https://techcrunch.com/2019/11/28/scientists-turn-undersea-fiber-optic-cables-into-seismographs/

Monitoring seismic activity all over the world is an important task, but one that requires equipment to be at the site it’s measuring — difficult in the middle of the ocean. But new research from Berkeley could turn existing undersea fiber optic cables into a network of seismographs, creating an unprecedented global view of the Earth’s tectonic movements.

Seismologists get almost all their data from instruments on land, which means most of our knowledge about seismic activity is limited to a third of the planet’s surface. We don’t even know where all the faults are since there’s been no exhaustive study or long-term monitoring of the ocean floor.