How Earthquakes Cause Tsunamis :



The illustration below shows a subduction earthquake (one where a denser plates shifts below its neighboring plate, at left). Energy is transferred and the displaced water forms a wave. As the wave travels and enters shallower water in the coastal area, it begins to increase in amplitude (National Geographic).

Tsunamis are not always colossal waves when they come into the shore. In fact, "... most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). Rather, they come in much like very strong and very fast tides (i.e., a rapid, local rise in sea level)." (USGS website) Nevertheless, there is destruction of life and of property by floating debris and impact of water. The tsunami produces a series of rushing waves and also a series of withdrawals.

     

Tsuanamis and Earthquakes-Basics- USGS

If you think about throwing a rock in water, a ripple is formed. It is the same principle involving a meteor or an earthquake, except they form bigger ripples. If you think about that ripple, it seems to disappear as time goes on--but in actuality, it does not stop. In the picture below we can see that a tsunami is very fast (the speed often compared with that of a jet) and it has a height of 20 inches.

However, we need to take into consideration the effect of the tsunami reaching the shore. The speed is diminished but the wave height is increased drastically.

http://observe.arc.nasa.gov/nasa/exhibits/tsunami/tsun_physics.html

To view a QuickTime visualization: Tsunami Visualization

For more information about earthquake formation see Tectonics