Earthquake swarm agitates California

LOS ANGELES — 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 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 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. 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,” California Institute of Technology 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.

While there’s only a slim chance any particular earthquake will trigger something far worse, experts say, it’s important to not completely relax.

When a swarm hit central Italy in 2009, according to seismologist Tom Jordan, one civil protection official sought to calm residents’ jitters by telling reporters: “The scientific community tells us there is no danger, because there is an ongoing discharge of energy. The situation looks favorable.”

A few hours after a magnitude 3.9 earthquake jolted the town of L’Aquila that April 5, a magnitude 6.3 struck, killing more than 300.