[S-VC28] Active Volcanism

[SVC28-P15] Debris avalanche deposits and the stratigraphy on the outcrop to north east from Mt.Unzen

*Daisuke Nagai¹ (1.Mt.Unzen disaster memorial hall)

Keywords:active volcano, Debris avalanche deposits, Unzen Volcano

Unzen Volcano is a stratovolcano which repeatedly grew through the formation of lava dome, the generation of pyroclastic flow and the mountain collapse. Pyroclastic flow deposits originating from Mt. Fugen have been found in the northeastern foot of Mt. Fugen (Kobayashi and Kato,1985,Kobayashi and Nakada,1991, Watanabe and Hoshizumi,1995). It is important to investigate what kind of activity the Unzen Volcano is going to do in the future and the past ejecta to consider future disaster prevention measures. In Nagai (2019), this paper reports on the debris avalanche deposit which has never been seen before in the Tateno area which is located about 5km in the northeast foot of Mt. Fugen. This time, another debris avalanche deposit was newly found in Ide no Kawamachi (About 7 km from Mt. Fugen), Mie district, northeast foot of the same mountain. The stratigraphy of ejecta was compared with that of outcrops in the Tateno area. The newly discovered debris avalanche deposit is found to be a newer age debris avalanche different from the one reported in Nagai (2019). This debris avalanche is thought to have occurred during a series of eruptions of the Kureishibaru pyroclastic flow at about 19000 years ago. There are unclear points, such as the identification of the source of the mountain collapse, which has become a subject of future research. We hope the results will be useful for future disaster prevention measures.