Relentless earthquake swarms rattle California. What does that mean for the Big One?
Mon, December 22, 2025
For more than a month, the Bay Area has been subjected to a seemingly ceaseless stampede of earthquakes — the latest in a series of seismic swarms that have rattled windows and raised fears across California.
The latest swarm has been centered in the East Bay suburb of San Ramon, where a magnitude 4 earthquake struck Friday night — the largest since the cluster began — and a magnitude 3.9 on Saturday night. Since Nov. 9, there have been at least 80 earthquakes of magnitude 2 or greater in the area, according to the U.S. Geological Survey.
While none of these quakes has been particularly potent — which was also the case in similar swarms recorded in Malibu, El Sereno and Ontario since 2024 — some fear the persistent seismic activity could foreshadow a back-of-mind concern for many Californians: the "Big One."
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But while experts acknowledge that some major earthquakes are preceded by more-modest temblors, they caution that the swarms by themselves probably don't offer a hint of when, or where, the next major quake will hit.
"There's gonna be a big earthquake in the Bay Area. We just can't say exactly when and where. So you should be prepared for that," said Annemarie Baltay, a U.S. Geological Survey seismologist.
The threat of a large earthquake always looms in California regardless of the appearance of small shakers.
There is a 60% chance of an earthquake of magnitude 6.7 or greater in the Los Angeles region by 2043, and a 72% chance of the same in the San Francisco Bay Area, according to 30-year estimates calculated by USGS in 2014.
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