FEW of us will ever forget waking up on Boxing Day 2004 to news of the massive tsunami in the Indian Ocean.
The tidal wave battered the shores of bordering countries, with Indonesia, Sri Lanka, India and Thailand the most severely affected.
The devastation was immense: fatalities were recorded in 11 countries, with the eventual death toll passing 22
5,000. In addition, 14,000 were posted missing and more than a million displaced.
The personal, community and economic impacts in the whole area are still being felt today.
The term tsunami was new to many of us in 2004. It is derived from the Japanese words tsu, meaning "harbour", and nami, meaning "wave", and refers to the seismic sea waves created by an underwater explosion, generally an earthquake or volcanic eruption.
Worldwide, tsunamis are not especially rare: there have been about 25 of them in the past 100 years, mostly in the Asia-Pacific region. The Boxing Day tsunami arose at the Sunda Trench, deep in the Indian Ocean where the Indo-Australian and Burma tectonic plates meet.
Here, the India Plate, which is drifting northwards at a rate of 6cm per year, slides under (or technically speaking, subducts) the Burma plate, creating a fault line where compressed energy had been building up over hundreds of years.
On 26 December, 2004, the sudden explosive release of this pent-up energy ripped open the ocean floor for 1,600km along a line starting near Aceh, the northernmost province of Sumatra, and running northwards towards the Andaman and Nicobar Islands, and drove the India Plate some 15m below the Burma Plate.
The ensuing earthquake, reaching a magnitude of 9.1 to 9.3 and lasting for eight to ten minutes, was one of the largest ever recorded.
This mega-thrust event caused waves up to 35m high to hit the surrounding coastlines, flooding up to two kilometres inland to well above the treetops and causing the devastation we witnessed on our TV screens.
Tsunamis cannot be predicted or prevented, and although a tsunami warning system based on monitoring seismic events under the ocean is in place in Japan and other Pacific Rim countries, where these events are relatively common, no such system existed in 2004 for countries bordering the Indian Ocean.
In fact, in the complete written histories of Indonesia and Thailand, which stretch back more than 400 years, there is no precedence for the 2004 tsunami.
But now, two groups of scientists have uncovered physical evidence of ancient tsunami deposits hidden in the geological record along the shorelines of Thailand and northern Sumatra. In both studies, scientists searched in swales: marshy pockets of peat between beach ridges that act as traps for sediment and are protected from erosion by wind, water, burrowing animals or interfering humans.
Here they identified a sheet of sand deposited by the 2004 tsunami and, by using earth cores, they found similar layers of light sand between the darker peat deposits below this, suggesting previous tsunamis in the area.
Determining the age of the buried sand layers can be a difficult task, but using radiocarbon dating of the organic matter in the layers below the sand, both teams reckon that the youngest tsunami to predate 2004 occurred around AD1300 to AD1400.
The oldest sand sheet found in Sumatra dated from AD780 to AD900, whereas the oldest identified in Thailand was about 2,200 years old.
These studies suggest that a tsunami as severe as that on Boxing Day 2004 may occur at 600-year intervals, and this information has important implications for the inhabitants.
If such a catastrophic event is unlikely to recur for several generations, they may, for example, decide not to invest in an early-warning system and perhaps also to rebuild their fishing communities close to the shore.
And although further geological surveys in other coastal areas and more accurate dating procedures are needed before these and other long-term decisions can be taken with any degree of confidence, tourists need not fear to return to this beautiful part of the world.
• Dorothy H Crawford is a professor of medical microbiology at Edinburgh University
The full article contains 706 words and appears in The Scotsman newspaper.
