Observations of Hydrothermal System and Preparatory Process of Phreatic Eruption: Recent Developments and Future Prospects

A phreatic eruption is a phenomenon in which water near the surface expands rapidly due to magma-supplied heat, ejecting the surrounding rocks. Recent studies of conceptual models, subsurface structures, pre-eruption processes, and eruption processes of phreatic eruptions are reviewed. These eruptions often occur in volcanoes with well-developed hydrothermal systems, where a low electrical resistivity layer is found near the surface using magnetotelluric surveys. The low resistivity layer indicates a low-permeability structure that acts as a pressure-confining cap on the hydrothermal system. In the brittle-ductile transition zone above deep magma, a sealing structure associated with quartz crystallization develops. Volcanoes with open conduits that connect magma reservoir and surface crater also have the potential for phreatic eruptions. A low-permeable sealing structure in the shallow part of the conduit plays an important role in eruptions of this type of volcano. Phreatic eruptions are prepared by an imbalance in the hydrothermal system, which is caused by increases of heat, volcanic gases, and fluids from the deep magma reservoir, and are triggered by depressurization of the aquifer due to the breakdown of the cap/sealing structure. In recent years, eruptive processes have been modeled using data from broadband seismograms and tiltmeters near vents. At Ontake, Hakone, and Aso volcanoes, slow crustal movements or very low-frequency earthquakes were observed just prior to phreatic eruptions. These phenomena result from crack opening due to the rapid vaporization of liquid water. Incremental seismic activities, low-frequency earthquakes, and expansion of volcanic edifice, and geochemical changes in volcanic gases and hot springs are identified as long-term eruption precursors. These precursors reflect the supply of new magma, related changes in volcanic gases, and increased fluid pressure in shallow hydrothermal systems. Several new techniques for monitoring volcanoes to detect temporal changes in resistivity, crustal deformation, and chemical composition of hot springs and groundwater have been developed for forecasting eruptions.