Most tsunami are caused by large earthquakes on the sea floor suddenly moving large bodies of water. The resulting waves move away from the source of the earthquake event at speeds of up to 900 kilometres per hour.
The Indian Ocean Tsunami of 2004 was caused by an undersea earthquake off the coast of Sumatra in Indonesia. Most of the earthquake activity around the world occurs near subduction zones - places where one tectonic plate is forced under another.
As the Australian plate slowly pushes under the Eurasian plate, massive stresses build up in the crust. These stresses also cause the Eurasian plate to be slowly forced upwards - part of the process that builds the mountains and volcanoes of Indonesia, as well as creating the many earthquakes felt in that region of the world each year.
When the stresses get too great, the plates will suddenly slip causing massive movements in the seafloor.
The part of the crust nearest to the fault zone rapidly moves upwards by a metre or so, lifting the entire body of water above it. A hundred kilometres away the opposite may happen: the seafloor drops and the ocean above it also falls.
These two movements (the sudden rise and fall of the seafloor hundreds of kilometres apart), combine to cause a series of tsunami waves which move away from the line of the fault in both directions.
As the tsunami moves across the open ocean it is almost undetectable on the ocean surface.
In this example, the tsunami waves are only about half a metre high but have a wavelength of 200 kilometres. Travelling at speeds of up to eight or nine hundred kilometres an hour (the speed of a commercial passenger jet), it will take each wave about 15 minutes to pass a slow moving ship.
As the tsunami leaves the deep water of the open ocean and approaches the shallower waters near the coast, it slows down and may grow in height depending on the shape of the seafloor.
A tsunami that is unnoticeable by ships at sea may grow to be several metres or more in height near the coast.
Our example tsunami is now 1.5 metres high with a wavelength of 100 kilometres and is moving at about 400 kilometres an hour.
Depending on whether the first part of a tsunami to reach the shore is a crest or a trough, it may appear as a rapidly rising or falling tide, and in some cases the tsunami may appear as a series of breaking waves. People near the beach may also hear a roaring sound, like an approaching train.
In this example, the first crest of our tsunami arrives without warning and inundates the beach and low lying land causing extensive damage.
After the first wave, the water will draw back and then the second and third waves will repeat the process at 15 to 20 minute intervals. The first wave may not be the biggest. Reefs and offshore islands may help to protect the coast from the devastating effect of a tsunami.
Earthquake generated tsunami animation