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 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.
“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.
Across California, there’s also a 48% chance of a magnitude 7.5 quake or greater by 2043, and a 7% chance of a magnitude 8 or greater quake.
It is true that after every earthquake, there’s a 1-in-20 chance it’ll be followed by a larger magnitude temblor. 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 quake on Oct. 6, 2024. But there have been fewer, much smaller quakes so far this year.
An earthquake swarm centered around El Sereno in Eastside Los Angeles also was detected last year, though that activity appears to have subsided following 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 Mount Diablo.
Including the current swarm and another last month, there have been nine earthquake swarms in the surrounding area since 1970. They have lasted anywhere from two to 42 days, with maximum magnitudes generally in the 3 to 4 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 Calaveras fault, which hasn’t ruptured in modern history.
Still, the Calaveras fault is one to watch for California. It’s considered one of the most likely faults to rupture in a major quake for the Bay Area, and has the potential to trigger a magnitude 6.7 quake or greater.
Also of concern are the Hayward fault and the notorious San Andreas fault, according to the USGS.
The smattering of small earthquakes in Southern California generated significant, not altogether unwarranted, attention last year.
In fact, 2024 ended up being a record of sorts for modest quakes in Southern California — there were 15 independent earthquake sequences that year with at least one magnitude 4 or higher earthquake, according to seismologist Lucy Jones. That was the highest total in 65 years, surpassing the 13 seen in 1988.
But last year’s figure probably didn’t mean much in the greater scheme of things. In 2025, there have been only five independent seismic sequences with at least one magnitude 4 or higher earthquake in Southern California.
That said, there have been situations when relatively modest quakes have preceded huge ones.
In 1857, a mammoth magnitude 7.8 earthquake ruptured along 225 miles of the San Andreas fault, stretching from Monterey County to Los Angeles County. That Big One hit at around 8:30 a.m. on Jan. 9, 1857, but it was preceded in Monterey County by a magnitude 5.6 earthquake an hour earlier, and a magnitude 6.1 earthquake an hour before that.
And then there were the 2019 Ridgecrest earthquakes in the Mojave Desert. A magnitude 6.4 earthquake ruptured July 4 at 10:33 a.m., but that was followed by a much larger magnitude 7.1 earthquake at 8:19 p.m. the next day.
There were also a few harrowing months in 1992. A magnitude 6.1 Joshua Tree temblor on April 22 of that year made scientists nervous because its epicenter was only about 6 miles away from the San Andreas fault. After the Joshua Tree earthquake, the Governor’s Office of Emergency Services issued an advisory for Southern California, warning people to be on alert for another, larger earthquake.
In the immediate aftermath, “nothing happened, but we kept on having all these aftershocks sitting there right near the San Andreas and making us uncomfortable and worrying about it,” Jones said. The aftershocks eventually migrated north, away from the San Andreas.
But those aftershocks crept toward the eventual epicenter of a far more powerful earthquake in the Mojave Desert — the magnitude 7.3 Landers earthquake of June 28, 1992. That quake resulted in aftershocks that also reactivated seismic activity around Joshua Tree.
“So we now had a continuous line of aftershocks coming all the way down to the San Andreas,” Jones said.
Hours after the Landers earthquake, a magnitude 6.3 earthquake struck in Big Bear, “which was on a different fault, but also the aftershocks to Big Bear extended down to the San Andreas in a different location,” Jones said.
The result was so alarming to scientists and officials that the Governor’s Office of Emergency Services issued what was then an unprecedented advisory urging people to stay off freeways, a call that came as freeway and bridge collapses from the 1989 Loma Prieta earthquake were still on the minds of many.
Nothing ended up happening on the San Andreas fault in 1992, however.
“It’s probably the closest we’ve come to a prediction, ever,” Jones said. “We were really pretty nervous about it.”
However, there have been other major earthquakes with no easily noticeable foreshocks, Jones said. Those include some of the most destructive in recent memory, such as the 1994 Northridge earthquake, the 1987 Whittier Narrows earthquake and the 1971 Sylmar earthquake.
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