Tsunami is an enormous sea wave caused by the sudden large-scale movement of the sea floor resulting in the displacement of large volumes of water. The cause may be an earthquake, volcanic eruption, a submarine slide or slump of sediment, which may itself have been started by an earthquake or tremor. The slipping of thousands of tons of rock from the sides of fjords may also cause a tsumani to trigger.
The effect of this sea floor movement in the open ocean may not be seen at all, as the resulting wave may only be one metre or less in height. However, because the whole depth of water is affected, there is a vast amount of energy involved, so when the waves reach shallow water or small bays, the effects can be devastating. The waves may travel at several hundred kilometres per hour, between 600 to 900kmph, and reach heights of around 15 to 30 meters. The devastation caused is clearly going to be terrible and there are many such instances in the record.
The word 'Tsunami' originates from the Japanese (tsu meaning harbour and nami meaning waves) where there have been many instances of destructive tsunami. In the year 1933, an earthquake triggered a tsunami with waves up to 27 metres high and thousands of people were drowned along the Japanese coast. The waves were actually recorded about 10 hours later in San Francisco, US, having crossed the Pacific Ocean. It appears that tsunami are generated by submarine earthquakes registering 8 or more on the Richter scale.