[JAKARTA] The Indian Ocean could be due another massive earthquake within the next 30 years — one that could rival the magnitude of the one that caused the Indian Ocean tsunami in 2004.
Researchers made the prediction after studying corals, which show rings of growth from which past sea levels can be inferred. Earthquakes push the land up, depressing the sea level in the area and preventing corals from growing upwards. Sea levels then rise as the land subsides, leaving the history of the earthquake imprinted in the coral growth patterns.
The scientists analysed coral growth over the last 700 years in the shallow reefs along the 700-kilometre 'Sunda megathrust' fault — a boundary between tectonic plates off the Mentawai islands in western Sumatra, Indonesia.
This section of the Sunda fault had been inactive for at least 50 years until a magnitude 8.4 earthquake in September 2007. Using the coral data the scientists found three previous sets of large earthquakes. If the cycles continue to be consistent — beginning every 200 years — the next earthquake is due within the next few decades.
Danny H. Natawidjaja, an author of the paper and a senior researcher at the Research Center for Geotechnology of the Indonesian Institute of Sciences, told SciDev.Net the probability of a large earthquake in western Sumatra in the next 30 years is more than 70 per cent.
"We call this scientific prediction a geological warning system," he said.
He added that the sea level off the Mentawai islands is rising at about one centimetre per year, which is to be expected prior to an earthquake as the land subsides and the fault accumulates strain between the two plates.
But M. Riyadi, head of the seismic and tsunami section at the Indonesian Meteorology, Climatology, and Geophysics Agency, said there is no way to predict the next earthquake precisely. For example, after the Indian Ocean tsunami, it was only three months before the next big earthquake hit the island of Nias in 2005.
"We have no precise prognosis on the return period, therefore scientific predictions are all welcome, but the most important thing is mitigation. We must be prepared whether the earthquake hits us in 30 years or as soon as next week," Riyadi told SciDev.Net.
The research was published in Science last month (12 December).
Science 322, 1674 (2008)