Abstract
Abstract Kusatsu-Shirane volcano, Japan, is known for its active phreatic eruptions. We have investigated its hydrothermal system by conducting audio-magnetotelluric soundings at 22 stations along a profile that extends across the volcano. The final two-dimensional model is characterized by two conductors. One is a 300- to 1000-m-thick conductor of 1–10 Ωm, which is located on the eastern slope and covered with 200-m-thick resistive layers of Kusatsu-Shirane lava and pyroclastics. This conductor indicates the presence of a Montmorillonite-rich layer of Pliocene volcanic rocks that may function both as an impermeable floor for the shallow fluid path from the peak to the hot springs to the east and as an impermeable cap for the deeper fluid path from the summit region to the foot of the volcano. The second conductor is found at a depth of 1–2 km from the surface, at the peak of the volcano, and its resistivity is as low as 1 Ωm or less. This low resistivity can be explained by fluids containing high concentrations of chloride and sulfate which were supplied from the magmatic gases. Micro-earthquakes cluster above this conductor, and the cut-off of the earthquakes corresponds to the top of the conductor. This conductor infers the presence of the fluid reservoir, and the upward release of these fluids from the reservoir through the conduit presumably triggers the micro-earthquakes at the peak area of the volcano. Crustal deformation modeling using GPS and leveling data of the past 10 years revealed that the center of the deflation coincides with the top of the second conductor, indicating that the fluid reservoir itself can be hosting the deformation.
Highlights
Electromagnetic methods are widely used for the imaging of active volcanoes because the resistivity is sensitive to the fluids and the clay alteration minerals (Ogawa and Takakura, 1990; Ogawa et al, 1992, 1998, 1999; Pellerin et al, 1996; Di Maio et al, 1998; Kagiyama et al, 1999; Fuji-ta et al, 1999; Matsushima et al, 2001; Muller and Haak, 2004; Caglar andIsseven, 2004; Manzella et al, 2004; Oskooi et al, 2005; Aizawa et al, 2005)
The surface 200-m-thick resistive layer corresponds to the Quaternary lava of Kusatsu-Shirane volcano, which is underlain by a 1000-m-thick conductive layer of lava that is older than Kusatsu-Shirane volcano
Audio-magnetotelluric soundings along a profile that extends across Kusatsu-Shirane volcano revealed the presence of two conductors
Summary
We use natural electromagnetic fields in the frequency range between 10 KHz and 1 Hz. Electromagnetic methods are widely used for the imaging of active volcanoes because the resistivity is sensitive to the fluids and the clay alteration minerals (Ogawa and Takakura, 1990; Ogawa et al, 1992, 1998, 1999; Pellerin et al, 1996; Di Maio et al, 1998; Kagiyama et al, 1999; Fuji-ta et al, 1999; Matsushima et al, 2001; Muller and Haak, 2004; Caglar andIsseven, 2004; Manzella et al, 2004; Oskooi et al, 2005; Aizawa et al, 2005). We interpreted the resistivity model using logging information obtained from 1300- to 1500-m-deep boreholes
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