Swarms of earthquakes shake California. What does it mean for the big one

For more than a month, the Bay Area has been hit by a seemingly nonstop stream of earthquakes — the latest in a series of seismic swarms that have rattled windows and raised fears across California.

The last swarm was stationed in the East Bay suburb of San Ramon, where a 4.0 magnitude earthquake struck on Friday night – the largest since the group began – and a 3.9 magnitude earthquake on Saturday evening. Since November 9, at least 80 earthquakes of magnitude two or more have occurred in the region, according to the US Geological Survey.

While none of these quakes were particularly strong — as was also the case in similar swarms recorded in Malibu, El Sereno, Ontario since 2024 — some fear that the ongoing seismic activity could herald a background concern for many Californians: the “big one.”

But while experts acknowledge that some major earthquakes are preceded by more modest earthquakes, they warn that swarms by themselves may not provide a hint as to when or where the next big quake will strike.

“There will be a big earthquake in the Gulf region,” said Anne Marie Baltay, a seismologist with the US Geological Survey. “We can’t say exactly when and where. So you have to be prepared for that.”

The risk of a major earthquake always looms in California, no matter how small tremors appear. There is a 60% chance of an earthquake of magnitude 6.7 or greater in the Los Angeles area by 2043, and a 72% chance of the same happening in the San Francisco Bay Area, according to a 30-year estimate calculated by the USGS in 2014.

Throughout California, there is also a 48% chance of an earthquake of magnitude 7.5 or greater by 2043, and a 7% chance of an earthquake of magnitude 8 or greater.

It is true that after every earthquake, there is a 1 in 20 chance that a larger one will follow. But traditionally, only half of earthquakes have easily detectable foreshocks, while the other half do not.

Southern California has provided some recent examples of swarms that eventually vanished. The Malibu area has been hit by three scary earthquakes recently — a 4.6 magnitude earthquake on February 9, 2024; 4.7 magnitude on September 12, 2024; And 4.1 degrees on March 9. But the situation has been calmer in the past six months.

Ontario suffered a wave of tremors a little over a year ago, culminating in a magnitude 4 earthquake on Oct. 6, 2024. But there have been far fewer smaller quakes so far this year.

A swarm of earthquakes centered around El Sereno on the East Side of Los Angeles was also detected last year, although this activity appears to have subsided after a 4.4 magnitude earthquake on August 12, 2024.

The cluster of small San Ramon earthquakes in recent weeks is also not very unusual. Earthquakes occur at the northern end of the Calaveras Fault where it extends to Mount Diablo.

Including the current swarm and one last month, there have been nine swarms of earthquakes in the surrounding area since 1970. They lasted from two to 42 days, and their maximum magnitude was generally in the 3 to 4 magnitude range, according to Baltay.

“This is still really consistent with these types of swarms,” Baltay said of the current activity.

Certainly, for residents who feel frequent small quakes, “it's really worrying…you don't know when it's going to end,” she said.

None of those previous swarms caused a major earthquake on the northern part of the Calaveras Fault, which has not ruptured catastrophically in recent history.

However, the Calaveras fault is a fault to watch in California. It is considered one of the faults most likely to rupture in a major earthquake in the Gulf region, and has the potential to cause an earthquake with a magnitude of 6.7 or more. In 1984, part of the Calaveras Fault ruptured and caused the 6.2 magnitude Morgan Hill earthquake, resulting in 21 people with minor injuries and $7.5 million in damage.

Also of concern are the Hayward Fault and the notorious San Andreas Fault, according to the USGS.

The small earthquakes that struck Southern California have aroused great, and not entirely unwarranted, interest in the past year.

In fact, 2024 ended up being a record year of sorts for even modest earthquakes in Southern California — there were 15 independent earthquake sequences that year with at least one quake of magnitude 4 or higher, according to seismologist Lucy Jones. This was the highest total in 65 years, surpassing the 13 figure set in 1988.

But last year's numbers probably didn't mean much in the larger scheme of things. In 2025, there were only five independent seismic sequences with at least one earthquake of magnitude 4 or higher in Southern California.

However, there have been cases where relatively modest earthquakes have preceded massive earthquakes.

In 1857, a massive 7.8 magnitude earthquake occurred along the 225-mile length of the San Andreas Fault, extending from Monterey County to Los Angeles County. That major quake struck about 8:30 a.m. on January 9, 1857, but it was preceded in Monterey County by a 5.6-magnitude earthquake an hour earlier, and a 6.1-magnitude earthquake an hour earlier.

(Raul Ranua/Los Angeles Times)

Then there were the 2019 Ridgecrest earthquakes in the Mojave Desert. A 6.4 magnitude earthquake occurred on July 4 at 10:33 AM, but this was followed by a much larger 7.1 magnitude earthquake at 8:19 PM the next day.

Animation does not reflect actual speed.

(Swetha Kannan / Los Angeles Times)

There were also a few horrific months in 1992. A 6.1-magnitude earthquake on April 22 of that year alarmed scientists because its epicenter was only about 6 miles from the San Andreas Fault. After the Joshua Tree earthquake, the Governor's Office of Emergency Services issued a warning for Southern California, warning people to be vigilant in anticipation of another, larger earthquake.

In the immediate aftermath, “nothing happened, but we kept having all these aftershocks up there near San Andreas and it made us uncomfortable and worried about it,” Jones said. Aftershocks eventually moved northward away from San Andreas.

But those aftershocks crept toward the final epicenter of a much stronger earthquake in the Mojave Desert — the 7.3 magnitude Landers earthquake on June 28, 1992. That quake triggered aftershocks that also reactivated seismic activity around Joshua Tree.

“We now have a continuous series of aftershocks reaching the San Andreas,” Jones said.

Hours after the Landers quake, a 6.3-magnitude quake occurred at Big Bear, “which was on a different fault, but the aftershocks to Big Bear also extended into the San Andreas at 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 highways, a call that came while the collapse of highways and bridges from the 1989 Loma Prieta earthquake was still on the minds of many.

A California Highway Patrol officer checks damage to cars that fell when the upper deck of the Bay Bridge collapsed onto the lower deck.

(George Nikitin/Associated Press)

However, nothing happened on the San Andreas Fault in 1992.

“This is probably the closest prediction we've ever come to,” Jones said. “We were very nervous about this.”

However, Jones said there have been other major quakes without easily noticeable tremors. These include some of the most devastating disasters in recent memory, such as the 1994 Northridge earthquake, the 1987 Whittier Narrows earthquake, and the 1971 Sylmar earthquake.

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