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

Abstract

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.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.

Remote Detection of the Electric Field Change Induced at the Seismic Wave Front from the Start of Fault Rupturing

“…A calculation at any point indicated that the sharp initial rise of the signal is conserved even at a distance of 30 km from the source (see [33, Figure 13]) by assuming electrical conductivity σ = 0.001 Sm −1 (the conductivity is around 0.01 ∼ 0.001 Sm −1 in central Japan [46]). The amplitude decreases by two orders but still can be detected by measurement system having high enough resolving power and S/N ratio, which is the case for the present special borehole measurement system.…”

Conductivity distribution and seismicity in the northeastern Japan Arc

“…Magnetotelluric observation data are briefly described here (see Fujinawa et al, 1997Fujinawa et al, , 1999. Broadband magnetotelluric data were acquired along six traverses (A, B, C, D, Copy right c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.…”

“…The remote reference technique (Gamble et al, 1979) was adopted in order to minimize the effects of noise. Several steps to improve data quality were taken, besides the remote reference technique, such as smoothing the impedance tensor in the frequency domain through the 1-D Bostic model.…”

“…In order to understand tectonics, seismicity, geology and metamorphic activity, an extensive magnetotelluric (MT) survey was conducted in the central part of the northeastern Japan Arc (Fujinawa et al, , 1999 following previous observations of Utada (1987), Ogawa (1992) and Nabetani and Fukuta (1995). A quasi-3-D georesistivity distribution was obtained by using broadband MT data corrected sub-surface distortion due to the 3-D heterogeneity.…”

“…A quasi-3-D georesistivity distribution was obtained by using broadband MT data corrected sub-surface distortion due to the 3-D heterogeneity. Some of the seismicity is explained on the basis of resistivity contracts, faults, or geological lineaments (Fujinawa et al, 1999). We used the relation of the conductivity data with the presence of saline fluid and partial melting (Jones, 1992;Simpson, 1998;Chen et al, 1996).…”