Tsunamis are often called tidal waves but they have nothing to do with tides and everything to do with Geology. The word Tsunami refers to harbor waves but they are certainly not restricted to harbors.
The Tsunami that destroyed so many lives on December 26 2004 was caused by the sudden release of tension built up at the junction of the Burma plate and the Indian plate in the form of an earthquake with the epicentre in the Sumatra straights. At first the earthquake was estimated to be 8.9 on the Richter scale but later estimates have increased this to 9.3. This is not the greatest earthquake ever measured, that dubious honour falls to the Chile earthquake of 1960 at 9.5, but it is still in the top five.
Earthquakes on land can frequently cause death and destruction to the communities surrounding the epicentre. One only has to look at the 2008 Sichuan earthquake in central China which measured on the 8.0 Richter scale and caused the deaths of over 650000 people. A Tsunami however can take this death and destruction thousands of miles across oceans to communities which hardly felt the initial quake.
The tectonic plates which make up the earths surface are moving against and underneath each other. At the oceanic ridges in the centre of the oceans new basaltic oceanic crust is added, this pushes the older crust away to create pressure against adjoining plates. Where an oceanic plate meets another plate it can be forced down into the semisolid mantle in a subduction zone, this is known as a mega-thrust. These zones are characterised by a deep oceanic trench next to either an island arc if the other plate is made of oceanic crust or large mountain ranges if it is being subducted under continental crust. Examples of island arcs are the island chains of Japan and Indonesia while the Rockies and the Andes of North and South America are evidence of the subduction of the Pacific plate under continental crust. If the plates meet at an oblique angle pressure is relieved by a slip-strike movement with the plates sliding against each other rather than one being subducted.
Studies have shown that the Indian plate is moving Northeast relative to the Burma plate at an estimated 5 cm per year at the Sunda Trench. This is not done gradually as the plates stick together and may show no movement for decades. The 2004 earthquake started the largest movement along a fault line ever recorded. It has been calculated that 1600km of fault line slipped approximately 15m. Beginning just of the coast of Aceh a rupture started 30 km below the seabed, this rupture moved north at a speed of 2.8 km/s for about 100 seconds. At this point there was a pause of another 100 seconds before the rupture continued North at a slightly slower speed of 2.1km/s for 5 minutes where it reached a plate boundary supporting slip-strike movement rather than subduction. This was fortunate as it lessened the speed of water displacement and so subjected the Northern part of the Indian Ocean particularly the low lying country of Bangladesh to a less severe Tsunami than that seen in the south.
The affect of this massive subduction of the Indian plate caused the sea floor of the Eastern Burma plate to rise displacing an estimated 30 km of water. The waves caused by this displacement were tracked as travelling at about 500km/h across the Indian ocean. While travelling across the deep ocean these waves were measured by remote satellites as being 60cm high. As they approached land the shallower seabed caused the waves to slow down but increase in height reaching a reported 30 meters. The waves first reached Aceh in Indonesia. The affects of the waves on the communities of Aceh were seen up to 4km inland, over 130000 Indonesians have been reported as killed.
Just over an hour later Tsunamis hit both Thailand east of the epicentre and Sri Lanka west of the epicentre. Although Thailand is far closer than Sri Lanka to the source of the Tsunamis the waves travelled more slowly in the shallower Andaman Sea than in the Indian Ocean. Tsunamis continued on to hit communities in India, Madagascar, the Maldives, parts of Africa and the Yemen.
Although this incident is often described as the Boxing Day Tsunami it was in fact a series of Tsunamis all started by the same earthquake. A series of waves were formed by the deformation of the sea bed and most reports are of three major waves hitting the affected coastlines with the third wave being the largest. The waves hit the coastlines in a series of retreat and rise cycles separated by about thirty minutes. Smaller waves continued to hit the already badly damaged communities for the rest of the day
In the Northern Indian Ocean Bangladesh was very lucky if waves similar to those seen in the South had hit its’ low lying coastline many people would have died as it was only two deaths were reported from this country.
The death toll from this disaster will probably never be truly known. There have been 184186 confirmed deaths but the estimated number is 230120. The government of Myanmar is believed to have significantly under reported the casualty numbers in that country.
There was such a high death toll because no one expected a Tsunami in the Indian Ocean. They have occurred there in the past but are rare compared with the Pacific Ocean. Many of the people living in the area were not aware of the meaning of the sudden withdrawal of the sea leaving the sea bed in view which preceded the Tsunami in several places. People went onto the sea floor picking up stranded fish while others just watched from the shore. If they had been aware and started to move to higher ground when the sea retreated many lives may have been saved. An Indian Ocean Tsunami warning system, similar to the one already in place in the Pacific, is in the process of being set up. It is unlikely that an early warning system would have done much to save most of those who perished in Indonesia as it was so close to the epicentre but many in the other affected countries may have survived.
Human interference with natural ecosystems of the Indian Ocean is thought to have removed some protection that used to surround many of the islands. Coral reefs have been dynamited to make shipping easier and to allow an increase in fishing for shrimp. In addition the Mangrove swamps and sand dunes that used to protect much of the coasts have been removed to make way for coastal homes and tourism developments. The Surin Island chain which still has a coral reef protection was in some way protected and did appear to have a lower loss of life than many similar but unprotected Islands.
One detail that came to light in the aftermath was the fate of aboriginal hunter-gatherer tribes living in the Andaman Islands. Anthropologists were worried about their fate as several tribes were already endangered by falling numbers. When aid workers reached these islands the aboriginals were found to be safe while settlers and tribes which had settled down to a farming existence had been badly impacted. It seems there was a folk law legend about a war between the god of the sea and the god of the forest when the sea withdrew they knew the sea god was preparing to attack the forest and the moved to the safety of higher ground.
