https://www.wired.com/story/scientists-spot-an-undersea-fault-using-fiber-optic-cables/

Working from a beachside shack on California's Monterey Bay, Nate Lindsey fired a stream of infrared laser pulses down a long fiber-optic cable extending onto the ocean floor. The miles-long cable had been there for a decade, transmitting data to and from scientific instruments on the seafloor, but Lindsey, a geoscientist at UC Berkeley, was trying something new. He and his team had disconnected the cable from all its usual sensors so they could use the fiber itself to sense vibrations on the ocean floor.

By monitoring how light beamed and bounced through the transparent fiber, Lindsey’s team was able to describe the texture and topography of the earth it was buried in. As they report in the journal Science today, the method led them to discover a new underwater fault 5 miles from the Monterey Bay coastline. The technique could potentially help monitor and characterize swaths of unmapped ocean floor, using the so-called dark fiber that telecom companies have already buried across oceans but do not actively use.