Various types of slow earthquakes (e.g., tectonic tremors and slow slip events) have been reported in tectonic zones, especially in the subduction zone. The tidal response of a tremor is considered to be strongly related to the weak friction state of the plate interface, and many studies have reported observational evidence of such correlation between tides and deep tremor activity. Here we used the modified frequency scanning method at a single station to detect micro tectonic tremors that have not been previously reported in southern Kyushu. In the early stage of the tremor activity, tremors are mostly modulated by slow slip events. In contrast, we found a seismic response to ocean tides during the later stage in the shallower part of the subduction zone. This might indicate that the tremors are triggered by tidal changes caused by fault weakening due to slow slip events as same as deeper condition.

A slow slip event (SSE) and tectonic tremors occurred approximately 1.5 month prior to the occurrence of the 2011 Tohoku‐Oki earthquake near the Japan Trench. Although we cannot completely rule out other sources of noise near the observation site, slow earthquakes would demonstrate the nucleation of a fast slip event from prior slow earthquake activity. Here we report on tectonic tremor activity detected from short‐period ocean‐bottom seismometer (OBS) data with a modified frequency scanning method that allows robust tectonic tremor detection by incorporating information from multiple frequency passbands in order to suppress false detections. We have identified at least five tectonic tremor sequences on the OBS records from the end of January 2011 to just before the occurrence of the Tohoku‐Oki earthquake in March 2011. To reveal the spatial distributions of tectonic tremors before the Tohoku‐Oki earthquake, we then calculated tremor energy, with corrected site amplification, from the identified tectonic tremor time window at each OBS that detected tremors. Possible sources of tremors were distributed in trenchward side of the SSE fault prior to the main shock. These tremors could be modulated by SSE to take into account consistent timing between tremor and SSE term, and consistency of spatial locations between estimated tremor sources and SSE faults. This spatiotemporal pattern in tectonic tremor activity demonstrates a significant separation of occurrence sources between tectonic tremors and regular earthquakes.

Dynamic triggering of large, detectable slow slip events (SSEs) is rarely observed, even though regional earthquakes often trigger tectonic tremors and very low frequency earthquakes. In this study, we investigate stress sensitivity of dynamic triggering of shallow SSEs in the Nankai Trough offshore of Kii Peninsula, Japan. We first identify additional shallow SSEs that have not been reported in previous studies and obtain a 15-year-long catalog of SSEs, some of which are triggered by passing seismic waves originating from large regional earthquakes. We then quantify dynamic and static stress perturbations on the plate interface induced by 19 candidate regional earthquakes using numerical simulations of seismic wave propagation. We find that SSE propensity to dynamic triggering depends mainly on the maximum Coulomb stress change and that relatively large dynamic stresses (>10-20 kPa) are needed to trigger a shallow SSE in the Nankai Trough. Regional earthquakes that can induce such large amplitude of dynamic stresses on the plate interface are relatively rare, which might explain the scarcity of dynamic triggering of large, detectable SSEs along the Nankai as well as other subduction zones. In addition, our analysis suggests that intraslab earthquakes can efficiently trigger SSEs in subduction zones via less-attenuated, slab-guided waves. Moreover, our results support the idea that an accretionary wedge in subduction zones promotes the dynamic triggering of shallow SSEs.