Ancient tsunami record in the Indian Ocean

DOI: 10.1038/4551183a

The 2004 Indian Ocean tsunami was not the first of its kind, according to research published in Nature this week. Two groups of scientists have found sedimentary evidence for possible predecessors to the 2004 event in Thailand and Sumatra, suggesting that the last similar-sized tsunami occurred in about AD 1400 — long before historical records of earthquakes in the region began.

There was no historical precedent for the 2004 Indian Ocean tsunami, either on the distant coasts it devastated or within its source area, which had gone hundreds of years or more without a similar disaster recorded. Sedimentary evidence of ancient tsunamis can also be hard to find — sand deposits are often destroyed by wind, running water and human or animal activity.

Brian Atwater and colleagues studied a grassy beach-ridge plain on an island north of Phuket, Thailand, where the 2004 tsunami reached maximum wave heights of 20 metres above sea level. A separate team led by Katrin Monecke looked the sedimentary record on coastal marshes in Aceh, northern Sumatra, where the waves reached up to 35 metres high. Both groups explored low areas between beach ridges called ‘swales’, which are known to trap tsunami sand between layers of peat and other organic matter. The teams looked at the contrast between the dark organic soil and the light-coloured tsunami deposits to identify a series of sand sheets, which they then dated with radiocarbon methods.

The results reveal a sand deposit beneath the most recent layer, from 600–700 years ago, which represents the last full-size forerunner to the 2004 event. The teams identified other older tsunamis, but the ages did not correlate on both beaches. The data suggest that the recurrence intervals of such destructive tsunamis in the Sumatra–Andaman Island region can span centuries, with the 2004 event separated from its youngest full predecessor by roughly 600 years. Such a long recurrence interval explains not just the lack of historical data, but perhaps also the enormity of the 2004 Sumatra–Andaman earthquake. The centuries-long time span between the separate tsunamis also adds to the challenge of preparing local communities for future tsunamis and maintaining their hazard awareness.

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