Intermittent Inflation on Azuma Volcano (Japan) Detected by PALSAR-2 and Sentinel-1 Data

The Azuma volcano is an active volcano on the central Japan. Since 2014, intermittent seismic activity beneath the volcano and emission of volcanic gas/steam over the geothermal area have been reported. Monitoring ground deformations at the volcano can yield an understanding of the shallow hydrothermal systems and mechanisms of phreatic eruptions. PALSAR-2 and Sentinel-1 data acquired in 2014-2020 were applied to multi-temporal InSAR analysis for extracting spatiotemporal variations in the deformations at the Azuma volcano. Satellite SAR data can detect deformation signals associated with hydrothermal activities even when the deformation signals appear only around the volcanic crater. The Azuma volcano is covered by dense vegetation except for a region of the geothermal area, called Oana crater, in summer and by heavy snow in winter. Therefore, Sentinel-1 data are expected to detect deformation signals from late spring to early autumn each year at the Oana crater. Time-series of PALSAR-2 line-of-sight (LOS) showed a wide range of deformations elongated in the east-west direction in 2015 and local deformations at the Oana crater in 2015 and 2019. The most part of deformation appeared at the Oana crater in 2015 returned into zero over approximately 2 years, while a deformation at another geothermal area about 300 m from the Oana crater maintained 2-3 cm of LOS change during the same time. Sentinel-1 LOS changes revealed detailed temporal variations of the deformations each year, that complemented the temporal characteristics of the deformations at the Oana crater detected by PALSAR-2 data. The observed local deformations at the Oana crater in 2015 and 2019 were reproduced by inflation of a prolonged sphere or a flat source (sill) tilting toward the southeast with a center 300 m below the surface. Inflation of a flat source (sill) at about 2 km below the surface (at sea level) can explain the spatial characteristics of the wide-ranging deformations in 2015. The timing of the deformation is largely consistent with a period of seismic activity at a shallower depth at the Azuma volcano. Consequently, we presume that the detected deformation was driven by hydrothermal activity at the Azuma volcano rather than magmatic activities.