Most tsunamis are triggered by a powerful earthquake in or near the ocean, in a subduction zone (a zone where plate tectonic forces push an oceanic plate down below the earth's crust). These earthquakes start under the ocean floor instead of on land. The waves begin far out in the ocean (the epicentre of the quake), and are quite small to start with (just as a land earthquake begins with small tremors in the ground). As they get closer to shore they get larger and larger, and these huge waves, when they reach the shore, are what we call tsunami.

Tsunamis can also be caused by volcanic activity in the ocean, or occasionally by a land movement known as a submarine landslide. They can also be caused by meteorite activity, but this is very unusual.

More tsunamis happen in the Pacific region than anywhere else, as this region has a large number of earthquakes in or close to the sea.