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2014年8月31日 (日)

日本海の津波 短時間での襲来に備えたい

The Yomiuri Shimbun
Sea of Japan coastal regions must be braced for tsunami onslaught
日本海の津波 短時間での襲来に備えたい

Even after an earthquake that is not very powerful, massive tsunami waves can strike areas on the coast of the Sea of Japan.

Robust disaster preparedness measures must be taken diligently to keep people from being caught off guard by a tsunami calamity.

Of the various types of earthquakes generated at the bottom of the Sea of Japan, which kinds of tremors should be considered likely to trigger a tsunami? The government on Tuesday announced projections regarding the maximum size of tsunami that should be anticipated to strike Sea of Japan coastal regions, and the arrival time of tsunami in each region. This is the first projection of its kind to be made by central government.

According to the projections, tsunami can occur at heights ranging from four to 12 meters in populous, low-lying areas along the eastern half of the Sea of Japan coastline, from Fukui Prefecture to Hokkaido. On the western half, from the northern part of Kyushu to Kyoto Prefecture, tsunami three to four meters high are said possible. Within the Sea of Japan region, there is an area in Hokkaido that the government estimates could be subject to tsunami as massive as over 20 meters high.

All these, however, are nothing other than estimations that have been calculated based on the current progress of relevant studies. There may be tsunami generated by underwater faults that have yet to be discovered. Constant vigilance must be maintained.

The projections are in line with a law enacted in 2011 for fortifying communities against tsunami disasters, in light of the Great East Japan Earthquake that year. The law makes it obligatory for municipalities facing the coast to strengthen tsunami countermeasures.

On the Pacific coast, each municipality has been proceeding with defensive preparations against huge tsunami by considering earthquakes such as those that might focus along the Nankai Trough in the Pacific Ocean.

However, in regions along the Sea of Japan, where no confirmed extensive seismic zones exist, there has been considerable variation in approaches to anti-tsunami measures among the 16 prefectural governments concerned, including Hokkaido.

Arrival within 1 minute

This compilation of tsunami projections by the central government is of high significance in prompting local entities to devise measures to tackle tsunami hazards.

What is important in mapping out countermeasures is to take into full consideration earthquake and tsunami characteristics peculiar to the Sea of Japan coast.

Underwater faults in the Sea of Japan are relatively close to coastal areas and are at shallower locations. This means that should a tsunami occur, it could become very large and could arrive at the coast within a short time. According to the projections, there could be tsunami in many areas that could reach the coast within a single minute after an earthquake. Given this, it is of paramount importance to conduct evacuations as swiftly as possible in the event of such a situation.

To alleviate damage from tsunami disasters, it is necessary for every local entity to work out a hazard map appropriate to accurate assessments of local characteristics. Local municipalities should address the tasks pragmatically in both practical and informative terms, including measures such as improvement of evacuation routes and disaster drills.

It has been pointed out in the projections that a tsunami occurring in the sea off a location between Hokkaido and Akita Prefecture could spread extensively because of the influence of undersea geological formations, possibly reaching as far as the Chugoku region.

There also are fears that a tsunami occurring in the Sea of Japan could rebound after reaching the coast of the continent, a phenomenon that would cause the tsunami to assault stricken areas in Japan repeatedly. That is to say, vigilance must be maintained even after a single tsunami has subsided.

Safety arrangements at nuclear power plants are also essential. There are many regions where nuclear power complexes have been located in a concentrated manner on the Sea of Japan coast, such as in Fukui and Niigata prefectures.

Learning from the Great East Japan Earthquake, power utilities have already made tsunami projections that are more stringent than the latest government projections, working out steps such as construction of seawalls and measures to ensure power sources in the event of immersion during a giant tsunami. Considering the menace of tsunami, the most thorough preparations for ensuring safety are essential.

(From The Yomiuri Shimbun, Aug. 29, 2014)Speech


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