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Tsunami Education & Awareness  

Why Talk About Tsunamis

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.

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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, contamination of drinking water, 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 the American Red Cross Mount Rainier 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:

WARNING: A tsunami was or may have been generated, which could cause damage; therefore, people in the warned area are strongly advised to evacuate. Indicates that a tsunami is imminent and that coastal locations in the warned area should prepare for flooding. A bulletin, initially based only on seismic information without tsunami confirmation, issued as a means of providing the earliest possible alert to at-risk populations. Warnings initially place a restricted area in a condition that requires all coastal areas in the region to be prepared for imminent flooding. Subsequent bulletins are issued at least hourly or as conditions warrant to continue, expand, or end the warning. In the event a tsunami has been confirmed which could cause damage outside the source region, the warning will be extended as appropriate.

WATCH: A tsunami was or may have been generated, but is at least two hours travel time to the area in watch status. Local officials should prepare for possible evacuation if their area is upgraded to a warning. An alert issued to areas outside the warned area. A bulletin, usually based only on seismic information with tsunami confirmation, issued as a means of alerting the population within 1 to 3 hours tsunami travel time beyond the tsunami warning area of an earthquake with the potential to have generated a tsunami that may affect the watch area. Subsequent bulletins are issued at least hourly or as conditions warrant to expand the watch area, upgrade it to a warning, or cancel the watch and warning. A Tsunami Watch is normally included in the text of the message that disseminates a Tsunami Warning.

ADVISORY: An earthquake has occurred in the Pacific basin, which might generate a tsunami. WC/ATWC and PTWC will issue hourly bulletins advising of the situation. A message issued when the Pacific Tsunami Warning Center has issued a warning for an event outside of the WC/ATWC Area of Responsibility (AOR) which poses no imminent threat to the AOR. A bulletin issued to areas not currently in either warning or watch status when a tsunami warning has been issued for another region of an ocean. An Advisory indicates that an area is either outside the current warning and watch regions, or that the tsunami poses no danger to the area. The WC/ATWC will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended.

INFORMATION BULLETIN: Information bulletins are issued for earthquakes which are not likely to trigger a tsunami dangerous to the Area of Responsibility (AOR). A bulletin issued to advise participants of the occurrence of an earthquake with the evaluation that a potentially destructive tsunami was not generated, or that an earthquake has occurred outside the TWC's AOR and could potentially generate a tsunami. If the evaluation indicates the possible generation of a destructive or nondestructive tsunami, an investigation will be initiated and additional tsunami information bulletins will be issued as warranted until the investigation is concluded.

INFORMATION MESSAGE: A message with information about an earthquake that is not expected to generate a tsunami. Usually only one bulletin is issued. Information messages issued when smaller earthquakes (less than the warning threshold) may be felt near the coastal areas. These messages are issued to assure coastal residents and emergency managers that there is no tsunami danger.

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.