Air-Fall Ash from the Main Crater of Asama Volcano on August 7, 2019, and its Water-Soluble Components

We collected volcanic ash immediately following the eruption of Mt. Asama on August 7, 2019, observed the characteristics of ash particles, and analyzed the water-soluble components. The volcanic ash consisted mostly of altered fragments, and no clear evidence of essential materials was found. The volcanic ash contained large amounts of water-soluble components, Cl and SO4 at concentrations of 8,710 mg/kg and 49,100 mg/kg, respectively. These results indicate that this eruption was caused by the phreatic explosion and that part of the volcanic edifice of Mt. Asama was fractured and emitted.

Surface deformation of Asama volcano, Japan, detected by time series InSAR combining persistent and distributed scatterers, 2014‒2018

Asama volcano is one of the most active volcanoes in Japan. Spatially dense surface deformation at Asama volcano has rarely been documented because of its high topography and snow cover around the summit. This study presents the first interferometric synthetic aperture radar (InSAR) observation of ground deformation at Asama volcano with 120 Sentinel-1 SAR images from both ascending and descending tracks and 20 descending ALOS-2 images acquired between 2014 and 2018. We exploited both persistent and distributed scatterers to overcome decorrelation of SAR signals and applied a three-dimensional unwrapping method to retrieve the displacement time series efficiently. Our observations reveal an asymmetric deformation around the volcano with two main deformation regions on the northeast and southeast flanks, respectively. The northeast flank (NEF) exhibits line-of-sight (LOS) extensions in all the three SAR datasets with maximum velocities of − 14, − 10, and − 12 mm/year for the descending ALOS-2, ascending, and descending Sentinel-1 measurements, respectively. The southeast flank (SEF) shows LOS extensions in the ascending observations and LOS shortening in the descending observations with velocities between − 12 and 9 mm/year. Decomposition of the LOS displacements reveals nearly pure subsidence at the NEF, while the SEF exhibits a substantial eastward component as well as subsidence. Comparisons of the vertical subsidence at two continuous GNSS stations near the summit crater with our InSAR observations indicate small discrepancies smaller than 4 mm/year. We interpreted that the subsidence at the NEF of Asama is primarily due to the hydrothermal activity, while the deformation at SEF is plausibly due to flank instability. We highlight that efforts should be taken to monitor the slope instability at Asama volcano in the future.

Feasibility Study on a Multi-Channeled Seismometer System with Phase-Shifted Optical Interferometry for Volcanological Observations

A new Phase-Shifted Optical Interferometry seismometer system was tested in terms of its feasibility for multi-channeled volcanological observations in two volcanos in Japan. The system is capable of both sensing ground motions and transferring its signals through optical means. The prototype of this system comprises three optical-wired stations and optical components, and was deployed in Sakurajima Volcano in 2016 and in Asama Volcano in 2017. The system successfully operated for 134 days in total and provided seismograms that are in good agreement with those obtained using conventional systems. Several obstacles for putting this system to practical use that need to be solved were found through tests. Their solutions will be explored in subsequent research.