The earthquake swarm that has shaken San Ramon for more than two months appears to be over, scientists said Monday. But when it comes to earthquake swarms, nothing is set in stone.
Since Nov. 9 there have been 91 earthquakes of magnitude 2.0 and above — the largest being a 4.0 on Dec. 19 — in the East Bay community. But no quake that size has happened since Jan. 10, when a 2.3 and 2.0 both occurred there.
“There’s been a gap for about two weeks,” said Sarah Minson, a research geophysicist with the U.S. Geological Survey’s Earthquake Science Center at Moffett Field. “It looks like the swarm may be done.”
The challenge, she noted, is that scientists are still learning about earthquake swarms, which are collections of dozens or hundreds of small usually harmless quakes. And they don’t have strict parameters for when swarms start or conclude.
“There isn’t a good way to officially define when swarms begin and end,” Minson said. “If there was more activity now we would probably call it a new swarm.”
David Schwartz, a geologist and scientist emeritus with the U.S. Geological Survey who lives in Danville, 10 miles from San Ramon, agreed.
“The swarm has been subsiding. It appears to have quieted down,” he said. “But these swarms aren’t like having a large earthquake on a fault followed by a series of aftershocks. They are still a bit of a mystery.”
Many of the quakes that have jostled San Ramon in recent months are so small that people living in the suburban Contra Costa County town of 85,000 didn’t feel most of them. And since the swarm began in November, the mini-quakes have caused no damage or injuries, San Ramon Mayor Mark Armstrong said Monday.
“A lot of people who never experienced earthquakes were worried,” said Armstrong, who worked as a supervisor at the Federal Emergency Management Agency from 2011 to 2018. “These weren’t that big of a deal. I could hear dishes rattle in our China cabinet a few times. But it was just a short jolt and it was over.”
According to the U.S. Geological Survey database, there were 471 earthquakes — some as tiny as magnitude .3 — that have occurred between Nov. 9 and Monday in a roughly 1-mile radius of San Ramon. Most were below 2.0.
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. They occurred in 1970, 1976, 2002, 2003 and 2015. None caused significant damage or injuries.
The last one, in 2015, had 90 quakes larger than 2.0 between Oct. 13 to Nov. 16, Minson noted.
Although quake swarms can increase the chance of a larger earthquake, it’s only by a very small amount, scientists said Monday. Most important, in none of the previous San Ramon swarms has a major earthquake been triggered on the Calaveras Fault or other nearby faults.
“These are mostly likely harmless,” said Roland Bürgmann, a professor of earth and planetary science at UC Berkeley and the UC Berkeley Seismology Lab. “But they are a good reminder that we should be ready for earthquakes.”
He noted that it can be difficult to know precisely when one swarm ends and another begins.
“It’s a tricky question,” Bürgmann said. “Some of the swarms in the past seemed to go away and then they came back. When do you call something a new sequence as opposed to being a continuation?”
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, perhaps influenced by changes in fluids miles below the Earth’s surface.
“It’s like dropping a piece of pottery and having pieces of different sizes spread across the floor,” he said in November when the latest swarm began. “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 added.
The 2015 swarm in the San Ramon area lasted 36 days and produced 654 tiny earthquakes, the largest a magnitude 3.6.
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.
San Ramon is special, Minson said, and swarms of small quakes happen there roughly once a decade on average.
“It’s a unique place in the Bay Area,” she said. “If you are there you should expect more of this as time goes on. And if you aren’t in the San Ramon Valley this isn’t very likely to happen.”
Armstrong, the mayor, said the swarm has been a topic of conversation for San Ramon locals, and something the city’s emergency responders have followed, but thankfully not much more than that.
“When I looked at some of epicenters on these they looked like they were right behind my house,” he said laughing. “It was a novelty. It was something to talk about. But it really wasn’t anything to be worried about.”
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