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津波は波浪ではなく、海面全体が盛り上る現象｡ 膨大な破壊力がある。



中　４分　170wpm　　　　　　　　　　　　　　　　　　　　　　　　　　　2014年　　　　　　　　　　　　　　　　　　　　　　　　
　
字幕 ： 開始後 で字幕On/Off､ で言語選択。文字の色やサイズﾞはｵﾌﾟｼｮﾝから｡
.　　　　 動画を見るとき、 でフルスクリーンに拡大すると見やすい。

★下記英文は ポップアップ辞書 が使えます｡
　　テキストはこちら⇒日英トランスクリプト （字幕はYouTubeの方が大きく見やすい）　　　　　

In 479 BC, when Persian soldiers besieged the Greek city of Potidaea(古代ﾎﾟﾃｨﾀﾞｲｱ), the tide retreated much farther than usual, leaving a convenient invasion route. But this wasn't a stroke of luck. Before they had crossed halfway, the water returned in a wave higher than anyone had ever seen, drowning the attackers.

The Potiidaeans believed they had been saved by the wrath of Poseidon. But what really saved them was likely the same phenomenon that has destroyed countless others: a tsunami. Although tsunamis are commonly known as tidal waves, they're actually unrelated to the tidal activity caused by the gravitational forces of the sun and moon.

In many ways, tsunamis are just larger versions of regular waves. They have a trough(谷） and a crest（山）, and consist not of moving water, but the movement of energy through water. The difference is in where this energy comes from. For normal ocean waves, it comes from wind. Because this only affects the surface, the waves are limited in size and speed. But tsunamis are caused by energy originating underwater, from a volcanic eruption, a submarine landslide, or most commonly, an earthquake on the ocean floor caused when the tectonic plates of the Earth's surface slip, releasing a massive amount of energy into the water.

This energy travels up to the surface, displacing water and raising it above the normal sea level, but gravity pulls it back down, which makes the energy ripple（波紋） outwards horizontally. Thus, the tsunami is born, moving at over 500 miles per hour（805km/h）. When it's far from shore, a tsunami can be barely detectable since it moves through the entire depth of the water. But when it reaches shallow water, something called wave shoaling（浅水変形）occurs. Because there is less water to move through, this still massive amount of energy is compressed. The wave's speed slows down, while its height rises to as much as 100 feet（30.5m).
　　（東日本大震災では岩手県大船渡市の綾里りょうり湾で局所的に遡上高 40.1m）

The word tsunami, Japanese for "harbor wave（港の波）," comes from the fact that it only seems to appear near the coast.

If the trough（谷） of a tsunami reaches shore first, the water will withdraw farther than normal before the wave hits, which can be misleadingly dangerous. A tsunami will not only drown people near the coast, but level buildings and trees for a mile inland or more, especially in low-lying areas. As if that weren't enough, the water then retreats, dragging with it the newly created debris, and anything, or anyone, unfortunate enough to be caught in its path.
　　（津波の谷が先に来ると、異常な引き潮になるが、そうなるとは限らないので危険）

The 2004 Indian Ocean tsunami was one of the deadliest natural disasters in history, killing over 200,000 people throughout South Asia. So how can we protect ourselves against this destructive force of nature? People in some areas have attempted to stop tsunamis with sea walls, flood gates, and channels to divert the water. But these are not always effective.

In 2011, a tsunami surpassed the flood wall protecting Japan's Fukushima Power Plant, causing a nuclear disaster in addition to claiming over 18,000 lives. Many scientists and policy makers are instead focusing on early detection, monitoring underwater pressure and seismic（地震の） activity, and establishing global communication networks for quickly distributing alerts. When nature is too powerful to stop, the safest course is to get out of its way.
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