[S-CG59] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG59-03] Topographic relief of the basement on the subducting oceanic plate along the entire Nankai Trough

*Kazuya Shiraishi¹, Yasuyuki Nakamura¹, Ryuta Arai¹, Tetsuo No¹, Yuka Kaiho¹, Ryo Miura¹, Ayako Nakanishi¹, Seiichi Miura¹, Gou Fujie¹, Shuichi Kodaira¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Nankai Trough, basement, oceanic plate, seismic reflection profile, subduction zone

We investigated seismic reflection profiles compiled along the entire Nankai Trough from 1997 to 2024, to elucidate topographic features of the basement on the subducting oceanic plate. We conducted a multi-year intensive survey campaign since 2018 to collect new seismic reflection data with dense survey lines covering most of the Nankai Trough subduction zone at intervals of 4 km or 8 km. We created a surface model of the subducting basement over a length of 730 km and a width of 150 km based on the consistent interpretation of reflection profiles with the vertical axis of reflection two-way time from many old and new datasets. Then, we converted the surface from the time domain to the depth domain using an existing regional 3D velocity model. The time-to-depth converted surface suggests detailed topographic features down to depths of 15 or 20 km compared with the previously proposed plate geometry model. The dense seismic reflection datasets enabled us to capture the topographic variation with a scale of a few kilometers for the most part. We divided the Nankai Trough subduction zone into three domains based on the topographic features of the basement on the subducting oceanic plate. The western domain (Hyuganada region) was characterized by a large ridge of the northern extension of the Kyushu-Palu Ridge and steep subduction towards Kyushu. The middle domain (Nankai region) was characterized by wide topographic reliefs composed of seamounts or ridges and linear depressions, which may have been formed by the past seafloor spreading of the Shikoku Basin and subsequent igneous activity before subduction. The eastern domain (Tonankai to Tokai region) was characterized by rough undulations with multiple ridges aligned in the west-east direction relevant to the Paleo Zenisu ridge. These divided domains correspond to the segments of the megathrust seismogenic zone along the Nankai Trough. However, the significant ridges were not always associated with the distribution of slow earthquake activities. Further investigations are necessary on other geological factors controlling spatiotemporal variation of seismic activities in addition to the topographic features of the subducting basement.