Tsunami (pronounced soo-NAH-mee) is a Japanese word. Tsunamis are fairly common in Japan and many thousands of Japanese have been killed by them in recent centuries.
Tsunami is a series of waves. Often the initial wave is not the largest. The largest wave may occur several hours after the initial activity starts at a coastal location. There may also be more than one series of tsunami waves if a very large earthquake triggers local landslides
• An earthquake generates a tsunami if it is of sufficient force and there is violent movement of the earth causing substantial and sudden displacement of a massive amount of water.
• A tsunami is not a single wave but a series of waves, also known as a wave train. The first wave in a tsunami is not necessarily the most destructive. Tsunamis are not tidal waves.
• Tsunami waves can be very long (as much as 60 miles, or 100 kilometers) and be as far as one hour apart. They are able to cross entire oceans without great loss of energy. The Indian Ocean tsunami traveled as much as 3,000 miles (nearly 5,000 kilometers) to Africa, arriving with sufficient force to kill people and destroy property.
• A tsunami may be less than a foot (30 centimeters) in height on the surface of the open ocean, which is why they are not noticed by sailors. But the powerful shock wave of energy travels rapidly through the ocean as fast as a commercial jet. Once a tsunami reaches shallow water near the coast it is slowed down. The top of the wave moves faster than the bottom, causing the sea to rise dramatically.
• Geological features such as reefs, bays, river entrances, and undersea formations may dissipate the energy of a tsunami. In some places a tsunami may cause the sea to rise vertically only a few inches or feet. In other places tsunamis have been known to surge vertically as high as 100 feet (30 meters). Most tsunamis cause the sea to rise no more than 10 feet (3 meters).
The Indian Ocean tsunami caused waves as high as 30 feet (9 meters) in some places, according to news reports. In other places witnesses described a rapid surging of the ocean.
Flooding can extend inland by a thousand feet (300 meters) or more. The enormous energy of a tsunami can lift giant boulders, flip vehicles, and demolish houses. Knowledge of the history of tsunamis in your area is a good indicator of what is likely to happen in a future tsunami event.
More info at:
http://www.geophys.washington.edu/tsunami/general/warning/warning.html
