This video excerpt from NOVA: “Japan’s Killer Quake” describes how an earthquake on the ocean floor can cause an enormous wave—a tsunami—on the ocean’s surface. When the Earth’s tectonic plates move and grind against each other, energy builds up that is released by an earthquake. That earthquake displaces the seafloor and zillions of gallons of water above it, creating a tsunami that races away from the center and builds into a deadly wave when it reaches land.