磐梯山1888年噴火によるサージ堆積物と被災記録

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Mt. Bandai (Bandai-san), famous for the large-scale collapse of a volcanic edifice triggered by a phreatic eruption on July 15, 1888, is an active volcano in Fukushima Prefecture. The phreatic eruption produced a pyroclastic surge, and which hit villages on the eastern foot of Mt. Bandai. Precious studies have focused on the surge deposit, and have showed the characteristics of the pyroclastic surge and yielded an eruption model. However, there are many paradoxes between the models arrived at and the disaster records. For example, there are the paradoxes of the timing of the generation, the number of flow units, and the depositional area of the pyroclastic surge. In this study, we compare the surge deposits with disaster records, and discuss the pyroclastic surges of the 1888 eruption. The 1888 Mt. Bandai eruption has six characteristics: (1) no significant precursor; (2) phreatic explosions produced pyroclastic surges; (3) the collapse of a volcanic edifice was triggered by proceeding explosions; (4) the main activity ceased within a few hours after the first eruption; (5) no amount of essential material in the 1888 deposit. The surge deposit of the 1888 eruption is made up of massive or cross-laminated beds with reverse or normal grading of pyroclastic materials. The deposit tends to distribute more thickly on the topographically low land. Sometimes accretionary lapilli and pieces of non-carbonizing wood are contained in the deposit. The depositional structure and the pattern of the size distribution of the deposit are in good agreement with those of a typical surge deposit. We found the surge deposit on the debris avalanche deposit, which gave us new evidence for understanding the past. Microscopic and X-ray diffraction investigations indicate that some layers in the deposit have different components, including the fresh fragments of basement granite. Based on this evidence, we concluded that the 1888 surge had plural flow units with different origins. Some of these flows had wet conditions, and the depositional temperature was relatively low. Since many inhabitants survived the disaster of the surge, many testimonies and records have been accumulated. According to these; (1) after the sound of the eruption, it took several dozen seconds for the surge to reach the foot of the mountain which was about 5km distant; (2) some victims were burnt; (3) some victims were overcome two times by the surges. Based on the character of the surge deposit and the summary of the victims' testimony regarding the timing, it was concluded that a surge was generated at least twice; before and after the collapse of the volcanic edifice.論文
2004-03-19

磐梯山1888年噴火によるサージ堆積物と被災記録

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