How does a tsunami occur?

The Japanese word 'tsunami' means 'harbour wave'. It is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean,
though it can occur in large lakes. Tsunamis are caused by undersea landslides or by 'seaquakes', that is an earthquake whose epicentre lies below the ocean. 

If the seabed is displaced up to several metres, the water layer of several kilometres in thickness above it also moves with it. The wave caused by this movement has a lot of energy-much more than a wave of the same height whipped up by the wind. This becomes apparent when the wave moves towards the coast.