Science
How does a tsunami happen?
A tsunami happens when a large, sudden movement of the seafloor — usually from an undersea earthquake — pushes a huge volume of water upward. That energy spreads as fast, low waves that grow into towering walls of water as they reach shallow coasts.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how a tsunami works.
Step by step
- 1Most tsunamis start with an undersea earthquake that shifts the seafloor.
- 2The displaced water spreads outward as a series of long, fast waves.
- 3In the deep ocean the waves are low and can travel as fast as a jet plane.
- 4As they reach shallow water near shore, they slow and pile up into great heights.
- 5Underwater landslides and volcanic eruptions can also trigger them.
Frequently asked questions
- Why is a tsunami so different from a normal wave?
- Normal waves are driven by wind at the surface; a tsunami moves the entire column of water from seafloor to surface, carrying vastly more energy.
- What's the warning sign of a tsunami?
- Often the sea suddenly draws far back from the shore before the wave arrives. Strong coastal shaking or an official alert means move to high ground immediately.
- How fast does a tsunami travel?
- In the open ocean it can race at 700–800 km/h — as fast as a jet — then slow dramatically as it nears the coast and grows in height.