Small quakes keep rattling Bay Area. What’s up?

[Earthquake, from B1] San Ramon, one of the largest cities in Contra Costa County and the surrounding Tri-Valley area, is no stranger to earthquake swarms, according to Anne-marie Baltay, a U.S. Geological Survey seismologist.

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Seismic swarms have occurred in 2002, 2003 and 2015 in the San Ramon area, and in 2018 around nearby Danville — with similar characteristics and magnitudes, Baltay said. However, none of those swarms were precursors to a large destructive quake on the Calaveras fault, which has not occurred in the Tri-Valley area in the modern historical record.

Earthquake swarms in the past in this area have lasted for as few as two days, but the longest one was 42 days.

“It may be unsettling for folks who live there,” Baltay said.

But there is generally little predictive value in earthquake swarms.

Scientists say there’s a 5% chance that any particular earthquake will be followed by an even larger one nearby over the next week.

For instance, the magnitude 7.8 earthquake that struck the San Andreas fault through Central and Southern California in 1857 was preceded an hour earlier by a far-more-modest magnitude 5.6 quake in Monterey County, and a magnitude 6.1 earthquake an hour before that.