COUNTING QUAKES The magnitude 7.2 El Mayor-Cucapah earthquake triggered numerous aftershocks across much of Southern California when it struck Baja California, Mexico, in 2010. A new study has identified more than a million tiny quakes recorded by seismometers in the region from 2008 to 2017, including hundreds of previously unnoticed aftershocks from the 2010 quake.
Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE)
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.
A broader swath of Earth rumbled with aftershocks triggered by the 2010 El Mayor-Cucapah quake in Baja California, just south of the U.S.-Mexico border, than had been thought. The original earthquake archive (left) showed aftershocks as far as 175 kilometers from the epicenter. A new analysis (right) reveals tiny quakes as far as 275 kilometers away triggered by the main shock, increasing the number of recorded aftershocks by 142 percent (red dots indicate above-average seismicity). That suggests that changes in stress on nearby faults aren’t the only ways that earthquakes interact. Other subtle shifts, such as changes in fluid pressure near the edges of the fault zone, could be responsible for how one quake can trigger another.
“Arguments for whether or not something is human-induced revolve around timing and location,” she says. “The thing is, very often there’s a delay, so that timing becomes ambiguous.” Being able to see whether an apparent delay between human activities and a big temblor is actually filled with tiny quakes — possibly revealing a continuous process — could be a game changer.
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