Rumbling rattles nerves, but not seen as warning

It is true that after every earthquake, there's a 1-in-20 chance it'll be followed by a larger magnitude temblor.

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But conventionally, only half of earthquakes have an easily detectable foreshock, while the other half do not.Southern California has provided some recent examples of swarms that ultimately fizzled out. The Malibu area had a trio of scary earthquakes recently — a magnitude 4.6 earthquake on Feb. 9, 2024; a magnitude 4.7 on Sept. 12, 2024; and a magnitude 4.1 on March 9. But it has been quieter in the last six months.

Ontario endured a spate of shaking a little more than a year ago, topping out with a magnitude 4.0 quake on Oct. 6, 2024. But there have been fewer, much smaller quakes so far this year.

An earthquake swarm centered in El Sereno on the Eastside also was detected last year, though that activity appears to have subsided after a magnitude 4.4 earthquake on Aug. 12, 2024.

The cluster of small San Ramon earthquakes in recent weeks is also not terribly unusual. The earthquakes are appearing on the northern end of the Calaveras fault as it runs into Mt. Diablo.

Including the current swarm and another last month, there have been nine earthquake swarms in the surrounding area since 1970. They have lasted two to 42 days, with maximum magnitudes generally in the 3.0 to 4.0 range, according to Baltay.

"This is still really consistent with these kinds of swarms," Baltay said of the current activity.

Certainly, for residents who feel the frequent small earthquakes, "it's really unsettling.... You don't know when it's going to end," she said.

None of those earlier swarms resulted in a major quake on the northern section of the Calaveras fault, which hasn't ruptured catastrophically in modern history.