抄録
A joint analysis of data obtained by conventional magnetotellurics and network magnetotellurics (band-width, 0.003-7, 680 sec) revealed detailed resistivity structure from the shallow crust to the upper mantle in the eastern part of Hokkaido, Japan, situated in the southwestern end of the Kuril Arc. The results are summarized as follows: (1) A conductive layer (a few to 10 Ωm), having a basin structure, is distributed widely to a maximum depth of about 6 km in the upper crust. Considering other independent studies, such as seismic reflection, gravity and drill core analyses, the bottom of this layer coincides with the boundary between the Tertiary and the Cretaceous formations. (2) A conductive layer (10-40 Ωm) situated in the lower crust extends from the volcanic front toward the backarc side, and is similar to feature with the Northeastern Japan Arc. (3) A highly resistive layer (5, 000-10, 000 Ωm) is analyzed in the upper to middle crust of the forearc side. Since the distribution of this layer is consistent with the high positive gravity anomaly region (227 mgal in maximum), the causative material may be common. A collisional tectonic event between the Eurasia plate and the Okhotsk Paleoland in the Cretaceous period may possibly be related with the existence of the layer, although the detailed tectonic implications are left to be solved. (4) The resistivity of the upper mantle is 40-100 Ωm. (5) The resistivity of the Pacific plate is estimated as 700-1, 000 Ωm, which is almost consistent with that of the Northeastern Japan Arc (500 Ωm).
