The Indian Ocean tsunami

26 December 2004

The impacts of the major earthquake and the ensuing tsunami in the indian Ocean on 26 December 2004 have been well documented(ISDR), and preliminary analyses are beginning to emerge. The notes here extend the tsunami section 6.7, page 149, of the book.

Useful web page with other links are found here (University of Buffalo) and here ( Wikipedia).

The earthquake

Although initially reported as 9.0 on the Richter scale, one later reanalysis has suggested a magnitude as 9.3 for the earthquake. This places it as one of the biggest earthquakes ever recorded, but slightly below the 9.5 of the earthquake off southern Chile on 22 May 1960. See page 150 and Figure 6.11.

The epicentre was at 30 19’ N, 950 51’ E, off the Indonesian island of Sumatra at a depth of 30 km below mean sea level. Tectonically it was part of the subduction process along the line where the India Plate dives beneath the Burma Plate, at the Sunda Trench. An estimated 1200 km of fault line slipped about 15 m along the subduction zone, in two distinct phases. The first was a rupture some 400 km long and 100 km wide that proceeded at 2.8 km/s to the northwest over 100 seconds. There was then a pause of about 100 seconds before the further northward propagation of the disturbance at slightly slower speeds of 2.1 km/s for a further five minutes the where the subduction changes to strike-slip horizontal plate motion. The seabed has been estimated to have risen by several meters displacing an estimated 30 km3 of water, which caused the tsunami. This reduction in ocean volume, if not compensated for elsewhere, would increase mean sea levels globally by 0.1 mm.

The tsunami

The resulting tsunami generated by this 1200 m of earthquake extent did not spread in the simple cylindrical way expected for a point source. Instead there was a focussing of wave energy at right angles towards the west and southwest. Locally amplitudes of 24 m or more have been estimated as the wave hit the shore of Ache. About 90 minutes later the wave hit the east coast of Sri Lanka, and the east coast of India. Many observers noted that the first wave was not the largest, and that repeated inundations occurred at intervals of 30 to 50 minutes. The reason for this periodicity is not known. The focussing of the tsunami wave meant that some nearby places such as the Cocos Islands, and fortunately low-lying Bangladesh experienced only very small impacts. Topography played a major role in the pattern of wave propagation. Because of the slower wave speed over the relatively shallow mid-ocean ridges, they tended to act as a wave-guide, producing some unexpected results like the 2.6 m peak-to-trough tsunami arrival reported from Manzanillo, Mexico. Kelvin wave propagation (see page 84 and figure 4.5) controlled by continental shelf boundaries refracted the waves around landmasses for example to reach the west coast of India in Kerala. Even the Queensland coast observed 0.2 m tsunami arrivals a day later, after Kelvin wave refraction around Australia. At even greater distances the tsunami was detected on the bottom-pressure DART buoys in the north and western Pacific Ocean; these buoys detected both the tsunami arrivals, some 22-30 hours after the earthquake, and the more rapid arrival of the seismic Raleigh waves an hour or so after the event.

The impacts

The immediate impacts of the tsunami flooding were extensive loss of life and of people displaced from their homes and livelihood. Estimates of fatalities vary from 230,000 to in excess of 300,000. Most were in the immediate locality of the earthquake, and were probably partly due to effects of the earthquake itself. Indonesia alone estimates more than 160,000 dead, with other major losses in Sri Lanka, India and Thailand. Displaced people probably numbered more than 1.6 million. The economics of rebuilding these communities has been described elsewhere. Inevitably it is the poorest people in small remote communities that depend on the sea whose lives have been changed most. Many of the bigger economic activities such as tourism and hotels had insurance and access to new capital from global sources. Nevertheless, the loss of some 9000 tourists caused an immediate reaction in terms of lower bookings and cancellations, even for places free of tsunami risk.

Environmental impacts include salt on arable land, loss of fresh water, and the destruction of rice, mango and banana plantations. In some cases it was noted that the presence of coastal vegetation including mangroves reduced the ferocity of the tsunami impact. Coral reefs were extensively damaged mainly by being smothered by sediment and debris. In most cases natural recovery is expected within 10 years.

And afterwards

The absence of a warning system, which could have saved many lives, was understandably criticised. Governments had not responded positively to proposals from scientists to extend the system already in operation in the Pacific Ocean. Now the Intergovernmental Oceanographic Commission of UNESCO has been charged with developing a system that will eventually be global. Member states have resolved to work together over the long-term to develop a system to warn of marine hazards of all kinds globally. At one extreme this means tsunami and storm surges; at the other it can mean pollution and rising sea levels. But the strong political will in the immediate aftermath of the tsunami is inevitably weakening, and the necessary resources are only slowly becoming available.

References

Bilham R. (2005) A flying start, then a slow slip. Science 308, 1126-1127.

Wilkinson C., Souter D. and J. Goldberg (2006) Status of Coral Reefs in Tsunami Affected Countries: 2005. Published by the Global Coral Reef Monitoring Network.

Titov V.V., Rabinovich A.B., Mofjeld H.O., Thomson R.E. and F I Gonzales (2005) The global reach of the 26 December Sumatra tsunami. Science, 309, 2045-2048.

Gower J., and Gonzales F., (2006) U.S. warning system detected the Sumatra tsunami. EOS 87 (10). 7th March 2006.