Permalink : http://hdl.handle.net/2115/38169

JaLCDOI : 10.14943/gbhu.72.387

A magnetotelluric (MT) survey was conducted in the Kuromatsunai area to identify resistivity structures and their features. A basin that formed in the Kuromatsunai lowland area during the Miocene contains a thick Miocene to Plio-Pleistocene sediments. On the west side of the lowland is an active fault zone that is experiencing ongoing, east-down displacement. Three magnetic field components and two horizontal electrical field components were observed over 20-40 hours at eleven sites, and magnetotelluric impedances were calculated for a frequency range of 320-0.00034 Hz. The strike direction of dominant resistivity structures was determined using a phase tensor estimation (Caldwell et al., 2004). A two-dimensional resistivity structure was obtained through a two-dimensional inversion with smoothness constraints and static shift corrections (Ogawa and Uchida, 1996). The results show that relatively resistive Plio-Pleistocene to upper Miocene strata occupy the shallow subsurface of the lowland. Conductive lower to middle Miocene strata underlie the Plio-Pleistocene units. Resistive granitic rocks present at greater depth below the lowland differ from the material present at similar depth outside of the lowland area. Granitic rocks are also exposed at the eastern end of the lowland. Induction vectors observed in the MT survey at Komagatake Volcano (Yamaya et al., 2004) suggest that telluric current flows toward the Japan Sea from the Pacific side of Funka Bay through a conductive layer at the back of the bay. It is likely that this conductive unit corresponds to the thick Miocene strata of the Kuromatsunai area.