Underwater earthquakes push water upward to create the initial movement, then gravity pulls the water downward, creating the horizontal force that forms the tsunami. The waves travel through the ocean in a similar fashion to ripples created by throwing a stone into a body of water.
Earthquakes occur often on the earth's crust that forms the sea floor; however, not all result in a tsunami. For a tsunami to form, the magnitude of the earthquake must measure a minimum of 7.0 on the Richter scale. Below this magnitude, the earthquake does not displace enough water to create the big wave. The movement must also lift or lower the sea bed. Sideways movement does not create the initial wave to start the process. The epicenter of the earthquake must be near the surface of the sea floor to cause the correct movement.
Tsunamis move very rapidly through deep water, but slow in speed as they reach the shore. While other waves travel on the surface of the water, the tsunami moves through it. While traveling though deep water, the wave is typically less than 3 feet high. However, as it reaches the shallow areas, the energy transfers to increase its height, according HowStuffWorks.com.