Norihiko Sumitomo

Resistivity structure around the hypocentral area of the 1984 Western Nagano Prefecture earthquake in central Japan

We carried out magnetotelluric measurements in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake occurred and earthquake swarms have been observed since 1976. Most of the earthquakes have focal depths shallower than 10 km. The depth of the hypocenters increases towards the west. Our purpose is to delineate the resistivity structures down to the focal depths of the earthquakes, because the resistivity structure is very sensitive to the free water in the crust. The regional strike was estimated as N60°E. We carried out a two-dimensional analysis over two profiles across the earthquake fault of the 1984 Western Nagano Prefecture earthquake: one is in the low seismicity region of the fault (A-A′) and the other, in the high seismicity region (B-B′). The resistivity structure along the A-A′ profile is reasonably homogeneous and shows a high resistivity of more than 300 ohm·m. The resistivity structure along the B-B′ profile has a clear boundary at the center of the profile. This boundary divides the structure along the B-B′ profile into two resistivity blocks and its location coincides with that of the earthquake fault. A conductor is detected at depths greater than 8 km to the northwest of the earthquake fault. The depth to the upper boundary of the conductor coincides with those of the seismic reflectors. This structure seems to be formed by the free water dehydrated from the magma.

Preliminary report on regional resistivity variation inferred from the Network MT investigation in the Shikoku district, southwestern Japan

The Network MT method was used in the eastern part of the Shikoku district, southwestern Japan, and a total of thirty-nine MT impedances (64 to 2560 sec) were obtained. These MT impedances had their values averaged over a triangular element, whose sides were a few kilometers long with geomagnetic observatory data from the Kakioka Geomagnetic Observatory. Well-determined MT impedances varied from north to south with the greatest differences being at the Median Tectonic Line, which is consistent with the surface geology in the area. In addition, very large or very small values of apparent resistivity were obtained in some triangular elements. These triangles were located on a cape or near an estuary, with effects of three-dimensionality clearly apparent. Stable MT impedances were not obtained for three groups of triangular elements: (1) those where one or two sides of the triangular element cross the coast; (2) those where the electric field was contaminated by severe artificial noise, these were mainly in the northwestern part of the survey area; (3) those where the triangles had an extremely acute- or obtuse-angle.A resistivity cross section was derived from the TM-mode data for a profile crossing the eastern part of the area. The shallower layer, which approximately corresponds to the crust, was divided into three blocks. Two resistive boundaries coincide with the geological tectonic lines and the strong horizontal contrast found at the Median Tectonic Line. The northernmost block is the most resistive, and the block to the south is the most conductive. Beneath these blocks, the subducting Philippine Sea plate was represented by a thick and highly resistive north-dipping layer. A highly conductive thin layer was found above the resistive layer on the southern side of the Median Tectonic Line. This layer is only found beneath the southern side of the Median Tectonic Line and is probably caused by pore water and/or sediment at the upper plane of the subducting Philippine Sea plate. Another conductive layer was found under the highly resistive north-dipping layer.The resistivity structure from the lower crust to the upper mantle is firstly obtained using the Network-MT method. However, further developments are needed in methods of data analysis, which are robust to artificial electric noise, in order to clarify the spatial distribution of MT impedances in the complete study area.

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

The ELF- and the ULF-MT surveys were carried out in the northern part of Miyagi Prefecture, northeastern Japan. This area is one of the most seismically active areas in this region, where hypocenters of microearthquakes are distributed on a fault plane at depths from 2 to 16 km. The aim of the present 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 observed data and decomposed to remove galvanic distortion, provided that the regional strike is N32?E to obtain the 2-D apparent resistivity and phase responses. The resistivity structure obtained by the inversion process using smoothness constraint shows that the relatively electrically conductive layer at depths from 4 to 10 km corresponds to the zone where the microearthquakes occur. The fact that the conductive zone correlates with the hypocentral zone is probably attributed to fluids in the crust. Another more conductive block is found at depths from 1 to 3.5 km and the bottom boundary of this conductor appears to restrict the uppermost depth where the microearthquakes occur. This subsurface conductor is interpreted as a marine sediment deposited during the Tertiary period. In the lower crust, the relatively conductive blocks (lower than 5Ω m) exist below a depth of 15 km.

The electrical structure across the Median Tectonic Line in east Shikoku, southwest Japan

The Median Tectonic Line (MTL) is one of the longest tectonic lines in Japan with the lateral movement. In order to investigate the subsurface structure around the MTL, we carried out the magnetotelluric soundings and the geomagnetic depth soundings in the northeastern Shikoku district, southwest Japan. We estimated the resistivity model at the depth from the surface to 10 km, which explains the observed apparent resistivities and phases. The most remarkable feature of the resistivity model is that the subsurface extension of the MTL has a north dip down to 5 km depth. This dipping interface also corresponds to a reflection seismic result. The north dipping MTL cannot be created by just a present strike slip motion. It requires that the horizontally compressive stress dominated in the early stage of the fault.