The same optic fibers that pulse with the world’s Internet traffic are now listening to the pulse of the planet, picking up earthquake tremors in better detail than traditional seismic networks do.
In a recent Science study, researchers used 15 kilometers of telecom fiber near Mendocino, Calif., to record the region’s biggest earthquake in five years—capturing in fine detail how the magnitude 7 rupture started, slowed and sped up, accelerating even faster than the speed of sound.
“This is almost as if you look at Saturn and say, ‘That’s a star.’ Then, you are given a new telescope and suddenly realize, ‘Oh, my God, there’s actually a ring around it!’” says Zhongwen Zhan, a geophysicist at the California Institute of Technology, who was not involved in the study.
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Optical fiber, one of modern science’s most remarkable inventions, is built to transmit light, which can carry encoded information with extreme efficiency. Even a small touch or bend can disrupt its flow, so telecom companies work hard to minimize environmental interference. “Yet what’s noise to telecommunications is data to us,” Zhan says.
The oil industry adopted this technology in the 1990s, deploying specialized fiber-optic cables to detect temperature, pressure and vibration during drilling. James Atterholt, a seismologist at the U.S. Geological Survey, hoped to adapt such observations to an actual earthquake. In May 2022 Atterholt and his team set up a device called an interrogator—“basically a big box with a laser and a computer,” he says—to send beams of light through an unused fiber on a coastal telephone cable. Depending on ground vibrations, tiny imperfections in the fiber reflected the light back every few meters, turning the thread into 2,800 mini seismometers.
On December 5, 2024, when the quake struck Cape Mendocino, Atterholt’s team was still monitoring the fiber-optic system. Its data revealed how the rupture moved eastward, slowed near a junction where three tectonic plates meet, and then accelerated to “supershear” speed, generating a sonic boom because it was traveling faster than the speed of sound. This was one of the clearest demonstrations of the complexity of a fault leading to supershear rupture, the researchers say. Recording comparable data with the existing seismometer network would require an even more enormous earthquake essentially right on top of the instruments.
Although this technology has been around for a while, “the actual demonstration of it in a proven case shows that it can improve earthquake early-warning systems,” says Brad Lipovsky, a geophysicist at the University of Washington, who was not involved in the study. Such a system would be especially crucial for coastal cities vulnerable to offshore quakes and tsunamis. Lipovsky and Zhan both also highlight the technology’s usefulness in extreme environments, such as Antarctica, where specially installed cable could monitor changing terrain and glaciers’ response to climate change.