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

[SCG59-02] Estimating three-dimensional resistivity structure in the Nankai Trough off Kumano, using marine magnetotelluric data

★Invited Papers

*Manaka Kuroda¹, Hiroshi Ichihara¹, Tada-nori Goto², Takafumi Kasaya³, Tetsuo Matsuno⁴, Keiichi Tadokoro¹, Hisanori Iwamoto⁵, Yusuke Ohta³, Yuya Akamatsu³, Kazuya Shiraishi³, Ryo Miura³ (1.Nagoya University , 2.University of Hyogo, 3.JAMSTEC, 4.Kobe University, 5.NME)

Keywords:Nankai Trough, Off Kumano, Resistivity, OBEM

The Nankai Trough is a subduction zone where the Philippine Sea plate is subducted under the Eurasian plate. The area off Kumano, located in the eastern part of the Nankai Trough, corresponds to the epicenter of the Tonankai earthquake. Recently, the occurrence of slow earthquakes has also been observed, and this area has become one of the most important regions for seismic studies. Pore fluids along the fault plane are considered to influence the occurrence of these earthquakes, but the detailed role of pore fluids has not been clearly elucidated. Rocks with a high pore fluid content tend to conduct electricity, so "Resistivity" is an important physical property to clarify the distribution of pore fluids.
The seafloor electromagnetic field observations were conducted off Kumano area at four sites in 2009, three sites in 2019−2020, four sites in 2020−2021 and five sites in 2024. The three-dimensional resistivity structure estimated based on the magnetotelluric (MT) method showed the low resistivity layer in the shallow part below the seafloor. This layer was also shown on the Philippine Sea plate, suggesting a high pore fluid content in pre-subduction marine sediments. In addition, focusing on the plate boundary (décollement) showed a trend towards higher resistivity in the deeper part than in the shallower part. This trend was correlated with the occurrence areas of large and slow earthquakes identified off Kumano area. In other words, the heterogeneous distribution of resistivity along the plate boundary may influence the occurrence of different types of earthquakes.