Tsunamis are large ocean waves generated by major earthquakes beneath the ocean floor or major landslides into the ocean. Tsunamis caused by nearby earthquakes may reach the coast within minutes. When the waves enter shallow water, they may rise to several feet or, in rare cases, tens of feet, striking the coast with devastating force. People on the beach or in low coastal areas need to be aware that a tsunami could arrive within minutes after a severe earthquake.
The tsunami danger period can continue for many hours after a major earthquake. Tsunamis also may be generated by very large
earthquakes far away in other areas of the ocean. Waves caused by these earthquakes travel at hundreds of miles per hour, reaching the coast several hours after the earthquake. The International Tsunami Warning System monitors ocean waves after any Pacific earthquake with a magnitude greater than 6.5. If waves are detected, warnings are issued to local authorities who can order the evacuation of low-lying areas if necessary.
Why prepare for tsunamis?
All tsunamis are potentially, if rarely, dangerous. Twenty-four tsunamis have caused damage in the United States and its territories in the past 200 years. Since 1946, six tsunamis have killed more than 350 people and caused significant property damage in Hawaii, Alaska, and along the West Coast. Tsunamis have also occurred in Puerto Rico and the Virgin Islands.
When a tsunami comes ashore, it can cause great loss of life and property damage. Tsunamis can travel upstream in coastal estuaries and rivers, with damaging waves extending farther inland than the immediate coast. A tsunami can occur during any season of the year and at any time, day or night.
How can I protect myself from a tsunami?
If you are in a coastal community and feel the shaking of a strong earthquake, you may have only minutes until a tsunami arrives. Do not wait for an official warning. Instead, let the strong shaking be your warning, and, after protecting yourself from falling objects, quickly move away from the water and to higher ground. If the surrounding area is flat, move inland. Once away from the water, listen to a local radio or television station or NOAA Weather Radio for information from the Tsunami Warning Centers about further action you should take.
Even if you do not feel shaking, if you learn that an area has experienced a large earthquake that could send a tsunami in your direction, listen to a local radio or television station or NOAA Weather Radio for information from the Tsunami Warning Centers about action you should take. Depending on the location of the earthquake, you may have a number of hours in which to take appropriate action.
from - http://geography.about.com/od/physicalgeography/a/tsunami.htm
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