Why Do Earthquakes Produce Tsunamis?

Not all earthquakes produce tsunamis — only those that cause significant vertical displacement of the ocean floor. This typically requires a large (magnitude 7.0+) shallow earthquake on a subduction zone fault, where one tectonic plate dives beneath another. When the overriding plate suddenly snaps upward (or drops), it pushes up (or pulls down) the entire water column above it. Since the ocean is relatively incompressible, the displaced water spreads outward as a tsunami. The wave energy is proportional to the area and magnitude of seafloor displacement. The 2004 Indian Ocean earthquake (magnitude 9.1) ruptured a 1,600-km segment of the Sunda Trench, displacing the seafloor by up to 15 metres over an area of about 100,000 square kilometres. Strike-slip faults (horizontal movement) and deep earthquakes generally do not produce significant tsunamis because they cause little vertical displacement of the seabed.