Researchers in Thailand and Indonesia wrote in two articles in Nature magazine that the tsunami hit around 1400, long before historical records of earthquakes in the region began.
"Tsunamis are something we never experienced before and after 2004, people thought it was something we would never experience again," Kruawun Jankaew of Thailand's Chulalongkorn University told Reuters by telephone.
"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."
The 2004 tsunami left 230,000 people either dead or missing across Asia, from Sri Lanka and India to Thailand, the Maldives and Indonesia. More than 170,000 of these victims were in Aceh province in Indonesia.
Jankaew's team studied a grassy plain on Phra Thong, an island north of Phuket in Thailand, where the 2004 tsunami reached maximum wave heights of 20 meters (65 ft) above sea level.
A separate team led by Katrin Monecke from the University of Pittsburgh looked at the sedimentary records on coastal marshes in Aceh, where the waves reached 35 meters.
They explored low areas between beach ridges called "swales" -- which are known to trap tsunami sand between layers of peat and other organic matter -- and discovered a layer of sand beneath the most recent layer (2004), from 600 to 700 years ago.
"Depending on where the depression is, it (the layer of the 1400 sand) can be 10 cm. But on higher ground, it can be two to five cm. Organic materials like bark and leaves, which contain carbon, were used for dating," Jankaew said.
The scientists are now trying to find out the scale of that catastrophe 700 years ago.
"We will look at the thickness and grain size of the sediment and we can calculate how fast the tsumani was, how far inland it went, and the floor depth," she said.
Jankaew said there are two more layers of sand under the 1400 layer but more studies would need to be done to date these.
Some experts blame the massive loss of lives in 2004 on ignorance of the region's tsunami history.
Very few people living along the coasts recognized natural tsunami warnings, such as the strong shaking felt in Aceh and the rapid retreat of ocean water from the shoreline that was observed in Thailand.
But on an island just off the coast of Aceh, most people safely fled to higher ground in 2004 because the island's oral history includes information about a devastating tsunami in 1907.