More than a million tiny earthquakes revealed in Southern California (and Baja)

In between the “big ones,” millions of tiny, undetected earthquakes rumble through the ground. Now, a new study uncovers a decade’s worth of such “hidden” quakes in Southern California, increasing the number of quakes logged in the region tenfold. Such troves of quake data could shake up what’s known about how temblors are born belowground, and how they can interact and trigger one another, researchers report online April 18 in Science.

The researchers used a technique called template matching to mine an existing archive of earthquakes, recorded by seismometers and other instruments in the region from 2008 to 2017. The team was searching for quakes of such small magnitude that their signals were previously too small to be separated from noise. The results boosted the number of earthquakes in the Southern California Seismic Network archive to 1.8 million.

Statistical analyses using this wealth of new data could help researchers suss out information about seismic activity that wouldn’t have been possible previously. “You can’t do statistics with small numbers,” says Emily Brodsky, a seismologist at the University of California, Santa Cruz, who wasn’t involved in the new study.

She likens the usefulness of tiny quakes to that of fruit flies: They’re like small but abundant laboratory model organisms. With large populations — whether of fruit flies or earthquakes — you can learn what’s robust and what’s a fluke; separating the two is a chronic problem in earthquake studies, Brodsky says.

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