Why do earthquakes on the ocean floor sometimes generate tsunamis?

Not every submarine earthquake makes a tsunami. The biggest tsunamis usually happen when a fault at a subduction zone lifts or drops the seafloor vertically over a broad area. That sudden movement displaces the water above it, creating waves that radiate outward at high speed. The 2004 Indian Ocean tsunami and the 2011 Tōhoku tsunami were both caused by major undersea megathrust earthquakes.