Thousands of people have died after an earthquake sent huge waves crashing into coastal resorts across south and east Asia. Dr Brian Baptie, a senior seismologist with the British Geological Survey, explained how the wave, or tsunami, was created.
In geological terms, what has happened?
Sumatra, or north-western Indonesia, is right on a plate boundary. The earth's surface is made up of lots of different tectonic plates and they are all moving around.
The plate that has the Indian Ocean on it is moving roughly north-east and colliding with Sumatra. And as that collision takes place, the Indian Ocean plate gets subducted underneath Sumatra, and as that plate is subducted it breaks up and that is what causes the earthquake.
This earthquake been one of the largest ever, one of the great earthquakes. There has been a rupture along a fault about 1,000km long, and that has generated a vertical displacement of about 10m. The displacement in the sea floor has generated this huge tsunami.
How does the wave develop?
There's a huge vertical displacement in the sea floor as a result of the earthquake and that displaces a huge volume of water.
You can imagine, if the rupture is 1,000km long with a 10m displacement in the sea floor you get hundreds of cubic kilometres of water and that results in a wave that travels through the ocean.
1960 - Chile, 9.5 magnitude
1964 - Alaska, 9.2
1957 - Alaska, 9.1
1952 - Russia, 9.0
2004 - Indonesia, 9.0
In the deep ocean the height of the wave can be a few metres, maybe 5-10m, and it travels at a few hundred kilometres per hour.
That means it travels relatively slowly compared with the seismic waves from the earthquake, and it has arrived quite a few hours later at surrounding coastal areas all around the Indian Ocean.
As the tsunami wave approaches the shore it slows down, because the water gets shallower and what that means is the wave increases dramatically.
When it hits the shoreline it can be 10-20m, and that is probably what has happened in this case.
Why was there no warning this was happening?
There is a tsunami warning system in place in the Pacific Ocean because there is a historical precedent of lots of earthquakes causing tsunamis like this, throughout the 20th Century.
But there is no real precedent for a tsunami like this in the Indian Ocean. So this is the first time this has happened, and there is no warning system as far as I know in the Indian Ocean.
Could there be more waves on a similar scale?
Unlikely there will be further tsunamis of the same size. What normally happens when you get a very large earthquake is you get aftershocks that continue for many days.
They are usually a bit smaller than the main shock, although it is not impossible that there could be another one. But there may be aftershocks and they may generate smaller tsunamis.