Geological research suggests the tsunami that killed 230,000 people in 2004 was not the first of its kind and the last tsunami of a similar size in the Indian Ocean occurred about 600 years ago.

Sedimentary evidence studied by two groups of scientists suggests that the last comparable tsunami may have swept over coastal areas in southeast Asia in about AD 1400, long before historical records were kept of earthquakes in the region.

The work by the scientists is to appear Thursday in two articles in the scientific journal Nature. The two research teams say they dug pits and took core samples of sediment in Thailand and Indonesia to arrive at their conclusions.

The long gap between massive tsunamis might help to explain the lack of historical data as well as the enormous scale of the huge undersea earthquake that spawned the deadly waves four years ago, researchers said.

They acknowledge that the infrequency of tsunamis of this magnitude makes it challenging for authorities to prepare communities along the Indian Ocean for such events.

A team led by Kruawun Jankaew of Chulalongkorn University in Thailand studied a grassy plain north of Phuket, Thailand, where the 2004 tsunami reached maximum wave heights of 20 metres.

The team discovered evidence of at least three major tsunamis that occurred in the 2,800 years preceding the 2004 tsunami. The team sampled more than 150 sites.

A second team led by Katrin Monecke of the University of Pittsburgh studied a marshy plain in Aceh, a province at the northern end of the Indonesian island of Sumatra hard hit by the 2004 tsunami. The waves there reached 35 metres.

The team also found evidence of previous tsunamis during the last 1,200 years in Aceh. The team sampled more than 100 sites.

"Tsunamis are something we never experienced before and after 2004, people thought it was something we would never experience again," Jankaew told Reuters news agency in a telephone interview from Thailand.

"But from this, we are able to identify that the place has been hit by a mega tsunami in the past. So even though it is infrequent for this part of the world, it still happens and there is a need to promote tsunami education for coastal peoples."

Both sets of researchers found deposits of sand, or "sand sheets," that were apparently left by the 2004 tsunami and ancient tsunamis. They looked at low areas between beach ridges known as "swales," which are said to preserve tsunami sand between layers of peaty soil.

The teams examined the contrast between the dark organic soil and the light coloured tsunami deposits of sand to identify a series of sand sheets. They then estimated the age of the sand sheets by carbon dating organic plant debris from above and below.

The results clearly reveal a layer of sand beneath the most recent layer of sand left by the 2004 tsunami. This second layer dates back to about 600 years.

The researchers said the AD 1400 sand sheet represents the last big forerunner to the 2004 tsunami. The dates of even older tsunamis identified by the researchers were not the same on both beaches and therefore were said not to be linked.

The researchers conclude that the incidence of extremely destructive tsunamis in the region can span centuries.

In a commentary in the same issue of Nature, Norwegian geologist Stein Bondevik said the findings are the "first steps towards compiling a record of ancient tsunamis in the Indian Ocean, and they should inspire and encourage geologists to continue the work. There is plenty to do."

Bondevik writes that the findings raise some issues.

"If events such as the 2004 tsunami happen only every 600 years or so, there are considerable implications for urban and coastal planning in the region. The inhabitants might consider the benefits of living close to the sea as greater than the risks of a catastrophic tsunami that will not return for many generations," he writes.

Bondevik suggests it may not make sense to maintain a warning system for deadly tsunamis if they occur so infrequently. "But smaller tsunamis may well happen more often, and a warning system should save lives during such events."

The tsunami on Dec. 26, 2004, killed people in 14 countries. In Sumatra and in Thailand, waves deposited sand more than a kilometre and a half inland, the researchers said.

With files from the Associated Press