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.

Mosquitoes and Fevers

As the rainy season approaches I have noticed that the Asian Tiger Mosquito, that striped flying monster which is the carrier or vector for both chikungunya and dengue, is increasing in number and range. Research also suggests that climate change is assisting the spread of this mosquito in many parts of the world.

And the chikungunya virus keeps mutating. We already know that the last Indian Ocean outbreaks were caused by a different strain of the virus. This strain differs from those involved in earlier outbreaks and makes the virus more likely to enter the cells of the Asian Tiger Mosquito and replicate after the insect has fed on the blood of an infected person. In fact, the symptoms are also changing. At a meeting of WHO last month it was suggested revising the definition of chikungunya fever because researchers have noticed the symptoms have changed.

This points to the fact that there needs to be timely and better identification of fever cases. But for prevention there needs to be early warning systems set up in countries like Seychelles. In La Reunion, where the chikungunya outbreaks caused massive health, economic and social problems, a surveillance system is now in place. This collects information on the mosquito density in every area which is then used in scientific models to predict where the next outbreak might occur. The system gives citizens and government advance warning.

The mosquito density is only a possible indication of the presence of the chikungunya virus and not a certainty, but it does give an advance warning for early action. In some countries better early warning systems especially based on gis (geographical information system) are under development. gis -based modeling can predict chikungunya-prone areas using information on the distribution of the mosquito vectors, rainfall, temperature, altitude, vegetation cover and urbanization.

More sophisticated data such as on the mosquitoes’ ability to transmit the disease and the susceptibility of people in different parts of the country can also be used in GIS-based models to predict the spread of the disease and help pinpoint the time when an outbreak could become an epidemic.

In Seychelles such GIS based modeling would be very useful. Both Dengue and chikungunya cause huge hardships, loss of productivity and losses of millions of Rupees. The country does have a GIS unit with good GIS maps. Perhaps donors should be approached to assist with setting up with a GIS-based early warning system. Putting this in place is not as simple as it sounds but is greatly needed because early action based on early warning is critical in preventing an epidemic.