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Three‐Dimensional Electrical Resistivity Distribution Beneath the Beppu–Shimabara Graben With a Focus on Aso Caldera, Southwest Japan Subduction Zone

2018; Wiley; Volume: 123; Issue: 8 Linguagem: Inglês

10.1029/2018jb015506

2169-9356

Maki Hata, Makoto Uyeshima, Yoshikazu Tanaka, Takeshi Hashimoto, Naoto Oshiman, Ryokei Yoshimura,

earthquake and tectonic studies

Abstract The Beppu–Shimabara graben, a tectonically active region comprising a N–S extensional stress field, is characterized by volcanically and seismically active regions in the Southwest Japan subduction zone. We determine the three‐dimensional electrical resistivity structure of the graben by inverting network‐magnetotelluric data. Our resistivity structure model indicates that the crust beneath the graben is a discontinuous electrically conductive belt that contains four significant conductive anomalies. The four anomalies in the graben are located from the east to west as the following arrangement: in the Hohi volcanic zone centered on Kuju volcano at depths of 0–39 km, at around the northwestern rim of Aso caldera at depths shallower than 7 km, at around the active Naka‐dake cone in Aso caldera at depths of 0–17 km, and in the northwestern part of the graben at depths shallower than 7 km. These conductive anomalies in the four locations mainly reflect the effects of various fluids: slab‐derived hydrothermal fluid, volcanic thermal fluid, aqueous fluid, and melt. The locations of the anomalies correspond with low seismicity areas in the graben, whereas resistive blocks of the model correspond with high seismicity areas, especially at the southwestern graben, the northeastern graben, and a border between Aso caldera and the Hohi volcanic zone. Our model suggests that the electrical resistivity distribution can contribute to evaluating whether each part of the graben in the regional extensional stress field has a high potential for the earthquake occurrence.