Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough.

The spatiotemporal characteristics of shallow slow earthquake activity are linked to the tectonic environments of shallow plate boundaries. In this work, the spatiotemporal variations of shallow very low frequency earthquake (SVLFE) activity along the Nankai Trough were investigated using a cross‐correlation analysis. The SVLFEs migrated or spread eastward along the strike direction of the trench during large SVLFE episodes. Migrations and clusters of SVLFEs suggest the occurrence of shallow slow slip events. The observed lateral variations in SVLFE activity patterns reflect the heterogeneous distributions of effective strengths at the shallow plate boundary along the Nankai Trough. Migrations and clusters of SVLFEs tended to be concentrated in the regions surrounding the stress accumulation peaks on the Philippine Sea Plate boundary. The stress accumulated in the transitional regions between high‐strength and low‐strength zones can be released by shallow slow earthquakes. Plain Language Summary: Shallow slow earthquakes with rupture speeds between those of regular earthquakes and stable sliding often occur in the shallower extents of expected megathrust rupture zones. Such shallow slow earthquakes are useful for monitoring the slip behavior of shallow plate boundaries. The spatiotemporal variations of shallow slow earthquakes are related to tectonic characteristics including stress accumulation, friction properties, and the presence of pore fluids. To investigate the spatiotemporal variations of shallow slow earthquakes occurring along the Nankai Trough, the activities of shallow very low frequency earthquakes (SVLFEs) were examined using template matching and cross‐correlation relocation analysis. It was found that activity patterns of SVLFEs differed in each analyzed segment due to the different structural characteristics of the shallow Philippine Sea Plate boundary. A detailed spatial distribution revealed that SVLFEs tended to cluster or migrate in the regions surrounding the boundary stress accumulation peaks on the Philippine Sea Plate boundary. The obtained results suggest that shallow slow earthquakes occur and migrate to release accumulated stress in transitional regions between high‐strength (strongly locked) and low‐strength (stable sliding) zones on the shallow plate boundary. Key Points: Matched filter technique revealed details of activity patterns of shallow very low frequency earthquakes (SVLFEs) along the Nankai TroughThe characteristics of SVLFE clusters and migrations reflect frictional properties and possible slow slip events on the plate boundarySVLFEs tended to cluster or migrate in the regions surrounding the shear stress accumulation peaks on the Philippine Sea Plate boundary [ABSTRACT FROM AUTHOR]