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.
There is general agreement that the recent swarm probably isn’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 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,” Llenos said, “you’re likely to increase the likelihood of a slightly larger earthquake happening.”