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2004 Boxing Day Tsunami

Read more about lessons learned by the New Zealand team from their week-long visit to Thailand.

What caused the Boxing Day tsunami in the Indian Ocean?

A magnitude 9.3 earthquake occurred on the seafloor off the western coast of Sumatra. It ruptured over a 1200km section of a fault under the sea, causing a huge area of the sea floor to be raised . This caused many hundreds of cubic kilometres of sea water to be lifted above its normal level. It spread out as a tsunami.

Towns on the coast facing away from the quake epicentre, such as Banda Aceh, suffered huge damage. How can this be?

Tsunami waves can travel around corners. As the huge energy of the tsunami moves through the ocean, its speed, direction, and behaviour are influenced by the depth and shape of the seafloor. The waves can be deflected and slowed by seamounts and other large structures on the sea floor. GNS scientists use computer models to show how tsunami waves propagate through the ocean and around New Zealand's coastline.

Did the Boxing Day tsunami waves reach New Zealand?

es. Waves up to half a metre in height were recorded at 15 coastal sites (mostly ports) between 18 and 25 hours after the earthquake occurred. The tsunami's path to New Zealand was long and winding. It went around the bottom of Australia and was reflected northward by the Campbell Plateau, in sub-Antarctic waters. Wave heights and arrival times around the New Zealand coast varied. This was no surprise to scientists as the ocean floor around New Zealand is highly variable. The tsunami waves were further modified when they entered New Zealand harbours, which are different shapes and sizes. The waves also reached the North Atlantic and North Pacific Oceans.

Why weren't people warned in the countries bounding the Indian Ocean?

There is a 24/7 tsunami-warning centre based in Hawaii for the Pacific Ocean, but the Indian Ocean does not have an equivalent. The Pacific warning centre was able to put out a warning in time to save lives in Kenya, but the protocols for informing many other countries were not in place. In the wake of the disaster, the UN is planning to install a warning system for the Indian Ocean. The project is being led by Reid Basher, a former New Zealander who now works for the UN in Bonn, Germany. But some places were too close to the tsunami source on 26th December for this type of system to have been effective, even if it had been operating then.

Case Study: Boxing Day Tsunami

Lessons the New Zealand team of scientists and engineers brought back from their week-long visit to Thailand in January 2005.

  • New Zealand has not experienced a natural disaster of this scale since European settlement, but could easily do so in the future.
  • A trigger in New Zealand could be a moderate (magnitude 6.5 or greater) earthquake within 200km of the coast.
  • A tsunami of comparable volume and momentum striking a populated New Zealand coastal area could easily overwhelm civil defence response and recovery arrangements.
  • A tsunami is not a single breaking wave. It is a raging torrent or wall of water that surges inland. The backwash from a retreating tsunami can produce as much damage as the forward surge.
  • When a backwash meets a subsequent incoming surge, hugely destructive vortexes of water can form. They can be hundreds of metres across.
  • Changes to the New Zealand Building Code are not considered necessary. However, planning requirements for critical facilities in low-lying areas should be a priority.
  • There is no single measure that can provide complete protection. Reducing the impact is best achieved through a range of measures.
  • Previous estimates of tsunami hazard in New Zealand, where based only on inundation, are likely to underestimate the risk as they do not account for momentum of the flows.
  • Trying to outrun a tsunami is not a recommended course of action.
  • Modern reinforced concrete buildings directly in the path of a large tsunami can suffer a surprisingly large amount of damage.
  • Uniform setback distances for coastal development are largely meaningless, as really big tsunamis can travel several kilometres inland.
  • Green belts, dunes, mangroves, and dense coastal trees can significantly reduce the impact of a tsunami.
  • Damage and destruction is accentuated when tsunamis reach the shore at high tide.
  • One of the keys to a quick recovery is access to government subsidised bank loans for small businesses. In Thailand, recovery loans were available at 1 and 2 percent.