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

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 and did not indicate any major resistivity variations parallel to the Nankai Trough. 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 and shallow slow slip events do not occur in the shallow, low resistivity layers, but at moderate depths where the resistivity along the plate boundary is moderate. Furthermore, the area of coseismic slip during the 1944 Tonankai earthquake was characterized by greater 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.