When a tsunami wave is in the deep ocean it usually goes unnoticed. The wavelenghts (the distance between the top of one wave and the top of the next wave) of Tsunami waves in the deep ocean can be up to hundreds of mile long.
As the waves comes closer to the shallow shoreline, the wave begins to slow down. The faster moving water behind starts to catch up to the slower moving water and the wave gets bigger and bigger. So a wave moving at 450 miles/hour that is 2-3 feet in the deep ocean can become a 100 foot wave moving 30 miles/hour into a town.
