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disasterous tsunami predicted
« on: December 12, 2008, 04:26:02 AM »
Frank Pope, Ocean Correspondent. The Times December 12, 2008

Ancient Indian Ocean coral points to another disastrous tsunami

The eastern Indian Ocean could be due for another earthquake rivalling the one that caused the deadly tsunami in December 2004, according to research on ancient corals in the area.

It has long been said that earthquakes occur in cycles, yet until now evidence has been hard to come by. Now a pioneering study of the corals off the west coast of Sumatra has revealed that the region's earthquakes during the past 700 years occurred in series of shocks that spanned decades.

Corals lay down growth rings every year, just like trees, and these record environmental changes including upheavals of the seabed. They show that the region's earthquakes usually come in a sequence.

An international team, led by Kerry Sieh, of Nanyang Technological University in Singapore, analysed cross-sections of ancient corals growing off the Sumatran Mentawai islands, which are on a 700km (435-mile) stretch of the “Sunda megathrust” fault. “Corals grow in bands about 1cm thick every year,” Dr Sieh said. “Slice through coral, X-ray it and date it and you can figure out exactly when the earthquake lifted that reef out of the water.”

Coral can grow up only to the level of the year's lowest tide, and when close to the surface it grows outwards to form flat, pancake-shaped heads. When sea levels rise - or the seabed sinks - the edges grow up towards the light, forming a ring-like “micro-atoll”. As the Indian tectonic plate pushes beneath the Burma plate, the strain accumulates, pulling the seabed downwards and forming micro-atolls of coral. Earthquakes occur when the strain becomes too great. The coral is then pushed above the water and growth is prevented. But the release does not occur all at once. The team discovered sequences that occurred in the 1300s, the late 1500s and the beginning of the 19th century. At least two of the three ancient sequences began with events that were smaller than their culminating events.

The “Mentawai Patch” had been locked for half a century before an earthquake in September 2007 (the December 2004 event was on a more westerly section). After analysing the historical build-up of strain through the coral sections, the authors predict more, and stronger, earthquakes in the area in the next few decades.

“We're publishing what was only theory before,” Dr Sieh said. “What we've been able to show is that the Mentawai Patch is not just one earthquake-failing, it's a sequence.”

The team discovered evidence that the cycle of earthquakes has been repeating itself for up to 2,000 years, but such ancient coral had been too badly eroded for accurate reading. “It's speculative,” Dr Sieh said, “and we chose not to put it in the paper because we didn't want to pollute the clean data we had, but it looks like such events go back to 200BC, with episodes in AD400 and AD1000.”

The lengths of the past three episodes, described in the journal Science, have varied from a few decades to a little over a century, which Dr Sieh said would make it impossible to say precisely when the next event would happen. But the team give a warning that the likely seismic shaking and possible tsunami could cause major losses of life and property. It is estimated that a quarter of a million people died in the 2004 tsunami.

The San Andreas fault in California was silent for 300 years before shuddering in 1857 and 1912. While the later event was recorded by literate inhabitants, the 1857 event was etched in nature alone: in tree growth rings.