Finding traces of ancient tsunamis

N. Gopal Raj
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Long ago:A tsunami similar to the one of 2004 appears to have occurred about a thousand years ago.— Photo: M. Moorthy
Long ago:A tsunami similar to the one of 2004 appears to have occurred about a thousand years ago.— Photo: M. Moorthy

The day after Christmas in 2004, a gigantic tsunami swept across the Indian Ocean, killing about a quarter of a million people and devastating coastal communities in a dozen countries, including India.

The tsunami had been unleashed by a massive undersea earthquake off northern Sumatra in Indonesia. A 1,500-km-long section of a fault, where the tectonic plate carrying the Indian subcontinent was pushing against the Burma plate, had suddenly ruptured, producing the third largest quake to be recorded in over a hundred years.

A tsunami similar to the one of 2004 appears to have occurred about a thousand years ago, according to C.P. Rajendran of the Centre for Earth Sciences at the Indian Institute of Science in Bangalore and his colleagues. Their finding was published recently in the Journal of Geophysical Research: Solid Earth .

Before 2004, there was “an assumption that the Bay of Bengal had no potential to generate such huge tsunamis,” Dr. Rajendran told this correspondent. There were no reliable historical records indicative of a tsunami that caused major loss of life on the east coast of India in the previous 500 years, he said.

Geologists have been looking closely at sediments deposited over centuries along coasts bordering the Indian Ocean for signs of anything like the 2004 tsunami. Such studies have been carried out on the India's east coast, Sri Lanka, Thailand and Sumatra.

Dr. Rajendran and his colleagues carried out field studies in the Andaman and Nicobar islands, an archipelago close to the fault that gave way in 2004. Their journal paper noted how debris left behind by that year's huge waves had supplied analogues that helped distinguish layers of sediment produced by earlier tsunamis. That included bits of coral as well as shells from shellfish and tiny sea-living organisms known as foraminifera. Pieces of pottery could be suggestive of human habitation nearby. Vegetation, including tree trunks, could be dragged inland by the waves. If such vegetation got trapped in swamp-like conditions, it could turn into peat over time.

Trenches and pits were dug to expose sediments that had been deposited over the last 2,000 years or so. Carbon dating made it possible to set an approximate period for a sediment layer from a tsunami.

On the islands, the scientists found sediments produced by “a major tectonic event accompanied by regional sea flooding” that happened during A.D. 770-1040. As the event had also left its imprint on the coasts of Tamil Nadu, Sri Lanka and Sumatra, they concluded that the tsunami which resulted in those sediments seemed comparable in size to that of 2004.

The islands also contained tsunami deposits from A.D. 1250 to 1450 that probably matched those reported previously from Sumatra and Thailand. It was conceivable a cluster of earthquakes had produced tsunamis that left those deposits. However, no such deposits had as yet been found on the Tamil Nadu coast or in Sri Lanka, raising questions about whether any of those tsunamis were truly comparable to that of 2004, they noted.



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