In 479 BC, when Persian soldiers besieged the Greek city of Potidaea, 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 and a crest, 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 outwards horizontally. Thus, the tsunami is born, moving at over 500 miles per hour. 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 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. The word tsunami, Japanese for "harbor wave," comes from the fact that it only seems to appear near the coast. If the trough 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. 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 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.
西元前479年,波斯士兵圍攻 波提狄亞的希臘城邦時, 當時的海潮退得比平時更遠, 剛好方便波斯士兵入侵。 但這並非好運。 就在穿越到一半前, 退去的海水卻更高打回來, 其浪高沒人見識過, 淹死了進攻的波斯士兵。 波提狄亞人相信是敵軍激怒了海神 而救了他們。 但真正救了他們的 其實是摧毀無數生命的自然現象: 海嘯。 雖然海嘯又稱為潮汐波, 但它們與受太陽、月球引力作用 所形成的潮汐活動並沒有關連。 許多方面看來,海嘯其實只不過是 放大版的正常海浪。 它們都有波谷及波峰, 靠的不是流動的海水, 而是海水中能量的移動。 其不同之處在於能量的來源。 一般海浪的形成, 風是其能量來源。 由於風的影響僅止於海平面, 海浪的大小和流速因此有限。 海嘯的形成 則是源於水面下的能量。 如火山爆發、 海底崩移、 或更常見的 海底地震, 地球表面的地殼板塊滑動時, 釋放大量能量至水中 而產生海底地震。 這些能量傳達到海面, 導致水位變化 高過一般海平面, 然而,地心引力造成的拉力, 使得能量以水平方向波動, 海嘯因此而生, 移動時速超過500英哩。 當海嘯距離岸邊較遠時, 很難被人察覺, 因為它移動的是整個從 海面到海底的水層。 然而,當海嘯到達淺水域時, 便出現波浪淺化效應。 因為可移動的海水變少, 不變的巨大能量被壓縮。 海浪的流速因此變緩, 浪高則可高達100英呎。 tsunami(海嘯)一詞 在日文意指「港口的波浪」, 是因為它似乎只出現在 靠近海岸的地方。 如果海嘯的波谷先抵達海岸, 在浪打過來前,海水會退得比平時更遠, 這可是有誤導性的危險。 海嘯不僅會淹沒岸邊的人們, 內陸一英哩或以上的建築物和樹木 也會被摧毀, 特別是低窪地區。 如果這還不夠糟, 當海水退去時, 將一併帶走新的殘骸、 以及不幸在它所經之處的 任何東西和人。 2004年的印度洋海嘯 是史上最致命的自然災害之一, 造成整個南亞地區超過20萬人喪生。 所以面對大自然的破壞力, 我們該如何自保? 有些地區的人們 試圖靠海堤、防洪閘門和渠道 來疏導海水,藉以抵擋海嘯。 但這些方法並非每次都奏效。 2011年的一次海嘯 便越過用來保護 日本福島核電廠的防洪牆, 導致核災事故, 同時帶走超過1萬8千多條人命。 許多科學家和決策者 將重點放在早期預警系統, 監測水面下壓力及地震活動, 並建立全球通訊網絡 以利快速發佈警報。 當大自然的力量 太過強大無法阻擋時, 最安全的作法就是別擋它的路。