Tsunamis have been historically referred to as tidal waves because as they approach land they take on the characteristics of a violent onrushing tide rather than the sort of cresting waves that are formed by wind action upon the ocean (with which people are more familiar). Since they are not actually related to tides the term is considered misleading and its usage discouraged by oceanoligists.
The most common cause of tsunamis is from a 7.5+ magnitude earthquake under the ocean floor. Often these earthquakes occur at boundary lines where tectonic or continental plates meet.
How Earthquakes Cause Tsunami
Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.
When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. The moving wave of the triggered tsunami begins traveling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline. Tsunamis travel swiftly across the open ocean.
Potential Tsunami Locations
Most tsunamis are caused by earthquakes generated in a subduction zone, an area where plate tectonic forces are forcing an oceanic plate down into the mantle. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".
Examples to remember
On the morning after Christmas, a 9.15 magnitude undersea earthquake took the lives of many by the triggering of its huge tsunami. Within hours of the earthquake, the coasts of Indonesia, India, Sri Lanka, Thailand, and other countries, were deluged with waves, some as high as 49 feet.
With over 310,000 people killed in a dozen countries, it has become known as one of the most deadliest disasters of modern history. To put that number in perspective, number two on this countdown for the worst tsunami was Krakatau, Indonesia. The 1883 Krakatau tsunamis killed over 36,000 people in its path. The casualties of this tsunami was more than eight times that number.
