Magnetotelluric investigations for the seismically active area in NorthernMiyagi Prefecture, northeastern Japan

The ELF- and the ULF-MT surveys were carried out in the northern part of Mi yagi Prefecture, northeastern Japan. This area is one of the most seismical ly active areas in this region, where hypocenters of microearthquakes are d istributed on a fault plane at depths from 2 to 16 km. The aim of the prese nt study is to investigate the relationship between electrical resistivity structure and the hypocentral distribution of microearthquakes in the area. The calculated impedance tensor at each site has been obtained from the ob served data and decomposed to remove galvanic distortion, provided that the regional strike is N32 degrees E to obtain the 2-D apparent resistivity an d phase responses. The resistivity structure obtained by the inversion proc ess using smoothness constraint shows that the relatively electrically cond uctive layer at depths from 4 to 10 km corresponds to the zone where the mi croearthquakes occur. The fact that the conductive zone correlates with the hypocentral zone is probably attributed to fluids in the crust. Another mo re conductive block is found at depths from 1 to 3.5 km and the bottom boun dary of this conductor appears to restrict the uppermost depth where the mi croearthquakes occur. This subsurface conductor is interpreted as a marine sediment deposited during the Tertiary period. In the lower crust, the rela tively conductive blocks (lower than 5 Omega.m) exist below a depth of 15 k m.