Marina, South of Market and Mission Districts at risk for earthquake liquefaction

By Adam Martin, Bay City News Service

February 24, 2006

An updated digital map released by the U.S. Geological Survey shows the regions of the Bay Area where the soil is most likely to liquefy during an earthquake, the USGS announced today.

The places that are most susceptible to this "liquefaction" of soil are areas where landfill has been placed over the San Francisco Bay, and areas along larger streams, according to Keith Knudsen, a senior engineering geologist with the California Geological Survey, which collaborated with the USGS on the mapping project.

During liquefaction, which is only one of the hazards that can occur during an earthquake, soil begins to act like a liquid and loses its ability to support buildings and other structures.

"Regions of man-made landfill fared poorly in the 1906 San Francisco earthquake and in the 1989 Loma Prieta earthquake. We can expect history to repeat itself in the next big Bay Area earthquake," Knudsen said.

Some of the Bay Area locales that have the highest risk of liquefaction include: the Marina, South of Market and Mission districts of San Francisco; parts of downtown Oakland and areas around the Oakland International Airport; areas along Alameda Creek in Fremont; and areas along the Coyote Creek and the Guadalupe River in San Jose, according to Knudsen.

Liquefaction can lead to what are essentially landslides, causing buildings to travel down even very gentle slopes and damaging infrastructure such as pipelines, roads and bridges, Knudsen said.

The USGS and the California Geological Survey also collaborated to update a second map that shows where soil deposits of different ages are located around the Bay Area. This soil deposit map provides the basic data for the liquefaction hazard map, and also provides information about where
the most severe earthquake shaking may occur.

Knudsen said the maps can be used by cities and counties for land use and planning purposes, as well as to help plan emergency response procedures.

The maps are also intended to help educate and inform the public about the risk of earthquake-related hazards.

"These are not maps meant to scare anyone," Knudsen said.

Preventive steps work best when it comes to potential earthquake hazards like liquefaction, Knudsen said. "The ideal case ... is that we catch people before they build their buildings" in high-risk areas, Knudsen said.

The two maps are more detailed, up-to-date versions of similar maps that were released about six years ago, according to Knudsen.

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