After smaller quake, 3.9-magnitude earthquake hits Inland Empire — followed by a dozen aftershocks

More than a dozen aftershocks followed a 3.9-magnitude earthquake that struck the Inland Empire Saturday — a quake that was felt by many and reported just 30 minutes after a 3.5-magnitude quake rattled the same area.

The first temblor was reported at 10:05 a.m. and had an epicenter about three miles southwest of Ontario, according to the U.S. Geological Survey. Soon after, at 10:34 a.m., a 3.9-magnitude quake shook the area.

Since the first earthquake, there were a total of 32 smaller quakes within the next three hours — including tiny 1.0-magnitude tremors and quakes of up to a 2.6 magnitude — all with epicenters located within two square miles of each other, USGS reports. And that includes more than a dozen aftershocks that followed the 3.9 quake.

Seismologist Dr. Lucy Jones described the collection of tremors as a "small swarm" that occurred in what's known as the Fontana Seismicity Lineation. Saturday's earthquake was the largest one in that particular area of San Bernardino County since 1953, when there was another 3.9-magnitude quake, according to Jones. 

"I've seen this type of cluster in Fontana dozens of times in my decades watching SoCal quakes," Jones wrote in a post on X.

Brian Olson, geologist with the California Geological Survey, said the 3.5-magnitude quake that struck first could be considered a foreshock given how significant the later one was, as that specific area hasn't seen such a large temblor in decades.

This year has been the most seismically active in Southern California since 1988, seismologists said following a string of earthquakes in the region earlier this summer. An area of Los Angeles County near Pasadena was struck by a 3.5-magnitude earthquake in June and another 4.4-magnitude temblor in August. 

While a 4.4 magnitude isn't necessarily high enough to cause serious damage, many across the county said they felt the shaking. At the time, South Pasadena Police Chief Brian Solinsky said there had been no reports of injuries or damage but that may have been due to how well buildings have been retrofitted.

"This one easily could have broken some windows," he said.

Just a week before, a larger 5.5-magnitude quake rattled a much wider stretch of Southern California. While it had an epicenter in Central California, near Bakersfield, people across areas of Los Angeles and Ventura counties felt the shaking. 

In fact, LA Mayor Karen Bass announced the LA Fire Department had deployed helicopters to survey the city for any damage.

It was followed by more than 400 aftershocks, including tiny tremors of less than 1.0 magnitude, over the next two days. 

"Each of those aftershocks could trigger another earthquake," said John Vidale, a seismologist at USC.

"This sequence had more aftershocks than most," Vidale said. "And so that also raises the odds of future activity a little bit."

Of the hundreds of aftershocks, two that happened within just an hour of the quake had a magnitude higher than 4.0.

"That's a pretty active sequence for this very short amount of time since the event," Jones said at the time.