Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaged by audiomagnetotellurics, and their implications for the hydrothermal system

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 m, 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 m 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.

Hydrothermal system beneath the crater of Tarumai volcano, Japan: 3-D resistivity structure revealed using audio-magnetotellurics and induction vector

“…They interpreted the upper part of the conductor as the impermeable layer, which had been hydrothermally altered and acted as the bottom of the crater lake. Nurhasan et al (2006) also referred to an altered, impermeable layer beneath the crater lake of Kusatsu-Shirane volcano. In case of the Tarumai, the aquifer formed by the bowl-shaped structure may be considered to be a sort of "buried crater lake."…”

“…MT soundings have been successfully used at many volcanic and geothermal areas to reveal hydrothermal convection systems and magma reservoirs (e.g., Matsushima et al, 2001;Aizawa et al, 2008;Kanda et al, 2008). MT soundings have also been used to locate volcanic fluid paths (e.g., Nurhasan et al, 2006). Such studies have generally analyzed MT data using inversions and assuming a two-dimensional (2-D) structure.…”

Two-dimensional resistivity structure of Unzen Volcano revealed by AMT and MT surveys

“…The resistivity structure described in the preceding paragraph is typical for an active volcano (e.g., Usu volcano (Ogawa et al, 1998;Matsushima et al, 2001); Galunggung Volcano (Wannamaker et al, 2004); Kusatsu-shirane volcano (Nurhasan et al, 2006); Asama volcano (Aizawa et al, 2008); Aso volcano ; Rotokawa geothermal field (Heise et al, 2008)). In general, such an extremely low resistivity is interpreted as low permeability clay due to hydrothermal alteration (e.g., Ogawa et al, 1998;Revil et al, 2002); it is believed that low permeability clay behaves as a sealing zone, and that high temperature fluids are maintained and circulate within the relatively high resistivity region (e.g., Björnsson et al, 1986;Ussher et al, 2000).…”

Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano

“…Conductive clay layers usually exhibit relatively low permeability (e.g., Jones and Dumas, 1993;Nurhasan et al, 2006;Aizawa et al, 2009a;Yamaya et al, 2009;Kanda et al, 2010). In particular, the top of such a clay layer can be sealed (i.e., impermeable), such that in the subsurface, any shallow, cold groundwater do not become wellmixed with deep, hot hydrothermal waters (Aizawa et al, 2009a).…”

“…• C (e.g., Ussher et al, 2000;Nurhasan et al, 2006;Uchida and Sasaki, 2006) means a high-temperature (∼200…”