Los Angeles: Researchers have warned that another large-scale tsunami could flood densely populated areas of western coastal Sumatra, Indonesia, in the next couple of decades.

Using samples of coral from the Mentawai islands, the researchers from Caltech University, University of Southern California (USC) and Indonesia conducted computer simulations of the 1797 and 1833 tsunamis, allowing them to evaluate worst-case scenarios for future tsunamis.

The computer modelling determined that two river valleys near Bengkulu, a coastal city of about 350,000 people, could experience flooding up to several kilometres inland.

"The computer models confirm a substantial exposure of coastal Sumatran communities to tsunami surges," said lead author Jose Borrero, a scientist at USC's Tsunami Research Center.

In December 2004, a 9.0 quake off the coast of northwest Sumatra triggered a tsunami that hit many countries in the Indian Ocean area and left more than 220,000 people dead.

The study found that the same fault that caused the 2004 tsunami extends farther southeast beneath the Indian Ocean, just off the southwestern coast of Sumatra.

The fault produced large earthquakes and tsunamis in 1797 and 1833, and researchers said the events appear to recur every 230 years on an average.

The study found that offshore islands could offer some protection to the larger city of Padang, but during the 1797 tsunami, the waves reportedly carried a 200-tonne ship and other vessels into the town.

"The population of Padang in 1797 and 1833 was a few thousand," Caltech geology professor Kerry Sieh said.

"Now it is about 800,000, and most of it is within a few meters of sea level," he said.

"We hope that these initial results will help focus educational efforts, emergency preparedness activities and changes in the basic infrastructure of cities and towns along the Sumatran coast," he said.

"When we tell people living along this 700-km section of the Sumatran coast that they will likely experience a big tsunami within the next 30 years, they ask for details," Sieh said.

"How much time after the earthquake will they have before the tsunami strikes? How big will the waves be? How far inland should they be prepared to run? What areas are likely to suffer tsunami damage? This paper is our first attempt to answer these important questions," he said.

The study was published in Monday's edition of the Proceedings of the National Academy of Sciences.