On the night of July 24, an explosive eruption occurred on Sakurajima in Kagoshima Prefecture, raising the eruption alert level to the highest level, "Level 5" for the first time.
While calling for strict caution, such as the evacuation of some residents on the island, the Japan Meteorological Agency also explained that "a large-scale eruption is not imminent."
What exactly does that mean?
And what are the outlooks and precautions for Sakurajima's future activities?
I will explain in detail from interviews with the Japan Meteorological Agency and experts.
After 8 pm on July 24, a big volcanic bomb ...
An explosive eruption occurred at the Minamidake summit crater on Sakurajima after 8 pm on the 24th.
A large volcanic bomb flew eastward of the crater beyond the previous warning range, reaching around 2.5 km.
The Japan Meteorological Agency released an eruption bulletin about 15 minutes after the eruption and raised the eruption alert level to "5".
The "eruption alert level" is announced by the Japan Meteorological Agency in five stages according to the status of volcanic activity.
Of these, this "5" is the highest level and will be announced not only when there is a danger around the crater, but also when an eruption that causes serious damage occurs in the area where people live.
The Japan Meteorological Agency called for strict caution against large volcanic bombs within approximately 3 km from the crater, and based on this information, Kagoshima City issued evacuation orders to some residents on the island.
Since the introduction of the alert level in 2007, it has been raised to "Level 5" for the second time since the 2015 eruption of Kuchinoerabu Island in Kagoshima Prefecture, and for the first time in Sakurajima.
What are the two "level 5"?
While strict caution is being called for, the Japan Meteorological Agency also explained that "a larger eruption that would affect a large area of the island is not imminent."
What does "a larger eruption than this time" mean when you raise it to the highest level 5?
In fact, Sakurajima is preparing two assumptions even at the eruption alert level "5".
Part 1. Explosive eruption
The first assumption is an explosive eruption like this one.
Sakurajima is known as a volcano that erupts frequently, but when a volcanic bomb flies more than expected and poses a danger to some areas of the island, it is raised to level 5 to call for evacuation of the residents of that area. increase.
So to speak, it is level 5 issued by "extension of normal volcanic activity".
Part 2. Large-scale eruption
Another assumption is a "large-scale eruption" in which a wide area of the island is damaged and volcanic ash affects a wide area including the outside of the island.
A typical example of the past was the 1914 eruption, which occurred about 100 years ago. In addition to the volcanic blocks and pyroclastic flows that accompanied the eruption reaching the residential areas of the island, a large amount of lava was generated from the hillsides on the west and southeast sides of the mountain. As it flowed out, Sakurajima became a land connection with the Osumi Peninsula.
Even if such an eruption is predicted, the Japan Meteorological Agency has decided to raise it to level 5, and the regional disaster prevention plan of Kagoshima City stipulates that residents should be evacuated from the island.
The Japan Meteorological Agency explains that "a large-scale eruption is not imminent" means that an eruption of a scale like the Taisho eruption is not immediately envisioned this time.
Damage occurs even if it is not a "large-scale eruption"
Even if it does not lead to a "larger eruption," it is not a situation that can be taken care of.
This is the first time that Sakurajima has been raised to level 5, but according to experts, it is said that explosive eruptions like this one have occurred repeatedly in the past.
Eruptions of this scale frequently occurred in the 1970s and 1980s, when eruptive activity was active.
In November 1986, a fountain with a diameter of about 2 meters and a weight of about 5 tons fell on a hotel building in what is now Furusato-cho, Kagoshima City, about 3 kilometers away from the crater.
In the eruption that occurred on June 4, 2020, large volcanic bombs scattered over 3 km from the crater.
The bomb fell to a location only a little over 100 meters away from the inhabited area.
However, the Japan Meteorological Agency confirmed that it was four days after the eruption and did not raise the level, but later explained that it was "a case of raising it to level 5."
Professor Masato Iguchi of the Volcanic Activity Research Center of Kyoto University, who has been studying the eruptive activity of Sakurajima for many years, said, "I recognize that this eruption is an explosive eruption that is common in Sakurajima. However, we should once again recognize that Sakurajima is an active volcano. "
Volcanic eruption What to do then?
When a sudden eruption occurs.
When "Eruption Bulletin" or "Eruption Warning" is announced.
How can I protect my safety?
This is a point you should know.
What is the outlook for future volcanic activity?
Residents are still evacuating in Sakurajima, and evacuation of 33 households and 51 people in some of Arimura-cho and Furusato-cho, where evacuation orders have been issued by Kagoshima City, has been confirmed. increase.
So what will happen to volcanic activity in the future?
The Japan Meteorological Agency is paying attention to crustal movement data.
From the 18th of this month, slight crustal movements indicating the expansion and uplift of the mountain body, which occur when magma rises from the underground, have been observed in Sakurajima, and then this eruption occurred.
Since the mountain body has continued to expand after the eruption, the Japan Meteorological Agency is carefully monitoring whether it will lead to an eruption like this one again.
In addition, the Japan Meteorological Agency explains that the eruption alert level will be lowered "if a large volcanic bomb or pyroclastic flow does not cause an eruption that affects the distance for 3 days, the level will be lowered."
However, since volcanic activity is very active, we are calling for never approaching a range of about 3 km from the crater.
Preparing for a large-scale eruption of the Taisho eruption class
In response to this eruption, Professor Iguchi of Kyoto University emphasizes the importance of preparing for a large-scale eruption of the Taisho eruption class.
Based on research on crustal movements so far, it is thought that the amount of magma beneath the Aira Caldera has returned to 90% of that before the "Taisho eruption."
Professor Iguchi said, "In the future, we should prepare for a large-scale eruption of the Taisho eruption class, and we must achieve advance evacuation instead of responding after the large-scale eruption occurs. We are prepared to respond properly when it is really dangerous. It is important to make. "