BioEd Online

The first pictures of the earthquake zone that triggered the Asian tsunami show how the undersea landscape has been transformed by the clashing of tectonic plates.

It is the first time that the sea floor has been observed so soon after such a serious earthquake, scientists say.

A British Geological Survey team, along with Tim Henstock and Lisa McNeill, marine geologists at the Southampton Oceanography Centre, UK, joined the Royal Navy's survey ship HMS Scott when it docked in Singapore in late January. They used the craft's sonar imaging system to map the deformed sea bed about 150 kilometres west of the Sumatran coast.

Coloured contour maps show large ridges up to 1.5 kilometres high, some of which have collapsed to create landslides several kilometres wide.

"From this we hope to understand better the geological processes that produced the earthquake," says McNeill. Their research may ultimately help to assess the risks from future earthquakes, she adds. The data will also be used to update navigational charts of the area.

Crashing plates

The earthquake started about 40 kilometres below the sea floor on 26 December 2004, caused by a sudden movement between the India and Burma tectonic plates. Where the plates meet, the India plate is steadily being forced downwards.

The earthquake was about twice as energetic as geologists initially thought, scientists revealed earlier this week. Seth Stein and Emile Okal, geologists at Northwestern University in Evanston, Illinois, say that the earthquake's magnitude was 9.3, not 9.0, on the moment magnitude scale¹. As the scale is logarithmic, an increase of 0.3 translates to a roughly twofold increase in magnitude.

This makes the earthquake the second largest ever recorded, beaten only by the 9.5-magnitude quake in Chile on 22 May 1960.