Tsunamis (the word is from the Japanese and most closely translated as ‘harbour wave’) are large-scale oceanic waves. Commonly (but inaccurately) called tidal waves, they have nothing to do with tides but are generated by significant displacement of water within the ocean. Although relatively rare, they are among the most potentially destructive of natural phenomena, capable of causing damage hundreds of miles from their source.
Definitions and Causes of Tsunamis
Tsunamis occur when some physical mechanism displaces a large amount of water. This mechanism is usually a major earthquake although a landslide (either underwater or beginning on land and entering the water), volcanic eruption, nuclear explosion or meteorite impact may have the same effect. Whatever the event, it must involve vertical displacement, so not all earthquakes will generate tsunamis. Major tsunami events are usually associated with earthquakes of magnitude 7 or greater.
When water is displaced vertically, it triggers a series of waves which radiate outwards from the source of the earthquake or impact. These can have very great wavelengths (the distance between two peaks or troughs) which may be hundreds of kilometres. The waves travel across the ocean at speeds of hundreds of miles an hour ( Federal Emergency Management Agency).
Despite their speed and the energy associated with them, tsunamis are of little danger in deep water where, if they are observable at all, they may show only as waves of around a metre in height. Once they reach shallower water, however, the complicated physics of wave motion comes into play. The tsunami will slow down (though it will still be moving at a significant speed) but its height will increase, generating waves of up to 30 metres.
Where Tsunamis Occur
Given the nature of their triggers, tsunamis can and have occurred in all the world’s oceans. They are mostly associated with the Pacific given the strongly seismic activity around the so-called Ring of Fire, and so there is a significant risk along all of the ocean’s coasts. Although tsunamis are rare in the Indian Ocean, it was here that the most significant and destructive event of recent years, the Boxing Day tsunami of 2004, took place.
Because of the vast distances which can be covered by the waves, they can have a significant impact many thousands of miles from their place of origin. The tsunami generated by the 2004 Boxing Day earthquake, for example, crossed the Indian Ocean to reach the east coast of Africa over 3000 miles away (Australian Government Bureau of Meteorology) , while an earthquake in Alaska in 1946 killed over 150 people in Hawaii (Earth Science Australia).
Tsunami monitoring systems are in place in many areas considered to be at risk. Using a combination of seismological data and information from tide gauges and ocean sensors, they can provide some indication about the possibility of a tsunami (New Scientist). Because there is no direct link between an earthquake size and the behaviour of a tsunami, however, it is not always possible to give an accurate warning of the scale or impact of a tsunami event.
Sources and Further Information
Australian Government Bureau of Meteorology “Tsunami Facts and Information” bom.gov.au, accessed 7 January 2011
Earth Science Australia “Tsunamis”, earthsci.org, accessed 7 January 2011
“Tsunami” Federal Emergency Management Agency, fema.gov, accessed 7 January 2011
Open University Course Team Waves, Tides and Shallow Water Processes Open University 1999
Will Knight “Tsunami Warning System is Not Just Sensors” New Scientist 4 January 2005
Join the Conversation