[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-P19] Detailed mapping of the subducting seamount and deformation structure in the Nankai Trough off the eastern Shikoku region deduced from densely acquired seismic profiles

*Yasuyuki Nakamura¹, Shuichi Kodaira¹, Yuka Kaiho¹, Tetsuo No¹, Kazuya Shiraishi¹, Gou Fujie¹, Seiichi Miura¹, Gaku Kimura² (1.Japan Agency for Marine Earth Science and Technology, 2.Tokyo University of Marine Science and Technology)

Keywords:Nankai Trough, seismic reflection survey, subducting seamount, slow earthquakes

Nankai Trough off Muroto is one of the areas of interest from the subduction zone seismogenesis and tectonics viewpoints. Subduction of a seamount was suggested southeast off the Cape Muroto from refraction seismic data analysis (Kodaira et al. 2000). Shallow very low frequency earthquakes have been observed around this seamount using the cabled seafloor observation network, DONET (Nakano et al. 2018), and onshore stations (Takemura et al. 2019). Deformation and rebuilding of the accretionary prism probably due to the subduction of the seamount have been reported seaward of the suggested location of the subducting seamount (Gulick et al. 2004).

We conducted a reflection seismic survey in the Nankai Trough off the eastern Shikoku region in December 2019 and January 2020 using R/V Kairei. A 5500m-long streamer cable with 444 channels of hydrophones and an air gun array with a volume of 7800 inch³ were used to obtain the seismic data along 21 and 6 survey lines running in dip and strike direction respectively. The total line length was ~ 2300 km, which covers the forearc area of the Nankai trough off the Kii channel to the east and off the Cape Muroto to the west, including seaward of the Tosa-bae. The pre-stack depth migration was applied to the data after the pre-conditioning, noise reduction, and iterative velocity analysis. Obtained depth migrated profiles show the characteristic structure in the subduction zone, e.g., top of the subducting Philippine Sea plate, decollement, fold-and-thrust deformation of the hanging wall sediments, bottom simulating reflection (BSR). The top of the Philippine Sea plate is traceable in most of the survey lines to create a detailed depth map of the subducting plate surface. The map shows a distinct shallow portion around the Tosa-bae, where the subduction of the seamount has been suggested. The map seems to show two peaks, not a single peak around the inferred seamount. The dip of the subducting plate is generally shallower off the eastern Shikoku than off the western Kii Peninsula. We can identify landward dipping clear reflections below the slope off the Cape Muroto, which are not clearly imaged off the western Kii peninsula and Kii channel. The decollement refection is clearly imaged with apparently negative polarity above the top of the subducting Philippine sea basement near the trough axis off the eastern Shikoku, however, the polarity of the decollement is positive off the Kii channel. These structural characteristics should be related to the plate boundary slip behavior difference between off Cape Muroto and off Kii peninsula.