A tsunami is a series of huge waves that occur as the result of a violent underwater disturbances, such as earthquakes or volcanic eruption. The waves travel in all direction from the of the disturbance. The waves may travel in the open sea as fast as 450miles per hour. As they travel in the open ocean, tsunami waves are generally not particularly large - hence the difficulty in detecting a Tsunami
Here is another diagram to explain more.
Tsunami
The 8.9 magnitude earthquake that hit the east coast of Japan had triggered a 23-foot wall of water to engulf the coasts of Japan. The tsunami swept everything in sight from cars, homes, boats, leaving a trail of destruction behind it.
What happened?
The dropping of the Pacific Plate sent a wall of water pushing up and building to 23 feet high as it approached shallow waters along the coast where its velocity is slowed and constantly growing in height. http://peoplepowerportal.blogspot.com/2010/09/dirty-business-of-menstruation.html
