So what caused the Japanese tsunami?

I know I know, “the earthquake, idiot”, but I remember that during the Indian Ocean quake and tsunami, there was a lot of talk about subduction plates and one moving under another and slip faults. I guess due to the looming disaster with the nuclear reactors, that was kind of overlooked this time.

Did one plate slip under another? Or was the 8.9 simply that strong as to have somehow shaken the water enough so that it came pouring on to land?

It has to do with the kind of earthquake as well as the energy released. An earthquake caused by plates sliding horizontally next to each other won’t be enough to cause a tsunami, but if the quake pushes the earth upwards, water is displaced, causing the wave.

Relevant Wiki link (with illustrations):
http://en.wikipedia.org/wiki/Tsunami#Generation_mechanisms

Pretty much this.

The wave needs to be produced from something pushing a LOT of water.

There are lots of earthquakes and most are in the ocean. Only a relative few produce a tsunami. Quakes can move sideways or down or lots of things. Depends on the fault. If you have a substantial amount of land moving up then wham…tsunami.

It hasn’t been overlooked at all. I’ve seen quite a few graphics and explanations about the shifting (or sliding under) of the plates. In this case, the Pacific Plate is sliding under the westernmost part of the North American Plate, causing it to spring up and throw the water upward. And yes, northern Japan is part of the North American plate.