ON THE SUPERCRITICAL FLUID UNDER THE BRITTLE-DUCTILE BOUNDARY IMAGED BY MAGNETOTELLURIC STUDIES IN NE JAPAN

Abstract

Supercritical fluid is a new geothermal resource. We have been conducting multiple wide-band magnetotelluric surveys in NE Japan including geothermal areas. We have found mid-crustal conductors which become shallower towards the known geothermal fields around Naruko-Onikobe-Kurikoma area, based on the three-dimensional modeling of resistivity structures. The top of the conductor is consistent with the cutoff depth of the crustal seismicity. Thus the conductor is located just below the brittle-ductile transition zone. The conductor is interpreted as a high salinity fluid bearing zone. The seismicity over the conductors suggests that the migration of the fluids into the brittle zone through the brittle-ductile boundary may induce seismicity by increasing the pore pressure in the brittle zone. The conductor below the brittle zone implies the existence of super critical fluids, and possible permeability, below the brittle-ductile boundary.