Resistivity structure of high-angle subduction zone in the southern Kyushu district, southwestern Japan

Magnetotelluric observations were carried out in the southern Kyushu district of southwestern Japan to investigate the characteristics of the electrical resistivity structure of a high-angle subduction zone. We constructed a 2-D resistivity model parallel to the subducting plate motion by using the inversion technique with the Akaike Bayesian Information Criterion (ABIC) smoothness constraint. The general features of the obtained resistivity structure are as follows: (1) a conductive block (below 1 Ω·m) is found beneath the volcanic zone and is widespread bilaterally below 40 km depth, (2) a resistive block (about 1000 Ω·m) distributes from 10 to 25 km depth in the forearc region and (3) a conductor (1 ∼ 30 Ω·m) is embedded beneath the resistive block, which may correspond to the negative Bouguer gravity anomaly observed in this region. We propose the following for the high-angle subduction zone: A serpentinized block is generated in the lower crust of the forearc region and a partial melting and hydrothermal fluid are well developed beneath the volcanic front.