The California Geological Survey has identified many Pacifica neighborhoods as likely areas for earthquake hazards. The maps detail where certain hazards, specifically landslides and soil liquefaction, could likely occur in the event of a significant earthquake.

Pacifica liquefaction zones

Image courtesy California Geological Survey

A new look at dangers associated with earthquakes reveals that much of Pacifica could be at risk of liquefaction in the case of a large temblor.

 

The city of Pacifica spans two of the state’s “quadrangles,” Montara Mountain and San Francisco South, each a roughly 60-mile zone that details 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. 

 

According to the state’s map, most of the Pacifica neighborhoods in Linda Mar, Rockaway and Sharp Park are inside a liquefaction zone. The zone in Linda Mar stretches from Linda Mar Beach east up San Pedro Creek and through Terra Nova and Oddstad boulevards. Meanwhile, most of the hills surrounding the city, including Montara Mountain, are marked as potential areas for 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 incorporated into development plans. The law also requires real estate agents and sellers to disclose if their property is within a mapped hazard zone. 

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. 

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 detection and ranging technology, commonly known as liDAR, which uses lasers to scan and map landscapes. 

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 and 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?’”

 

August Howell is a staff writer for the Review covering city government and public safety. Previously, he was the Review’s community, arts and sports reporter. He studied journalism at the University of Oregon.

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