Earth Planets Space, Vol. 57 (No. 3), pp. 209-213, 2005

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

Takafumi Kasaya¹, Tada-nori Goto¹, Hitoshi Mikada¹, Kiyoshi Baba¹, Kiyoshi Suyehiro¹, Hisashi Utada²

¹Japan Agency of Marine-Earth Science and Technology, Natsushima-cho 2-15, Yokosuka 237-0061, Japan
²Earthquake Research Institute, University of Tokyo, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, Japan

(Received November 30, 2004; Revised February 17, 2005; Accepted February 21, 2005)

Abstract: 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 -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.
Key words: Nankai trough, 1944 Tonankai earthquake, Magneto-telluric, OBEM, Resistivity, Conductor, Fluid, Kii Peninsula.