Pegging the steep slopes of Montara Mountain and the Santa Cruz Mountains as areas where earthquake-induced landslides could occur, new Seismic Hazard Zone maps released by the California Department of Conservation last week are ushering in new requirements for those hoping to build structures in San Mateo County zones deemed to be at risk for such damage.

Showing the risk earthquake-induced landslides and liquefaction pose for property owners in cities as far north as Pacifica and as far south as Woodside, studies of the Montara and Woodside quadrangles were among five Seismic Hazard Zone maps the California Geological Survey created in an effort to mitigate the damage stemming from the movement of unstable land after a major earthquake.

Tim McCrink, a seismic hazard program manager with the California Geological Survey, a division of the Department of Conservation, explained the purpose of the map is to trigger a site-specific investigation of areas which state officials believe have a high probability of landslide and liquefaction, a phenomenon in which the soil temporarily moves like quicksand.

He said the Seismic Hazards Mapping Act was passed in the state Legislature following the 1989 Loma Prieta earthquake, during which soil liquefaction and landslides caused significant damage. McCrink explained the maps trigger a process by which local jurisdictions require those proposing to build in zones with potential for liquefaction and landslides to hire licensed geologists to investigate the sites and mitigate possible hazards.

By informing those proposing new projects of potential dangers on their sites, officials are hoping the effort to map zones for investigation can stem additional damage that can follow a serious earthquake.

“If you spend a few thousands of dollars when something is constructed, you save yourself hundreds of thousands of dollars down the road,” he said. “It saves us incredible amounts of money down the road and it saves lives.”

McCrink said the maps add another dimension to local building codes and safety regulations aimed at ensuring homes are built to withstand the shaking effects of an earthquake. Though shaking accounts for the majority of an earthquake’s damage, McCrink noted the Seismic Hazard Zone maps ensure studies are done to assess the liquefaction and landslide hazards in a given area and require developers or homeowners proposing significant changes to their homes to mitigate possible hazards on their properties.

He said cities and counties are expected to require those who apply for building permits in a designated zone to complete a site-specific study. If the study finds significant hazards on the site, the applicant will be required to include mitigation measures in their plans, which could include retaining walls and other slope stabilization measures for landslides and foundation reinforcement and dewatering for liquefaction, noted McCrink.

Those looking to sell their homes in an Earthquake Zones of Required Investigation, or EZRI, will be required to disclose at the time of sale that their property lies within such a zone, he added.

Earthquake zones

Including portions of Burlingame, Hillsborough, Millbrae, Pacifica, San Bruno and unincorporated San Mateo County, the map the department drew of the Montara Quadrangle identified beaches and alluvial plains along the shores of the San Francisco Bay and the Pacific coastline as Earthquake Zones of Required Investigation, or EZRI, for liquefaction hazard. The map also designated the slopes of the Montara Mountain and ridges surrounding the San Andreas rift valley as EZRI for earthquake-induced landslides.

The map the department released of the Woodside Quadrangle — which includes portions of Belmont, Portola Valley, Redwood City, San Carlos, Woodside and unincorporated San Mateo County — pegged alluvial plains extending to the San Francisco Bay and sections of the San Andreas rift valley as zones that should be studied for potential liquefaction hazard. Land encompassing the slopes of the Santa Cruz Mountains was identified by the map as a zone to be studied for potential earthquake-induced landslides.

Property values

Though some have voiced concerns about whether Seismic Hazard Zone maps may decrease the property value of homes found to be within the zones, McCrink said studies completed on the implications of earthquake fault zone maps showed site-specific studies of potentially at-risk zones bolstered the confidence of those buying property since they were informed about the possible risks and required mitigation measures.

Having mapped quadrangles in and around Los Angeles as well as in Contra Costa and Alameda counties, among other regions, state officials are setting their sights on mapping two other quadrangles covering portions of San Mateo County as they continue to carry out the Seismic Hazards Mapping Act, said McCrink.

McCrink hoped residents across the state would view the effort as an opportunity to inform themselves about properties they own or are thinking of purchasing, noting the department has created a web-based tool allowing resident to check whether specific parcels are in hazard zones.

“People should not alarmed … if they find that their particular property [or] their home … is in one of these zones,” he said. “It’s a chance to get more informed.”

Visit conservation.ca.gov/cgs/geohazards/eq-zapp to view individual parcels affected by Seismic Hazard and Earthquake Fault zones.

(650) 344-5200 ext. 106

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