Monday, June 8, 2009

Tsunamis- Past and Present

1993 – I was with a Dutch coral reef geologist at Anse Boileau looking at ancient reef deposits when a local resident showed us sandy sediments inland that had been exposed by some excavations. The geologist believed these were reef sediments deposited there many years ago by a huge storm or upheaval.

Now, scientists have found layers of similar sandy sediment dropped by a 600 year old tsunami under more recent layers deposited in 2004 in Thailand’s Phra Thong Island. These findings have been published in the journal Nature.

The two sandy sediments in Thailand are similar in thickness, suggesting a tsunami 600 years ago similar to that in 2004. Both tsunamis were caused by earthquakes. The longer the intervals between tsunami events, the more stress that can build up at the tectonic plate boundary and the larger the earthquake will be.

Were the sediments we found at Anse Boileau deposited 600 years ago by a tsunami or earlier by another event? Indeed a tsunami did hit Seychelles in August 1883 when the massive explosions and collapse of the volcano of Krakatoa generated large waves. These waves destroyed 295 towns and villages in the Sunda Strait in Western Java and Southern Sumatra drowning 36,417 people.

The Krakatoa tsunami, being of volcanic origin was only destructive locally in Indonesia, but in Seychelles the effect was apparently seen through a tidal effect that temporarily emptied the Victoria harbor area and left fish and other sea life stranded. If it did generate waves in Seychelles that were similar to those of the 2004 tsunami is not well known.

The 2004 tsunami, unlike the one from the Krakatoa disaster, was caused by an earthquake off the western coast of northern Sumatra, Indonesia. The resulting tsunami devastated the shores of Indonesia, Sri Lanka, South India, Thailand and Seychelles.

With the new discoveries in Thailand, there is evidence that a similar earthquake-generated tsunami occurred 600 years ago and may have hit countries like Seychelles. This is important because it will build more knowledge about the time intervals between these earthquakes.

But tsunamis can also be detected by signs that appear in the natural environment. In Seychelles and elsewhere coastal animals like crabs and turtles disappeared hours before the 2004 tsunami. The sea gypsies of Indonesia who had ancestral knowledge of previous tsunamis saw the signs and saved coastal villages by giving them early warning.

Both science and indigenous knowledge are useful in making sense of the natural world. This proves that we must look at nature in a more holistic fashion and we must quiz local people for whatever knowledge they may have.

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