Story Asia

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On the 26th of December, 2004, many people in South Asia and the Far East suffered from a devastating natural disaster which is known within the international scientific community as, ‘The Great Sumatra-Andaman Earthquake’, and its deadly twin, ‘The Asian Tsunami’. For those who live in the region, the entire episode is commonly referred to as, ‘The Tsunami’. As the 4th anniversary of ‘The Tsunami’ approaches, it seems appropriate to analyse how it might have come to pass.

The word ‘tsunami’ is derived from two Japanese words: ‘tsu’ meaning ‘harbour’ and ‘nami’ meaning ‘wave’. To understand how a tsunami can occur, it is, perhaps, necessary to understand a little about earthquakes as well. The earth is covered with several pieces of hard rock which fit together like a jigsaw puzzle. Each piece is called a ‘plate’ and they move and push against each other; in other words, these plates interact with each other along boundaries called, ‘faults’. Oceanic plates slide under continental plates. When an oceanic plate tries to slide underneath the continental plate, the continental plate is dragged down. After being repeatedly pulled down, the continental plate can snap to its former shape. This is a large vertical movement of the earth’s crust and is called a ‘subduction earthquake’. It vertically displaces the water above it from its equilibrium position by either elevating the sea floor or making it subside. In an attempt to regain its equilibrium, seawater is pushed to the surface. This generates a wave which can create a tsunami. ‘The Great Sumatra-Andaman Earthquake’, which measured between 9.1 and 9.3 on the Richter scale, was an example of a ‘megathrust’ earthquake. It generated a tsunami which was able to travel across oceans.

The reason that tsunamis are able to travel great distances is because the rate at which a tsunami loses its energy is inversely proportional to its wavelengths. As such, it loses little energy. This allows it to travel at great speed and over a long distance. To put this in perspective, let’s look at normal waves. On any given day, these waves are wind-generated and they might have a wavelength of about 150 metres. It takes about 10 seconds for this wave to oscillate to a complete cycle. A tsunami in the deep ocean has a wavelength of about 200 kilometres. This wave travels at well over 800 km/h and its oscillation at any given point takes 20 minutes to an hour to complete a cycle.

As the tsunami approaches the coast, the wave is compressed and its speed is slowed down considerably because the waters become shallow. The tsunami can slow down its speed to less than 80 km/h, have a wavelength that is about 20 kilometres and an oscillation that takes mere minutes. To those watching from the shore, the resulting wave appears as a rapidly rising tide or even a series of breaking waves.

A tsunami loses its energy the moment the wave hits the shore because it has encountered friction and turbulence from the sea bed. However, it has still reached the coast with so much energy stored in it that it has the potential to strip beaches of sand, tress and vegetation. As we saw with ‘The Tsunami’, the force of the natural disaster was enough to destroy buildings; hundreds of thousands of lives were lost and millions of people were displaced. For many in South Asia and the Far East, since that Boxing Day in 2004, life has never been the same.

Aneeta Sundararaj can be contacted via editor@howtotellagreatstory.com