Local Galvanic Distortions in the Central Part of North-Eastern Japan (Part 1)

Abstract

Wide-band (20,000 Hz to 0.002 Hz) magnetotelluric (MT) observations have been conducted since 1990 in the central part of the Tohoku district in the northeastern part of Japan to investigate the deep electrical resistivity structure. MT data were obtained on three east-west-trending MT transects about 140 km long and separated by about 20 km. In this paper we investigate three-dimensional galvanic distortions on the three transects using the Groom-Bailey tensor decomposition technique. We have focused on the regional strike direction and are not considering static shifts in the data. The decomposition method worked well at most sites but failed at some sites suggesting considerable regional three-dimensionality there. We found that the north-northwest to south-southeast direction is the predominant regional strike. This direction coincides with the regional geological strike being nearly perpendicular to the subduction direction of the Pacific plate. The induction vectors are generally in agreement with the regional strike direction deduced by the decomposition method on the southernmost line, but they deviate systematically on northernmost and middle lines suggesting conductivity contrasts in these areas.

Studies of the Georesistivity Structure in the Central Part of the Northeastern Japan Arc

“…The application of the transient electromagnetic measurements (TEM) to help to understand static shift is underway and will be reported upon soon (Fujinawa et al, 1997). According to the experimental results to date we can say that the distortion of the resistivity profile due to the static shift does not generally exceed ±0.3 decades in general agreement with the statistical evaluation of the effect at about 140 sites by Meju (1994).…”

“…The full analysis of static shift and local distortions, however, are beyond the scope of the present report and are treated in other papers (partly in Kawakami et al, 1997). The application of the transient electromagnetic measurements (TEM) to help to understand static shift is underway and will be reported upon soon (Fujinawa et al, 1997).…”

“…Our concern in the present work is shallower than Moho at about 30 km, and thus a 2-D modeling is assumed to be appropriate for these data. More detailed analysis of the three-dimensional effects due to 3D galvanic distortion (Bahr, 1987;Bailey, 1989, Groom andBahr, 1992;Jones et al, 1993) also suggests that the crust can be treated as a two-dimensional to a fairly good approximation (Kawakami et al, 1997).…”

“…An impedance distortion analysis conducted on the three transects (Kawakami et al, 1997) show that there are surface distortions which could not be neglected but that are small enough to assure that the two-dimensional treatment is valid and the regional strike is near north-south.…”

“…3. The vertical elongation of SE apparently penetrating through the Moho should be reconsidered by fully taking account of the effect due to the surface heterogeneity of the resistivity distribution such as partially tried in Kawakami et al (1997).…”