Three-dimensional resistivity structure in the Nankai Trough off Kumano inferred using marine magnetotelluric investigations

Abstract Megathrust and slow earthquakes are known to occur in the area off Kumano, along the eastern part of the Nankai Trough, Japan. Pore fluids along the fault surface play an important role in earthquake occurrence, but the detailed fluid distribution remains unknown. In this study, based on seafloor electromagnetic field observations, we estimated the three-dimensional resistivity structure, which reflects the fluid distribution off Kumano. The optimal three-dimensional resistivity model showed a low-resistivity layer at shallow depths below the seafloor. The low-resistivity layer matches geological features observed in seismic reflection surveys, including forearc basin fill, the frontal prism, and underthrust sediments. Along the plate boundary (décollement), the resistivity increased markedly with increasing depth. Comparison with slow earthquakes along the plate boundary reveals that shallow very low-frequency earthquakes, shallow slow slip events, and tremor only occur at depths < 10 km and on the conductive zone of the plate boundary. Furthermore, the area of coseismic slip during the 1944 Tonankai earthquake was characterized by higher resistivity than the slow earthquake epicentral area. These findings indicate that the pore-fluid distribution along the plate boundary may influence earthquake occurrence and type. Graphical Abstract