3:30 PM - 3:45 PM
[SVC34-07] Reconsideration of 1882AD eruption record in Kusatsu-Shirane Volcano
Keywords:phreatic eruption, Kusatsu-Shirane Volcano, Shirane Pyroclastic Cone, 1882AD
Reconstructing the detailed progression of an eruption using the records of people who directly experienced the eruption and reinterpreting them using modern volcanology is beneficial for volcanology and disaster prevention, as it allows us to summarize phenomena that are difficult to record as strata with a high degree of time resolution. Kusatsu-Shirane volcano has more than 10 records of eruptions in the 20th century, all of which were phreatic eruptions (Japan Meteorological Agency, 2013), but there are very few records have been reconstructed using modern volcanological knowledge, especially from older eruptions. In this report, we directly investigated documents from the time of the 1882 eruption as much as possible, cross-referenced descriptions from multiple documents, and reconstructed the progression of the eruption based on reliable information after discarding uncertain content.
Based on information from elders, it is known that the previous eruption prior to 1882 occurred in 1805 (Yagi, 1931). The "Joshu Kusatsu Onsen Daizu," an illustrated map of Kusatsu Onsen created in 1810, shows fumarolic gas and smoke at Kusatsu-Shirane, but these appear to have ceased thereafter, and are not depicted in the "Joshu Kusatsu Onsen no Zenzu" of 1879. There were water basins at Mizugama and Yugama craters and grass and small trees grew up to the shore (Ohashi, 1914, etc.), which suggests that no eruptions occurred between 1810 and 1882, and that volcanic gases were not emitted in quantities or concentrations sufficient to impede plant growth.
Records of precursors to an eruption include rumbling near the summit beginning in July 1882 (Ohashi, 1914). The 1882 eruption began at about 1pm on August 6th, when a new crater formed at the bottom of the Yugama crater, and at least five explosions occurred by the next morning. During the eruption between 1pm and 2pm, ash fell in a muddy rain-like manner in a northwesterly direction, and rock blocks 0.3-1 m in diameter that were ejected at the same time mainly left impact craters on the slopes of the Shirane pyroclastic cone. During the eruption on the night of the 6th, ash fell in a southerly direction, possibly due to a change in wind direction. There had been rain near the summit until the day before the eruption, and thick fog on the 6th (Nauman, 1893), so the initial muddy rain may have erupted along with the rain. After six days of eruptions that erupted ballistics and volcanic ash from the crater accompanied by cocktail jet smoke, the eruptions gradually changed to steam eruptions, but eruptive activity around the crater continued for more than one month. The eruption caused syneruptive-spouted lahar flows, which flowed into rivers, washing away many bridges and causing damage to fisheries downstream, which was the greatest disaster for the people at that time.
Based on records indicating that eruption deposits reached a maximum thickness of 1.3 m within the Yugama crater and records of ash fall on vegetation 5 km northwest of the crater (Nauman, 1893), we estimated the total volume of eruption products. The volume of ejecta was estimated using data from the 2014 eruption of Mt. Ontake and methods such as Legros (2000) were used to estimate the total volume, and it was confirmed that the volume was within the same order of magnitude as previous report (on the order of 106 m3).
Based on information from elders, it is known that the previous eruption prior to 1882 occurred in 1805 (Yagi, 1931). The "Joshu Kusatsu Onsen Daizu," an illustrated map of Kusatsu Onsen created in 1810, shows fumarolic gas and smoke at Kusatsu-Shirane, but these appear to have ceased thereafter, and are not depicted in the "Joshu Kusatsu Onsen no Zenzu" of 1879. There were water basins at Mizugama and Yugama craters and grass and small trees grew up to the shore (Ohashi, 1914, etc.), which suggests that no eruptions occurred between 1810 and 1882, and that volcanic gases were not emitted in quantities or concentrations sufficient to impede plant growth.
Records of precursors to an eruption include rumbling near the summit beginning in July 1882 (Ohashi, 1914). The 1882 eruption began at about 1pm on August 6th, when a new crater formed at the bottom of the Yugama crater, and at least five explosions occurred by the next morning. During the eruption between 1pm and 2pm, ash fell in a muddy rain-like manner in a northwesterly direction, and rock blocks 0.3-1 m in diameter that were ejected at the same time mainly left impact craters on the slopes of the Shirane pyroclastic cone. During the eruption on the night of the 6th, ash fell in a southerly direction, possibly due to a change in wind direction. There had been rain near the summit until the day before the eruption, and thick fog on the 6th (Nauman, 1893), so the initial muddy rain may have erupted along with the rain. After six days of eruptions that erupted ballistics and volcanic ash from the crater accompanied by cocktail jet smoke, the eruptions gradually changed to steam eruptions, but eruptive activity around the crater continued for more than one month. The eruption caused syneruptive-spouted lahar flows, which flowed into rivers, washing away many bridges and causing damage to fisheries downstream, which was the greatest disaster for the people at that time.
Based on records indicating that eruption deposits reached a maximum thickness of 1.3 m within the Yugama crater and records of ash fall on vegetation 5 km northwest of the crater (Nauman, 1893), we estimated the total volume of eruption products. The volume of ejecta was estimated using data from the 2014 eruption of Mt. Ontake and methods such as Legros (2000) were used to estimate the total volume, and it was confirmed that the volume was within the same order of magnitude as previous report (on the order of 106 m3).