[S-VC28] Active Volcanism

[SVC28-04] Evaluation of volcanic activity using "Non-eruptive ground deformation"

*Masato Iguchi¹ (1.Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University)

Keywords:Sakurajima volcano, ground deformation, volcanic gas

Sakurajima volcano is one the active volcanoes in the world. In recent 65 years, it has repeated eruptions at the summit crater of Minamidake and Showa crater 500 m east of the Minamidake since October 1955. The eruptive activity stayed at the Minamidake from 1955 to 2005 with vulcanian eruption, strombolian eruption and continuous ash emission, and then the eruption center was shifted to the Showa crater in 2006. The eruption at the crater was mostly vulcanian type with more frequent, but smaller than the Minamidake crater. At the end of October 2017, the eruption center returned to the Minamidake and the following eruptive activity is divided into 6 episodes; 1) November 2017, 2) January 2018, 3) February – September 2018, 4) October 2018 – July 2019, 5) September 2019 – June 2020 and 6) after August 2020. Sequence of vulcanian eruptivity during the episodes is analyzed based on seismic, ground deformation, infrasound, SO₂ discharge rate and mass eruption rate from ash fall deposit. A characteristic inflation – deflation patter is identified in the ground deformation records. It corresponds to eruptive events (vulcanian type with significant air-shock of >10 Pa and less infrasonic type) with effusion of volcanic ash and non-eruptive phenomena associated with gas emission. Time series of volume change of pressure source inducing non-eruptive ground deformation (NEGD) are examined, comparing with volume change of pressure source by eruptive events, mass eruption of volcanic ash, and discharge rate of SO₂. The episode 5 has the largest number of vulcanian eruption and mass eruption of volcanic ash among the episodes. The sequence of the episode 5 was commenced by an eruption on September 3, 2019 and mostly terminated with a strong eruption, which ejected volcanic bomb up to a distance 3.3 km from the crater, on June 4, 2020. The activity of NEGD events is dominant in the initial stage of the episode and declines accompanying relatively larger ratio of effusion of volcanic ash. This characteristics is shown by partitioning of NEGD events in the total volume change of pressure source and apparent density which is defined by mass eruption of volcanic ash to the total volume change. The gas-rich magma is emitted in the initial stage in September 2019 to February 2020 and ash-rich magma is ejected in the latter stage during the period from March to June 2020. This is supported by SO₂ discharge rate of 2000 – 4700 ton/day in the initial stage and 700 - 2600 ton/day in the latter stage. The transition of magma from gas-rich to ash-rich is identified during the episode 3, secondly active.