NR 2006-11
April 17, 2006

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


SACRAMENTO – The California Geological Survey (CGS) today released three preliminary maps showing areas in and around the South Bay with potential for earthquake-induced landslides and liquefaction. These Seismic Hazard Zone maps designate areas where new development and construction must implement special precautions to protect life and property in the event of a large quake. The maps will become official after a 90-day public review period.

“Once we define the areas with the highest potential for these hazards, it’s relatively simple and cost-effective to take pre-construction measures to minimize the risk to life and property,” said State Geologist John Parrish, head of CGS, a branch of the state’s Department of Conservation.

Communities covered by the maps include Morgan Hill, Menlo Park, Palo Alto, and Redwood City. Each map covers an area about 62 square miles in size known as a “quadrangle.”

Earthquakes of magnitude 5.5 or greater can trigger liquefaction or landslides. Liquefaction occurs when water-saturated, sandy soil is violently shaken and temporarily loses its ability to support structures. Liquefaction caused underground gas pipes to rupture in San Francisco’s Marina District during the 1989 Loma Prieta earthquake; the leaking gas fueled a big, hard-to-extinguish fire. The temblor also produced landslides that blocked Highway 17 for days. The Seismic Hazards Mapping Act was passed the year after Loma Prieta.

Seismic Hazard Zone maps are aimed at new development, as beginning construction with safety features in mind is usually easier and more cost effective than retrofitting an existing building for the impacts of liquefaction or landslides. The maps create Zones of Required Investigation. For development within a zone, the local building department must require -- before permits are issued -- that a registered geologist investigate the site for evidence of liquefaction or landslide potential. If such evidence is found, design modifications must be made in the planning stage. Examples of these modifications include deep foundations in liquefaction zones or slope stabilization in landslide zones.

Property sellers and real estate agents must inform potential buyers if property they're selling is in a seismic hazard zone, as is the case when property is in a designated flood zone.

“As the centennial anniversary of the Great San Francisco Earthquake reminds us, big quakes are inevitable in California,” said Chuck Real, the Supervising Geologist who heads the Seismic Hazards Zonation Program for CGS. “The good news is that new construction within our zones will be better prepared for the next ‘big one’ than it would otherwise have been.”

For those in existing homes or structures, the Governor’s Office of Emergency Services, American Red Cross and the Federal Emergency Management Agency have all released publications about becoming more earthquake-ready, which should be available at local public libraries.

The Seismic Hazard Zonation Program has identified 345 California communities as high-risk areas for liquefaction and/or landslides; 167 have been zoned. There are now 112 Seismic Hazard Zone maps covering all or portions of eight counties. Upon the official release of the newest three maps, CGS will have completed 22 seismic hazard zone maps in the greater Bay Area.

A closer look at what the new maps show:

Mt. Sizer Quadrangle: Covers southeastern Santa Clara County and consists almost entirely of the rugged terrain of the Diablo Range, except for a few square miles of flat land in the southwest corner partially occupied the city of Morgan Hill. The rest of the quadrangle consists mainly of sparsely populated land.

The map designates Zones of Required Investigation for liquefaction and earthquake-induced landslides zones only for the southwestern half of the quadrangle because mountainous terrain and projected land use in the northeastern half make urbanization unlikely. Zones for the liquefaction hazard generally are limited to a few narrow canyon bottoms and stream valleys, such as Packwood and Hoover valleys. Zones for earthquake-induced landslides, however, cover most of the area subject to seismic hazard zonation because of the predominance of steep slopes combined with relative low rock strength.

Palo Alto Quadrangle: Covers eastern San Mateo County and northwestern Santa Clara County. Most of the lower half of the map area consists of rough terrain of the Santa Cruz Mountains while most of the northern half is occupied by Santa Clara Valley and salt evaporation ponds at the south edge of San Francisco Bay. Included in the study area are the cities of Atherton, East Palo Alto, Los Altos, Los Altos Hills, Menlo Park, Palo Alto, Portola Valley, Redwood City, San Carlos, and Woodside.

Most of the area covered by zones for liquefaction lies within Santa Clara Valley in the northern third and along the northeastern margin of the quadrangle. The zone encompasses large parts of the cities of Palo Alto, East Palo Alto, and Redwood City. Less extensive zones for liquefaction encompass all of Portola Valley as well as most of Francisquito, Los Trancos, Madera, and lesser stream canyons and valleys in the Santa Cruz Mountains. Earthquake-induced landslide zones affect only a small fraction of land within the quadrangle. The largest of these covers much of the steep-sloped highlands southwest of Portola Valley. Most of the remaining areas consist of relatively small, isolated zones in the hilly terrain between Portola Valley and Highway 280.

Mountain View Quadrangle: This is a revised release that addresses a previously unevaluated area of about one square mile within San Mateo County. The entire area -- low-lying marine wetland along the shore of San Francisco Bay -- is a potential liquefaction zone. Previously released and remaining in official form are those areas of the map covering Santa Clara (June, 2002) and Alameda (July 2003) counties.