Why Talk About Tsunamis?
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(PDF of this Page) Twenty-four tsunamis have caused damage in the United States and its territories during the last 204 years. Just since 1946, six tsunamis have killed more than 350 people and caused a half billion dollars of property damage in Hawaii, Alaska, and the West Coast. As a tsunami nears the coastline, it may rise to several feet or, in rare cases, tens of feet, and can cause great loss of life and property damage when it comes ashore. 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.

What Are Tsunamis, and What Causes Them?

Tsunamis are ocean waves produced by earthquakes or underwater landslides. The word is Japanese and means "harbor wave," because of the devastating effects these waves have had on low-lying Japanese coastal communities. Tsunamis are often incorrectly referred to as tidal waves, but a tsunami is actually a series of waves that can travel at speeds averaging 450 (and up to 600) miles per hour in the open ocean. In the open ocean, tsunamis would not be felt by ships because the wavelength would be hundreds of miles long, with an amplitude of only a few feet. This would also make them unnoticeable from the air. As the waves approach the coast, their speed decreases and their amplitude increases. Unusual wave heights have been known to be over 100 feet high. However, waves that are 10 to 20 feet high can be very destructive and cause many deaths or injuries.

Tsunamis are most often generated by earthquake-induced movement of the ocean floor. Landslides, volcanic eruptions, and even meteorites can also generate a tsunami. If a major earthquake is felt, a tsunami could reach the beach in a few minutes, even before a warning is issued. Areas at greatest risk are less than 25 feet above sea level and within one mile of the shoreline. Most deaths caused by a tsunami are because of drowning. Associated risks include flooding, water contamination, fires from ruptured tanks or gas lines, and the loss of vital community infrastructure (police, fire, and medical facilities).

From an initial tsunami generating source area, waves travel outward in all directions much like the ripples caused by throwing a rock into a pond. As these waves approach coastal areas, the time between successive wave crests varies from 5 to 90 minutes. The first wave is usually not the largest in the series of waves, nor is it the most significant. Furthermore, one coastal community may experience no damaging waves while another, not that far away, may experience destructive deadly waves. Depending on a number of factors, some low-lying areas could experience severe inland inundation of water and debris of more than 1,000 feet.

Learn whether tsunamis have occurred in your area by contacting your local emergency management office, National Weather Service office, or American Red Cross chapter. If you are in a tsunami risk area, learn how to protect yourself, your family, and your property.

Awareness Information

The West Coast/Alaska Tsunami Warning Center (WC/ATWC) is responsible for tsunami warnings for California, Oregon, Washington, British Columbia, and Alaska.

The Pacific Tsunami Warning Center (PTWC) is responsible for providing warnings to international authorities, Hawaii, and U. S. territories within the Pacific basin. The two Tsunami Warning Centers coordinate the information being disseminated.

All tsunamis are potentially dangerous, even though they may not damage every coastline they strike. Damaging tsunamis are very rare. Our coastlines are vulnerable, but tsunamis are infrequent. Understand the hazard and learn how to protect yourself, but don't let the threat of tsunamis ruin your enjoyment of the beach.

The WC/ATWC and PTWC may issue the following bulletins:

Be familiar with the tsunami warning signs. A strong earthquake lasting 20 seconds or more near the coast may generate a tsunami. A noticeable rapid rise or fall in coastal waters is also a sign that a tsunami is approaching.

Tsunamis most frequently come onshore as a rapidly rising turbulent surge of water choked with debris. They are not V-shaped or rolling waves, and are not "surfable."

Tsunamis may be locally generated or from a distant source. In 1992, the Cape Mendocino, California, earthquake produced a tsunami that reached Eureka in about 20 minutes, and Crescent City in 50 minutes. Although this tsunami had a wave height of about one foot and was not destructive, it illustrates how quickly a wave can arrive at nearby coastal communities and how long the danger can last.

In 1957, a distant-source tsunami generated by an earthquake in the Aleutian Islands in Alaska struck Hawaii, 2,100 miles away. Hawaii experienced $5 million in damages from that tsunami.

Plan for a Tsunami

Develop a Family Disaster Plan. Please see the "Family Disaster Plan" section for general family planning information. Tsunami-specific planning should include the following:
  • Learn about tsunami risk in your community. Contact your local emergency management office or American Red Cross chapter. Find out if your home, school, workplace or other frequently visited locations are in tsunami hazard areas. Know the height of your street above sea level and the distance of your street from the coast or other high-risk waters. Evacuation orders may be based on these numbers.
  • If you are visiting an area at risk from tsunamis, check with the hotel, motel, or campground operators for tsunami evacuation information and how you would be warned. It is important to know designated escape routes before a warning is issued.

If you are at risk from tsunamis, do the following:

Assemble a Disaster Supplies Kit

Please see the section "Disaster Supplies Kit" for general supplies kit information. Tsunami-specific supplies should include the following:

How to Protect Your Property

Media and Community Education Ideas

What to Do if You Feel a Strong Coastal Earthquake

If you feel an earthquake that lasts 20 seconds or longer when you are on the coast:

What to Do When a Tsunami WATCH is Issued

What to Do When a Tsunami WARNING is Issued

Return home only after local officials tell you it is safe. A tsunami is a series of waves that may continue for hours. Do not assume that after one wave the danger is over. The next wave may be larger than the first one.

What to Do After a Tsunami

For information pertaining to emergency planning and response in your own state, please see our state pages:
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