Detection of Shallow Tectonic Tremors in Hyuga-nada, Nankai Trough, Using the Newly Established N-net OBS Network

Monitoring tectonic tremors is crucial for understanding stress release in subduction zones and assessing megathrust earthquake risk. The Hyuga-nada region, located at the western edge of the Nankai Trough, Japan, is a key area for investigating how slow earthquakes relate to megathrust events and complex subduction structures. To better understand these interactions, we detected shallow tectonic tremors by applying our machine-learning workflow to newly established offshore N-net data. Between July 2024 and September 2025, two major tremor episodes were identified: one immediately after the August 2024 Mw 7.0 earthquake and another three days before the January 2025 Mw 6.7 event. These tremors occurred approximately 50 km offshore from the mainshocks and showed a complementary distribution to the 2024 coseismic slip. Tremor activity was enhanced in fluid-rich plate-boundary zones but suppressed in drained areas, indicating that structural heterogeneity governs tremor generation.