Slow earthquakes are slip events on faults that have longer durations than ordinary earthquakes. These events often generate seismic waveforms characterized by dominantly lower frequency than ordinary earthquakes. Over the past two decades, research in geodesy and seismology has revealed a variety of slow earthquake phenomena, such as slow slip events (SSEs), very low-frequency earthquakes (VLFEs), low-frequency earthquakes (LFEs), and tectonic tremor (tremor). Tremor is often considered to represent a swarm of LFEs (Ide et al., 2007;Shelly et al., 2007).Along the Nankai Trough, southwest Japan, deep LFEs and tremor were first identified in the early 2000s at depths of 30-40 km in the deep extension of the megathrust seismogenic zone on the plate boundary (e.g., Katsumata & Kamaya, 2003;Obara, 2002). This type of tremor has been observed in subduction zones worldwide (Obara &

7We estimate the locations of three tremor sequences, denoted A, B, and

On 27 September 2014, Ontake volcano, in central Japan, suddenly erupted without precursory activity. We estimated and tracked the source locations of volcanic tremor associated with the eruption at high temporal resolution, using a method based on the spatial distribution of tremor amplitudes. Although the tremor source locations were not well constrained in depth, their epicenters were well located beneath the erupted crater and the summit. Tremor sources were seen to descend approximately 2 km over a period of several minutes prior to the beginning of the eruption. Detailed analysis of the time series of tremor amplitudes suggests that this descent is a robust feature. Our finding may be an important constraint for modeling the 2014 eruption of Ontake volcano as well as for monitoring activities on this and other volcanoes.

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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 $${\sim }\,130$$ ∼ 130 km in northeast–southwest and $${\sim}\,50$$ ∼ 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