|MadSci Network: Earth Sciences|
Good question! Tsunamis generally form from an underwater earthquake or other disturbance (such as an underwater landslide) that happens in fairly deep water. The disturbance briefly changes the level of the water above it over a broad region. At its point of origin, and as it moves across the ocean, a tsunami wave can be as little as a few inches high. However, as they approach the shore, tsunamis are focused by interaction with the ocean bottom. They change from being very broad and low to being very narrow and tall - the typical "tidal wave" we see in the movies.
If the originating earthquake were on the shore instead of under the water, the surface of the water would pushed sideways(generating typical waves)instead of being raised or lowered (generating a possible tsunami).
The key to a tsunami is the large area that is affected - this means that a large volume of water is involved. Also, a very broad wave can go farther without peetering out (formally known as being dampened) than a narrow wave can. A wave generated by an on-shore earthquake would not get far before it gradually faded away due to friction, wide resistance, etc. Tsunamis, on the other hand, can travel thousands of miles before they hit a shoreline, without losing too much of their energy.
Try the links in the MadSci Library for more information on Earth Sciences.