Subducting seamounts are thought to increase the normal stress between subducting and overriding plates. However, recent seismic surveys and laboratory experiments suggest that interplate coupling is weak. A seismic survey in the Japan Trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude M approximately 7. Both observed seismicity and the pattern of rupture propagation during the 1982 M 7.0 event imply that interplate coupling was weak over the seamount. A large rupture area with small slip occurred in front of the seamount. Its northern bound could be determined by a trace of multiple subducted seamounts. Whereas a subducted seamount itself may not define the rupture area, its width may be influenced by that of the seamount.

Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events.

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Hydration of the subducting oceanic plate determines the amount of water transported from Earth’s surface into its interior, and plate bending-related faulting (bend faulting) just prior to subduction is considered to promote hydration. Bend faulting shows significant spatial variation, but its contribution to hydration is still poorly understood. Here we present the results of controlled-source seismic surveys around the junction of the Japan and Kuril trenches. We found structural changes caused by bend faulting before subduction differed distinctly between both trenches and were well correlated with plate hydration after subduction, suggesting the bend faulting controls spatial variations in plate hydration. Differences in bend faulting are closely related to the angle between the current trench and the ancient spreading ridge, and the hydration is more extensive where this trench-ridge angle is oblique in the study area. Thus, we propose this angle is a major factor controlling plate hydration.