Ash Transport and Deposition from 2019-2020 Eruption of Aso Volcano, Japan

Tephra fallout from the eruption causes social damages (electric power facilities, agricultural products, health, aircraft, etc.), environmental impact (air pollution, temperature decreasing, etc.) and secondary disasters (removing by wind, lahar, etc.). To prevent and mitigate these disasters, it is necessary to understand the tephra transport and deposition (TTD) process. However, TTD process is still on debate despite more than four decade of research due to various complications (diversity of eruptions, uncertainty of eruption parameters, aggregation, etc.). Here, in order to discuss tephra transport and deposition process from the plumes on Dec. 15, 2019 (plume Dec15) and on Feb. 21, 2020 (plume Feb21) eruption of Aso volcano (Japan), we conducted the observation of eruption, direct sampling of ash particles within 3 km from the vent, as well as grain-size analyses, SEM observation and tephra simulation analyses using Tephra2. The white-gray colored plume Dec15 gently rose up to 1300 m above crater rim and horizontally flowed to outside of the caldera rim by wind, releasing aggregated ash. The gray colored plume Feb21 gently rose diagonally up to 800 m and started advection not horizontally but descended gently, releasing non-aggregated ash almost within the caldera. The aggregated particles were 100–250 μm in diameters and consist of mainly 5–7 ϕ particles. The aggregated fine particles gained terminal velocities up to several orders of magnitude greater than their individual fall velocities. The deposits include 26.8 wt.% of aggregated particles that is notable and affect to the tephra transport and deposition. Based on the observation, grain-size analyses and Tephra2 simulation, the fine particles segregated from lower plume at early stage and spreading current at Dec. 15, 2019. The spreading plume Feb21 subsided gently possibly due to the depletion of fine particles. The production of fine particles attribute to segregation from plume at early stage, aggregation within plume and ash dispersal to distal area.