Sonar shows undersea upheaval caused by tsunami quake

Picture / Reuters, Royal Navy

LONDON - The first underwater images of the seabed off Indonesia where an earthquake struck on Boxing Day show the colossal scale of the geological forces that sent a devastating tsunami across the Indian Ocean.

Three-dimensional pictures taken by a sonar on the Royal Navy vessel HMS Scott reveal a ruptured sea floor of submarine canyons and mountains created by the movement of one tectonic plate beneath another at the rate of 6cm a year.

It was the sudden release of pressure built up over decades of tectonic movement that triggered the earthquake, which some geologists have re-evaluated as three times more powerful than previously thought.

Images released yesterday by the Navy show how the flat Indian tectonic plate (shown in blue) is slipping under or "subducted" beneath the crumpled Burma plate (shown in green and yellow), which is being pushed up in the process.

Steve Malcolm, commanding officer of HMS Scott, said the seabed at the earthquake epicentre looked like a "rumpled carpet" when viewed on the sonar screen.

Data from the sonar is being analysed by the marine geologists Lisa McNeill and Tim Henstock, of the Southampton Oceanography Centre. It is the first time the sea floor has been studied so intensely after such a large earthquake.

Dr Henstock said: "There are features which we would think are something like the Grand Canyon would look. You can see huge piles of mud maybe a few hundred metres thick; there's a lot of evidence of activity at the subduction zone."

The scientists estimate some of the ridges are up to 1500m high Some of the ridges have collapsed to produce huge landslides of mud and rock several kilometres long. Seismologists have calculated that the epicentre of the earthquake was some 40km below the sea floor.

They initially registered it as 9.0 on the Richter scale. But, seismologists from Northwestern University in Evanston, Illinois have recalculated the earthquake as 9.3, which would make it three times larger - because the scale is logarithmic - and the second-largest recorded earthquake.

Seth Stein, who reanalysed the seismograms, said: "The rupture zone was much larger than previously thought. The initial calculations that it was a 9.0 earthquake did not take into account what we call slow slip, where the fault, delineated by aftershocks, shifted more slowly."

It was the forces generated by the quake that lifted huge volumes of water to generate the tsunami, Dr Stein said.

"The additional energy released by the slow slip along the 745-mile (1200km) long fault played a key role in generating the tsunami."

Dave Long of the British Geological Survey, said an early-warning system in the Indian Ocean would give better protection against a tsunami caused by an earthquake.

"A tsunami warning system simply tells you the earthquake had happened and that a tsunami is going to come through later. The advantage of this sort of information is that we can model in advance, if we have a certain type of earthquake, what might happen."

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