Resistivity image of the Philippine Sea Plate around the 1944 Tonankai earthquake zone deduced by Marine and Land MT surveys

The Nankai Trough is an active convergent region in southwest Japan and mega-thrust earthquakes have repeatedly Occurred in some areas of its plate-boundary interface. Generation of mega-thrust earthquakes is inferred to be related to the existence of water. The resistivity Structure is very sensitive to the existence of water. For that reason, it is important to obtain the resistivity image around the rupture area of mega-thrust earthquakes. We carried out land and marine magnetotelluric surveys in the Kii Peninsula and the offshore Kii Peninsula where the 1944 Tonankai earthquake occurred. We constructed a 2D resistivity model using an inversion technique. The modeled resistivity structure portrayed the Philippine Sea Plate as a resistive region. However, its resistivity becomes more conductive as the plate subducts, showing 10 Omega-m around the down-dip limit. These characteristics are considered to relate to the water. Therefore, we infer that water might control the generation of mega-thrust earthquakes.