Calculating the threat of tsunami

Box 1 | What caused the Indian Ocean tsunami on 26 December 2004?

The tsunami that struck coastal regions of the Indian Ocean on the 26 December 2004 killed more than 289,000 people and left many more injured or without homes. The surge of sea water that caused the destruction was the result of the largest earthquake in the world for forty years. The main quake measured 9 on the Richter scale and was followed by a series of aftershocks of between 5.7 and 7.3 in intensity. Less severe earthquakes are often measured in the region.

The earthquake was caused by the movement of two of the earth's tectonic plates. The oceanic India plate is moving north at an average of 6 cm per year and is being forced under the continental Burma plate. However, on the 26 December the plates suddenly shifted 15 meters over a length of 1200 km on the ocean floor. The epicenter of the earthquake was off the west coast of northern Sumatra, Indonesia. The shift in the plates on the ocean floor created a tsunami, which spread across the Indian Ocean.

Unfortunately, there is no early warning system for tsunamis in countries surrounding the Indian Ocean as there is in the Pacific. The earthquake was detected by many monitoring stations, but there are no tidal buoys or wave sensors in place to detect potential tsunamis that might follow a quake. Although the tremors were felt in neighboring countries, victims were unaware of the approaching tsunami. Apart from the immediate devastating effects of the tsunami, the long term effects on agricultural land due to salt water, sources of fresh water and the psychological impact on the survivors is not yet known.

Box 2. The disappearance of an ancient civilisation
Box 3. The warning system in the Pacific

Related sites
The structure of the Earth (Back to basics, Australian Academy of Science)
Enormous earthquake in Indonesia causes tsunami (Geoscience Australia)
Searching for Australian earthquakes no longer a shaky experience (Geoscience Australia)
Recent earthquakes (Geoscience Australia)
Tsunamis in Western Australia (West Australian Seismicity)
Tectonic setting (West Australian Seismicity)
Descriptive model of the July 17, 1998 Papua New Guinea tsunami (United States Geological Survey)
Most recent tsunamis (West Coast and Alaska Tsunami Warning Center, USA)

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Page updated February 2005.