Ocean floor sediments may trigger off megaquakes and tsunamis

Washington, September 12 : Scientists in the US has found compelling evidence that ocean floor sediments may be the cause of megaquakes, including the earthquake and tsunami that struck the Indian Ocean on December 26, 2004.

According to a report in Discovery News, the scientists include David Scholl, an emeritus scientist at the United States Geological Survey, and his team, who have been looking closely at subduction zones.

These areas are where the ocean crust grinds beneath an adjacent tectonic plate, plunging in fits and starts into the mantle.

The zones are infamous for their catastrophic temblors, called 'megathrusts,' which often generate powerful earthquakes and devastating tsunamis.

The great magnitude 9.2 earthquake that shattered the Indian Ocean in 2004 occurred along a subduction zone, with the ensuing tsunami killing nearly a quarter of a million people.

The largest quake ever recorded, which shook southern Chile at magnitude 9.5 in 1960, was also born from subduction.

Curiously, those areas and others around the world share another trait.

The trenches formed by subduction are packed with sediment. Washed from eroding mountain ranges, the detritus can fill trenches with up to five kilometres of mud.

The researchers believe that where the sediment is thickest is where worst earthquakes inevitably strike.

"About 75 per cent of large earthquakes happen where large amounts of sediments accumulate," said Scholl.

In the 1980s, scientists first proposed that large stacks of sediment on subducting plates may increase the maximum power of earthquakes.

Ocean plates are full of cracks and bumps and the irregular topography can prevent the plate from slipping during an earthquake, limiting its power.

But, if enough stacked sediment smoothes out the bumps, the earthquake can propagate over long distances.

The sediment-earthquake connection remained a fringe theory, until the 2004 Sumatra-Andaman earthquake. Aided by a thick layer of sediment, a 1600-kilomtre-long region of the fault slipped all at once.

At the time, conventional wisdom among geologists held that a fault's potential for an earthquake greater than magnitude 8.2 was simply a matter of time.

"Then there was a magnitude 9.2 quake in a place we never expected it. Now it seems sediments may have played a role," said Professor Emile Okal of Northwestern University.

Following the devastation in Sumatra, Scholl and his colleagues compared a database of sediment thicknesses in trenches around the world, with records of great earthquakes of magnitude 8.2 or greater since the late 1800s.

They found a compelling correlation between sediments in trenches and strong earthquakes around the world.

ANI