Earthquake swarm shakes the East Bay

Residents of an East Bay suburb have spent the past 10 days dealing with a contradictory trend — not recurring rainstorms or the end of the baseball season. Earthquakes. Lots of them.

Since November 9, 83 earthquakes have occurred within a roughly one-mile radius around the city of San Ramon. Earthquakes, which geologists call swarms, are small in size, ranging in magnitude from 0.6 to 3.8 magnitude.

Six of them, including Monday night at 10:47 p.m., had temperatures of 3.0 or higher — generally significant enough for most people to feel them. Residents have certainly noticed, making some wonder if a larger quake is on the way.

“You don't know when the next one is going to come,” said Neeta Jain, who was helping her children get ready for bed during Monday's jolt. “You just hope the house holds up.”

Typically, earthquake swarms occur near volcanoes or geothermal fields.

But the geology beneath the San Ramon Valley — which runs roughly from Walnut Creek to Dublin along Interstate 680 — is a complex mix of small faults, many without names, between the Calaveras Fault and Mount Diablo, which come together to sometimes cause bouts of small earthquakes, scientists say.

There have been five other large earthquake swarms in the San Ramon Valley since 1970, some lasting up to a month.

Although earthquake swarms can increase the chance of a larger quake, it is only by a very small amount, scientists said Tuesday. Most importantly, in none of the previous cases did a major earthquake occur.

“These things turn on and off,” said David Schwartz, a geologist and scientist emeritus with the USGS who lives in Danville. “It usually doesn't lead to bigger earthquakes. But it scares people really bad.”

Other major earthquake swarms occurred in the region in 1970, 1976, 2002, 2003, and 2015.

Schwartz said small earthquakes do not occur on the Calaveras Fault, a large fault that runs from Hollister through San Jose to Danville.

Instead, he said, they occur on a series of small, unnamed faults that form a complex geological landscape between Mount Diablo and the Calaveras Fault, with pressure and strain coming from several larger faults in different directions.

“It's like dropping a piece of pottery and scattering pieces of different sizes on the ground,” he said. “That's what we have in the San Ramon Valley. A fractured area with a lot of little faults. And sometimes they light up.”

He added that small earthquake gusts do not relieve pressure on the Calaveras Fault or make a large earthquake less likely.

“This event shows that there is a high level of stress in the area, and that small faults are interacting with it,” Schwartz said. “Fluids and water may play a role. But there's nothing here that says it relieves pressure on the Calaveras Fault.”

No one knows how long the earthquake swarm will last.

The last major earthquake to hit the San Ramon area, in 2015, lasted 36 days and triggered 654 small quakes, the largest of which was a magnitude 3.6, said Roland Borgmann, a professor of Earth and planetary sciences at UC Berkeley and the UC Berkeley Seismology Laboratory.

In many cases, subterranean fluids – such as magma from a volcano – can cause regular waves of earthquakes in some parts of the world by changing the pressure in rock cracks deep beneath the Earth's surface.

In San Ramon, there are no volcanoes. Water likely contributes, he said. Although the area has received regular rain over the past month, Borgman said the rain probably did not cause these quakes because they occur 3 to 5 miles below the surface, too deep for rainwater to seep in in such a short time.

He noted that other earthquake swarms have occurred since 1970 in the San Ramon Valley during months without rain, such as August.

“This area is special,” said Sarah Minson, a research geophysicist at the USGS Earthquake Science Center at Moffett Field. “There's probably something complex going on with the cracks and fault geometry there that allows fluids to flow in a way that causes a lot of small earthquakes.”

And for people who are anxious?

“Maybe people shouldn't be worried,” Minson said. “Will this cause a massive earthquake? This place has had swarms before and nothing bigger has happened.”

“There are a bunch of small cracks that allow fluids to flow and cause these small earthquakes,” she added. “It doesn't look like the behavior you see with large earthquakes.”

Scientists say other places in California occasionally see swarms of quakes, including Geysers in Sonoma and Lake counties, Mammoth Lakes in the Sierra Nevada, and Brule in Imperial County near the California-Mexico border.

“Usually these groups come and go and are gone within a few weeks,” Borgman said.

For now, San Ramon residents are trying to take it easy.

“They're a little unnerving,” Jain said. “But it's very quick and very short, and then nothing really happens. No power goes off, no lights flicker, and everything is fine. There's just a little bit of shaking and shaking.”

Another resident, Dustin Lopez, was watching television to calm down when he felt a “quick jolt” Monday night. But the 43-year-old wasn't bothered.

“This seemed lower than what we saw last week,” he said. “It was enough to make my son run into the room and say, 'Was that jumping across the house?'

He knows that he lives in earthquake country. The swarm is a reminder of that, even though it has been 36 years since the last major earthquake hit the Bay Area, the Loma Prieta quake in October 1989.

“At some point, there's going to be a big, big earthquake,” Lopez said. “I think you just have to realize that it might happen, and hopefully you'll be prepared when it does happen.”

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