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Observation of Submarine Earthquake Swarm Activity in the Tokara Islands Region Using Distributed Fiber-optic Sensing on a Submarine Telecommunication Cable

Description: 

Since June 2025, earthquake swarm activity has intensified in the Tokara Islands region of the southwestern Ryukyu Islands, Japan. On July 2, a Mw 5.7 earthquake and significant offshore crustal deformation occurred, resulting temporal evacuation of the residents of nearby islands. However, because the source area is located offshore and conventional seismic stations are limited to the islands, it has been difficult to resolve the detailed spatial distribution and temporal evolution of the activity. To address this observational gap, we applied distributed fiber-optic sensing to an existing submarine optical fiber communication cable connecting the islands. Two fiber cores of the Akuseki Island–Takarajima submarine telecommunication cable were connected to distributed fiber-optic sensing systems (distributed acoustic sensing and Rayleigh-based strain sensing), and continuous observations were conducted from 16 September 2025 to 13 January 2026. This experiment represents the first application of distributed fiber-optic sensing to monitor ongoing earthquake swarm activity in the Tokara Islands region using operational telecommunication infrastructure. Extremely frequent seismic activity, at a rate of approximately one event per minute, was detected by distributed acoustic sensing. Most of the observed earthquakes were shallow crustal events. The observed earthquakes are often not detected by the island-based seismic network, including of those occurred at very shallow depths beneath a spatially limited offshore area, suggesting a possible link to submarine volcanic processes. In parallel, Rayleigh-based strain observations of the submarine cable revealed tidal but non-stationary strain variations, which are likely to reflect temperature changes in the seafloor water. These results demonstrate that distributed fiber-optic sensing using submarine telecommunication cables provides a novel and highly effective means of monitoring offshore seismic and volcanic activity in island regions, and continuous utilization of such infrastructure is expected to significantly enhance regional hazard assessment and safety.

Session: Fiber-Optic Sensing Applications in Seismology and Environmental Science - V

Type: Oral

Date: 4/17/2026

Presentation Time: 04:30 PM (local time)

Presenting Author: Eiichiro Araki

Student Presenter: No

Invited Presentation: Yes

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