Detailed mapping of the plate interface in the Nankai Trough subduction zone off Kii channel revealed by densely obtained seismic reflection profiles

Abstract:

Subduction of the topographic relief, rough seafloor and seamounts, has been thought to play an important role on the variation of the plate boundary slip behavior. Recently the slow earthquakes were reported to occur around the subducted seamounts in the Nankai Trough. We conducted seismic surveys in the Nankai Trough between off Cape Shionomisaki and Cape Muroto. Forty-four seismic reflection profiles with 4km separation were acquired across the trough axis. In addition, eight profiles along the trough axis were also acquired. Time-migrated profiles were interpreted to map the top of the subducting oceanic crust. The preliminary depth map of this horizon was calculated by a simple depth conversion using a one-dimensional velocity model estimated from refraction surveys in this area. The topography of the subducting oceanic crust is less undulated off the western Kii Peninsula, however it shows a distinct high southeast off the Cape Muroto, where a subduction of the seamount has been inferred. The horizontal scale of this topographic high is estimated to be smaller than 20 km. We also recognized other small topographic highs of the subducting oceanic crust off the eastern Shikoku island, further southeast of the previously inferred subducting seamount. The area with topographic highs of subducting oceanic crust has been reported to show shallow very-low frequency earthquake (sVLFE) activity. On the other hand, the sVLFE has been less active off the western Kii peninsula, where no distinct topographic features were identified in the top of subducting oceanic crust. We will conduct detailed structural mapping including the deformation of the hanging wall sediments and plate boundary fault with pre-stack depth migration currently in progress.