Most tsunamis are caused by earthquakes at destructive plate boundaries. Here, an oceanic plate is subducted into the mantle beneath a continental plate. This movement causes friction, which in turn causes the plates to stick. Energy accumulates, like that of a compressed spring. When the energy exceeds the friction, the plates snaps back into position. This movement thrusts the water above causing a wave to form. The waves can travel large distances. When the waves reach shallower water the following happens:
Water retreating is a sign that a tsunami is approaching a coast. Shortly after this happens, the waves reach the shore. This is actually the trough of the wave following behind.
The main effect of tsunamis is coastal flooding. The force of such large waves can also destroy coastal property.