Residents of a suburban East Bay community have spent the past 10 days dealing with a jarring trend — not repeated rain storms or the end of baseball season. Earthquakes. Lots of them.
Since Nov. 9 there have been 83 earthquakes in a roughly 1-mile radius around the town of San Ramon. The earthquakes, which geologists call a swarm, are small, ranging in size from .6 magnitude to 3.8.
Six, including one Monday night at 10:47 pm were 3.0 or above — generally large enough for most people to feel. Residents have definitely noticed, leaving some to wonder if a bigger quake is on the way.
“You just don’t know when the next one’s going to come” said Nita Jain, who was helping her children get ready for bed during Monday’s shaker. “You just hope that the house holds up.”
Normally, earthquake swarms happen near volcanoes or geothermal fields.
But the geology under the San Ramon Valley — which runs roughly from Walnut Creek to Dublin along Interstate 680 — is a complex mix of small faults, many of them without names, between the Calaveras Fault and Mount Diablo that combine to occasionally trigger the flurries of small quakes, scientists say.
There have been five other significant earthquake swarms in the San Ramon Valley since 1970, some lasting as long as a month.
And although quake swarms can increase the chance of a larger quake, it’s only by a very small amount, scientists said Tuesday. Most important, in none of the previous cases has a major earthquake been triggered.
“These things turn on and turn off,” said David Schwartz, a geologist and scientist emeritus with the U.S. Geological Survey who lives in Danville. “They typically have not led to larger earthquakes. But they do scare the heck out of people.”
The other major quake swarms in the area occurred in 1970, 1976, 2002, 2003, and 2015.
Schwartz said that the small quakes are not occurring on the Calaveras Fault, a major fault that runs from Hollister through San Jose to Danville.
Rather, he said, they are happening on a series of small, unnamed faults that form a complicated geological landscape between Mount Diablo and the Calaveras Fault, with pressure and stress coming from multiple larger faults in different directions.
“It’s like dropping a piece of pottery and having pieces of different sizes spread across the floor,” he said. “That’s what you have in the San Ramon Valley. A fractured-up area with a lot of small faults. Sometimes they light up.”
Nor do the flurries of small quakes relieve pressure on the Calaveras Fault or make a big earthquake less likely, he said.
“This event shows there’s a high level of stress in the area, and small little faults are reacting to it,” Schwartz said. “Fluids and water may play a role. But there’s nothing here saying it is relieving stress on the Calaveras Fault.”
Nobody knows how long the quake swarm will continue.
The last significant quake swarm in the San Ramon area, in 2015, lasted 36 days and produced 654 tiny earthquakes, the largest a magnitude 3.6, said Roland Bürgmann, a professor of earth and planetary science at UC Berkeley and the UC Berkeley Seismology Lab.
In many cases, underground fluid — such as magma from a volcano — can set off regular flurries of earthquakes in some parts of the world by changing pressure in the cracks of rocks deep below the earth’s surface.
In San Ramon, there are no volcanoes. So it’s likely that water is contributing, he said. Although the area has received regular rain over the past month, Bürgmann said the rain probably didn’t trigger these earthquakes because they are happening between 3 and 5 miles below the surface, too deep for rainwater to percolate in such a short time.
Other earthquake swarms since 1970 have occurred in the San Ramon Valley during months when there was no rain, like August, he noted.
“This area is special,” said Sarah Minson, a research geophysicist with the U.S. Geological Survey’s Earthquake Science Center at Moffett Field. “There is probably something complex happening with the cracking and geometry of the faults there that is letting fluids flow in a way that is causing lots of little baby earthquakes.”
And for people who are worried?
“People probably shouldn’t be concerned,” Minson said. “Is this going to cause a ginormous earthquake? This place has had swarms before and nothing larger happened.”
“There are a bunch of little cracks that are letting fluid flow and causing these small quakes,” she added. “It doesn’t look like the behavior you see with large earthquakes.”
Other places in California have occasional quake swarms, scientists say, including the Geysers in Sonoma and Lake counties, Mammoth Lakes in the Sierra Nevada, and Brawley in Imperial County near the California-Mexico border.
“Usually these clusters come and go and are over in a few weeks,” Bürgmann said.
For now, San Ramon’s residents are trying to take it all in stride.
“They are a little bit nerve-wracking,” Jain said. “But they’re so quick and short, and then nothing really happens. The power doesn’t turn off, the lights don’t flicker, everything is perfectly fine. There’s just a little wiggle and a shake.”
Another resident, Dustin Lopes, was watching television to wind down when he felt a “quick jolt” Monday night. But the 43-year-old was unfazed.
“This one felt like less than what we’d experienced last week,” he said. “It was just enough to make my son run in the room and say, ‘Was that you jumping across the house?’”
He knows he lives in earthquake country. The quake 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 in time, there will be a big, major quake,” Lopes said. “I think you just have to realize that that may happen and hope that you’re ready for when it does.”
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