By Marion Softky
Almanac Staff Writer
New geologic maps highlight still another hazard that people preparing for the Big One may need to think about: liquefaction.
Detailed maps of the Bay Area, released February 24 by the U.S. Geological Survey and the California Geological Survey, show the susceptibility of soils in different areas to soften and become liquid when shaken by a big earthquake.
Bright red areas, mostly fringing the Bay, signal where fill or loose, moist soils might turn to mush.
In 1989, buildings in the Marina District of San Francisco sagged into the ground and broke, while sections of the Cypress Freeway collapsed, squashing cars and killing people.
“Soil behaves like a liquid,” said geologist Keith Knudsen with the California survey. “Structures can settle; on a slope or a stream bank, a whole mass of land can slide down a slope, even a very gentle slope.”
In San Mateo and Santa Clara counties, most of the flatland bordering the Bay is marked yellow, for moderate susceptibility to liquefaction. The hills are gray for very low risk.
Most striking is the bright red band — denoting higher risk — stretching up San Francisquito Creek from the Bay to where the creek crosses Sand Hill Road just below the Stanford golf course.
About 500 to 600 meters wide, “these are the natural levees along the creek,” said geologist Carl Wentworth of Ladera, one of the authors of the maps. “They have built up over several thousand years of flooding.”
Not to panic, Dr. Wentworth said. “For most houses, liquefaction is not a big deal; 80-90 percent of the damage from earthquakes is from shaking.”
More at risk from liquefaction are lifelines, such as roads, highways and bridges, or gas, water, and sewer lines, Dr. Wentworth said. “They can even float to the surface.
“But the agencies responsible are very aware of the risk. They’re first in line.”
Advice
Dr. Wentworth carries earthquake insurance and advises others to consider it.
An earthquake is coming, so people should take basic precautions, Dr. Wentworth advises. First, make sure your house is tied to its foundation, the water heater is strapped in place, and heavy objects won’t fall and hurt someone. “There’s not much you can do about liquefaction,” Dr. Wentworth said. But people who are concerned should consult an expert.
INFORMATION
The new geologic maps can be viewed at the map sales office of the U.S. Geological Survey at 345 Middlefield Road in Menlo Park, in Building 3, next to the flag pole; or on the Web at earthquake.usgs.gov (click on regional, Northern California, and New geologic maps.) The maps on the Web are purely geologic and don’t include roads and political boundaries. These can be viewed at the map sales office.
