Victorian-style houses lean dramatically to the right after an earthquake.
Little temblors like those detected in the new data are much more numerous than the building-toppling quakes like the one that ripped through San Francisco in 1906. Credit: The U.S. National Archives

Every 3 minutes. That’s how often an earthquake struck Southern California from 2008 to 2017, new research published in Science shows.

Scientists have discovered over 1.6 million previously unknown earthquakes, most of them tiny, by mining seismic records. These results, which constitute the most comprehensive earthquake catalog produced to date, reveal in detail how faults crisscross the Golden State and shed light on how one earthquake triggers others.

“Having a better earthquake catalog is just like having a better microscope,” said Robert Skoumal, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif., not involved in this study. “We are able to take a closer look at the location of faults, how those faults rupture, and how they interact with each other.”

Small and Numerous

A tenet of earthquake science motivated Zachary Ross, a seismologist at the California Institute of Technology in Pasadena, and his collaborators: Earthquake catalogs are always incomplete. That’s because small earthquakes, many of which are too tiny to feel, are always lurking below the limit of detectability. And these little temblors are much more numerous than the building-toppling, highway-churning beasts that make headlines.

“For every magnitude unit you go down in size, you get about 10 times as many.”

“For every magnitude unit you go down in size, you get about 10 times as many,” said Ross.

Ross and his colleagues used data from over 500 seismometers in the Southern California Seismic Network to tease out small, previously unrecorded earthquakes.

They used a technique called template matching, which involves using the seismic waveforms of known earthquakes as templates and then looking for matches in seismic data collected nearby.

“The shaking that’s recorded…will look almost the same,” said Ross. “They’re seeing all the same rocks as they’re traveling along.”

Down to the Noise

“We’re basically at the noise level of the instrumentation.”

Ross and his team combed through a decade of seismic records using over 280,000 earthquakes as template events. They found over 1.6 million new earthquakes as small as magnitude 0.3. Such low levels of ground shaking can also be caused by construction-related vibrations, ocean waves, and nearby aircraft, said Ross.

“We’re basically at the noise level of the instrumentation.”

Using small differences in the arrival times of seismic waves from an earthquake, the scientists calculated the hypocenter of each new event. This information, along with an earthquake’s timing and magnitude, allowed Ross and his colleagues to assemble detailed maps of Southern California’s earthquakes.

Video by Caltech

The new earthquake catalog does a far better job of tracing fault lines and revealing how earthquakes trigger others compared with older records, said Ross.

—Katherine Kornei (@katherinekornei), Freelance Science Journalist


Kornei, K. (2019), More than a million new earthquakes spotted in archival data, Eos, 100, Published on 19 April 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.