MENLO PARK — The release of new quake-risk maps by the California Geological Survey this week means developers will need to perform additional assessment and mitigation when building on parts of the Peninsula.
One new map describes the liquefaction risk — that is, the risk of land turning into liquid during shaking — for areas of Redwood City, Menlo Park and East Palo Alto in a significant earthquake, according to Anne Rosinski, engineering geologist with the CGS. Once those maps are delivered to each city developers are legally required to perform soil and site studies to determine their property’s quakerisk and include mitigation measures in their development proposals.
When Redwood City officials saw preliminary CGS maps six months ago, which show that most of the land between the Bay and Alameda de las Pulgas has the potential for liquefaction, they disputed the findings, according to city engineer John Lynch.
“We thought the area was too large. It covers most of Redwood City,” said Lynch, who hasn’t seen the final maps yet. “I never heard a thing back from them.”
Despite their concerns, however, city officials said most developers are already required to do soil testing as part of the environmental review process.
Geological Survey officials say the map shows potential for liquefaction, but that isn't a guaranteed risk. “This doesn’t mean there will be catastrophic damage — each earthquake is a roll of the dice,” Rosinski said.
However, the San Andreas Fault is poised to bring a 7.0-magnitude quake to the Peninsula that could cause an estimated $1 billion in damage, kill 500 people and make 300,000 to 400,000 people homeless, according to USGS geologist Tom Brocher.
The data was released Monday in a press conference devoted to the lessons geologists learned from the Oct. 17, 1989, Loma Prieta earthquake. The Seismic Hazards Mapping Act, adopted in 1990, requires the CGS to publish maps on regional liquefaction and landslide risks — and requires property owners to disclose those risks to potential buyers once maps are completed.
The CGS is still mapping the rest of San Mateo County, a project that could be finished in “a couple of years,” depending on financial resources, according to Keith Knudsen, a senior engineering geologist with the CGS. U.S. Geological Survey geologist Tom Brocher anticipated that those new maps would look similar to USGS hazard maps that show Bayside zones are susceptible to liquefaction while hilly regions are at risk for landslides.
Post-Loma Prieta studies have triggered more than $30 billion in seismic retrofits in the Bay Area that should help prevent future quake damage, according to USGS geologist Tom Holzer.