Constraining the Effect of Bathymetric High Subduction in an Accretionary Wedge: Evidence From the Tokai Area of the Nankai Trough, Japan

Abstract The Tokai area in the eastern Nankai Trough is characterized by the subduction of a rough plate interface caused by bathymetric highs related to the adjacent Izu‐Bonin arc and fault fabrics in the oceanic crust. However, the effect of this process on the structural development of its accretionary wedge and megathrust seismicity is still poorly understood. To get better insight into Tokai area we apply full‐waveform inversion to legacy wide‐angle seismic data exploiting available signal frequency up to 15 Hz. From this model we extract pseudo‐reflectivity attributes, which are complementary to the section obtained with Kirchoff pre‐stack depth migration of streamer data. The high‐resolution images reveal detailed geological characterization of the subducting oceanic crust, the geometry of the megathrust and splay faults, and the structure of the wedge. The latter is characterized by imbricate fans, out‐of‐sequence thrusts, and duplexes that significantly contribute to its thickening. We compare these features to published structural models and propose that the increased structural complexity in the Tokai segment is due to fault reactivation triggered by the subduction of the bathymetric highs. We suggest that this fault reactivation also controls the shallow seismogenic behavior of this region. We show that the evolution of the forearc basin is related to the ongoing underplating process and out‐of‐sequence thrusting within the wedge. We compare our seismic imaging results with bathymetric, magnetic, and gravity data, revealing no evidence of large ridges beneath our profile, which previous studies have proposed as a cause of the persistent Tokai seismic gap.