Sept. 10, 2008 -- Ocean floor sediments may cause Earth's biggest earthquakes. The planet's most powerful quakes happen in subduction zones, where 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 devastating tsunamis: The great magnitude 9.2 quake that shattered the Indian Ocean in 2004 occurred along a subduction zone, with the ensuing tsunami killing nearly a quarter of a million people in a stroke. 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 three miles of mud. And where the sediment is thickest is where worst earthquakes inevitably strike. "About 75 percent of large earthquakes happen where large amounts of sediments accumulate," said David Scholl of the Unites States Geological Survey. It's more than a coincidence. Scientists first proposed that large stacks of sediment on subducting plates may increase the maximum power of earthquakes in the 1980s. Ocean plates are full of cracks and bumps like seamounts, and the irregular topography can prevent the plate from slipping during a quake, limiting its power. But if enough stacked sediment -- half a mile or more -- smoothes out the bumps, the quake can propagate over long distances. |
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