Forearc segmentation due to a ridge collision; 3D structure of the eastern Nankai, Japan

Large earthquakes and tsunamis are major hazards associated with subduction zones, whose segmentation controls rupture extent and event magnitude. However, the mechanisms governing segment size and shape remain poorly understood. Proposed controlling factors include variations in the geophysical-geological properties of the upper plate, lower plate, and megathrust interface. The Nankai Trough, Japan’s well-studied subduction zone, is anticipated to host future megathrust earthquakes within decades. Its eastern forearc is subdivided into the Tonankai and Tokai segments, both underthrust by the Philippine Sea (PHS) Plate. This study presents a new structural interpretation of 3D seismic reflection data acquired in 2021-22 focusing on the boundary between these segments. We investigate geological and geophysical features potentially influencing segmentation. High-resolution seabed and subsurface data identify three prominent topographic highs with active fold-and-thrusting in the inner forearc sediments and accretionary wedge, and plate interface. These features trend NNE–NE in the Tonankai segment and ENE in the Tokai segment, across a major arcuate structure (syntaxis). Paleo-Zenisu Ridge on the PHS plate—a backarc remnant of the Izu-Bonin arc—collides with the southern end of this NW-trending syntaxis and appears to impose geometric control on the segment boundary.