A magnitude 3.3 earthquake was felt in parts of Southern California on Sunday afternoon, one in a series of quakes to hit the Riverside County area.

The latest quake occurred at 4:36 p.m. and was centered on Glen Avon, south of Fontana. The seismic activity continued into Monday morning with dozens of new small quakes.

According to the U.S. Geological Survey, the quakes were felt across swaths of the Inland Empire, a region that has had hundreds of small quakes over the weekend, including one that registered a magnitude of 3.1.

On Friday, seismologist Lucy Jones wrote on Twitter that small earthquakes can be felt if they’re shallow — only 1 mile underground instead of 5 miles or more. She called the area a “perennial hotspot” of small, clustered quakes and nothing out of the ordinary for California.


“This is just a place where the earth sputters along instead of letting go all at once,” she said.

This is the Fontana trend - a perennial hotspot of small quakes, including a M3.1 in Jan 2018 and a M3.6 in July 2018. And when it has quakes, they are usually in clusters like this. In other words, ordinary, common California quakes https://t.co/NcjXSupiTv — Dr. Lucy Jones (@DrLucyJones) May 30, 2019

It has been almost five years since California experienced its last earthquake of magnitude 6 or stronger. But scientists say the calm period will end, with warnings about seismic dangers ranging from the Big One on the San Andreas fault to catastrophes that could come from lesser-known faults, such as the Hayward or Newport-Inglewood.

Researchers also recently discovered that the state has experienced 10 times more earthquakes than previously known. They documented 1.8 million earthquakes in Southern California over the last decade, the vast majority so small that they had been undetectable to computing systems. Previously, only 180,000 earthquakes were on record for the last 10 years.


Scientists are now better able to identify previously undiscovered faults, detect patterns of moving earthquake swarms — like the one in Riverside County — and identify faint clusters of foreshocks that occur before a larger earthquake.