Between 2008 and 2017, Southern California was hit by 1.8 million earthquakes, 10 times more than previously thought, said a new study.
Seismologists at the California Institute of Technology found approximately 180,000 earthquakes had been recorded during that time.
Data showed the region experiences 495 quakes a day, or roughly one every three minutes. However, the reason these quakes are just being discovered is they're too small to notice.
"It's not that we didn't know these small earthquakes were occurring," said Zachary Ross, lead author of the study who will join Caltech's faculty in June as an assistant professor of geophysics, in a statement. "The problem is that they can be very difficult to spot amid all of the noise."
The study was published Thursday in the peer-reviewed journal Science.
Researchers say the temblors are tough to find because seismic data also includes background noise such as building construction and shaking from traffic.
To find the quakes, seismologists used a technique called "template matching," where an easily-identifiable earthquake signal is used as a template to find matching data indicating a temblor. Researchers also used an array of powerful computers to scan the earthquake catalog and verify the new earthquakes.
Scientists said most of the smaller temblors found were between negative magnitude 2.0 and 1.7.
In a statement, Michael Gurnis, director of the seismological laboratory and geophysics professor at Caltech, said the study "has opened a new window allowing us to see millions of previously unseen earthquakes and this changes our ability to characterize what happens before and after large earthquakes."
Follow Brett Molina on Twitter: @brettmolina23.
This article originally appeared on USA TODAY: Southern California experiences 'hidden' earthquakes every 3 minutes, says study