage,” Young said.

With this new map, the California Geological Survey has issued 91 official Seismic Hazard Zone Maps, 73 for Southern California and 18 for the Bay Area. Another 15 maps are in various stages of public review. Each map covers about 60 square miles.

DOC/California Geological Survey 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.

Color copies of official maps can be purchased through DOC's California Geological Survey at (213) 239-0878, (916) 445-5716, or (415) 904-7707. The maps also can be viewed and downloaded on the Web here.