Tsunami: role of mega thrust confirmed

Published in The Hindu on December 22, 2005

tsunami
The vertical displacement of the seafloor runs for a length of 750 miles. — Photo: NASA

One year after the killer waves struck havoc that killed hundreds of thousands of people, the answers for what actually caused the tsunami have been documented by Discovery Channel. The study proved conclusively that it was a mega thrust — a large-scale rupturing and vertical displacement of the earth’s crust — that caused the tsunami.

The Discovery team of 27 scientists had spent 17 days on board the ship The Performer in May this year in the Indian Ocean to explore the ocean floor off the coast of Sumatra to find the causal factors that created the unprecedented killer waves.

First time ever

This is the first time ever that anyone had ventured three miles beneath the ocean surface to find the answers and explore the effects of an earthquake soon after it had happened.

The India plate moving at the rate of 5cm every year is diving (subducting) deep under the Burma plate. The continuous subduction of the India plate results in stress build up in the Burma plate. Beyond a point, the stress that is built up is released producing earthquakes.

The team explored the areas northwest of the earthquake’s epicentre as the seabed had moved the most in this region during the earthquake. “After surveying these areas, we found evidence that the seafloor had been lifted by nearly 40 feet,” noted Prof. Kate Moran, the lead scientist of the expedition and a professor at the Department of Ocean Engineering, University of Rhode Island, U.S.

In search of landslides

Though a mega thrust has been cited as the causal factor, the possibility of landslides produced as a result of earthquake either independently or being an additional factor in causing the tsunami is real. Particularly, as a vast chain of mountains 2000 miles long with peaks taller than the Rockies runs along the length of the fault. The mountain chain has been created by tectonic forces.

Good possibility

The Discovery Channel team spent the first few days looking for signs of landslides. “A shallow focus earthquake (10 km below the seafloor) and having a magnitude of 9.3 has a good possibility of triggering landslides,” pointed out Dr. R.K. Chadha, Scientist at the National Geophysical Research Institute, Hyderabad.

“Our task was to see if landslides (on this mountain chain) were also caused and if so, to ascertain if they were of recent origin (caused by December 26 quake),” said Dr. Baban Ingole, Marine Biologist at the National Institute of Oceanography, Goa.

Dr. Ingole was the only Indian scientist on board The Performer.

Large earthquakes cause vibrations or massive ground shaking that liquefies and turns the sediment found on the mountain chain into a fluid. “We expected to see many such underwater landslides caused by liquefied sediments,” Prof. Moran noted. But they found none.

The scientists did find some evidence of landslides such as fractures and debris that had moved down from the mountain chain. But all these features ultimately turned out to be older and not caused by the December 26 quake. Another evidence was the one-metre long umpellula, a deep-sea animal looking more like a plant and found on the seafloor. The umpellula being a fragile animal, would have been destroyed by landslides, had there been any, caused by December 26 quake.

“We also found sponges and other organisms,” Dr. Ingole said. “Sponges like the umpellula are anchored to the ocean floor and these two animals take a long time to grow.” The team found that umpellula was much older than the December 26 quake; the landslides were not of recent origin.

Deep sea undisturbed

The umpellula study tells another tale altogether. Fragile marine organisms so close to the fault were left intact; humans, flora and fauna thousands of kilometres away from the epicentre were destroyed by the killer waves.

Ocean bottom in the deep sea is placid and tsunami waves are but only ripples they are generated and move away from the epicentre at a speed of 750 km per hour at a water depth of 5000 metres.

With no trace of landslides seen, the scientists turned their attention to find evidences of the vertical displacement of the crust caused by the mega thrust. Large-scale vertical displacement of the oceanic floor should leave behind many trails. And the expedition team found many. On the ocean floor, the team has recorded many cracks that serve as indicators of seafloor movement. But the clinching evidence is the vertical wall of the cliff that rises abruptly from the ocean floor directly above the fault line for a height of 40 feet.

The recent origin of the vertical displacement of seafloor comes from the rough saw-toothed edges seen at the top of the cliff.

The rough edges would have been smoothened had it not been of recent origin.

The 40-feet vertical displacement of the seafloor runs for a length of 750 miles. “We had predicted the vertical movement of the seafloor to have happened in the first 500-odd kms of the fault though rupturing had happened for a length of 1300 km,” Dr. Chadha said.

Coral proof

Another proof of the vertical displacement comes from the islands formed by the mountain chain jutting out of the ocean at some places. The mountain chains are in turn formed as a result of volcanic eruptions along the subduction zone. These islands have coral reefs growing in shallow waters.

The vertical shift of the seafloor lifted the mountain chain and in turn the islands. It has been recorded by other scientists and the Discovery team that large tracts of coral reefs now lie above water and exposed in these islands. Some areas in the islands have been pushed up by about 10 feet, the team found.

Apart from confirming the causal factor, the scientific expedition has also come out with a model to predict which areas are prone to tsunamis and how badly they would get affected. Discovery Channel is airing the programme `unstoppable wave’ at 8 pm on December 25 and 26. It is by far one of the well-documented scientific evidences of a vertical displacement of the crust that caused the tsunami.

 

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