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.
公元前479年波斯士兵围攻 希腊城市波提狄亚时, 潮水比平时退得更远, 留下了一条便捷的入侵路线。 但这不是意外的好运。 士兵们还没走到一半, 高过以往见过的 回流的水波 就淹死了这些进攻者。 波提狄亚人认为他们得救于 波塞冬的愤怒。 但真正救了他们的可能是 已摧毁过无数他人的同样的现象: 海啸。 虽然海啸通常被称为潮汐波, 它们实际与太阳和月亮引起的 潮汐活动无关。 在许多方面,海啸只是 比正常波浪还大的波浪。 它们有波谷和波峰, 并不是流动的水, 而是水的能量运动。 区别在于这种能量是从哪里来的。 正常的海波 来自风。 由于风只影响表面, 波浪的大小和速度是有限的。 但海啸是由 水下的能量引起的, (比如)火山爆发ˎ 海底滑坡ˎ 或最常见的, 地球表面的板块滑动引起的 海底地震 向水里释放出大量的能量。 这能量向上行进到表面, 排开水且使水面高于 正常的海平面, 但重力把它拉下来, 使得能量向外横向传播。 因此,海啸就出现了, 以每小时500英里以上的速度行进 当海啸远离海岸时, 海啸几乎不能被探测到, 因为海啸通过整个水深移动。 但当海啸到达浅水地带, 波变浅的情况就发生了。 因有较少的水移动, 这巨大的能量被压缩。 水波的速度慢了, 但水波的高度升起高达100英尺。 海啸这个词, 日语叫“港波, 来源于 它似乎只出现在海岸附近这一事实。 如果海啸的低谷先到达海岸, 在波浪撞击之前,水会比正常情况下退出的距离更远, 这种情况既危险又有误导性。 海啸不仅会淹死靠近海岸的人, 还会把内陆一英里或更远距离的建筑物和树木冲平, 特别是在低洼的地方。 好像这还不够, 水然后再退下来, 随之脱下来的还有新产生的碎片, 以及路途中极不幸 被冲走的任何东西或人。 2004年印度洋发生的海啸 是历史上一次最致命的自然灾害之一, 在整个南亚地区死了超过200,000人。 因此怎样才能保护我们 抵抗这种大自然的破坏力呢? 某些地区的人们试图 用海堤、防洪门 和渠道疏导水来阻止海啸。 但这些并不总起作用。 2011年的海啸超越了 保护日本福岛电厂的防洪墙, 引起核灾难, 造成超过18,000人丧生。 许多科学家和政策制定者 却集中在早期发现, 监测水下压力和地震活动, 并建立全球通信网络 以快速发送警报。 当自然太强大而无法阻止它时, 最安全的做法是为其让道。