[S-CG55] Oceanic plate as inputs to subduction zone: from oceanic spreading ridge to subduction trench

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

*Gaku Kimura¹, Yasuyuki Nakamura², Kazuya Shiraishi², Gou Fujie², Shuichi Kodaira², Asuka Yamaguchi³, Rina Fukuchi⁴, Yoshitaka Hashimoto⁵ (1.Tokyo University of Marine Science and Technology, 2.JAMSTEC, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Dia Consultants Co., Ltd., 5.Kochi University)

Keywords:Nankai Trough, slow earthquake, seamount subduction, Philippine Sea plate, Tosa-Bae, subduction

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.