Royal Navy survey ship reveals seabed fractures and landslides caused by earthquake, now reclassified as second largest ever recorded (By David Adam and John Aglionby in Jakarta - Thursday, February 10, 2005 - Source: The Guardian)
The first images of the seabed at the focus of the massive earthquake that caused the Boxing Day tsunami were released yesterday by scientists working on the Royal Navy survey ship HMS Scott. Though it is too early for experts to be sure what generated the Indian Ocean waves, the images have given them important clues.
The sonar images show a section of the ruptured fault about 43 to 50 miles long, running roughly north-south.
Different colours indicate depth below the sea surface.
To the right and marked in purple is the Indian plate, which slides underneath the Burma plate at about 2cm a year along the sharp line marking the plate boundaries, where the deep blue begins.
Scientists think the lighter blue region of Burma plate was pushed up about 10 to 20 metres when the earthquake struck, forcing the water above into the destructive waves.
Seismologists at Northwestern University in Illinois have upgraded the quake to magnitude 9.3, three times bigger than previously thought and the second largest ever recorded.
Dr Russell Wynn, of the Southampton Oceanography Centre, said the jagged undersea mountains marked in green and yellow on the Burma side of the plate boundary probably formed over thousands of years. "If you were standing on the seabed and looked at those mountains then they would be higher than Ben Nevis," he said.
Little research has been done in the region and there are no maps of what the fault looked like before the earthquake, making it difficult for experts to pinpoint how it moved.
There are signs of recent activity: towards the extreme right of the purple Indian plate, near the top of the picture, are two landslide blocks, each about 2km (1 miles) across and 100 metres thick.
The earthquake probably shook them loose; the right hand edge of the light blue region is marked with slip scars.
Dr Wynn said: "An area the size of a small town fell away from the slope and broke up into bits. Slabs of material travelled about 10km down the slope on to that flat bit of Indian plate and are just lying there. We don't know for definite that was caused by the recent earthquake but it's a very fresh looking feature."
One of the researchers on board HMS Scott, Dr David Tappin of the British Geographical Survey, said the images he had seen showed seabed movement, landslides and tectonic plate movement.
"There has been a lot of movement over a distance of about 1,000km and there is evidence that there has been several phases of movement," said Dr Tappin.
"But one of the key aspects of this is that the area is unknown. The information we're acquiring is the first from this area because there has not been very much scientific research done off the west coast of Sumatra."
Dr Tappin said the interpretations were still very preliminary and would require extensive analysis of further information that his team and other researchers working elsewhere at sea and on land were collecting.
His data has been compiled in the last two weeks by numerous sonar soundings of the seabed taken at depths ranging from 1,500 metres to 5,000 metres and spread over 5,000 square miles.
(Article Link: http://www.guardian.co.uk/tsunami/story/0,15671,1409648,00.html)
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