New maps spell out seismic hazards

“These maps designated areas where, during the planning stage, construction might have to incorporate design elements to protect life and property in the event of a large earthquake,” said State Geologist of California John Parrish in a prepared statement. “We know that, sometime in the future, large, damaging earthquakes will strike in a populated area.” 

The hope, Parish adds, is that the detailed information about potential hazards in these zones will enable newly built structures to better withstand earthquakes. 

“We’re trying to get ahead of the hazard,” said Tim McCrink, program manager at the California Geological Survey. 

Each map covers an area, described by geological survey officials as a “quadrangle,” about 60 square miles in size. The map detailing the Montara Mountain quadrangle pinpoints a liquefaction zone extending west of San Francisco International Airport to the fringes of the San Andreas Fault. 

It also identifies liquefaction zones around Pacifica State Beach, San Pedro Valley, El Granada and the Half Moon Bay Airport. Nearly three-quarters of the map — including most of the Golden Gate National Recreation Area and San Pedro Valley County Park — is zoned for landslides. 

McCrink said that if geotechnical studies find hazards in these zones, prospective builders would need to design foundations deep and sturdy enough to withstand the shifting mosaic of loose soil. Similarly, slope stabilization might be required in landslide zones. 

“After the earthquake, if liquefaction occurs, that structure on top of the foundation can also be re-leveled so it will not fail,” he added. 

Any earthquake of a 5.5 magnitude or higher can prompt landslides or liquefaction. And while the tremors themselves contribute to most of the destruction caused by earthquakes, landslides and liquefaction led to major damage during the 1989 quake that rattled Loma Prieta. The Seismic Hazard Mapping Act, which requires the California State Geologist to create regulatory maps charting earthquake zones, was passed the next year. 

The maps will become official following a 90-day review period for local governments — and then a 90-day window for geological officials to make additional recommendations of their own. Printed versions of the maps will be available at local planning and building departments. 

“We’re providing an opportunity for cities or counties, or even engineering geologists who might be familiar with information that we may have missed, (to provide feedback),” said McCrink. “This is a chance to get more information to us so we can refine the map to be more accurate.” 

But gauging exactly when an earthquake might occur becomes more difficult. McCrink said that because geotechnical experts have no way of measuring the stresses among fissures winding deep beneath the ground, it’s hard to predict when a particular fault might rupture. 

“What they do is look at the history of ruptures along various faults in the Bay Area and make a forecast,” he said. “There’s a certain amount of uncertainty, but, based on what we know about those faults, they start using probabilities.”

According to the United States Geological Survey, there is a 72 percent probability of one or more magnitude 6.7 earthquakes, or higher, in the Bay Area between 2014 and 2040.