Published online 29 October 2008 | Nature | doi:10.1038/news.2008.1193

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Ancient tsunami uncovered

Indian Ocean disaster of 2004 was biggest in more than 600 years.

tsunamiThe Indian Ocean mega-tsunami left the region devastated.Koen Broker / Alamy

Geologists have found evidence for the most recent predecessor of the Indian Ocean mega-tsunami thought to have killed more than 220,000 people in 2004.

Two teams independently conclude that the last time a tsunami of similar size hit the region was around AD 1300–1400. "This is crucial information for improving disaster preparedness", says Jörn Lauterjung, a physicist at the national German Research Centre for Geosciences in Potsdam, who co-ordinates the implementation of the German-Indonesian Tsunami Early Warning System for the Indian Ocean.

Each team analysed more than 100 sediment cores collected during fieldwork in 2006 and 2007, and found traces of several tsunamis that may have occurred during the last 2,500 years. But only the medieval event, whose age was determined by radiocarbon dating of organic material in the sediment, correlated between the two study areas near Phuket, Thailand, and in Aceh on the Indonesian island of Sumatra.

The next big hit

The results, published this week in Nature1,2, suggest that Indian Ocean tsunamis on that scale occur every 600–700 years. Models of how seismic stress builds up in the Sunda Trench, which forms a seam between two tectonic plates on the floor of the Indian Ocean, give similar frequency estimates. One of the studies also found evidence of a large tsunami in AD 780–990.

Kruawun JankaewResearcher Kruawun Jankaew sits in a pit that shows evidence for four tsunamis less than 2,500-2,800 years old. The 2004 tsunami laid down the light-colored sand layer at top.Brian Atwater

Katrin Monecke, a geologist formerly at Kent State University in Ohio, and her colleagues searched around Aceh for sediment records undisturbed by erosion or agriculture. Like the team in Thailand, the team took cores in ridges and in low marshy areas called swales, where sandy tsunami deposits are preserved between darker layers of peaty soil.

"Many people in Aceh still live in tents, and many are still in a state of shock," says Monecke. "But when we explained what we were doing they were very interested and supportive, lending us their boats or helping us find paths in areas where no good maps are available."

Risk worth taking?

The German-Indonesian early-warning system is intended to prevent a similar tragedy in the future. After four years of intense preparation and testing, the system will be officially launched on 11 November in Jakarta by Indonesia's president Susilo Bambang Yudhoyono.

Lauterjung points out that the rarity of mega-tsunamis does not limit the system's usefulness. Although the Indian Ocean accounts for only 5% of global tsunamis, smaller local events occur every few years near seismically active regions from Indonesia to Oman. On 17 July 2006, for example, a magnitude-7.7 earthquake off Indonesia triggered three-metre high waves that killed hundreds of people along the south coast of Java.

However, as it is likely that no large tsunami will strike again for centuries, many fishermen and their families will consider the risk of living close to the sea preferable to the disadvantage of moving farther inland.

In Aceh, the northernmost province of Sumatra that was most heavily hit by the 2004 tsunami, there are no written accounts or social memory of past tsunami catastrophes. This might explain why most people in the area failed to run to higher ground when they felt the shock waves, unlike those on more frequently affected islands. The mega-tsunami killed more than 150,000 people in Aceh alone.

Maintaining an early warning system for that long will be a challenge, says Stein Bondevik, a geologist and tsunami researcher at the University of Tromso in Norway.

Europe's soft underbelly

Although tsunami warning systems are already running, or being installed, in the Indian and Pacific Oceans, experts warn that the Mediterranean coast is still unprotected, despite the fact that the Hellenic arc — an area of explosive volcanism in the eastern Mediterranean — is thought to see mega-tsunamis at roughly the same frequency as Indonesia and Thailand.

The issue will be addressed next week at a meeting in Athens, Greece, of the United Nations Eductional, Scientific and Cultural Organization's Coordination Group for the Tsunami Early Warning and Mitigation System in the North-Eastern Atlantic, the Mediterranean and Connected Seas.

"If there was a submarine earthquake in the Aegean, who would issue a warning? Who would warn the Middle East?" says Costas Synolakis, director of the Tsunami Research Center at the University of Southern California in Los Angeles. "Four years since losing a quarter million people [in Asia] the underbelly of Europe remains blissfully unprotected." 

  • References

    1. Jankaew, K. et al. Nature 455, 1228–1231 (2008).
    2. Monecke, K. et al. Nature 455, 1232–1234 (2008).
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