17:15 〜 18:30
[MIS13-P06] Sedimentary process of ash deposits from long-term activity of repeated vulcanian eruptions at Sakurajima volcano, Japan
キーワード：火山灰、堆積物、ブルカノ式噴火、桜島火山
Ash deposit from a long-term activity of repeated vulcanian eruptions are important for understanding the development of a volcanic edifice. In this study we discussed the sedimentary system of ash deposits produced by long-term vulcanian activity from Showa crater of Sakurajima volcano, Japan, based on the grain size distributions of falling ash from individual eruptions. We sampled falling ash particles from 21 individual vulcanian eruptions between March 3–11, 2010. The grain size distributions of the ash samples were unimodal, bimodal, and trimodal characteristics. The median particle size of the main subpopulation decreases with distance from the Showa crater. The low standard deviation of the grain size of the main subpopulation implies deposition from transient volcanic plumes (Walker, 1971). For the finest subpopulation in the polymodal samples, the median particle size is very fine ash (<30 μm) which is difficult to fall as individual particles due to low terminal velocity, indicating that this subpopulation was originated from ash aggregates. Overall, the sedimentary process of the repeated vulcanian eruptions at Sakurajima volcano is explained by accumulation of ash particles from transient plumes and ash aggregation, and the latter controlled the behavior of the very fine ash. We consider the model proposed in this study reflects the long-term activity of repeated vulcanian eruptions, based on the following: (1) The vulcanian eruptions considered in this study were typical of Sakurajima volcano; (2) the Md–distance relationship of main subpopulation from this study are similar to those for the March 26 2016 vulcanian eruption (Miwa et al. 2020) and the July 28–August 3 2013 vulcanian eruptions at Sakurajima volcano (Gabellini et al. 2020). Deposition of ash particles during long-term vulcanian activity generates a close-concentric spatial distribution of ash grain size and thickness around the crater (Imura, 1995), and builds the volcanic edifice, along with the eruptive products from large-scale eruptions. Year-long monitoring of ash accumulation should be suggested for future research.