Japanese Agency Observes Tsunami with Submarine Optical Cable

JAMSTEC used a submarine optical cable to detect a tsunami near Torishima, leveraging changes in water pressure and cable shape.By The Japan Times
June 7, 2024

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.

It successfully observed a tsunami between a point some 60 km off the cape and an area near the cape as it continued a survey on slow earthquakes that had occurred along the Nankai Trough since January 2022.

Currently, the fiber-optic cable cannot estimate the height of a tsunami. However, Takashi Tonegawa, a senior researcher at JAMSTEC, said, “If we understand the physical characteristics of the optical fiber and subseafloor structure in advance, the height of a tsunami can be calculated.”

“We may be able to use (the fiber-optic cable) for a tsunami early warning in the future,” he said.

The Meteorological Agency deploys seismographs and water pressure gauges, installed at certain intervals, along with the Chishima and Japan trenches and the Nankai Trough in the Pacific Ocean. There is no such observation network in the Sea of Japan.

If put into practical use, a tsunami and earthquake observation system using a submarine fiber cable might cost less than setting up seismographs and water pressure gauges.

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