History in a sheet of sand

 
Last Updated: Saturday 04 July 2015

A tsunami at its giant best<sc (Credit: ART BY HOKUSAI) 600 years ago, a tsunami hit the Indian Ocean. Some lessons

THE Indian Ocean Tsunami System Programme, costing US $16. 6m, ended in March this year, amidst hopes that it would check the devastation that occurred in 2004. The 2004 tsunami--the deadliest in recorded history--killed over 280,000 people in 11 countries. The giant killer wave was triggered by an earthquake measuring as high as 9.3 on the Richter scale and shook the entire planet by 1 cm. There was no official documentation of a tsunami of that magnitude previously. As these raised questions about the effectiveness of warning systems, two teams of researchers studied sedimentary evidence left behind by the giant wave, in Thailand and Sumatra, to establish a frequency of such events.

Tsunami deposits huge amounts of sand in affected areas but they are rarely preserved. The researchers examined areas that act as sediment traps. These areas, called swales, are low lying tracts of land between beach ridges. Swales are marshy and hold deposits of peat and organic matter.

The researchers dug pits in such swales at more than 150 sites in Phra Thong, Thailand and Meulaboh, Aceh and looked for deposits of sand. They used carbon dating method to figure out the age of the sand sheet and found that the last full-sized predecessor to the 2004 tsunami occurred about 600 years ago. The two studies were published in the October 30, 2008, issue of Nature.

The authors studied the implications of coastal planning and highlighted the significance of protecting coral reefs and mangroves for better chances at survival. For example, coral reefs protected people on the Surin Island chain of Thailand's coast while in Indonesia, Sri Lanka and Bangladesh, where reefs were destroyed to make way for shrimp farms, people were badly affected.

The researchers also suggested that communities should sustain awareness of tsunami hazards, rather than try and predict the next occurrence. Awareness signs include taking cognizance of a noticeable fall or rise in the water level, foaming and heavy underwater turbulence. On Thailand beach, a 10-year-old who had studied about tsunami, recognized the signs and warned others.

Such awareness also saved lives on Simeulue Island in northern Sumatra which was very close to the epicenter. According to an official report by the district government, only 7 out of a total population of over 78,000 died. The tribals correctly interpreted the severe ground shaking and fled to higher grounds. This act was not considered spontaneous but based on information handed down through generations about a tsunami that occurred in 1907.

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