They are caused usually by earthquakes below the sea-floor, submarine landslides or volcanic explosions. The diagram given below shows the generation of a tsunami:
During an earthquake, sometimes there is a sudden subsidence or upheaval of the sea floor, due to displacement of blocks. Accordingly, all the water at the epicentre of the earthquake is lifted or dropped for an instant giving rise to a sea-wave of several hundred kilometers long but only a few metres high in the open sea. This sea- wave moves at a speed of 750 to 800 kms per hour.
Even though the speed of the wave slows down drastically as it moves through shallow coastal water, the height of the wave rises to 30 or 40 metres on approaching the coast. This wave of water, as a very large and fast wave, hits the shore but because of its extremely long-wave length it does not withdraw quickly as the normal waves do.
Its long duration and great height cause great damage to the entire coast and many deaths by drowning in low-lying coastal areas. It is thought that coastal flooding which occured in Japan in 1703, with an estimated life loss of 1,00,000 persons, may have been caused by seismic sea waves.
In the year 1952, a strong earthquake with the epicentre at the northern tip of Kuriles trough gave rise to a Tsunami that reached the shores of Kamchatka and Kuriles and caused enormous destruction,