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After the Loma Prieta earthquake in October 1989, the first story of this three-story building in San Francisco s Marina District failed when ground shaking was intensified by liquefaction.

If a major earthquake rattles the South Bay, the sandy soils alongside San Jose's two largest rivers pose the highest risks to modern businesses and homes in Santa Clara County, according to new seismic maps by the U.S. Geological Survey.

The maps reveal the hidden danger along San Jose's ancestral Guadalupe River and Coyote Creek flood plains — now home to such places as Cisco Systems, the Highway 101/87 interchange, part of Mineta San Jose International Airport, the San Jose Mercury News and many homes and apartment buildings.

(Click on image to enlarge.)

The new maps, based on 100 borings in valley soil, provides the most detailed look yet of the South Bay's seismic safety, said USGS researcher Thomas L. Holzer. Moreover, unlike previous maps, this map offers the statistical probability of liquefaction — the phenomena when damp sand turns liquid-like during an earthquake. The new maps, available on the Web, allow property owners to zoom to street level for a close-up of their risk.

"We are giving the odds of this actually occurring on your property,'' Holzer said.

The good news, he said, is that much of the valley is safer than previous studies presumed, because it is built on relatively stable mud. While seismic shaking can be amplified by mud, mud does not dangerously destabilize through liquefaction, like sand, he said.

"There's more peace of mind when you get farther away from these creeks,'' Holzer said. "The hazard is smaller.''

The USGS team calculated a 30 to 40 percent risk of liquefaction along the two old San Jose waterways in a 7.8 magnitude earthquake along the San Andreas Fault, about the same size of the famed 1906 earthquake.

The risk drops to 10 to 20 percent in a 6.7 magnitude earthquake along the Hayward Fault, the same size of the 1868 quake, or a 6.9 magnitude quake along the Calaveras Fault.

Smaller creeks, such as Stevens Creek, San Francisquito Creek or Los Gatos Creek, do not pose a significant hazard, he said.

Even construction along the bay on man-made fill is relatively safe, by comparison, because it stands on mud and not sand, he said. Furthermore, with the exception of Alviso, there is little construction in the baylands.

The Santa Clara Valley, at the south end of San Francisco Bay, is a giant trough that has been subsiding and filling with sediments for millions of years.

Today the Guadalupe River and Coyote Creek are generally well-behaved, confined within concrete levees to reduce risk of flooding.

But once they meandered along the valley, carrying vast volumes of water and sand. Over time, the water subsided but the sand stayed behind — and then humans built upon it. The water table beneath those flood plains remains very high.

Engineers at the San Jose airport will study the new data, according to spokesman David Vossbrink. But he said that large sunken concrete pilings support construction of a new garage and terminal. The A terminal, recently renovated, meets current standards; the older C terminal is slated for demolition. The concrete runways are less vulnerable than a building, Vossbrink added.

One high-risk section of the South Bay's highway system — the 87-101 interchange — was built in 2005 using state-of-the-art construction, said Caltrans spokeswoman Lauren Wonder.

At Cisco, "we are reviewing the USGS report and are not able to comment yet on its findings,'' said spokesman Terry Alberstein. Similarly, Mercury News president and publisher Mac Tully said, "We'll need to do some research to better understand the implications for our facility."

The new map is just a starting place for a property owner, the USGS's Holzer said. The next step is to hire a geotechnical engineer to conduct soil borings to identify a property's specific risk.

Even if built on sand, structures can be stabilized by steel beams, supported by sunken pillars, he said.

Sand can also be solidified through injections of grout and other chemicals, he said.

It's too early to tell how the new findings could impact property owners' earthquake insurance rates. The research, published in next month's issue of the Bulletin of the Seismological Society of America, must first be reviewed by California's seismic hazard mapping program.

"We've just pinpointed the new areas where we need to pay attention,'' Holzer said. "For a specific site, it's important to get more detailed information.''

Contact Lisa M. Krieger at lkrieger@mercurynews.com or (408) 920-5565.