Areas of WG in liquifaction zones, state officials say

Newly released map could affect building requirements in zone

By Kate Carter

Sections of Willow Glen are at greater risk of land movement during a sizable earthquake than previously thought, according to a preliminary map of area liquifaction zones released Aug. 17 by California's Department of Conservation.

The map covers areas of downtown and West San Jose, Santa Clara, Campbell, Sunnyvale, Los Gatos and Saratoga. It shows a large area to the north and east that is at higher risk of becoming unstable during a 6.0-magnitude earthquake or higher. Water near the ground's surface could mix with sandy soil to temporarily become quicksand and cause damage to structures and underground infrastructure, said Don Drysdale, spokesman for the department's Division of Mines and Geology.

"Most of the flatland in San Jose is in a liquifaction area," Drysdale said, referring the area's high water table and sandy soil. "It's a slightly greater risk. But 95 percent of the damage [from an earthquake] comes from the shaking."

The zone boundary runs through Willow Glen in a curve from the Los Gatos Creek west of Bird Avenue, to the intersection of Lincoln and Curtner avenues and over to Foxworthy Avenue west of the Kirk Community Center. Areas to the west of the boundary are considered to be at normal risk; areas to the east have a higher liquifaction risk. Areas along the Los Gatos and Dry creeks are also at greater risk. But Drysdale said the map only gives a general impression where liquifaction could occur and "is not specific to a block or a house."

Drysdale said cities will use the map, once it becomes official early next year, to require slightly higher reinforcement standards for new construction in the liquifaction zone. Sellers of property in the zone are also required to inform buyers of the risk, he said.

Most California cities already have stringent building requirements, Drysdale said, which is why devastating earthquakes in other parts of the world are not as great a threat here. The greatest danger from liquifaction, he said, would come from systems below the ground that could be damaged and cause problems like gas leaks or electrical surges and outages. Liquifaction was a cause of damage to San Francisco's Marina district, which is built on landfill, after the 1989 Loma Prieta earthquake, he said.

The map is one in a series that should cover most of the state's earthquake-prone areas in the next 10 years, said Jack McMillan, outreach coordinator with the state division. The state legislature passed the Seismic Hazards Mapping Act in 1991, and a team of geologists has mapped much of Southern California since 1997, he said. The state released maps of Milipitas, Calaveras and East San Jose earlier this year, and another covering areas of Los Gatos, Cupertino and Mountain View will be released next year.

The team uses information in county and city building department files, as well as from water districts and agencies like Caltrans, to find information about subsurface conditions in the top 50 feet of soil, McMillan said. Geologists plot that information and use it in formulas to calculate area's risks for liquifaction and landslides, he said.

The maps indicate that 80 to 90 percent of liquifaction could occur within the zones, but they don't define how great the hazard is in any one particular area, McMillan says. The maps are important because construction within the zones triggers extra attention and investigation by city and county planning and building staffs to seek extra ways to support the at-risk structures, he said.

Several other organizations, such as the Area of Bay Area Governments and the United States Geological Survey have also released maps to determine seismic risks locally, McMillan said. The state maps, though, are the only ones that must under state law be considered by government agencies, he said.

"The first trial of how accurate these maps are is when the first earthquake hits," McMillan said.

The division is taking public comment on the map during the next three months, either written or in public at the state Mining and Geology Board's monthly meeting. For address and meeting information, visit www.conservation.ca.gov.

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To view the preliminary map, visit www.consrv.ca.gov/dmg/shezp/maps/p_sjosw.htm. It can also be purchased from BPS Reprographics Service, 415.512.6550.

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