Electrical image of subduction zone beneath northeastern Japan

We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Ω m, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80-100 km depth and takes an overturned form toward the back arc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent back-arc volcanoes in the system.