A research team, including Principal Researcher Eiichiro Araki of the Research Institute for Marine Geodynamics at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in collaboration with the NTT Access Network Service Systems Laboratories, conducted observations using distributed acoustic sensing (DAS) and other fiber-optic sensing technologies. The study utilized submarine telecommunications fiber-optic cables laid in the Tokara Islands region of Kagoshima Prefecture, where earthquake swarm activity intensified in June of last year. The observations, carried out from September of last year to January of this year, revealed that active seismicity has been ongoing. The results were presented at the Seismological Society of America (SSA) 2026 Annual Meeting held in Pasadena, California.
The Tokara Islands region saw a surge in earthquake swarm activity starting in June of last year. On July 2, an earthquake with a moment magnitude of 5.7 occurred, leading some residents of nearby islands to evacuate for a time. The focal area of the swarm is located offshore, but the observation network is limited to the islands. It has been difficult to understand the causes and progression of the activity.
To address this, the researchers attempted to gain a detailed understanding of sub-seafloor activity by applying distributed fiber-optic sensing to the submarine cables laid for inter-island communication. The team connected DAS and other distributed fiber-optic sensing equipment to the submarine cable between Akusekijima and Takarajima, conducting observations from September 16 of last year to January 13 of this year.
At the start of the observation period, extremely frequent seismic activity was observed, occurring at a rate of approximately one event per minute. By determining earthquake hypocenters from the DAS data obtained during this period, over 15,000 earthquakes were identified, most of which were shallow crustal events.
While activity near Suwanosejima and Takarajima was also captured, the majority of earthquakes occurred on the southwest side of Akusekijima, persisting with occasional bursts of intensification. Based on the magnitudes observed via DAS, the study was able to comprehensively record earthquakes with a magnitude of 0.25 or greater.
Of these recorded events, only 434 were determined by the Japan Meteorological Agency using the existing island-based network, demonstrating that DAS observation using submarine cables allows for extremely high-sensitivity seismic monitoring. Furthermore, some of the observed activity was found to occur within limited areas at very shallow depths beneath the sea floor.
According to the researchers, these events may be generated by processes that are different from the numerous other earthquakes, and future studies are needed to examine their potential links to submarine volcanic activity or sub-seafloor hydrothermal activity. The results of these observations indicate that distributed fiber-optic sensing, such as DAS using submarine telecommunications cables, is highly effective for monitoring submarine seismic and volcanic activity in island regions like the Tokara Islands.
The researchers suggest that in the future, continuous monitoring could allow for a detailed understanding of seismic activity in various island areas, potentially contributing to regional safety.
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.