Pouring through 10 years' worth of Southern California seismic data with the scientific equivalent of a fine-tooth comb, Caltech seismologists have identified nearly two million previously unidentified tiny earthquakes that occurred between 2008 and 2017.

The new data reveal that there are about 495 earthquakes daily across Southern California occurring at an average of roughly three minutes apart.

Previous earthquake cataloging had suggested that approximately 30 minutes would elapse between seismic events.

This 10-fold increase in the number of recorded earthquakes represents the cataloging of tiny temblors, between magnitude 1.7 and 2.0, made possible by the broad application of a labor-intensive identification technique that is typically only employed on small scales.

The problem is that they can be very difficult to spot amid all of the noise," says Zachary Ross, lead author of the study and postdoctoral scholar in geophysics, who will join the Caltech faculty in June as an assistant professor of geophysics.

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 Institution of Oceanography at UC San Diego.

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