11:15 〜 11:30
[MIS23-03] 御嶽山2014年噴火における火山岩塊噴出条件の粒径依存性
キーワード：御嶽山、火山岩塊、噴出条件、噴出速度
Ballistic blocks are solid to molten rock fragments ejected during explosive eruptions that move in atmosphere along ballistic trajectories (Taddeucci et al., 2017). The sudden eruption at Mount Ontake on September 27, 2014, killed 58 people, and the casualties were mainly caused by the impact of ejected ballistic blocks (Tsunematsu et al., 2016). Therefore, it is necessary to assess ballistic blocks’ hazard based on estimating the ejection conditions of ballistic blocks ejected during the 2014 Mount Ontake eruption to avoid such a high number of casualties. Thus, we estimated the ejection conditions of ballistic blocks by comparing the observed distribution of the ballistic impact craters on the ground (Kawaguchi et al., 2021MS) with simulated distributions of landing positions using the numerical model, Ballista, which is able to calculate the trajectories and landing position of ballistic blocks (Tsunematsu et al., 2016). We varied input parameters as follows, the rotation angle and the direction angle from 10° to 50° at 10° intervals, and an ejection speed ranging from 140 m/s to 190 m/s at 10 m/s intervals. To assess consistency between the distribution of the impact craters and simulated distributions, we set two areas (200 × 200 m), area 1 and area 2. We calculated the proportion of the number of the impact craters in area 1 to that in area 2. We derived a proportion of 1.698 from the calculation, then considered the simulated distributions that yielded a proportion of 1.698 ± 0.1 from the same calculation to have consistency with the distribution of the impact craters. As a result, we obtained a rotation angle of 20°, a direction angle of 20°, and an ejection speed of 120 m/s. This estimation is consistent with that in Tsunematsu et al. (2016), the rotation angle of 20°, the direction angle of 20°, and the ejection speed of 145-185 m/s. Considering that ballistic blocks ejected during the eruption have various diameters, we assessed the control of the ejection conditions of ballistic blocks on their diameters by comparing the spatial distribution of the ballistic blocks’ diameters obtained from field surveys with simulated distributions of landing positions. We focused on the ejection velocity from among the ejection conditions. Therefore, we varied the ejection speed from 50 m/s to 500 m/s at 10 m/s intervals. We divided the spatial distribution of ballistic blocks’ diameters into three groups, smaller than 15 cm, 15-30 cm, and larger than 30 cm to better understand the characteristics, and used each of the three distributions for comparison. As to ballistic blocks with diameters larger than 30 cm, we compared the farthest travel distances of ballistic blocks with simulated travel distances to assess consistency. As a result, we obtained the ejection speed of ballistic blocks with diameters larger than 30 cm of 120-130 m/s. As to ballistic blocks with diameters smaller than 15 cm and 15-30 cm, firstly we estimated the ejection speeds by comparing the farthest travel distances of ballistic blocks with simulated travel distances. Secondly, we created histograms of the number of ballistic blocks versus the distance from the crater. Based on these histograms, we calculated differences between the mean and median values of distances from the crater in order to assess consistency between the spatial distribution of ballistic blocks’ diameters and the simulated distributions. As a result, we obtained the ejection speeds of ballistic blocks with diameters smaller than 15 cm to be 260-270 m/s, and that of ballistic blocks with 15-30 cm diameters to be 160 m/s. These results suggest that the smaller the diameter is, the larger the ejection speeds of ballistic blocks becomes.