Tsunamis are powerful waves caused by underwater earthquakes or eruptions that can crash over ten stories tall into a coastline.
If you were sailing far from shore, how would you feel about a tremendous tsunami heading your way?
Actually, you wouldn’t notice a tsunami at all. In deep water, a tsunami raises the ocean’s surface only about one or two feet—much less than ordinary ocean waves. What’s more, it takes up to an hour for a single tsunami to roll by. How can a wave be so deadly to a coastline, but so tame in the open ocean?
The energy of a wave has more to do with how deep the water churns than how tall the wave rises. Ordinary waves can be rather tall, but they don’t go very deep. What’s more there’s never much distance between one ordinary wave’s crest and the one following it. The distance between waves is called “wavelength,” and it’s another secret of a tsunami’s strength.
While ordinary waves only tug at the surface water, tsunamis churn the water all the way to the deepest ocean floor! They also have very long wavelengths—a hundred miles or more. Although tsunamis raise the surface only a foot or so in deep water, they carry a lot of energy.
When a tsunami approaches a coastline, the water gets shallower. This causes friction at the front of the wave and slows it down. The faster moving back of the wave piles into the slower front, squeezing the whole hundred mile tsunami like an accordion. Just as a rug will crumple and rise if it’s scrunched by an opening door, the tsunami gets taller. By the time it hits the beach, it can be a hundred feet high!
译文：
海啸是由海底的地震或者火山喷发引起的足以把一个十层高楼淹没的强大海浪。
如果你正远离海岸航行，这时遭遇了海啸，是什么样的感觉呢?
事实上，你根本不会注意到海啸。在深海中，海啸只会掀起一或两英尺的海浪-远远低于普通的海浪。更重要的是，一次海啸最多一个小时就过去了。为什么对于海岸如此致命的海浪在公海中如此温驯呢?
海浪的力量与水层波动的深度而不是海浪的高度有关。常波可能很高，但是它们水体波动的深度并不深。更重要的是常波前后的波峰之间并没有太大的距离。波之间的距离被称为“波长”，这是海啸力量的另一个秘密。
尽管常波只是在水表面生成，但是海啸可以在最深的海底引起水层波动使其产生。它们同样有很长的波长-一百英里或更多。虽然海啸通过深海中的水层波动只能提升水平面一英尺左右，但是它们带来了大量的能量。
当海啸接近海岸线时，水的深度逐渐变浅，导致波前摩擦，使其速度变慢。快速前进的波浪与前面速度较慢的波浪重叠，像手风琴那样挤压这些整个百里长的海啸，正如由于门开时被压住而弄皱卷起的地毯，海浪就越来越高。当它到达海滩的时候，它可以达到一百英尺。