Relentless earthquake swarms rattle California. What does that mean for the Big One?

Seismic swarms have rattled the Bay Area for more than a month, centered in San Ramon where a magnitude 4 and a 3.9 struck Friday and Saturday nights. Since Nov. 9, the U.S. Geological Survey recorded at least 80 earthquakes of magnitude 2 or greater in the area. Similar low-magnitude swarms occurred recently in Malibu, El Sereno and Ontario. Some residents fear a larger event, but seismologists caution swarms alone do not indicate when or where a major quake will occur. Thirty-year USGS estimates give a 72% chance of magnitude 6.7+ in the Bay Area and a 60% chance in Los Angeles by 2043; statewide chances include 48% for magnitude 7.5+ and 7% for magnitude 8+.

"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."

"While none of these quakes have 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." 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."

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