Originally Posted by kathyblack
And I notice the size of the wave face in Samoa is much bigger and more violent than the photos in northern Tonga. Why?
I think it's the shape of the harbor. Pago Pago is that V shape. My husband tells me it's because Tonga is so much farther away from the center of Earthquake.
You are more or less correct. In the open ocean, tsunamis are very fast, very long wavelength waves. Generally a boat in mid ocean would not even notice a passing tsunami, since they are no higher than ordinary ocean waves. Because they have such long wavelength, the energy is spread out over a big area. They can maintain that energy over great distances.
When they approach land, they begin to "feel" the bottom. The wave slows down, which decreases the wavelength, and the water
"piles up" increasing the height. (This is similar to what happens to ordinary surf, only on a much bigger scale.) Because the wavelength decreases the energy gets concentrated into a much much smaller area. The exact size and shape of wave one sees when a tsunami gets to shore depends mostly on the shape of the bottom in that area. The local bottom configuration is much more important than the distance to the earthquake.
For more information, go to the Pacific Tsunami Warning Center Frequently Asked Questions (FAQ)