[S-CG52] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG52-P02] North-south changes in the shallow crustal structure of the Japan Trench outer rise - Results of the Hakuho Maru KH-23-6 survey -

*YUE SUN¹, Jin-Oh Park¹, Gou Fujie², Tetsuro Tsuru³, Kazutaka Yasukawa⁴, Erika Tanaka⁵, Mari Hamahashi⁶, Syed Idros⁷, Hironori Otsuka⁸, Katsura Kameo¹, Masanari Ashida¹, Ryoji Toda¹, YU FAN¹, Yuqi Lyu¹, Noboru Yamakawa¹, Konosuke TERUI¹, Qichao Wen¹, YIGE YANG¹, YANG ZHAO¹, Ayana Tochikawa³, Hayato MATSUSHITA³, Ichika Matsunaga³, Yoshiki Horiuchi⁹, Masayuki Toizumi⁹, Yufu Suzuki¹⁰ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.JAMSTEC (Japan Agency for Marine-Earth Science and Technology), 3.Tokyo University of Marine Science and Technology, 4.Frontier Research Center for Energy and Resources (FRCER), The University of Tokyo, 5.Kochi University, 6.Yamaguchi University, 7.Kyushu University, 8.Kobe University, 9.Nippon Marine Enterprises, Ltd., 10.MOLMEC)

Keywords:Japan Trench, Shallow crustal structure, Seismic reflection survey, Outer rise

In the Japan Trench, the Pacific plate is subducting beneath the North American plate. Buckling of the subducting plate creates a shallow extensional stress field on the seaward side of the Japan Trench, forming an outer-rise zone. There, normal faults develop and form the holst graben structure. The movement of the normal faults causes the outer-rise earthquakes. It is known that after a large trench earthquake, outer-rise earthquakes occur nearby. However, no large outer-rise earthquakes have occurred since the 2011 Tohoku earthquake, and it is increasingly important to clarify the actual mechanism of outer-rise earthquake occurrence. Park et al. (2021) proposed that fluids play an important role in seismic activity and that fluid circulation exists in the Japan Trench Outer Rise, with normal faults as entry and exit points.
As part of the investigation of the spatio-temporal scale of large-scale fluid circulation in the Japan Trench Outer Rise, the KH-23-6 research survey was conducted in October 2023 by the scientific research vessel Hakuho Maru. During this cruise, seismic reflection surveys were conducted using airgun shots (two GI guns, 355 cu. in.) and the receiver of a 288-channel streamer cable (total length: approximately 2000 m). The main survey line ran north-south, almost parallel to the trench axis. These lines connect the existing east-west line, with the main objective of elucidating spatial changes in the crustal structure of the Japan Trench seaward in a north-south direction.
Preliminary results show that 1) the shallow crustal structure on the seaward side of the Japan Trench is complex in a north-south direction. 2) In the vicinity of the pseudofaults proposed by Nakanishi (2011) (discontinuities in magnetic anomaly lines, ca. 39°N), the seafloor surface is relatively flat, but the depth of the acoustic basement changes drastically. Differences in shallow crustal structure can be seen on either side of the pseudofaults.