Large earthquakes and tsunamis are major hazards at subduction zones, where segmentation governs rupture extent and magnitude. However, the mechanisms that define segment boundaries remain unclear. In the eastern Nankai Trough, which is anticipated to host future megathrust earthquakes, the forearc is divided into the Tonankai and Tokai segments. Here we use newly acquired 3D seismic reflection data (2021–2022), to image structures across the segment boundary. High-resolution profiles reveal three seafloor topographic highs associated with active folding and thrusting in forearc sediments, the accretionary wedge, and along the plate interface. These features trend NNE–NE in Tonankai but ENE in Tokai, across an arcuate syntaxis. Collision of the Paleo-Zenisu Ridge, a remnant of the Izu–Bonin arc on the Philippine Sea Plate, appears to impose the syntaxis structure of the upper plate. Our results demonstrate that the forearc structural complexity exerts a primary control on margin segmentation and associated hazards. Complex subduction zone interfaces control plate segmentation along the Nankai Trough with the quieter Tokai segment presenting the greater risk for megathrust earthquakes and tsunamis, according to structural mapping using 3D seismic reflection data.
Kimura et al. (Tue,) studied this question.