The California Geological Survey last week released new hazard maps for San Mateo and Contra Costa counties that detailed where landslides and soil liquefaction could likely occur in the event of a significant earthquake.
The CGS’s Seismic Hazard Zone maps found “significant” liquefaction zones in parts of San Mateo County, particularly in Half Moon Bay, Miramar and San Bruno. The state has already mapped most of the Peninsula, including Montara Mountain, Woodside and San Mateo. But La Honda and San Gregorio are two notable rural areas that don’t have data accessible yet.
Each map, a roughly 60-mile zone called a “quadrangle,” accounts for three types of geologic issues caused by earthquakes: a fault rupture, landslide and liquefaction, which describes the process when seismic tremors cause soil to mix with groundwater and behave like quicksand.
The state agency identifies most of the city of Half Moon Bay as inside a liquefaction zone. Its quadrangle is 74 square miles, and the liquefaction zone spans the city’s entire coastline and more, including most of the neighborhoods up to Pilarcitos Creek, including El Granada, Miramar and rural areas like Purisima Creek Redwoods Preserve. The map also identifies fault zones on both the east and west sides of the Half Moon Bay Airport, and more than half of Montara Mountain’s quadrangle is at risk of earthquake-induced landslides.
The CGS maps were drafted in February but became official on Sept. 23. Land management agencies and cities use hazard maps to identify properties that require site-specific studies before breaking ground on new development. Under the 1990 Seismic Hazards Mapping Act, local jurisdictions can withhold permits at a site within a hazard zone until geological surveys are complete and used in development plans. The law also requires real estate agents and sellers to disclose if their property is within a mapped hazard zone.
Erik Frost, a CGS senior engineering geologist, said the maps don’t automatically mean there is a hazard waiting to happen in each zone. Rather, it’s to signal more work is needed if anyone plans to build on the land. He stressed the maps shouldn’t be a substitute for site-specific research.
“These zones are simply areas where there’s a higher probability that the hazard exists. It’s not 100 percent probability,” Frost said. “A site-specific study says, ‘Is there actually a hazard here? If so, how significant of a hazard is it, and how do we engineer around it?’”
The state agency’s data traces back to the 1989 Loma Prieta earthquake, when a quake with a 6.9 magnitude rocked the Bay Area, killing 63 and injuring more than 3,700. The CGS says earthquakes of magnitude 5.5 or greater on the Richter scale can trigger landslides or liquefaction. Max Marshall, an engineering geologist, said much of the work creating these maps involved pairing data from county soils reports with GIS mapping programs, including light and detection ranging technology, commonly known as lidar, which uses lasers to scan and map landscapes.
For residents looking to buy and develop property inside a hazard zone, an earthquake can be devastating if the right insurance policies aren’t in place. Generally, earthquake insurance does not cover landslides for homes or businesses. This means specialized coverage, commonly referred to as “difference in conditions” can be expensive, said Janet Ruiz, a spokeswoman at the Insurance Information Institute.
Though the CGS’s public data is available for insurance companies, it’s not always used when selling policies. California Earthquake Authority, for example, uses the state’s Uniform California Earthquake Rupture Forecast, according to CEA spokeswoman Sarah Sol.
“The issuance of new maps for this area does not have any immediate impact on CEA insurance premiums because we take a consistent statewide look at earthquake risk and rely on the best available science on California earthquake probabilities as a whole when setting our rates,” Sol wrote in an email to the Review.