Analyzing volcanic-like earthquakes with distributed acoustic sensing using a short segment of the Tongan seafloor telecommunications cable

Shunsuke Nakao, Mie Ichihara, Masaru Nakano, Taaniela Kula, Rennie Vaiomounga, Masanao Shinohara

The 2022 Hunga Tonga-Hunga Haʻapai (HTHH) eruption highlighted the need for monitoring submarine volcanoes. Distributed Acoustic Sensing (DAS), utilizing existing seafloor cables offers a promising solution. We analyze a one-week DAS dataset recorded in February 2023, one year after the eruption, using a 30-km segment of the domestic telecommunication cable in Tonga. The previous study (Nakano et al., 2024) analyzed the data for relatively large local earthquake events with clear P- and S-phases. This study focuses on unclear and small events, plenty of which are included in the data. Our objective is to create a comprehensive event catalogue and distinguish HTHH-related activity from tectonic earthquakes at the Tonga Trench. We introduce a new method to identify temporally sustained events and distinguish them from brief incoherent noise. We define a new parameter to represent the duration of sustained signal energy, with which we automatically pick events. This method successfully detects a total of 770 events, many of which are missed by conventional methods that detect the sudden increases in amplitude. The result also reveals a stable seismicity rate of approximately 110 events per day. To determine their origin, we estimate the apparent slowness of each event using a robust method combining 2D Normalized Cross-Correlation and linear fitting. We find more events with positive slowness values, which correspond to arrivals from the HTHH volcano direction, than those from the Tonga Trench (negative slowness). The result suggests that a significant fraction of the detected small earthquakes originate from the HTHH volcanic system, indicating that the volcano or its surrounding magmatic system maintains a high level of seismic activity one year after the major 2022 eruption.