09:45 〜 10:00
[MIS13-04] 桜島火山における放出岩塊のハイスピード映像観測と数値モデル
キーワード：放出岩塊、爆発的噴火、桜島火山、数値モデル
Sakurajima is one of the most active volcanoes in Japan and in the world, erupting explosively. Vulcanian eruptions are the most frequent eruption type in Sakurajima, and ballistic blocks are often ejected during vulcanian eruptions. In Sakurajima, a ballistic block ejected during the eruption on June 4, 2020 and it landed in the resident area called “Yuno”, 3.4 km from the vent. This was an exceptional event that a block traveled so long distance (longer than 2.5 km) after the block landed also in the Yuno area in 1987. The impact crater due to the ballistic block was six meters in diameter. Moreover, near the block landing location, another block penetrated the roof of the hut. Thus, people living near the volcano have risks being hit by blocks, and it is important for assessing ballistic hazards using numerical models.
Recently, numerical models of ballistics were suggested by Tsunematsu et al. (2016), Biass et al. (2016) and Bertin (2017), while the models cannot express some characteristics of ballistic transport. Especially, the model consists mainly of gravity and air drag terms, but some phenomena, such as that larger particles travel shorter distance than smaller particles, are not reproduced by these models. It is necessary to elucidate another force by observations of experiments or real eruptions. In fact, based on the hypothesis that the ejection of ballistic projectiles is affected by the gas flow near the vent, we carried out experiments. However, we could not verify our hypothesis. Therefore, we have observed the vulcanian eruption in Sakurajima volcano and a numerical simulation result using a model Ballista (Tsunematsu et al., 2016) is compared with the block trajectories.
Campaign observation was carried out on Mt. Harutayama in Sakurajima from January 25 to January 29, 2021 using two highspeed cameras installed on a roof terrace of the Harutayama branch of Sakurajima Volcano Research Center. Mt. Harutayama is located approximately 2.7 km from the Sakurajima active vent “A” and “B”. One highspeed camera is for a monochrome shooting with an 800 mm zoom lens and another is for a color shooting with a 135 mm zoom lens.
During the campaign observation, the weather was sometimes unstable and sometimes the Harutayama region was covered by the ash clouds due to the Sakurajima continuous ash ejection. We successfully shoot an explosion at 16:48 (JST) on January 28. A seismic monitor showed the explosion waves, and the switch was turned on at the same time as the sound of the explosion. Although the color highspeed camera did not record any block as the magnification of this camera was not enough, the monochrome camera recorded seven blocks traveling in the air.
The vertical speed of these blocks was ~10 to 50 m/s, which was much smaller than the ejection speed recorded during the Strombolian eruptions (Taddeucci et al., 2017). As the crater wall around the vent A and B was ~400 m higher than the bottom of the craters, the blocks travelled this vertical interval. The smaller velocity was probably due to the velocity decay during the travel of this height difference. In our presentation, we would like to present the result of the image analysis of the explosion and discuss the ballistic transport mechanism based on the comparison of these result with simulated trajectories.
Recently, numerical models of ballistics were suggested by Tsunematsu et al. (2016), Biass et al. (2016) and Bertin (2017), while the models cannot express some characteristics of ballistic transport. Especially, the model consists mainly of gravity and air drag terms, but some phenomena, such as that larger particles travel shorter distance than smaller particles, are not reproduced by these models. It is necessary to elucidate another force by observations of experiments or real eruptions. In fact, based on the hypothesis that the ejection of ballistic projectiles is affected by the gas flow near the vent, we carried out experiments. However, we could not verify our hypothesis. Therefore, we have observed the vulcanian eruption in Sakurajima volcano and a numerical simulation result using a model Ballista (Tsunematsu et al., 2016) is compared with the block trajectories.
Campaign observation was carried out on Mt. Harutayama in Sakurajima from January 25 to January 29, 2021 using two highspeed cameras installed on a roof terrace of the Harutayama branch of Sakurajima Volcano Research Center. Mt. Harutayama is located approximately 2.7 km from the Sakurajima active vent “A” and “B”. One highspeed camera is for a monochrome shooting with an 800 mm zoom lens and another is for a color shooting with a 135 mm zoom lens.
During the campaign observation, the weather was sometimes unstable and sometimes the Harutayama region was covered by the ash clouds due to the Sakurajima continuous ash ejection. We successfully shoot an explosion at 16:48 (JST) on January 28. A seismic monitor showed the explosion waves, and the switch was turned on at the same time as the sound of the explosion. Although the color highspeed camera did not record any block as the magnification of this camera was not enough, the monochrome camera recorded seven blocks traveling in the air.
The vertical speed of these blocks was ~10 to 50 m/s, which was much smaller than the ejection speed recorded during the Strombolian eruptions (Taddeucci et al., 2017). As the crater wall around the vent A and B was ~400 m higher than the bottom of the craters, the blocks travelled this vertical interval. The smaller velocity was probably due to the velocity decay during the travel of this height difference. In our presentation, we would like to present the result of the image analysis of the explosion and discuss the ballistic transport mechanism based on the comparison of these result with simulated trajectories.