HONG KONG, Dec 11 (Reuters) - A study of Indonesian reefs showed corals record cyclical environmental events and could predict a massive earthquake in the eastern Indian Ocean within the next 20 years, researchers said on Thursday.

The study of corals off Indonesia's Sumatra island showed they have annual growth rings, like those in tree trunks, which record cyclical events such as earthquakes, the scientists said.

"If previous cycles are a reliable guide we can expect one or more very large west Sumatran earthquakes ... within the next two decades," Kerry Sieh, professor at the California Institute of Technology's Tectonics Observatory, told reporters in Singapore.

Scientists said the earthquake could be similar to the magnitude 9.15 earthquake which sparked the devastating 2004 tsunami and left 230,000 people either dead or missing across Asia.

More than 170,000 of those victims were in Aceh on the northwestern tip of Sumatra.

Sieh said while Thailand and Sri Lanka were unlikely to be affected, people in Sumatra should be prepared.

"The tsunami could be at five metres in Padang (in Sumatra). This is a worse case scenario," he said.

Sieh, whose team's research was published reported in Science journal, said corals off Sumatra's Mentawai Islands showed a major earthquake had occurred every 200 years since 1300.

"When earthquakes push the seafloor upward, lowering local sea level, the corals can't grow upward and grow outward instead," the researchers wrote in Science.

Earlier this month, Sieh and his colleagues reported in the journal Nature that an area off Sumatra that has been the source of disastrous earthquakes, still carried a lot of pent-up pressure that could result in another strong quake. (Reporting by Matthew Webster in Singapore and Tan Ee Lyn in Hong Kong; Editing by Nita Bhalla and Sophie Hares)

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