Map showing earthquake density in Southern California. Courtesy Scripps

Seismologists from Scripps Institution of Oceanography and Caltech have identified hundreds of thousands of previously unidentified tiny earthquakes that occurred in Southern California between 2008 and 2017.

The study, published Thursday online in the journal Science, expands the earthquake record for the region and period of time by a factor of 10—from about 180,000 recorded earthquakes to more than 1.8 million.

The new data reveals there are an average of 495 earthquakes daily across Southern California, occurring at roughly three minutes apart—a much faster rate than thought.

The researchers used new computational techniques to analyze old seismic data and uncovered tiny temblors, between magnitude 1.7 and 2.0, that were difficult to spot amid background “noise” like automobile traffic or building construction.

“It’s not that we didn’t know these small earthquakes were occurring. The problem is that they can be very difficult to spot amid all of the noise,” said Zachary Ross, lead author of the study and a postdoctoral scholar in geophysics at Caltech.

Ross collaborated with Egill Hauksson, research professor of geophysics at Caltech, as well as Daniel Trugman of Los Alamos National Laboratory and Peter Shearer of Scripps.

“Seismologists have always been blessed with an abundance of data from networks of seismic stations all over the world,” said Shearer. “Only now are our computational tools beginning to take full advantage of all the information in the seismograms.”

The researchers relied on an array of 200 powerful graphics processing units that worked for weeks on end to scan for patterns in the old data and detect tiny quakes

The work was supported by the National Science Foundation and the U.S. Geological Survey.

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