A tsunami is an unusually large wave triggered by a major force (usually an earthquake) applied to the ocean. In the open ocean a tsunami does not have a particularly large amplitude, but its wavelength is enormous. As it approaches land, the amplitude of a tsunami can increase dramatically, leading to catastrophic flooding.
Several sites, some more media-rich than others, present conclusion about the physics of tsunamis. The origin of tsunamis is well understood (though, because most are caused by earthquakes, impossible to predict). Any short, sharp excitation of sufficient size can cause a tsunami; as in many wave-bearing media, the specific details of the excitation are not as important as its total size.
My reconstruction of the major practical difficulties in tsunami warning is
- Since the tsunami has a small amplitude in the open ocean, it is difficult to detect before it nears shore, and
- Whether a small amplitude open-ocean tsunami turns into a large amplitude wave near shore (called “shoaling”) depends critically on the details of the shoreline and the direction of approach of the wave. During the time between the triggering of a wave and its arrival on land, there is often not enough information to precisely calculate the threat to populated areas and then to communicate an appropriate warning. After the fact it is possible to reconstruct the path of the wave, but this is small consolation to the survivors.