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[SVC32-P02] A study on the relationship between shrinkage of Ninoike and slush flow after the 2014 eruption of Ontake volcano
Keywords:avalanche, drone, volcanic ash
The thickly deposited volcanic ash causes various phenomena and disasters even many years after the eruption. The proximal pyroclastic density current (PDCs) during the 2014 eruption of Ontake volcano thickly deposited in and around Ichinoike crater. However, the PDCs did not reach Ninoike lake, and the thickness of the volcanic ash fall around Ninoike lake was several centimeters (Maeno et al., 2016). Therefore, the area of Ninoike lake immediately after the eruption did not change much from before the eruption. After that, Ninoike was reclaimed by the influx of volcanic ash from the Ichinoike crater and the movement of volcanic ash within the Ninoike crater, and the area has been shrinking year by year. Under such circumstances, on May 21, 2019, a relatively large avalanche occurred at the Ichinoike barranco, which is located on the southwest side of the Ninoike crater. It also damaged a part of the Ninoike sanso (mountain hut) and completely destroyed the water supply facility (Kunitomo et al., 2019). This study reports the shrinkage of Ninoike lake after the eruption and discusses the causes of the avalanches and slush flows that occurred during this process.
Since 2017, when the Ontakesan Volcano Labatory of Nagoya University opened, we have been observing changes in Ninoike by visual observation and photography at Kengamine and the Ninoike crater. For the three years from 2020 to 2022, changes in the area of Ninoike lake were investigated using aerial photographs taken by drones. The drone used (Mavic 2 pro) was purchased as a hobby, and full-scale drone surveying has not been conducted. Using rocks and buildings as landmarks, the size of the photographs for each year were adjusted to determine changes in the area of Ninoike lake. In addition, the positions of rocks that can be confirmed from drone photographs were measured by PPK (Post Processing Kinematics, using the GEONET Mitake as the base station) using a commercially available inexpensive dual-band GNSS receiver (DG-PRO1RWS). The position accuracy of PPK is about several centimeters. As of September 13, 2022, the area of Ninoike (the area of the depression) is about 650m2. The area of Ninoike immediately after the eruption was about 6500m2 (calculated from JGI's 2014 aerial photograph), so it has shrunk to about 10%. In addition, the color of the lake, which looked milky green until around 2020, will appear ocher after 2021, and the depth of the depression is thought to be shallow. Currently, the area of Ninoike is shrinking at a pace that is halved every year, and if this trend continues, it may disappear in a few more years (Fig.2). The shrinkage of Ninoike seems to have accelerated since around 2018-2019. The causes are thought to be the increase in volcanic ash inflow from Ichinoike due to the heavy rainfall in 2018 and subsequent increase in precipitation, and the stirring and movement of volcanic ash at the bottom of the Ninoike crater due to the slush flow in 2019.
The May 21, 2019 avalanche occurred under these circumstances. According to interviews with local residents, no avalanche has ever occurred at the Ichinoike barranco, and they have never experienced an avalanche of this magnitude. The question is why a large avalanche occurred at the Ichinoike barranco only in 2019. The direct cause of the avalanche occurrence in 2019 is the rain just before. Normally, the water that accumulates in Ichinoike crater flows under the scree of the barranco and reaches the bottom of the Ninoike crater without affecting the snow layer above, so it does not reach the avalanche. However, in 2019, it is thought that the avalanche was triggered by the flow of water stagnating in the scree and blowing out to the bottom of the snow layer. It is difficult to explain by meteorological conditions, and it is thought that the volcanic ash that flowed in from Ichinoike blocked the flow of water under the scree. Since 2020, there have been no avalanches at Ninoike, even during heavy rains during the thaw season. It is speculated that the slush flow and subsequent movement of volcanic ash within the Ninoike crater removed the damming condition.
(Acknowledgments) Mr. Yoshiyuki Sawada of Otaki Village and the people of Mitake Branch of Kiso Town provided with photographs after the avalanche and information on Ninoike.
Maeno et al. (2016) Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal pyroclastic density current and fallout deposits. EPS,68: 82.
Kunitomo et al.(2019) Slush avalanche occurred in the Ninoike crater of Ontake volcano, Programme and abstracts the Volcanological Society of Japan, P121. ( in Japanese )
Since 2017, when the Ontakesan Volcano Labatory of Nagoya University opened, we have been observing changes in Ninoike by visual observation and photography at Kengamine and the Ninoike crater. For the three years from 2020 to 2022, changes in the area of Ninoike lake were investigated using aerial photographs taken by drones. The drone used (Mavic 2 pro) was purchased as a hobby, and full-scale drone surveying has not been conducted. Using rocks and buildings as landmarks, the size of the photographs for each year were adjusted to determine changes in the area of Ninoike lake. In addition, the positions of rocks that can be confirmed from drone photographs were measured by PPK (Post Processing Kinematics, using the GEONET Mitake as the base station) using a commercially available inexpensive dual-band GNSS receiver (DG-PRO1RWS). The position accuracy of PPK is about several centimeters. As of September 13, 2022, the area of Ninoike (the area of the depression) is about 650m2. The area of Ninoike immediately after the eruption was about 6500m2 (calculated from JGI's 2014 aerial photograph), so it has shrunk to about 10%. In addition, the color of the lake, which looked milky green until around 2020, will appear ocher after 2021, and the depth of the depression is thought to be shallow. Currently, the area of Ninoike is shrinking at a pace that is halved every year, and if this trend continues, it may disappear in a few more years (Fig.2). The shrinkage of Ninoike seems to have accelerated since around 2018-2019. The causes are thought to be the increase in volcanic ash inflow from Ichinoike due to the heavy rainfall in 2018 and subsequent increase in precipitation, and the stirring and movement of volcanic ash at the bottom of the Ninoike crater due to the slush flow in 2019.
The May 21, 2019 avalanche occurred under these circumstances. According to interviews with local residents, no avalanche has ever occurred at the Ichinoike barranco, and they have never experienced an avalanche of this magnitude. The question is why a large avalanche occurred at the Ichinoike barranco only in 2019. The direct cause of the avalanche occurrence in 2019 is the rain just before. Normally, the water that accumulates in Ichinoike crater flows under the scree of the barranco and reaches the bottom of the Ninoike crater without affecting the snow layer above, so it does not reach the avalanche. However, in 2019, it is thought that the avalanche was triggered by the flow of water stagnating in the scree and blowing out to the bottom of the snow layer. It is difficult to explain by meteorological conditions, and it is thought that the volcanic ash that flowed in from Ichinoike blocked the flow of water under the scree. Since 2020, there have been no avalanches at Ninoike, even during heavy rains during the thaw season. It is speculated that the slush flow and subsequent movement of volcanic ash within the Ninoike crater removed the damming condition.
(Acknowledgments) Mr. Yoshiyuki Sawada of Otaki Village and the people of Mitake Branch of Kiso Town provided with photographs after the avalanche and information on Ninoike.
Maeno et al. (2016) Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal pyroclastic density current and fallout deposits. EPS,68: 82.
Kunitomo et al.(2019) Slush avalanche occurred in the Ninoike crater of Ontake volcano, Programme and abstracts the Volcanological Society of Japan, P121. ( in Japanese )