Why do some earthquakes cause tsunamis?

Earthquakes that rupture beneath the ocean can trigger tsunamis if they displace the seafloor vertically. The most dangerous ones usually occur at subduction zones, where one plate dives beneath another and the seafloor can jump up or down during a megathrust quake. The water disturbance spreads as a long-wave train across the ocean and can travel at jetliner speeds in deep water. When those waves reach shallow coastal areas, they slow down and grow much taller. The 2004 Indian Ocean earthquake and the 2011 Tōhoku earthquake and tsunami are major modern examples.