Dan Muramatsu, Takeshi Matsushima
Research is reviewed on subsurface structures and the history of volcanic activity since the 20th century at Ebinokogen Iwo-yama volcano. The resistivity structure and hydrothermal system at Iwo-yama are revealed with electromagnetic, geodetic, and seismic surveys. The shallow low-resistivity layer distributed at around 200-700 m beneath Iwo-yama is considered to be a low-permeability clay-rich layer, above which there is a mixing zone of groundwater and volcanic fluids, and below which there is a reservoir of volcanic fluids observed as a pressure source of ground deformation. A general conceptual model in which a low-resistivity layer dominated by altered clay causes pressurization as a cap structure has been reported in relation to other hydrothermally active volcanoes, and the basic structure is understood to produce phreatic eruptions. Future work may include clarifying fluid supply systems from deep (> 4 km below sea level) to shallow (< 0 km below sea level) parts of the volcano and the internal structure of the shallow low-resistivity layer. Iwo-yama produced significant fumarolic activity from the early 20th century until the 1980s, but this activity declined from the mid-1990s and disappeared in 2007. A quiet period continued until 2013; then, shallow seismicity and ground deformation with seismic tremors began to be observed in 2014, suggesting the occurrence of fluid intrusion. Fumarolic activity resumed in December 2015. The fumarolic area expanded intermittently during the period 2016-2017, followed by a phreatic eruption in April 2018. Significant hydrothermal activity continued mainly at the Iwo-yama South Crater after the 2018 phreatic eruption, including repeated small eruptions in the period 2022-2023. Understanding the patterns and mechanisms of hydrothermal activity and transitions by continuing multi-parametric geophysical observations is essential for evaluating the potential for phreatic eruptions.