Ground deformations associated with an overpressurized hydrothermal systems at Azuma volcano (Japan) revealed by InSAR data

Abstract

Abstract Inflations at active volcanoes are indicators of overpressure in the subsurface, which is known to be a phenomenon that precedes eruptions. Volcanic overpressure is induced by the injection of magmatic fluids, accumulated magma, or heat supply from greater depths. Azuma volcano (Japan) has experienced several episodes of volcanic unrest with increase in seismicity at the depth of the hydrothermal system, implying a potential increase of phreatic eruptions. The time-series of interferometric synthetic aperture radar data, associated with the unrest episodes occurring in 2014–2015 and 2018–2019, revealed spatiotemporal variations of inflation episodes, centered around Oana crater, the most active fumarole of Azuma volcano. The modeled best-fit geometry of the elongated pressure source for the local deformation has the same dip as the overlying corresponded with the topographic slope direction and angle around Oana crater, suggesting the existence of topography-correlated layered structures within the hydrothermal system. In contrast, the broader deformation associated with the 2014–2015 unrest was explained by the overpressure of a horizontal flat source at 500 m below sea level, showing similar depth of the top as the conductive low-resistivity or low-viscous body suggested by previous studies. The unrest episodes were thus interpreted as mainly resulting from the supply of magmatic fluids or the volumetric thermal expansion of the hydrothermal system, caused by heat transfer from greater depths. Our study aids in understanding the shallow structure of this volcanic system and contributes towards evaluating the potential for forthcoming eruptions in Azuma volcano.