Structure of the Tokai segment from the integrated high-resolution seismic imaging: A case study from the eastern Nankai Trough

Abstract

Subduction zones around the world remain one of the most important targets of geophysical investigations. This is because the megathrusts at convergent margins are the sites of the largest earthquakes ever recorded. The factors that control these earthquakes, as well as the evolution of accretionary wedges are still subject of geological and geophysical interpretation. Given that subduction zones are generally located offshore, approaches from the field of marine geophysical imaging - in particular seismic acquisition and processing techniques- are the methods of choice to investigate these sites. In this study we combine the results of waveform modeling, inversion and migration methods applied to the ocean-bottom seismometer (OBS) data and multi-channel seismic (MCS) data acquired in the Tokai segment (TS) of the eastern Nankai Trough. We use high-resolution velocity model derived with full-waveform inversion (FWI) and its attributes together with the reflectivity profile derived with Kirchhoff pre-stack depth migration (K-PSDM) to build a data-driven geological interpretation of the region. We focus on the nature of the subducting oceanic crust, geometry of the megathrust and splay faults, subducting front, complex accretionary wedge and the paleo-prism acting as a backstop. In particular we are able to image in great detail the complex deformation associated to the subduction of seamounts. We perform waveform and ray modeling in the FWI velocity model to analyse wave propagation within the accretionary wedge, making it possible to associate different structures directly to the arrivals in the OBS data. We support our seismic-derived results with bathymetry, magnetic and gravity data and show that they are consistent with our geologic interpretation of the structural elements that shape the Tokai segment.