That displaced an absurd amount of water, and water carries energy very efficiently. Imagine jumping into a bathtub--the water reacts pretty violently. So you've got tons and tons of water, moving towards shore. As it gets closer to shore, it picks up speed for awhile, because there's less room for the water to be in, like when you put your thumb partly over a spurting hose. But eventually it starts to slow down due to friction with the ocean floor. Here's where things get ugly: the water may be slowing down, but its amplitude is increasing. (Amplitude refers to the height of this slow-brewing wave or series of waves.) So it slows down a bit, but that doesn't make this any less dangerous, because the wave is getting taller at the same time it's scrunching together. Then it his the shore, and it's a tsunami. A huge goddamn wave.