Underwater earthquakes are the most common tsunami instigator. To understand them, we have to delve into plate tectonics, which suggests that a series of huge plates makes up the lithosphere, or top layer of the Earth. These plates make up the continents and seafloor. They rest on an underlying viscous layer called the asthenosphere.
Think of a pie cut into eight slices. The piecrust would be the lithosphere and the hot, sticky pie filling underneath would be the asthenosphere. On the Earth, these plates are constantly in motion, moving along each other at a speed of 1 to 2 inches (2.5 to 5 centimeters) per year. The movement occurs most dramatically along fault lines (where the pie is cut). These motions can produce earthquakes and volcanism, which, when they occur at the bottom of the ocean, are two possible sources of tsunamis.
When two plates come into contact at a region known as a plate boundary, a heavier plate can slip under a lighter one. This is called subduction. Underwater subduction often leaves enormous "handprints" in the form of deep ocean trenches along the seafloor.
In some cases of subduction, part of the seafloor connected to the lighter plate may "snap up" suddenly due to pressure from the sinking plate. This results in an earthquake. The focus of the earthquake is the point within the Earth where the rupture first occurs, rocks break and the first seismic waves generate. The epicenter is the point on the seafloor (or other part of the Earth's surface) directly above the focus.
When this piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force transfers to the water. The energy pushes the water upward above normal sea level. This is the birth of a tsunami. The earthquake that generated the Dec. 26, 2004, tsunami in the Indian Ocean had a magnitude of 9.1 -- one of the biggest in recorded history.