Scientists are quite “concerned” about the huge earthquake swarm that has been shaking southern California in recent weeks, and right at this moment bubbling tar is literally coming up through the streets in one section of Los Angeles. None of this means that a major seismic event is imminent, but it is certainly not a good sign either.
The seismic storm that unleashed more than 1,000 small earthquakes in San Bernardino and Riverside counties these last three weeks elicited what has become a typical reaction in quake country.
To some, the “swarmageddon” 40 miles east of downtown Los Angeles brought fear that a bigger threat was coming. To others, as long as they didn’t feel a shake, it was easy to just put it out of their minds.
California has small quakes all the time — a magnitude 3 every other day on average. But not all of them act the same, and some bring more danger than others.
As officials install more seismic sensors as part of the state’s early warning system, experts are getting an increasingly better look at California’s smaller earthquakes.
There is general agreement that the recent swarm probably wasn’t a precursor to a catastrophic quake. But other small quakes — especially ones near major fault lines like the San Andreas — are potential warnings.
“I would redefine normal as: You should still be prepared for a large earthquake,”
U.S. Geological Survey research geophysicist Andrea Llenos said.
“We do know a big earthquake is going to happen” — just not when and where.
The Fontana earthquake swarm about 40 miles east of downtown Los Angeles began on May 25 and has produced more than 1,000 earthquakes.
The last time earthquake scientists were especially concerned in California about a large triggered earthquake was nearly three years ago.
On Sept. 26, 2016, a rapid succession of small earthquakes — the strongest a trio measuring above magnitude 4.0 — began rupturing under the Salton Sea close to the San Andreas fault.
Scientists worried those quakes could set off a domino effect, reawakening the southern San Andreas from its long slumber. That fault is capable of producing a magnitude 8.2 quake.
Their worst fears didn’t materialize.
But “any time you have an increase in the number of small earthquakes, you’re likely to increase the likelihood of a slightly larger earthquake happening,” Llenos said.
Small earthquakes on the southern edge of the San Andreas fault warrant concern, as they could trigger a major earthquake on California’s longest fault. (Lorena Elebee / Los Angeles Times)
There have long been myths associated with small quakes.
“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. We’re going to have the Big One,’” seismologist Lucy Jones said.
“The idea that little earthquakes make the Big One less likely doesn’t work. And the idea that it makes it certain to happen doesn’t work,” Jones said. “Any earthquake has a slight increase in the chance of having something” worse coming next, Jones said. “But mostly, it’s really small.”
(There’s only a 5% chance any particular earthquake will be followed up by something larger.)
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The Fontana earthquake swarm that began on 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. That’s why, Llenos said, “it’s probably not going to affect the likelihood of larger earthquakes happening.”
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. There was the 2015 Fillmore swarm in Ventura County, for example, with more than 1,400 quakes maxing out at magnitude 2.8.
According to Jones, there’s nothing particularly more ominous about swarms versus a single, small temblor.
Places that have fluids moving around underground, where magma can heat up groundwater, are more likely to have swarms. 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.
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. But it’s not particularly close to any major faults.
Other swarms are more concerning.
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. One swarm in 2015 generated 4,000 quakes over five months, according to the Berkeley Seismology Lab.
Still, that activity is occurring close to the Calaveras fault — capable of producing an earthquake as big as the magnitude 7 along the East Bay’s Hayward fault in 1868.
“Just because it’s something we haven’t had in the past doesn’t mean it’s not going to happen in the future,” Llenos said.