Tsunami - How and where do these undersea earthquakes occur?

The earth is made up of several pieces of hard rock that fit together a bit like a jigsaw.  These are called tectonic plates and they move very slowly.  Oceanic plates are denser/heavier than continental plates and so they slide under the continental plates.  Where this happens it is called a subduction zone.  There are subduction zones off Chile, Nicaragua, Mexico and Indonesia.  These areas are prone to earthquakes, which happen when the plates suddenly move against each other.