Abstract
In order to clarify the subsurface pre-Tertiary basement structure at the eastern margin of the Niigata plain, we carried out remote sensing, geological, electromagnetic and gravity surveys at Muramatsu town, Niigata Prefecture. The study area contains distinctive topographic features : a NNE-trending active reverse fault (the Muramatsu Fault) and an asymmetrical tectonic relief (the Atago hill) whose eastern margin is bounded by the Muramatsu Fault. Resistivity and gravity analyses reveal that the pre-Tertiary basement is characterized by resistive and high-density zone, and the overlying Tertiary to Quaternary sediments correspond to conductive and low-density zone. Based on the two-dimensional resistivity and density structures, a significant gap of the upper surface of the basement is inferred below the Atago hill, which is attributed to a west-dipping fault with a remarkable throw of about 1,400 m. The west-dipping fault is probably interpreted to have formed as a normal fault during rifting stage due to extensional stress. This subsurface normal fault seems to be connected to the surface active reverse fault (the Muramatsu Fault), indicating the reactivation of the former normal fault as a reverse fault under the present compressional stress. The tectonic relief of the Atago hill is likely to reflect the asymmetrical deformation of the Tertiary to Quaternary sediments due to the reactivation of the same fault.
