City officials will lobby state and federal geologists Monday to show Foster City on upcoming state earthquake hazard maps as less prone to liquefaction than current federal maps indicate.
In February, the U.S. Geological Survey released an update of its regional “liquefaction susceptibility” maps, which detail areas where the soil is prone to liquefying during an earthquake, a phenomenon with disastrous results for buildings. As in past maps, areas in which the groundconsists of landfill on Bay mud received the highest risk rating.
Nearly all of Foster City is built on landfill, except for a few pre-existing surfaces whose liquefaction risk was downgraded on the 2006 maps, and the city was given a high risk rating. City officials, however, argue that their town is built on “engineered landfill,” designed to withstand earthquakes — a distinction the USGS maps don't make.
Official maps of liquefaction and landslide-prone zones can impact real estate prices and earthquake insurance rates.
“I would like due diligence. Do you think its fair that they don’t do more detail?” Mayor Linda Koelling said.
The maps are intended as regional maps, USGS spokesman Tom Brocher said. More detailed studies are expensive and typically done — and paid for — by whomever wants them, he said.
City officials hope the California Geological Survey will do more detailed work. In 1990, the state Assembly mandated the creation of state seismic hazards maps because of the 1989 Loma Prieta quake. CGS is due to begin work on maps of San Mateo County at the end of this year or later, CGS spokesman Don Drisdale said. He did not know if types of landfill will be differentiated on those maps.
“It’s a little bit of a sticky point. This is the first time, to my knowledge, that this has come up,” Drisdale said.
Engineered landfill is not automatically safer, according to Michael Clark, senior engineering geologist with the engineering firm Kleinfelder. It is designed to resist liquefaction, but its ability to do so is based on the specifications followed and the diligence of the contractor, he said. Foster City officials have said their landfill was built rigorously, noting the city's good performance during the Loma Prieta quake.