Spatiotemporal evolution of tremor activity near the Nankai Trough trench axis inferred from the spatial distribution of seismic amplitudes

Slow earthquakes have gained importance due to their proximity to the focal regions of megathrust earthquakes. Among slow earthquakes, tectonic tremors have the highest dominant frequency and are thus best resolved. Here, we estimated the locations of tectonic tremors off the Kii Peninsula, southwestern Japan, from December 2020 through January 2021 using the seismograms of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). The study area is adjacent to the Nankai Trough, where large megathrust earthquakes are known to occur. We successfully estimated the locations of 3578 tectonic tremor events within an area of ∼ 130 km in northeast–southwest and ∼ 50 km in northwest–southeast directions along the trench axis. Tremor activity differed between the northeastern and southwestern areas of the focal region, which were separated by a central region of markedly low activity. During the study period, tremor activity began at the northeastern edge of the focal region, and expanded to the southwest along the trench axis until reaching the central low-activity region. Renewed tremor activity later began at the southwestern edge of the low-activity region and migrated southwest along the trench axis. We also detected two distinct events similar to rapid tremor reversals that migrated to the northeast, the first of which may have been triggered by the combined effects of teleseismic surface waves and Earth’s tides. Such detailed locations of tectonic tremors can be used as a proxy of the stress state in the accretionary prism and/or along the plate boundary in the Nankai Trough. Graphical Abstract