Sep 16, 2022 - News

A reality check on SF Bay Area earthquakes

Illustration of a seismograph reading in the shape of an exclamation point.
Illustration: Brendan Lynch/Axios

Small earthquakes, like the recent tremors in the San Francisco Bay Area, are quite common and indicate there are faults accumulating stress that needs to be released, two earthquake experts told Axios.

Driving the news: A 4.4-magnitude quake with an epicenter 2 miles north of Santa Rosa shook the SF Bay Area on Tuesday, followed by a 4.3-magnitude aftershock.

  • On Thursday, a 2.9-magnitude quake had an epicenter 2 miles east-southeast of Berkeley.

Why it matters: There's collective anxiety around when the "big one" will hit. The U.S. Geological Survey predicts that a 1906-type earthquake occurs at intervals of about 200 years.

  • That early 20th century earthquake, the most powerful in Northern California's recorded history, was a magnitude 7.9.
  • But the "real threat" to the Bay Area is from smaller magnitude earthquakes of about 7.0 on the Hayward, San Andreas or Rodgers Creek faults expected before 2032, per the USGS.
  • The 1989 Loma Prieta earthquake, for example, was a magnitude-6.9 quake that caused extension damage to parts of the SF Bay Area, resulting in 63 deaths, 3,757 injuries and about $6 billion in damage.
What they're saying:

Of the recent tremors felt around the Bay Area, Austin Elliot, research geologist at the U.S. Geological Survey, said, “It would require hundreds of magnitude 4s to release the stress that a magnitude 7 releases. Big earthquakes are inevitable, and these smaller ones should count as lucky reminders to get prepared."

Ross Stein, former scientist at the USGS and CEO at seismic analytics company Temblor, agrees, noting: "Not only are little earthquakes not good in terms of their ability to relieve the stresses [of the faults], the little earthquakes are the seeds.

  • And sooner or later, one of those little babies is going to manage to grow into something big."

Yes, but: While aftershocks are common, "only a small fraction of earthquakes are directly followed by a larger one," Elliot explains.

  • There's about a 5% chance of an aftershock that is larger than the initial quake, and "that likelihood decays rapidly within a few days," Elliot says.

Between the lines: Much of what we all love about the SF Bay Area, Stein says, is thanks to the area's earthquake faults.

  • The San Andreas fault, for example, "has uplifted the coast ranges, given us our wonderful climate" and created Napa Valley, Silicon Valley and Lake Tahoe, Stein says.
  • The SF Bay Area would be "flat as a pancake if it weren't for earthquakes, because they've created and sculpted the landscape for us," Stein says.

Of note: Many residents were notified of the 4.4-magnitude quake via the MyShake early earthquake warning system.

What's next: There are ways to live safely with earthquakes, Stein says.

  • Making sure where you live is seismically retrofitted is a good start, he says. In San Francisco, the city has required certain types of buildings to retrofit their foundations in preparation for the next big earthquake.
  • Stein has also equipped himself with an international orange whistle, an earthquake emergency kit for his car and home, extra water on-hand and created a communication plan with his family.
  • "You can take precautions that make you live as safely as you can with earthquakes," he says. "They're infrequent but the consequences can be really large and the goal is to be ready."
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