sunamis are surface waves that sweep the oceans when earthquakes occur at the bottom of the sea. The height and the range of these waves, which spread out from their place of origin – the ‘focus’ or the hypocentre – depend on the force of the quake. The duration of a tsunami can vary from three minutes to three hours.
The length of the wave can be up to 200 kilometres and the speed of the waves can reach up to 800 kilometres per hour. They could rise up to 30 metres or more and are most severe in confined beaches such as bays, gulfs and harbours. The more devastating aspect of the tsunami on Galle can be traced to the fact that oncoming waves rose to great heights due to the constriction caused by the harbour. The term tsunami is the Japanese word for ‘harbour wave’.
To picture what happens to cause a tsunami, imagine a stone being dropped into a basin full of water. The ripples or waves caused would spread out from the point of impact, in circles. In the case of a tsunami, the waves move out in circles – the only differences being that the disturbance emanates from below sea level, rather than above; and that the ‘pulses’ are not mere waves but massive walls of water. The disturbance underwater is a convulsive shuddering of the seabed, caused by an earthquake that originated on the ocean floor, which churned the water into a mighty swirl, giving rise to the massive waves. In this case, the quake registered 9.0 on the Richter scale.
The point at which the quake originated – the hypocentre – was somewhere a few kilometres north of the island of Sumatra, which is on the same latitude as Sri Lanka but lying about 1,500 kilometres to the east. Between this point and our country, there is only the open sea, with no obstruction by way of intermittent landmasses to mitigate the ferocity of the tsunami – so the island sustained the full impact of the tsunami.
We are told that the quake erupted at around 6 a.m., and the waves caused by it traversed an expanse of 1,500 kilometres in less than two hours – much faster than the speed at which an aircraft would cross the same distance.
We have learned to live without earthquakes, as we are situated far away from regular earthquake belts of the world. Geographers presently suggest that earthquakes frequently originate along the lines of convergence in ‘tectonic plates’ that form the surface of the earth.
The structure of the interior of the earth is such that it is covered by a number of layers of rock – the outermost being the lithosphere, which is also the crust of the earth.
This shell is broken into slabs or plates, called ‘tectonic plates’, which number about 23 major plates – about 120 kilometres thick under continents and about 65 kilometres deep under the oceans. These converge along lines called fault lines.
These plates can move along the fault lines, as the lithosphere lies over a layer called the asthenosphere, which is one per cent molten – not entirely solid, and therefore acts as a lubricant. This movement, when it occurs – which is not often – is a slow grind caused by the release of pressure, allowing the underlying hot rocks to turn into lava, leading to volcanoes and earthquakes.
The tectonic plates can move along fault lines in three different directions: they could rub against each other; they could move away from each other; or they could crash into each other – in which event, one plate goes under the opposing one. This is called ‘subduction’, which could cause severe quakes – as would have happened on 26 December.
On many previous occasions, strong waves were reported off our eastern coast, which went unnoticed by the public. It is also believed that such high-intensity quakes do not occur frequently at the same place. So it should be safe for us to resume our normal work without the fear of a recurrence. Quakes, however, do not occur only along fault lines. They do happen in mid plates, though infrequently.