life is like a vapor
The water that gets sucked away is basically going into the surge. Watch a normal beach wave some time, you will see that as a wave builds, the water underneath drops. Once there is not enough water underneath (usually at the beach), the wave breaks.
--Eric Fri Jan 14 12:18:47 2005
Huh, it's interesting to me that a pile of water can act as an attractive force, so to speak.

I guess it's like how I'm surprised evaporation can "actively" draw heat away from something.
--Kirk Fri Jan 14 13:08:33 2005
If you think of a barometer, which is a tube of mercury (water works fine for barometers, except that water barometers are 9.8m tall) sitting in a pan of mercury. As you evacuate the tube, the mercury rises in the tube and the level in the pan falls. The same thing happens with the ocean, but the causative agent in this case not air pressure but a landmass that starts moving from side to side, effectively compressing the ocean. The ocean is compressed, but since water isn't very compressible, the water is pushed upwards into the tsunami wave.

In the deep water at its highest point, the tsunami was about two feet high and probably insensible to most sailors it passed under, and it was down to some 9 inches before it reached Sri Lanka. However, the energy held in that wave is channeled upwards as the water becomes shallow, and so a 2 foot wave becomes 60 feet.

So I wouldn't call it an attractive force (not to say you are), but it's just the gravity pulling the water downwards, oceanwards, as the water level drops.
--LAN3 Fri Jan 14 19:00:43 2005
I understand how a short little thing becomes a monster as the depth decreases; it's the way it drains the water away from the shore that I'm having trouble wrapping my mind around...
--Kirk Sat Jan 15 12:42:58 2005
Tidal Wave same as little waves, just bigger. If you don't get that, you didn't grow up near an ocean, did you? ;)
--Frau Erin Sat Jan 15 14:37:15 2005
Actually, I spent most of my summers at Ocean Grove, New Jersey.

But it never seemed liked the water was "sucked away"...I guess I just assumed that if the water was lower right before the wave, that was just the normal, non-wave height of the water at that time...
--Kirk Sat Jan 15 14:45:12 2005
Like any wave, the normal magnitude is halfway between the lowest and the highest points, not merely the lowest.

More importantly, the 60-foot is made of a motherlode of water that has to come from somewhere. It used to be counterintuitive to me is that waves are just mechanical movement of water, but the water doesn't actually go anywhere when the wave passes; hence, the water that was next to Sumatra isn't the same water that smacked Thailand, even though the energy of the wave passed through both.

When boating, my dad taught me this by "chip-chucking," where you throw a wood chip into the water and you can use it as a frame of reference when you can't see any others at that point. It's also used to calculate speed, based on the time it takes to pass the known length of the boat. I think I've mostly revised my intuition about these things, thanks to a coffee-can full of wood-chips.
--LAN3 Sun Jan 16 02:13:01 2005
Waves have a peak and trough phase. Water does not always "suck out" in a Tsunami ... it just depends on whether the trough phase of the wave happens to hit first.
--kPa Thu Jan 20 22:36:39 2005

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