Tsunami is the Japanese word for "demon wave." Tsunamis are better known as "tidal waves." Tidal waves are caused by earthquakes under the water. Sometimes they are caused by very large icebergs that break off the cliff and fall into the water. Tsunamis usually occur in the Pacific Ocean. Today satellites watch over the Pacific Ocean for Tsunamis to get warnings out to the people. In Japan, they line their beaches with large, special concrete blocks called "tetrapods" to break up and decrease the damage the Tsunamis.
You know that earthquakes cause Tsunamis. But what causes earthquakes? The Earth has four main layers. The outermost layer is called the crust. The crust is not whole, but made up of large pieces called plates. The plates move very slowly. Sometimes the plates get squeezed and pushed past each other. This causes the Earth to move and shake. This is an earthquake. One famous earthquake is the "Great San Fransisco Earthquake."It happened in 1906. It caused massive damage.