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

The January 2022 eruption of the Hunga Tonga-Hunga Ha'apai (HTHH) volcano highlighted the critical challenges in monitoring remote submarine volcanic activity. Distributed Acoustic Sensing (DAS) utilizing existing seafloor telecommunications cables offers a promising solution to bridge this observational gap. We analyzed a one-week DAS dataset recorded in February 2023, approximately one year after the eruption, using a segment of a domestic telecommunication cable in Tonga.While a previous analysis of this dataset focused on relatively large events with clear phases, our objective was to comprehensively detect small and unclear seismic signals to evaluate the post-eruption activity. We developed a new "duration-based" detection method that identifies temporally sustained energy increases in the array's median power, effectively suppressing spatially incoherent noise. This method successfully detected 770 discrete events, revealing a stable seismicity rate of approximately 110 events per day, significantly more than those detected by conventional triggering algorithms.To distinguish the origin of these events, we estimated the apparent slowness of the signals using a robust method combining 2D Normalized Cross-Correlation and linear fitting (RANSAC). The results showed that most events have positive apparent slowness values, corresponding to arrivals from the direction of the HTHH volcano, rather than the negative apparent slowness corresponding to tectonic earthquakes from the Tongan Trench. These findings indicate that the HTHH volcano or its surrounding magmatic system maintained a high level of seismic activity even one year after the large 2022 eruption. This study demonstrates the capability of DAS to monitor subtle volcanic seismicity in submarine environments where traditional sensors are absent.