A tsunami is an entirely different kind of wave than a normal wave. A normal wave is transverse, and a tsunami is a pressure wave. The first bobs the water up

A tsunami is an entirely different kind of wave than a normal wave. A normal wave is transverse, and a tsunami is a pressure wave. The first bobs the water up and down and the wave moves forward, the second shoves water ahead of it, displacing it upward into the wave. The tsunami won't break when the column gets short, but rather transfer all of the energy of moving a deep water column to a shallow one. That's what makes them dangerous.