Seismicity Observation at Hyuga-Nada Subduction Zone by Ocean Bottom Seismometer from 2015 to 2024

The Nankai Trough, extending from Suruga Bay to offshore Kyushu, marks the subduction boundary where the Philippine Sea Plate descends beneath the Eurasian Plate. This region is seismically active, with Hyuga-nada experiencing a magnitude 7.1 earthquake in August 2024, though no historical record exists of magnitude greater than 8 events there. Slow earthquakes, including low-frequency tremors and very low-frequency earthquakes (VLFEs), frequently occur at the plate boundary. Marine seismic networks deployed from 2015 to 2024 in Hyuga-nada used Long-Term Ocean Bottom Seismometers (LT-OBS) to monitor seismic activity. Precise event relocations combined 1D and 3D velocity structures, using double-difference seismic tomography method and then determine the first-motion focal mechanisms to refine hypocenters and classify earthquake mechanisms. Results show seismicity concentrated at depths of 14–20 km, with diverse focal mechanisms dominated by strike-slip and normal fault types. Most earthquakes likely originate within the subducting Philippine Sea Plate rather than the plate boundary. Additionally, the spatial-temporal distribution of swarm events of regular earthquakes and VLFEs suggests that their seismicity may be related, with VLFEs potentially triggering subsequent regular earthquakes.