The California Geological Survey has released five official hazard zone maps covering parts of San Mateo County, including large swaths of the Coastside. The maps may influence the region’s development for years to come.

Issued on Thursday, the maps pinpoint areas where an earthquake has the potential to trigger landslides and liquefaction, a geological phenomenon that temporarily transforms loose soil into a fluid suspension resembling quicksand. The maps also designate specific zones that require geotechnical studies before any construction or development is allowed. 

Tim McCrink, program manager for the California Geological Survey, said that the purpose of the hazard zone maps is to pinpoint areas where site-specific studies are necessary. 

“The best way to figure out if the hazard is on the parcel that you’re interested in is to get a geotechnical engineer out there,” said McCrink. “Then they (dig) some bore holes and trenches as part of a very specific investigation to see if the hazards exist on the site.” 

Each map covers an area, referred to as a “quadrangle,” totaling around 60 square miles. The map outlining the Montara Mountain quadrangle identifies a liquefaction zone that extends west of San Francisco International Airport to the San Andreas Fault.

It also pinpoints liquefaction zones near Pacifica State Beach, San Pedro Valley, El Granada and the Half Moon Bay Airport. Nearly three-quarters of the Montara Mountain quadrangle could be prone to landslides. 

“Most development projects are going to have that geological review or technical study done anyway,” said McCrink. “The zones are focused on what the expected hazard might be. If a consultant sees that a property is in a liquefaction zone, they’re going to design their investigation to address that. They’re going to be looking for different types of soil at different depths and density.” 

The maps themselves are an aftershock of the 1989 earthquake that rattled Loma Prieta, near Santa Cruz. According to geological survey officials, an earthquake of a 5.5 magnitude or higher can catalyze landslides and liquefaction. While shaking causes most of the mayhem during a major earthquake, both landslides and liquefaction caused significant damage during the Loma Prieta quake. 

The Seismic Hazards Mapping Act, which mandated the regulatory maps across the state, was passed the following year. 

Over the past few decades, McCrink said, the maps have helped local jurisdictions to identify potential hazards, and any development changes required, before construction occurs. 

“One of the things we’ve found after major earthquakes is that cities and counties don’t have the resources to issue permits when people want to rebuild after an earthquake,” he said. 

“That whole process of investigation and mitigation tends not to happen. So people rebuild in the same bad spot, without those, which just sets them up for disaster in the next earthquake.

“If you identify the hazard and fix it beforehand, your community is going to be far more resilient,” he added. “And the cost of coming back is going to be infinitely smaller.” 

For more information, and to see the maps themselves, visit  the website, conservation.ca.gov/cgs.

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