Regarding the explosive eruption that occurred on Sakurajima in Kagoshima City on the early morning of the 25th, the Japan Meteorological Agency initially announced an eruption bulletin saying that "a pyroclastic flow has flowed down 1.8 km" and expanded the range requiring caution to near the residential area. Did.
However, as a result of investigating the site, he changed his view that it was "not a pyroclastic flow but a part of the eruption that flowed downwind", and returned the range requiring caution to the previous "about 2 km from the crater". It was.
Regarding the explosive eruption that occurred at the Minamidake summit crater on Sakurajima at around 1: 9 am on the 25th, the Japan Meteorological Agency issued an "eruption bulletin" at 1:52 am, about 40 minutes after the outbreak, at 2:40 am. The warning around the crater was announced again, and the area requiring caution, which was about 2 km from the crater, was expanded to about 2.4 km from the crater near the residential area.
At that time, the Japan Meteorological Agency said, "The camera confirmed that the pyroclastic flow had flowed down about 1.8 km in the southwest direction."
However, when the staff of Kagoshima Local Meteorological Observatory investigated it locally, there was no high temperature part that could be confirmed after the pyroclastic flow flowed down, and no trace of the tree being knocked down was found.
For this reason, the Japan Meteorological Agency changed its view that the phenomenon of pyroclastic flow this time is "thought to be part of the eruption that flowed downwind", and it is necessary to be cautious after reissuing the crater area warning at 3:30 pm. The range has been returned to the previous "about 2 km from the crater".
Regarding this response, the Kagoshima Local Meteorological Observatory said, "Since a quick judgment is required for the pyroclastic flow under the limited observation system, this time it was judged to be a pyroclastic flow by visual inspection of a surveillance camera. As a result, it was not a pyroclastic flow. When considering the safety of residents, it is possible that similar cases will occur in the future. Which data should be focused on, such as seismometers and air vibration meters, in order to make accurate judgments at an earlier stage? We will scrutinize it thoroughly. "
Expert "There are few cases and evaluation is difficult"
Professor Masato Iguchi of the Volcanic Activity Research Center of Kyoto University, who is familiar with the eruptive activity of Sakurajima, said, "When the pyroclastic flow flows down 1.8 km from the crater, it will be the longest in Sakurajima and requires a considerable amount of magma, but this time the ground. There was little change in the fluctuation data. Also, looking at the video, the speed was slow for a pyroclastic flow, and there was doubt about the evaluation of the pyroclastic flow. "
Regarding the change in the view that the Japan Meteorological Agency "is considered to be part of the eruption that flowed downwind," "there are few cases of pyroclastic flow, and it is difficult to correctly evaluate the phenomenon from the beginning. It is important to be on the highest alert immediately after the outbreak, so even if the result is wrong, it is important to be vigilant. "
He added, "At the scale of the eruption at Minamidake, which usually occurs, it is unlikely that the pyroclastic flow will flow down to 1.8 km, but it can occur in large-scale eruptions, so be careful when activities increase." I am.