Resistivity structure around the hypocentral area of the 1984 Western Nagano Prefecture earthquake in central Japan
© 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. 2002
Received: 18 April 2001
Accepted: 2 September 2001
Published: 20 June 2014
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.