The Japan Agency for Marine-Earth Science and Technology, or JAMSTEC, said Thursday that it observed a tsunami that occurred off Torishima, an island in the Izu chain in the Pacific, in October last year using the long-distance fiber-optic cable it placed for research purposes on the seabed off Cape Muroto in Kochi Prefecture.

A tsunami changes water pressure in the sea and the submarine topography, slightly altering the shape of the fiber-optic cable. This creates a gap in the round-trip time of a laser light transmitted through the cable from a base station on the ground, making it possible to observe tsunami and earthquakes.

The cable extends some 100 kilometers southward from an area near Cape Muroto and can identify a section of several to several tens of meters whose shape changed due to an earthquake or tsunami.