NR 2004-34
October 15, 2004

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


SACRAMENTO – On the eve of the anniversary of Loma Prieta, which killed 63 people and caused an estimated $6 billion in damage, Department of Conservation’s California Geological Survey released three new official Seismic Hazard Zone maps. These maps show the risk of liquefaction and landslides in portions of Alameda and Santa Clara counties.

Speaker pro Tempore Leland Yee, PhD (D-San Francisco/Daly City), has taken an active interest in the CGS mapping program and seismic safety in general.

“The Seismic Hazard Mapping Program is a good example of the state’s commitment to earthquake preparedness,” he said. “It is vitally important that the legislature be committed to taking every step possible to ensure public safety in the event of an earthquake.”

The Seismic Hazard Mapping Program is also working in three other areas of Santa Clara County, as well as three in Alameda County. The program recently began its first work in San Mateo County, and is in the process of producing maps that establish zones in the communities of Atherton, Menlo Park, Palo Alto, Portola Valley and South San Francisco.

“These maps improve public safety 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,” said Michael Reichle, Acting State Geologist and head of the California Geological Survey. “By identifying the areas where liquefaction and landslides are more likely to occur, local planners and the building community can take steps to minimize the danger.”

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.

Liquefaction – which occurs when water-saturated, sandy soil is violently shaken 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.

“Loma Prieta is sometimes referred to as the `geologists’ earthquake’ because it demonstrated the effect poor soils can have on the level of earthquake damage,” said Charles Real, the Supervising Geologist who oversees the mapping program at the Department of Conservation.

Cities and counties use the maps to regulate development. If property is located in a Seismic Hazard Zone, where liquefaction or earthquake-induced landslides are deemed more likely to occur, local government must withhold development permits until the level of hazard has been determined by a detailed geotechnical investigation at the construction site. If hazards are present, mitigation measures are incorporated into development plans. 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.

The program has identified about 345 California communities as high-risk areas for liquefaction and/or landslides; 159 have been zoned. To establish its priority list, the program looks at the level of seismic hazard in each locale as well as the amount of new development going on. Work is planned in Marin, Contra Costa, Sonoma and Solano counties once Alameda, Santa Clara and San Mateo counties are complete.

Geographic Information System technology is used to integrate the information into a three-dimensional view of the ground and shallow subsurface. Here is a closer look at what is shown on the latest Seismic Hazard Zone maps:

Niles Quadrangle, Alameda County: Part of the city of Fremont covers the western half of the quadrangle, and the map also covers portions of Newark, Union City and Pleasanton as well as the unincorporated community of Sunol. The liquefaction zone is concentrated in the southwestern quarter and central part of the quadrangle. An earthquake-induced landslide zone covers about 36 percent of the quadrangle, where the combination of steep slopes and weak rocks in the East Bay Hills have produced widespread landslides.

Morgan Hill Quadrangle, Santa Clara County: The southeastern end of Santa Clara Valley and parts of the cities of Morgan Hill and San Jose are located in this quadrangle. The liquefaction zone of required investigation covers the valley floor, the lowlands along Las Animas Creek and San Felipe Creek, and the bottoms of other creek canyons such as Llagas Creek. Approximately 35 percent of the quadrangle – confined to hills and mountains, with virtually none in the valley -- lies within the landslide zone.

Milpitas Quadrangle, Alameda County: The Santa Clara portion of this quadrangle was mapped in 2001. This quadrangle lies along the southeastern margin of San Francisco Bay and includes portions of the cities of Fremont, Milpitas, Sunnyvale, Santa Clara and San Jose. The boundary between Alameda and Santa Clara counties crosses the northern half of the quadrangle, along Scott Creek and Coyote Creek. Nearly the entire area west of Interstate 680 is designated as a liquefaction hazard zone. Some of the hilly areas in the northeastern part of the quadrangle are zoned for earthquake-induced landslides.

The Seismic Hazard Mapping Program released its first maps for review in October 1996. At the moment there are 108 official maps covering more than 160 communities. Mapping has been completed in Los Angeles, Orange, Ventura and San Francisco counties, and the program’s emphasis has shifted from Southern California to the Bay Area.

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.