蔵王火山1895年噴火における投出岩塊の噴出条件の推定

抄録

Ballistic projectiles are large pyroclasts (＞0.1 m in diameter) traveling through the air without being affected by the flow of gas. This phenomenon is harmful (and potentially fatal) when a volcanic eruption suddenly occurs as the ballistic velocity is quite high, sometimes reaching several hundred meters per second. Therefore, it is important to simulate the trajectory of ballistic projectiles in an affected region. We have estimated the ejection conditions of the 1895 Zao eruption by visually comparing simulated results using a numerical model called “Ballista” to actual block distributions obtained from field observations and aerial photographs. Interestingly, around Goshikidake (northeast of the Okama crater) the farther blocks were from the crater, the larger the block size was. The ejection direction was estimated to be 120° from the north (southeast direction), because the deposit blocks are spatially dense in this direction. The ejection angle was estimated to be 10°, and the ejection velocity was estimated to be 110-120 m/s. The estimated eruption velocity of the 1895 Zao eruption was similar to that of the 2014 Ontake eruption and within the range of small vulcanian eruptions. Although we often worry that a magmatic eruption will occur after a phreatic eruption, it is also possible that a vigorous block emission will occur with a considerably high ejection velocity during a phreatic eruption.