One swarm in 2015 generated 4,000 quakes over five months, according to the Berkeley Seismology Lab. In the Bay Area, the San Ramon Valley has had many swarms over the last several decades that haven’t resulted in large earthquakes, Llenos said. What to do before - and during - a big one » But it’s not particularly close to any major faults. There’s an ongoing earthquake swarm around the town of Cahuilla, about 20 miles east of Temecula in Riverside County, that started in 2016 and is moving westward and getting shallower, probably triggered by the movement of groundwater. They include the Salton Sea geothermal field in Imperial County, the Coso volcanic field of Inyo County, Mammoth Mountain in Mono County and the Geysers geothermal field in Lake, Mendocino and Sonoma counties. Places that have fluids moving around underground, where magma can heat up groundwater, are more likely to have swarms. There was the 2015 Fillmore swarm in Ventura County, for example, with more than 1,400 quakes, maxing out at magnitude 2.8.Īccording to Jones, there’s nothing particularly more ominous about swarms versus a single small temblor. Most swarms aren’t cause for concern and can be thought of simply as “a bunch of small earthquakes that are more of an irritant than otherwise,” Caltech seismologist Egill Hauksson said. That’s why, Llenos said, “it’s probably not going to affect the likelihood of larger earthquakes happening.” The Fontana-area earthquake swarm that began May 25, with its largest event a magnitude 3.2, was much less of a concern - it’s quite a ways away from the San Andreas and San Jacinto faults, two of California’s scariest. (There’s only a 5% chance any particular earthquake will be followed up by something larger.) “Any earthquake has a slight increase in the chance of having something” worse coming next, she added. And the idea that it makes it certain to happen doesn’t work,” Jones said. “The idea that little earthquakes make the Big One less likely doesn’t work. We’re going to have the Big One,’ ” seismologist Lucy Jones said. “Half the people are saying, ‘Oh, they’re having a lot of earthquakes - it gets rid of the energy, it makes us safer.’ And half of them are going, ‘Oh, my God, we’re having earthquakes. There have long been myths associated with small quakes.
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