According to Professor Yeh at Oregon State University, as reported in the New York Times, http://www.nytimes.com/2011/03/13/weekinreview/13water.html, the energy going into the Sendai tsunami was a bit less than that of an exploding atomic bomb. That surprised me; I would have thought it to be much more.
Usually we think of atomic bombs as being aimed at population centers or strategic targets. But could an atomic bomb actually cause a tsunami? If so, it would seem that it might be able to cause more damage that way than in a direct attack.
The energy in the tsunami is considerably less than the total energy released by the earthquake. According to the Wikipedia article on the Richter scale, a 9.0 earthquake releases the energy equivalent of 474 megatons of TNT. The biggest nuclear weapons today have a yield of about 500 kilotons, or about 0.1% of the energy yield of the entire quake. There used to be nuclear weapons in the 25 megaton range (or about 5% of the quake’s energy).
There were underwater nuclear tests before they were banned by treaty. I don’t know of any case where a test caused a tsunami. I don’t see any reason why an underwater explosion couldn’t.
I don’t think that using a nuclear weapon to create a tsunami would be an effective tactic for a few reasons:
The creation of a tsunami by an underwater disturbance is not a sure thing. Most underwater earthquakes don’t cause tsunamis.
Deployment would be difficult. You’d have to get a bomb down to the bottom of the ocean near the enemy’s coast.
It would be useful only against coastal targets.
It would waste a lot of the bomb’s energy. Consider the current tsunami - while it’s true that it caused a lot of destruction, how much more destruction would there be from a nuclear weapon of equivalent energy detonated over land? The A-bomb that destroyed Hiroshima was relatively small, and it caused a lot more damage than the tsunami did.
I’m not sure you could form a tsunami with a (single) nuclear explosion. While it would certainly displace a lot of water, the resulting wave might have very different characteristics. An earthquake releases its energy across a wide area over a period of several second, while a nuclear explosion is localised and almost instant. A tsunami wave has a very small amplitude (wave height of only a few inches) in open water and a very long wavelength (tens to hundreds of kilometres). I’d imagine an underwater nuclear explosion would produce a wave with a very large amplitude and a relatively short wavelength.