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.

"We knew that the earthquake was a major player in causing the tsunami," said oceanographer Kathryn Moran, a colleague of Grilli's at URI.

"But we also thought that the earthquake might have triggered underwater landslides that contributed to the tsunami waves."

Along with other researchers, the two launched an investigation into the role landslides played in the disaster.

"We were able to identify a large landslide [from the sonar measurement] and a fracture or a rip in the seafloor," Moran said. The scientific team called those two locales the Landslide Site and the Ditch.

Using an unmanned underwater vehicle, the researchers probed up to 3 miles (4.8 kilometers) beneath the Indian Ocean's surface at those two sites and several others.

Grilli said the landslide "could have caused waves of between 5 and 10 meters [16 and 33 feet] on the nearby coast."

But the survey by the robotic submersible indicates that the slide didn't contribute to last December's destruction.

For one thing, "there were living animals growing on the landslide scars," Moran said. "They had to be older than last December 26."

The landslide, Grilli said, "is certainly a lot older than one year. It could be thousands of years old."

"We now know that this tsunami wave was solely caused by the seafloor uplift," Moran concluded.

Bottoms Up

At the Ditch, however, there was evidence of recent tectonic violence.

"There was no life there," Moran said. "There was a rip that was up to 12 meters [39 feet] high."

"We're interpreting it as a rip that formed on December 26. It could be a crack that formed on the seafloor as a result of the uplift," she said.

Moran and Grilli presented their team's findings and analysis this month at a meeting of the American Geophysical Union in San Francisco.

Sieh, the Caltech geologist, discussed his group's latest findings at the same meeting.

This year his team looked at shallow corals on islands off the west coast of Sumatra to assess how much the Earth's crust had moved vertically on December 26.

Corals can grow only as high as the water level at low tide. Following a quake, their height above the water's surface indicates how far the Earth's crust got pushed up.

Sieh found that after the December quake the seafloor had risen permanently by about five feet (one and a half meters) at a field site near the southern end of the rupture.

Furthermore, by extracting information from the corals about past uplift events, Sieh found evidence that another powerful quake could soon hit the southern half of Sumatra.

The critical portion of the Sunda megathrust has ruptured about every 240 years for at least the last 700 years, Sieh said. The most recent quake occurred in 1833.

Grilli agrees with this conclusion. "There could be a large earthquake in the not-so-distant future off of southern Indonesia," he said.

The ocean engineer added that as little as a quarter of an hour after a quake, 33-foot (10-meter) waves could slam the low-lying coast, which offers no high ground to retreat to.

"[There's] a lot of work to be done in terms of warning systems," Grilli said. And to shelter coast dwellers, he added, "you need to actually build solid structures."

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