资源环境科技发展态势分析平台

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.

Plain Language Summary Phreatic eruptions result from the sudden release of pressurized fluid into the air. Imaging the pressurized region is important to predict future phreatic eruptions. We studied Iwo-yama volcano, Kirishima, Japan, where recent volcanic unrest has led to a small phreatic eruption. By imaging the subsurface electrical resistivity structure using the magnetotelluric technique, we have identified a low-permeability clay-rich layer at depths of 200-700 m below the surface. Earthquake hypocenters and one of the sources of surface uplift are located beneath this clay layer, so we conclude that the layer caps deep hydrothermal fluids and allows the buildup of pressure beneath it. If this clay-rich layer suddenly breaks, or if the fluid supply through the deeper conductor suddenly increases, a larger phreatic eruption could occur in the future. Imaging and monitoring of such clay-rich layers is important to our understanding of shallow activity beneath volcanoes.