California

Multiple earthquakes have hit Central California. What does this mean for the Big One?

Multiple earthquakes have rattled Central California in the last few days.

The activity, so close to the 30th anniversary of the devastating Loma Prieta earthquake, has left people wondering: Is it time to worry about the Big One?

The short answer to that is “No,” according to Annemarie Baltay, a seismologist with the U. S. Geological Survey.

Baltay said it’s unlikely the area will see a large earthquake such as a magnitude-8 temblor. But she advised that people to think about how prepared they are for future earthquakes.

What does this series of earthquakes mean?

The series of quakes began Tuesday with a magnitude-4.7 earthquake in Tres Pinos near Hollister and continued through Thursday.

“I think those earthquakes are earthquakes we expect to happen,” Baltay said

She noted that earthquakes with a magnitude of 4 or larger generally occur in that area on average about once every two years, with 11 happening in the last 20 years. That area is an approximately 12-mile radius around Tuesday’s earthquake.

“It confirms and reinforces all of our modeling and understanding of earthquakes in the region,” Baltay said. “It reminds us that these faults are active and behaving the way we expect them to behave.”

In the approximately 30-mile radius around Tuesday’s quake, which stretches from around Gilroy to Big Sur, south to King City and east to the Central Valley, there have been 23 earthquakes with a magnitude 4 or greater in the last 20 years, Baltay said.

The largest of those was the Tuesday quake, Baltay said.

Earthquakes happen when the different sides of a fault get stuck as they move past each other. Stress builds up, and when they release and start moving again, that causes an earthquake.

The section of the San Andreas fault where the Tuesday earthquake occurred is “creeping,” which means stress doesn’t build up quite so much and it appears to move at a smoother pace, Baltay said. That’s why earthquakes greater than a magnitude 4.0 occur in that area with some regularity.

In Central California, Baltay said Tuesday’s earthquake appears to have been the main event, while a 3.4-magnitude quake northwest of Pinnacles on Wednesday and a 3.6-magnitude temblor near Pinnacles on Thursday look like aftershocks.

Baltay said that’s a pretty typical sequence, and aftershocks can happen if the fault doesn’t move as much as it should have. She said it’s unlikely a bigger quake could happen in the area.

“We don’t have evidence that this part of the fault can participate in a much larger earthquake, but we do see these characteristic, magnitude 4.5-ish events,” Baltay said.

Are all the earthquakes that happened this week related?

The earthquakes that rattled the Bay Area on Monday are not related to the series of quakes in the Central California area, Baltay said. All of the faults are part of the San Andreas fault system, but the quakes occurred far away from each other.

“The fact that they occurred within a day of each other is statistically random,” Baltay said.

Can we expect a big earthquake anytime soon?

A 30-year forecast put out by the the USGS in 2014 shows that a magnitude 6.7 earthquake is about 72% likely to occur in the Bay Area between now and 2043, Baltay said.

“More likely than not you will experience it in the next 25 years, so be prepared,” Baltay said. She added that an earthquake of that size would likely occur along the Hayward Fault, which runs along the East Bay.

In general, a huge earthquake such as a magnitude-8 quake is “incredibly unlikely,” Baltay said.

She noted that the San Andreas Fault did experience a magnitude-7.9 earthquake more than a century ago, during the 1906 San Francisco earthquake. But in general, an earthquake of that size is improbable.

“These are chaotic systems and we can never say for certain what will happen,” Baltay cautioned.

What can I do to be prepared?

“Take the opportunity to think about what you should do right now,” Baltay said, advising that people try not to get too scared when an earthquake occurs.

“Practice ‘Stop, drop and hold on,’ ” Baltay said.

She said people should think about the places where they spend their time and what they would do if an earthquake happens when they’re at work, for example, or another place where they spend a lot of time.

Baltay also said people should talk to their children about what to do if an earthquake hits, and think about how families will communicate if modern communications systems go down.

She added that people should be sure to have a three days’ supply of food and water in their home and figure out a plan for their pets.

“You should view these earthquakes as a reminder that we live in earthquake country, and be prepared,” Baltay said.

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Gabby Ferreira is a breaking news and general assignment reporter at The Tribune in San Luis Obispo. A native of Houston, Texas, she was a reporter in Tucson, Arizona; Indianapolis, Indiana; and Palm Springs, California, before moving to San Luis Obispo County in 2016.
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