Seismologists from Caltech are using fiber optic cables to monitor and record the aftershocks from the 2019 Ridgecrest earthquake sequence in greater detail than previously possible. Thousands of tiny aftershocks are occurring throughout the region each day, an unprecedented number of which will now be able to be tracked and studied.

The nascent technique involves shooting a beam of light down a "dark," or unused, fiber optic cable. When the beam hits tiny imperfections in the cable, a miniscule portion of the light is reflected back and recorded.

In this manner, each imperfection acts as a trackable waypoint along the fiber optic cable, which is typically buried several feet beneath the earth's surface. Seismic waves moving through the ground cause the cable to expand and contract minutely, which changes the travel time of light to and from these waypoints. By monitoring these changes, seismologists can observe the motion of seismic waves.

"These imperfections occur frequently enough that every few meters of fiber act like an individual seismometer. For the 50 kilometers of fiber optic cable in three different locations we've tapped into for the project, it's roughly akin to deploying over 6,000 seismometers in the area," says Zhongwen Zhan, assistant professor of geophysics, who is leading the effort.

The project was launched just days after the two large earthquakes struck the Ridgecrest area. Zhan called around, searching for unused fiber optic cable that would be long enough and close enough to the seismically active region to be useful. Eventually, the manager of the Inyokern Airport, Scott Seymour (who had also offered the use of the fiber network around the airport), connected Zhan with Michael Ort, the chief executive officer of the California Broadband Cooperative's Digital 395 project. The project aims to build a new 583-mile fiber network that mainly follows the U.S. Route 395, which runs north-south along the eastern side of the Sierra Nevada, passing near Ridgecrest.