DAS observations of oceanic volcanism with a Tonga seafloor cable: challenges and future perspectives

Eruptions may occur at poorly instrumented volcanoes. In the case of submarine volcanoes, the rapid installation of monitoring systems is almost impossible, even though both scientific and societal demands are very high. When the Hunga volcano, a submarine volcano in Tonga, produced a massive eruption on January 15, 2022, no seismic stations were operating in Tonga, and no tide data were available between the volcano and the inhabited islands. After the event, the Tongan government and international collaborators discussed improvements and planned the installation of permanent seismic stations across the islands of Tonga. Although this effort is progressing, the remoteness of the islands still prevents rapid deployment. In addition, spatial coverage remains poor due to the limited accessible land areas. The domestic submarine telecommunication cable in Tonga, which runs along the volcanic arc, appeared to be an ideal solution to this problem.

Although we initiated this plan shortly after the eruption, we conducted the first DAS observation for one week in February 2023, one year after the eruption. This was possible because the cable had not yet been repaired following damage caused by the eruption. Only a ~30 km section from the land station in Tongatapu was available, with its offshore end located about 40 km from the Hunga volcano. The first challenge was to extract useful information under these limitations. We successfully located 17 local earthquakes, one of which occurred directly beneath the Hunga volcano (Nakano et al., 2024). In addition, we developed a novel method to extract unclear low-frequency events, detected approximately 700 such events, and estimated their apparent propagation speeds (Nakao et al., 2026).

Subsequently, we planned a second DAS observation using the fully restored domestic cable, which passes near the Hunga volcano. The main challenge was to conduct observations without disrupting telecommunications, as the Tonga cable system does not include dark fiber. A new technology, wavelength division multiplexing (WDM), provided a solution. We carried out observations from August to December 2025. Although only limited raw data were available, the results provided a new perspective on the oscillatory environment of the seafloor along the active volcanic arc of Tonga (Nakao et al., this meeting). The next challenge is how to extract useful information from these data and share it with stakeholders in a timely manner. If such a framework can be established, it would enable the rapid deployment of monitoring systems on the seafloor, significantly enhancing disaster mitigation and advancing volcanological research.

Volcanic seismic observations have both similarities to and differences from tectonic earthquake observations. Based on our previous studies of active volcanoes on land and beneath the ocean in Japan, we propose the use of DAS observations for monitoring oceanic volcanism and welcome further input from the DAS research community.

This study used the data obtained by the collaboration with FiberSense Ltd., Tonga Cable Ltd., Tasmania University team led by Prof. Rebecca Carey, JICA, and Tonga Geological Services. This research was supported by JST and JICA (SATREPS: No. JPMJSA2309).