The risk of a possible, massive disaster must not be left unattended.
This can also be said of Mt. Fuji, an iconic mountain of Japan. It is an active volcano. It is important, first of all, to assess what sort of disaster could occur if Mt. Fuji should erupt.
A working-group panel of the Central Disaster Management Council has begun studying the possible impacts of an eruption of Mt. Fuji on the Tokyo metropolitan area. On the basis of discussion to be held by experts, the panel is set to work out, within about one year, the risks posed by a massive fall of volcanic ash and disaster reduction measures that could be taken.
There is no precedent, the world over, for a large city, underpinned by advanced technologies, to be struck by a large-scale eruption. It is important to make a factual study based on estimated data.
Many volcanoes in Japan are ones whose volcanically active period is at least 500,000 years. The formation of Mt. Fuji is believed to have started 100,000 years ago. Mt. Fuji became the nation's highest mountain within a relatively short period. The mainspring of its activity is a magma supply underground.
Over the past 3,200 years, Mt. Fuji has experienced more than 100 eruptions. Seventy or more confirmed craters have been found, either at its summit or around its base. Most of them are reckoned to have been generated by small-scale eruptions, but at least seven are believed to be ones that occurred on a large scale.
The most explosive eruption on record was the Jogan eruption, which occurred from 864 to 866, emitting 1.4 billion cubic meters of lava and dividing nearby lakes. It was followed in scale by the 1707 Hoei eruption, which spewed out massive amounts of volcanic ash -- equivalent to 700 million cubic meters of lava. The ash from this eruption fell and accumulated even in what is now central Tokyo.
Ash a major hazard
The working-group panel will advance its discussion on the basis of these historic examples. It is only reasonable for the panel to assume a maximum-scale eruption.
The Cabinet Office, the government of Yamanashi Prefecture -- home to part of Mt. Fuji -- and other entities concerned in 2004 drew up hazard maps, the first of their kind, showing possible impacts from lava flows and pyroclastic flows on the surrounding areas, on the basis of the eruption records of Mt. Fuji.
As these studies have advanced, however, more serious situations have been pointed out. A typical example is discoveries of new traces of craters. Such traces have been confirmed even outside areas where experts had previously thought eruptions might occur.
It is important for the results of the latest studies to be reflected in the relevant measures. Local governments concerned have already started reexamining their hazard maps.
Also, regarding the spread of volcanic ash, detailed analytical examples concerning the Hoei eruption have increased. That eruption lasted for 16 days, but when the volume of ash emitted and the wind direction are taken into account, it has been found that a future eruption of Mt. Fuji on an equivalent scale could result in an even larger amount of ash falling on the Tokyo metropolitan area than at the time of the Hoei eruption.
According to case studies of ash fall -- in Japan or abroad -- which the Cabinet Office has collected, automobiles are unable to run on roads covered in about 10 centimeters of ash, and about 5-centimeter accumulation would damage the water supply. Even as little as about 1 centimeter of ash fall could cause power outages, due to its accumulation on power transmission cables.
To protect the vital functions of the metropolitan area, knowledge and information must be rallied.
(From The Yomiuri Shimbun, Sept. 18, 2018)
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