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Written in the sand — tsunamis can strike twice

The 2004 Indian Ocean tsunami was not the first of its kind in the region, according to research published in Nature this week. Two multinational groups of scientists provide sedimentary evidence for possible predecessors to the 2004 event in Thailand and Sumatra, which suggests that the last similar-sized tsunami occurred about AD 1400.

There was no historical precedent for the 2004 Indian Ocean tsunami, either on the distant coasts it devastated or within its source area. Sedimentary evidence of ancient tsunamis can also be hard to find — in such dynamic environments tsunami sand deposits are often destroyed by wind, running water and human or animal activity.

Brian Atwater from the University of Washington in Seattle, USA, and his colleagues studied a grassy beach-ridge plain on an island north of Phuket, Thailand, where the 2004 tsunami reached maximum wave heights of 20 meters above sea level. A separate team, led by Katrin Monecke from the Kent State University in Ohio, USA, examined the sedimentary record on coastal marshes in Aceh, northern Sumatra, where the waves reached up to 35 meters high.

Both research teams explored low marshy areas — marshes have previously revealed evidence of ancient tsunamis elsewhere in the world. These areas lie between beach ridges called ‘swales’, which are known to trap tsunami sand between layers of peat and other organic matter. The teams compared the layers noting the differences between the dark organic soil and the light-colored tsunami deposits. They were able to identify a series of sand sheets, which they then dated with radiocarbon methods. They also considered biotic evidence, including marine and brackish-water diatoms, which provide a useful indicator of tsunami deposits. The Atwater team also noted historical accounts from sea voyages in the AD1300–1400s.

Both teams report 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, which — as Atwater and colleagues describe — expended centuries’ worth of plate convergence. And, as Moecke and colleagues note, 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 and preparedness.

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