Structural Characteristics of Subducting Oceanic Ridges in the Eastern Nankai Trough Region Derived from FWI of OBS Data

The eastern part of the Nankai Trough is a region of intense tectonic activity where the Philippine Sea Plate subducts beneath the Eurasian Plate. Large interplate earthquakes, such as the Tonankai and Tokai earthquakes, have repeatedly occurred in this area. A notable feature of the region is the paleo-Zenisu Ridge – a ridge subparallel to the trough axis – believed to be subducting beneath the accretionary prism. Previous studies have suggested that the extent of the paleo-Zenisu Ridge is closely linked to the distribution of rupture zones for megathrust and slow earthquakes. However, the precise extent and topography of the paleo-Zenisu Ridge remain poorly constrained.

Within the long-term collaborative project we aim to reconstruct the detailed seismic velocity structure of the eastern Nankai Trough using several dense 2-D wide-angle OBS datasets to better constrain the condition of the paleo-Zenisu Ridge and evaluate its critical role in the region’s tectonic and seismic dynamics. In this study, we apply first-arrival traveltime tomography and time-domain full-waveform inversion to recover high-resolution velocity models along two parallel OBS profiles (spaced ~20 km apart) acquired by JAMSTEC in the eastern Nankai Trough. These profiles integrate OBS data from two legacy seismic dasets (KR07-10_B and KR12-12_Z04; ~4.8 km OBS spacing) with recently acquired coincident profiles (KM23-13), where OBS instruments were deployed between the receiver positions of the legacy surveys. This integration results in datasets consisting of 80 and 97 OBS per profile, with an improved spacing of ~1.6 km.

The dense OBS spacing of the combined datasets enables stable FWI application up to 8 Hz, allowing for detailed reconstruction of the underlying structure of the accretionary wedge and subducting oceanic crust. Our results, combined with other high-resolution velocity models from the Tokai area of the Nankai Trough, reveal variations in the topographic relief of the paleo-Zenisu Ridge along the subduction front axis. These findings suggest a non-uniform impact of the subducting ridge on the overlying wedge, as well as variations in stress distribution, fluid migration, and seismic coupling along the subduction interface. By leveraging additional OBS profiles and advanced inversion techniques, this study enhances our understanding of the paleo-Zenisu Ridge and its role in shaping the tectonic framework of the eastern Nankai Trough.