This study focuseson the results of an audio-frequency magnetotelluric (AMT) survey across theJigokudani valley, Tateyama volcano, Japan, to investigate the spatialrelationship between the distribution of electrical resistivity and geothermalactivity and to elucidate the geologic controls on both its phreatic eruptionhistory and recent increase in phreatic activity. The AMT data were collectedat eight locations across the Jigokudani valley in September 2013, with highquality data obtained from most sites, enabling the identification of anunderground 2D resistivity structure from the transverse magnetic (TM) modedata. The data obtained during this study provided evidence of a largeconductive region beneath the surface of the Jigokudani valley that isunderlain by a resistive layer at depths below 500 m. The resistive layer iscut by a relatively conductive region that extends subvertically toward theshallow conductor. The shallow conductive region is divided into an uppermostslightly conductive section that is thought to be a lacustrine sediment layerof an extinct crater lake containing hydrothermal fluids and a lower sectioncontaining a mix of volcanic gases and hydrothermal fluids. The lowpermeability of the clay zone means that the uppermost clayey sediments allowthe accumulation of gases in the lower section of the conductive region, suggestingthe existence of a cap structure. The deep resistive layer likely consists ofunits similar to the granitic rocks that are widely exposed throughout theJigokudani valley. We suggest that the relatively conductive zone thatseparates these granitic rocks represents a high-temperature volcanic gasconduit, given that the most active fumarole in the Jigokudani valley liesdirectly along the trajectory of this path.
Kaori Seki ---Departmentof Earth and Planetary Sciences, Tokyo Institute of Technology
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发表于 2015-9-13 00:45
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