Still, no one had been able to prove the sediment-earthquake connection, which remained a fringe theory until the 2004 Sumatra-Andaman quake. Aided by a thick layer of sediment, a 1,000-mile-long region of the fault slipped all at once. "On December 26, 2004, the whole theory of how megathrust earthquakes form fell under a cold wet shower with Sumatra," said Emile Okal of Northwestern University. 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. Multiply the time since the last earthquake by the speed at which the plates are subducting, and you have your answer. "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 Okal. Following the devastation in Sumatra, Scholl and his colleagues compared a database of sediment thicknesses in trenches around the world to 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 quakes around the world. Okal stressed Scholl and his team's theory will not entirely solve the mystery of how and why great earthquakes form, though. "The other 25 percent of earthquakes that don't occur where there are lots of sediments is a big problem," he said. Large temblors in Kamchatka, Russia, and southern Peru, for instance, occurred in sediment-poor trenches. The exceptions complicate any efforts to predict where the next big quake -- and tsunami -- might strike based on the amount of sediment in subduction zone trenches. "If you want to be the Devil's advocate, you have a lot to talk about," Okal said. Related Links: |
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