California’s Napa Fault is more powerful than previously known

Now, an expert with the United States Geological Survey says the fault that caused the 2014 quake is longer and more dangerous than previously thought.

In soon-to-be-published research, USGS geologist Belle Philibosian concluded that the West Napa Fault stretches from just north of St. Helena and past Calistoga — about 9 miles farther north than previously thought.

She also concluded that the 37-mile fault, if it ruptured in its entirety, is capable of producing a 7.0 magnitude earthquake — which has about twice as much energy as a 6.8 magnitude earthquake that was previously believed to be the fault’s maximum.

“The northward extension was a little bit of a surprise,” Philibosian said. “We didn’t necessarily anticipate that there was an additional section of fault.” 

Philibosian believes that the 2014 earthquake put more stress on the newly revealed northern part of the fault, which stretches along the western edge of Napa Valley and did not rupture.

“That doesn’t necessarily mean an earthquake is imminent, but it increased the likelihood that it would happen sooner,” she said.  

But UC Davis geology Professor Michael Oskin, who has also studied the West Napa Fault, believes that the 2014 earthquake was not strong enough or positioned in such a way as to put the northern portion under significant stress. 

“It was a relatively small event compared to the size of the West Napa Fault, so there’s nothing to that which should cause great alarm,” Oskin said. 

Still, he said, the 2014 quake is “useful as a reminder that that fault exists and it can do damage and that it can do more damage than what it did in 2014.”  

Philibosian estimated that each part of the West Napa Fault produces significant earthquakes of at least magnitude 6, rupturing several kilometers or more of the fault, every thousand years to every several thousand years. Smaller earthquakes can happen more frequently, she said, but faults like West Napa typically release most of their energy through earthquakes of magnitude 6 or higher, rather than many small ones. 

For the next significant earthquake, “sooner might mean within a century — that might be relatively soon as far as this fault is concerned — but it could also be tomorrow. That’s the uncertainty that we deal with,” she said.  

Philibosian said that if the entire West Napa Fault ruptured at once, in a magnitude 7 earthquake, the impacts to the Bay Area would be similar to the 6.9 magnitude Loma Prieta earthquake of 1989, which was centered in the Santa Cruz Mountains and caused a large section of Interstate 880 in Oakland to collapse.

Oskin agreed that the West Napa Fault is less active than the better-known San Andreas or Hayward faults, or the Rodgers Creek Fault, which is partly in Sonoma County and connects to the Hayward Fault. But he did not have an estimated timeline of how often the West Napa Fault may produce large earthquakes. 

“The earthquake process is not a clock,” he said. “It is very nonuniform in terms of the size and timing between events.”

Philibosian spent years combining laser topography data, aerial photos and field observations to create a more detailed and accurate map of the West Napa Fault. Oskin also studied the data earlier, and drew the same conclusion — that an unmapped northern extension of the fault exists. 

Beyond mapping the northern part of the fault, Philibosian created a more detailed image of the already known part of the fault. She hopes landowners proactively take precautions when building in areas along the fault. (California’s laws prohibit building on top of active faults — but it is not retroactive if structures were built on top of faults before the law came into effect in 1972.)

Philibosian’s new mapping has yet to be published. She expects to complete her work within the next year, though there are various internal editing processes it must then go through before it enters the USGS fault database. 

Reach Clare Fonstein: clare.fonstein@sfchronicle.com