Noriaki Kawakami

2-D georesistivity structure in the central part of the northeastern Japan arc

Wide-band (0.002?20,000 Hz) magnetotelluric measurements (MT) observations have been conducted along three traverses in the central Tohoku district of the northeastern Japan arc at 86 observation sites since 1990 in order to image the electrical resistivity structure. We used the impedance tensors fully corrected for the three-dimensional galvanic distortion effects including static shift effects in order to refine previous 2-D models (discussed in Fujinawa et al., 1997). The subsurface 3-D effects are found to be generally small with the result of a slight difference between the present model and the previously reported one. p]The modeling results indicate that the crust is homogeneous without an enhanced conductivity zone in the lower crust, in general agreement with results in the northern part of the Tohoku district. The refined resistivity profiles delineate more clearly two near-surface conductive anomalies located in the fracture zone between the Dewa Hill and the Central Basin Range, and in the zone between the Kitakami and Abukuma River regions. Conductors in the crust west of the Sekiryo Mountain Range generally correlate well with mapped faults or pre-Tertiary tectonic lines. Several buried faults are also suggested from the conductivity data. The electrical resistivity distribution and known active faults are integrated to better understand the seismo-tectonics and geologic regime associated with the subduction processes in the Japan arc region.

Conductivity distribution and seismicity in the northeastern Japan Arc

Wideband magnetotelluric (MT) observation data were obtained from 91 sites along six transects in the central part of the Japan Arc. Here, a quasi-3D georesistivity distribution, in addition to other geophysical and geological parameters, is used to better understand seismicity in the region. We found that high seismicity in the Central Mountain Range is due to relatively poor fluid saturation caused by volcanism. The high conductive fracture zone in the west of the Central Basin without big earthquakes is characterized by a low Poisson ratio. It is suggested that strains cannot be accumulated in those regions. The Miyagi-ken-hokubu region with a frequent occurrence of large earthquakes, is conductive—high Poisson ratio—high Vp suggesting that the zone is relatively fluid-rich and brittle.