Iwo-yama volcano, part of the Kirishima Volcanic Complex, has recently shown signs of unrest. We conducted a hypocenter relocation of shallow earthquakes and broadband magnetotelluric measurements around Iwo-yama. Three-dimensional inversion of magnetotelluric data revealed an electrically conductive layer that is interpreted as a hydrothermally altered clay-dominated unit. Shallow earthquakes occur beneath this layer, suggesting that it controls the location of seismicity. The base of the layer corresponds to the depth of a pressure source identified by a leveling survey. These observations suggest that the supply of high-temperature fluids has increased over time beneath Iwo-yama, causing an increase in pore pressure beneath the clay-rich layer and resulting in tectonic earthquakes and ground inflation. Increased upwelling of fluids through a fracture in the clay-rich layer may have caused a vigorous liquid-gas phase transition near the surface, which in turn might have led to the small phreatic eruption on 19 April 2018.
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