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Comment by ajays | Hacker News Reader

Color me educated. I used to think that a tsunami was a giant wave which would splash, and then it's gone. I didn't know/realize how slow-moving but MASSIVE this was.

So what happens to all this water? Does it eventually go back into the sea the way it came in? Or is this the default sea-level now?

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From the wiki article, by the time a large tsunami reaches the shore, it has a wavelength of ~20km, taking ~1-2 minutes to reach the peak of the wave.

And yeah, in the long term, most of the water will go back out to sea (or evaporate off). Of course, areas that were below sea-level before hand, will now be big ponds until they're drained, but by and large, the water will withdraw (which does extra damage!).

> areas that were below sea-level before hand Some areas became below sea-level _after_ the quake. The land sank a maximum of 70cm.

Yeah, I think a good number of people don't understand how tsunamis work, even here in Hawaii. During an emergency, radio and TV broadcasters here try to explain that tsunamis are more like solid walls of water - i.e. the entire ocean lifting up six feet, but of course, people don't listen. That makes it very dangerous when there's a tsunami warning (like last Thursday), because people think, oh, it's just like surfing! six feet is not even that much!, which means that some people don't evacuate. Plus there are always some geniuses that insist on sitting next to the shoreline, and the police have to shoo them away.

Even "wall of water" is misleading, as that's not what it is at all. A horizontal wall perhaps, but not a vertical one like in the finale of Abyss.

One way I've used to explain it is to imagine you're on a boat at sea and a tsunami passes under you. You wouldn't even notice.

The boat would slowly rise a few feet and stay there for a few minutes as this huge mass of water passed under it, then slowly drop back down to its original height, and no one would be the wiser.

It's not until the mass of water hits the shore and keeps piling on for several minutes, that people can see what it is.

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