Fig. 4

a Polarization data from all regions with iso-depth contours of the Pacific and Philippine Sea slabs. The iso-depth lines of the subducting Philippine Sea and Pacific plates are indicated by red and blue broken lines, respectively (Nakajima and Hasegawa 2007). Numbers indicate line depths in kilometers. The north end of the subducting Philippine Sea plate, estimated from seismic tomography, is indicated by a thick broken black line. The yellow lines are the locations of faults that ruptured during the 1891 Nobi earthquake. The rose diagrams show the number of the polarization directions for each part. The boundaries of each part are shown by green dashed line. b Schematic figure of subducting oceanic plates in the Japan region. The NE–SW polarization directions observed in the northeastern part of a and ENE–WSW directions observed in the southwestern part are consistent with the movement directions of the subducting oceanic slabs. c Schematic figure of the region beneath the 1891 Nobi earthquake. The heterogeneous structures were inferred from magnetic conductivity studies. The inferred fluid-rich area is shaded blue. The brown square indicates the region of the 1891 Nobi earthquake. Heterogeneous structures appear to correspond to anisotropic regions in the mantle wedge