The Sunnyvale Sun
News
Time to get in some traction on liquefaction
By HUGH BIGGAR
There is a potential monster lurking beneath Sunnyvale--one that can swallow buildings whole and burp sand and water into the air.
The creature from the deep is not an animated being, but a geological presence known as liquefaction. The phenomenon creates particular vulnerability for Sunnyvale during an earthquake, in addition to intensely shaking ground.
Geologists define liquefaction as a process during earthquakes in which certain types of soil become liquid.
Other liquefaction zones in Santa Clara County include a mile-wide section along Guadalupe Creek and the Mineta International Airport in San Jose.
"When you look at the liquefaction maps, it's like throwing paint on Sunnyvale," said public information officer John Pilger of newly released liquefaction maps displaying colored swaths where the city is most vulnerable. "I don't want to be here during an earthquake." Pilger, a former spokesman for the Federal Emergency Management Agency, is also emergency preparedness commissioner in Milpitas.
According to the maps released by the Association of Bay Area Governments and the U.S. Geological Survey, Sunnyvale's eastern side stretching to the bay is where liquefaction is most likely to occur. The process would require a powerful earthquake to generate the ground waves involved in liquefaction, a not unlikely prospect, experts say.
Scientists currently predict a 62 percent chance of a powerful earthquake in the Bay Area by 2032. It could happen at any time, and make the 7.1 Loma Prieta quake in 1989 seem like a gentle hug in comparison.
"A powerful earthquake is a very serious concern," says Dick Volpe, a civil engineer with the Santa Clara Valley Water District. "The U.S. Geological Survey has done a commendable job of presenting the hazards in clear terms," he said of maps showing earthquake-shaking intensity in different Bay Area cities. The maps for Sunnyvale show more than half of the city is in the highest two categories for ground-shaking.
In part, those hazards are due to Sunnyvale's location near several seismic faults and on the edge of a basin of soft ground extending 35 miles in the Santa Clara Valley, from Sunnyvale west to the mountains and from Palo Alto to Los Gatos. Cupertino sits at the center of that basin and is likely to experience the most intense shaking since the oval-shaped Cupertino Basin concentrates seismic shocks and intensifies them.
Like Cupertino, Sunnyvale has additional soft soil worries of its own, though a different nature.
Soft, sandy soil, especially on the eastern side of Sunnyvale extending to the bay shore, causes the ground to liquefy during severe shaking. As a part of that process, ground soil to within 30 feet of the surface can turn to quicksand. When it gives way, it takes buildings and cars with it. The ground can erupt in what are called "sand boils." Water and sediment spew up out of the ground.
An example many will remember
A vivid example of liquefaction occurred in the 1989 Loma Prieta earthquake in San Francisco's hard-hit Marina District. Much of the Marina District had been built on former bay land that had been filled in. This loose ground soil liquified and collapsed in the Loma Prieta earthquake.
According to Sunnyvale city documents, 9,356 acres of Sunnyvale's 12,362 urban acres are in "areas of moderate, high, or very high liquefaction susceptibility."
"Unfortunately, there is nothing you can really do about liquefaction [occurring]," Pilger says, "though when new buildings are constructed, we make sure the structure is sound based on the characteristic of the soil."
Despite the uncertainty of liquefaction, Pilger encourages Sunnyvale residents to do all they can to prepare.
"Sunnyvale has a number of things in place to prepare for an earthquake," he says, pointing out the city's emergency preparedness drills and emergency response drills. "SNAP is our first line of defense," he adds of the Sunnyvale Neighborhoods Actively Prepare program.
Run by Sunnyvale's Office of Emergency Services, SNAP offers classes to help individuals prepare for such disasters as earthquakes, floods, fires and hazard materials leaks. The classes help neighborhoods organize and provide training in such areas as emergency communications, how to turn off household appliances such as gas and light rescue skills.
City is doing what it can
"It helps you prepare for different types of incidents and helps you think about how to be self-sufficient," Pilger says, pointing out people can expect to be on their own for a least 72 hours. There is also a Sunnyvale Amateur Radio Emergency Service to help provide the city's public safety department with information during a disaster.
"Emergency services can support only so much," Pilger says. "There are only six fire stations in the city, so services are going to go to the high priority areas."
To prepare for a possible powerful earthquake, the water district's Volpe also encourages residents and business owners to do the basics, such as creating earthquake kits for residences and cars, strapping water heaters to the wall, knowing how to turn off gas and water at the source and securing heavy furniture. If the earth starts to move, taking shelter beneath a heavy table and moving with it will help protect you from falling masonry such as fireplace bricks, say preparedness experts. That secured hot water can be a 40-gallon source of water.
"It is much more soothing to stick your head in the sand and not do anything," Volpe says, "but the old Boy Scout rule applies: Better safe than sorry."
Pilger agrees.
"The reality is we live in earthquake country," he says. "It may not happen in our lifetimes, but that is a gamble I personally am not will to take."
To view the Association of Bay Area Government simulation maps and to prepare a home or business for an earthquake, visit www.quake.abag.ca.gov.
What is liquefaction?
Liquefaction occurs when intense seismic ground shaking causes sandy and silty soil to lose strength and behave as a fluid rather than solid ground for a short period. As a result, voids can occur in the soil layer and lead the void space to collapse. Such liquefaction mostly takes place in areas with young (less than 10,000-year-old) soil with loose sediment and a high water table.
The U.S. Geological Survey is holding an open house on liquefaction and other natural hazards June 3 and June 4. The events allow the public to learn more about the science of natural hazards such liquefaction and earthquakes. The free open house runs from 10 a.m. to 4 p.m. at 345 Middlefield Road, Menlo Park. To learn more, visit www.openhouse.wr.usgs.gov.
