[S-CG55] 沈み込み帯へのインプット：海洋プレート誕生から沈み込み帯まで

[SCG55-05] Rejuvenated extension of the Philippine Sea plate and its effect on subduction dynamics in the Nankai Trough

*木村 学¹、中村 恭之²、白石 和也²、藤江 剛²、小平 秀一²、山口 飛鳥³、福地 里菜⁴、橋本 善孝⁵ (1.東京海洋大学・海洋資源環境学部、2.海洋研究開発機構、3.東京大学大気海洋研究所、4.株式会社ダイヤコンサルタント、5.高知大学)

キーワード：南海トラフ、スロー地震、海山沈み込み、フィリピン海プレート、土佐ばえ、沈み込み

The Nankai Trough, Japan, is a subduction zone characterized by the recurrence of disastrous earthquakes and tsunamis. Slow earthquakes and associated tremor also occur intermittently and locally in the Nankai Trough. The causal relationship between slow earthquakes and large earthquakes is important to understanding subduction zone dynamics. The Nankai Trough off Muroto, Shikoku, near the southeast margin of the rupture segment of the 1946 Nankai earthquake, is one of three regions where slow earthquakes and tremor cluster in the Nankai Trough. On the Philippine Sea plate, the rifting of the central domain of the Shikoku Basin was aborted at ~15 Ma and underthrust the Nankai forearc off Muroto. Here, the Tosa-Bae seamount and other high-relief features, which are northern extension of the Kinan Seamount chain, have collided with and indented the forearc wedge. In this study, we analyzed seismic reflection profiles around the deformation front of accretionary wedge and stratigraphically correlated them to drilling sites off Muroto. Our results show that the previously aborted horst-and-graben structures, which were formed around the spreading center of the Shikoku Basin at ~15 Ma, was rejuvenated at ~ 6 Ma to ~3.3 Ma and has remained active since. The reactivated normal faulting has enhanced seafloor roughness and appears to affect the locations of slow earthquakes and tremors. Rejuvenated normal faulting is not limited to areas near the Nankai Trough, and extends more than 200 km into the Shikoku Basin to the south. This extension might be due to extensional forces applied to the Philippine Sea plate, which appear to be driven by slab-pull in the Ryukyu and Philippine trenches along the western margin of the Philippine Sea plate.