http://www.sciencemag.org/news/2018/06/seafloor-cables-carry-world-s-internet-traffic-can-also-detect-earthquakes

Seafloor cables, such as this link between the United States and Spain, can serve as seismic sensors.

Some 70% of Earth's surface is covered by water, and yet nearly all earthquake detectors are on land. Aside from some expensive battery-powered sensors dropped to the sea floor and later retrieved, and a few arrays of near-shore detectors connected to land, seismologists have no way of monitoring the quakes that ripple through the sea floor and sometimes create tsunamis.