Resistivity structure across Itoigawa-Shizuoka tectonic line and its implications for concentrated deformation

Abstract

We investigated the deep crustal resistivity structure across Itoigawa-Shizuoka Tectonic Line (ISTL), one of the most dangerous active intraplate faults in Japan, by use of wide-band magnetotelluric (MT) method. The 28 MT stations were aligned perpendicular to the ISTL. A two-dimensional model was created in transverse magnetic (TM) mode where electric currents flow in N60°W-N120°E directions. The model showed good correlations with the surface geology. In particular, we found a thick (∼6 km) surface conductor to the east of ISTL which corresponds to the heavily folded sedimentary layer. The Japan Alps to the west of the ISTL is characterized by the resistive upper crust, where the pre-Tertiary rocks crop out. The Japan Alps is underlain by a conductor below 15–20 km depth, which is consistent with the low seismic velocity anomaly. We also found a localized shallow conductor corresponding to the Mt. Tateyama volcano. The most important feature is the conductor in the mid-crust directly under the area of active folding to the east of the ISTL. This may imply a localized zone of fluids because of the enhanced porosity in a shear zone. The recent seismicity clusters in the resistive crust underlain by the conductor, and this suggests the fluid involvement in earthquake generation processes.