Seafloor Study Traces Culprits Behind Indian Ocean Tsunami

Ben Harder and Anna Petherick
for National Geographic News
December 23, 2005

By inspecting the seafloor off the Indonesian island of Sumatra, scientists have developed a detailed geological picture of how the earthquake that caused the deadly December 26, 2004, tsunami unfolded.

Based on their findings, the researchers predict that another huge quake and devastating tsunami could be coming soon.

The quake occurred when an 800-mile (1,300-kilometer) segment of an undersea fault line suddenly ruptured.

The fault, called the Sunda megathrust, stretches from Burma to Australia. It lies along a subduction zone, where one tectonic plate slips beneath another, shoving it upward.

The rupture traveled at a rate of 1.4 miles (2.2 kilometers) a second, said Stéphan T. Grilli of the University of Rhode Island (URI). The ocean engineer has worked on computer models of that day's events.

"It took about ten minutes for the rupture to unzip the fault from south to north," Grilli said.

The rupture "was able to go north because the strains had been building up for centuries," said Kerry Sieh, a geologist at the California Institute of Technology.

"They were ready to be released like a spring," said Sieh, who has long studied the region's seismology.

Vertical motion, or uplift, of 20 feet (6 meters) or more along the fault caused the water column above to rise and then ripple outward.

The ripples formed the tsunami that devastated coastal communities on Sumatra, as well as in India, Malaysia, Sri Lanka, and Thailand.

Landslide Results

In February a British naval vessel carrying sophisticated sonar equipment surveyed the ocean floor off Sumatra. The study identified evidence of massive underwater landslides.

Continued on Next Page >>




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