I once had to give an oral presentation on tidal waves and tidal bores while in the U.S. Navy so I've had a fairly good grasp of the origins and mechanisms of these kinds of waves, aka by the Japanese word "tsunami", a word which seems more appropriate since "tidal wave" is something of a misnomer as these kinds of waves have nothing to do with the Earth's tides.
The force which often powers these destructive waves is the vertical displacement of the sea bottom and that's exactly what happened with the March 10 Japan earthquake. A long stretch of ocean bottom displaced, the energy from this displacement causing extremely long waves that slow down and pile up in height as they reach shorelines and then blast ashore, rolling forward and obliterating everything in front of them.
Though technology has come a long way in predicting the heights of predicted tsunamis and where they might strike the proximity of the sub sea earthquake off the coast of Sendai, Japan resulted in these waves, which can travel at speeds exceeding 500 MPH across the ocean's surface, reaching Japan's shorelines very quickly.
This amateur video footage is a grim reminder of the destructive power of these waves: Amateur video footage of tsunami
I hope that scientists and oceanographers can use this and other footage, as well as the earthquake event itself, to further enhance their tsunami warning capabilities.
Having watched this and other tsunami footage from this earthquake event, I was, despite my understanding and knowledge of tsunamis, absolutely appalled and greatly saddened at the horrific destruction and loss of human life these waves have caused.