The term Tsunami comes from the Japanese meaning of Harbor (“Tsu”) and Wave (“Nami”). Tsunamis have a much smaller amplitude (wave height) offshore, and a very long wave length, often hundreds of miles long.
This is why most times they are not noticed at sea. They form only slight swells usually around 300 - 400mm above the normal sea surface. They can occur at any state of the tide and can flood coastal areas if the wave's surge is big enough.
Tsunami is also called a Tidal Wave by some; this term is wrong. They are not related to the coming and going tides. This being said, it may also be wise to mention that the word tsunami meaning wave and Harbor are also brought into question and are wrong in the sense that tsunami, are not limited to just the harbors.
One suspected cause for the forming of a Tsunami is the sudden movement of the Earth's Plate below the surface of the ocean.
Above is a rather simple chart showing what may happen that will cause the Tsunami to form. As you can see the water has no resistance and as it's displace by the energy the wave becomes unsettle building speed and growing over long distances as it approaches the landfall.
As you can see in the picture above the wave grows in intensity as it approaches or nears land fall.
Tsunami can cause much death and destruction to those that are caught unawares of it.
The tsunami that hit Indonesia also caused much death and lots of damage. The picture below shows some of the aftermath.
The share power and force of the surge can move very quickly and as evidence from this picture it can flatten and destroy everything in its path.
It is believed that earth quakes out at sea are the cause of many tsunami.
