The Sumatra-Andaman Islands earthquake of 26 December 2004 was the largest for forty years and was followed by the most destructive tsunami experienced in recent history. In a little over three hours the tsunami swept across the Indian Ocean at almost the speed of sound, killing nearly a quarter of a million people. The disaster is unique in the number of countries affected and the geographic spread of damage. The largest death toll was in the Sumatran city of Banda Aceh, when the first of a sequence of waves arrived approximately 22 minutes after the initial eruption. In Sri Lanka, nearly 2000 kilometers from the source, it is their greatest natural disaster. A disproportionate number of the dead were women and children.
About 1% of the casulties were tourists, mainly from northern Europe. The majority of these occurred in the west facing Thai resorts where locals and tourists died in similar numbers.
The earthquake had a magnitude in excess of 9.1. Earthquakes of this magnitude are thought to be one of the side effects of subduction, the process by which one of the large rigid plates making up the earth's surface rides under another.The Boxing Day earthquake occurred in the Sunda subduction zone, where the Indo-Australian plate is sliding beneath the Eurasian plate along a gigantic fault zone almost 3500 miles long. This is not a smooth process, and along the subduction zone there are regions where locally progress is interrupted, perhaps for hundreds of years, while strain energy builds up. The earthquake was the consequence of the sudden and catatrophic release of this energy as the interrupted subduction process finally took place. It was the consequent displacement of land mass rather than the earthquake itself that resulted in a redistribution of the Indian Ocean.
A tsunami is not a movement of water as such, it is the transmission of a huge pulse of energy through the water. Out in the open ocean virtually no wave is seen and a ship on the surface would probably not be aware of the passage of the energy pulse. It is only at the interface between land and sea that the energy is converted into the movement of water. It is also not a single event; the subduction can generate a series of pulses and intervening land masses can deflect and reinforce the energy in a difficult to predict pattern. In Sri Lanka the second wave was the deadliest, in parts of Thailand it was the third.
Countries to the east of the earthquake on Boxing Day 2004 experienced a negative wave, those to the west a positive wave. A negative wave follows a retreat-rise-retreat pattern, with the sea sometimes receding large distances before the first wave comes. Tilly Smith, 10 years old, was on Maikhao beach in Phuket, Thailand with her parents and realised from a recent geography lesson that the retreating sea indicated a tsunami was coming. She managed to persuade her parents of the imminent danger and the beach was evacuated safely. A positive wave follows a rise-retreat-rise pattern and there is no warning of its arrival.
A tsunami is a relatively common occurance in the Pacific Ocean where a sophisticated warning system alerts vulnerable populations to the risk. It is a much rarer event in the Indian Ocean and there is no man-made warning system. Such a system has two components, detecting a potential threat and having the means of contacting the right people to ensure evasive action is taken. It happens that the Pacific warning system did detect the Sumatra-Andaman 2004 earthquake and the direction of a likely tsunami but had no one to contact.
The last major tsunami in the Indian Ocean caused by subduction occurred in 1833, and although it devasted Sumatra the direction of the wave was away from the larger land masses to the west. However it is reported that the indigeneous tribes of the remote Andaman and Nicobar islands have an oral tradition that associates earthquakes with large movements of water - on the 26th December 2004 the islanders moved to higher ground as soon as the tremors were felt and there was no reported loss of life.