As around 70% percent of the Earth is covered by water, it is obvious that a large number of earthquakes would occur underwater. What would happen in this situation? Often the result is the destructive tsunami.

'Tsunami' is a word from the Japanese language, and is the scientific term used to describe a 'seismic sea wave', or a wave triggered by seismic activity. Although these waves are often called 'tidal waves', this is incorrect and confusing, as these waves have nothing to do with tides whatsoever.

Scientists have proposed a few theories describing what triggers a tsunami. One theory is that the wave is set off by the earthquake tilting or disturbing the sea floor. Another theory is that an underwater landslide or volcanic eruption may be responsible for a tsunami.

The destructive power of tsunamis is immense. Tsunamis can travel hundred of kilometres through the ocean from where they originated. They have been recorded going at speeds of around 725 to 800 kilometres per hour (For comparison, the cruising speed of a Boeing 747 jet airliner is 940 kilometres per hour), but may only be half a metre or so in height. However, when the wave reaches the more shallow waters of the coast, the height of the wave begins to dramatically increase. Tsunamis of a height of around 15 metres have been recorded. When a tsunami hits a coastal settlement, the results can be devastating - the Japanese town of Sanriku suffered this fate in 1896.