Originally Posted by RichardInSF
Had the earthquake been mid-Pacific, the tsunamis could have reached both the California and Japan coastal areas -- but many hours later. People should have been evacuated immediately to higher land on hearing of the earthquake; if the epicenter was far enough away (I don't know if it was) there could well have been enough time to get to safety. The Japanese are very aware of this and take prompt action in case of nearby earthquake, for example.
According the reports that I read it took the wave about an hour to reach Thailand and about an hour and a half to reach Sri Lanka. Although not all of the loss of life could have been prevented I'm sure the numbers would have been MUCH smaller had people known to move inland (and they had plenty of time to do it).
Over recorded history most deaths caused by Tsunamis have been due to ignorance of the fact that you should run (not walk) away from the water should you feel an earthquake in a costal area.... same goes if you learn of a recent quake in a costal area that is in a direct line of sight of your current position.
If there is a shallow (less than 100Km depth) quake in a costal area (especially if the epicenter is underwater) there WILL be a Tsunami. The magnitude of the Tsunami will be dictated by the size of the quake, its location, and underwater topography. The wave will slow as the water gets shallow and all of that velocity will be transformed into amplitude - translation - the wave gets TALLER and slower as the water gets shallower.