On 26 December 2004, a massive tsunami inundated the coastline of southeast Asia with no warning. The wave left behind death and destruction on a scale virtually unprecedented in living memory. Devastation crossed political borders and was worsened by a lack of preparedness: in many of the hardest-hit regions there were no historical records of a tsunami of similar magnitude.

Government attention soon turned towards better early-warning systems for future tsunamis. These efforts, however, were hampered by a lack of knowledge about the history of tsunami activity in the region. Now, two teams have reconstructed the millennial-scale history of tsunamis in Thailand (Nature 455, 1228–1231; 2008) and the Indonesian island of Sumatra (Nature 455, 1232–1234; 2008).

Credit: USGS

As the flood waters receded, they left behind a thick layer of sand on many beach plains and ridges. A team led by Kruawun Jankaew of Chulalongkorn University, Thailand, dug and drilled into the marshy sediments of the Thai island of Phra Thong, hoping to identify similar sand layers. Their efforts were rewarded with two such deposits. Using radiocarbon dating of the shell and organic material that sandwiched the layers, they conclude that the older tsunami occurred approximately 2,800 years ago, and that the younger layer was deposited sometime around or immediately after AD 1300–1450.

Meanwhile, a team led by Katrin Monecke of Kent State University, Ohio, was searching the depressions between beach ridges in Aceh, Sumatra, where they uncovered a thick sand layer. Their radiocarbon measurements placed the timing of deposition between AD 1290 and 1400, roughly corresponding to the Thai deposits. The team's transects also revealed another large tsunami event from the period AD 780–990.

The two groups' findings suggest that tsunamis such as the 2004 event have a recurrence interval of approximately 600 years. However, the studies were not designed to detect tsunamis of smaller magnitudes, such as the one that followed the 28 March 2005 earthquake, which still caused extensive damage to buildings and infrastructure.

Both teams stress that despite the apparently low risk of a large tsunami event in the near future, there is still a need to maintain adequate warning systems and evacuation plans to protect coastal residents from the unpredictable seas.