NR 2005-16
August 16, 2005

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


Note: The maps can be viewed here.

SACRAMENTO – The Department of Conservation’s California Geological Survey today released two new regulatory Seismic Hazard Zone maps. These maps show the risk of liquefaction and landslides in portions of Santa Clara County and – for the first time -- San Mateo County.

“These Seismic Hazard Zone maps are good examples of the state’s commitment to earthquake preparedness,” said Mike Chrisman, Secretary for Resources. “They are important tools in the effort to protect public safety.”

Liquefaction and landslides are potential side effects of earthquakes in the magnitude 5.5 or greater range that can cause destruction over and above the damage done by shaking

“These maps identify the areas where liquefaction and landslides are more likely to occur,” said State Geologist John Parrish, head of the California Geological Survey. “With that knowledge, local planners and the building community can take steps to minimize the danger by ensuring that new construction takes into account not only that we’re in earthquake country, but also that there are concerns other than shaking.”

The Mindego Hill Quadrangle map encompasses about 59 square miles of mainly mountainous terrain partly within and to the south of the communities of Portola Valley in San Mateo County and Palo Alto and Los Altos Hills in Santa Clara County.

Most of the area shown on the map remains undeveloped, and a substantial part of it is parkland. About 64 percent of the area covered is zoned for earthquake-induced landslide potential. Only a fraction of the area covered – the channels and narrow floodplains of Sausal, Los Trancos, Adobe and other creeks – is zoned for liquefaction potential.

The Castle Rock Ridge Quadrangle map encompasses about 60 square miles of mainly rugged terrain in the Santa Cruz Mountains; however, the zoned area is limited to one-third of the map, about 20 square miles of land in Santa Clara County.

Of the zoned area – including land partly within and southwest of Saratoga and Monte Sereno -- about 65 percent is designated for landslide potential because of the widespread occurrence of steep slopes, low rock strength and expected high ground acceleration in the event of a strong earthquake. Only a small amount of the area – along Saratoga, Lyndon and San Tomas Aquinas creeks -- is designated as a liquefaction zone.

Liquefaction – which occurs when water-saturated, sandy soil is shaken violently and temporarily loses its ability to support structures – caused underground gas pipes to rupture in San Francisco’s Marina District during the Loma Prieta earthquake. Leaking gas fueled a spectacular, 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.

If the site-specific geotechnical studies conducted prior to development reveal that liquefaction or landslide hazards are present, design changes to better protect life and property during future earthquakes -- such as deep foundations in liquefaction zones and slope stabilization in landslide zones – are required before new developments are approved and built.

The program has identified about 345 California communities as high-risk areas for liquefaction and/or landslides; 160 have been zoned. There are now 109 Seismic Hazard Zone maps covering all or portions of eight counties.

To establish its priority list, the California Geological Survey looks at the level of seismic hazard in each locale as well as the amount of new development going on. The program is continuing to work in Santa Clara and Alameda counties, and also is in the process of producing maps that establish zones in the communities of Atherton, Menlo Park, Palo Alto, and South San Francisco.

In addition to studying and mapping earthquakes and other geologic phenomena, the Department of Conservation regulates oil, gas and geothermal wells; ensures reclamation of land used for mining; administers agricultural and open-space land conservation programs; and promotes beverage container recycling.