Emergency and Risk Management

The Tsunami Hazard

A tsunami (pronounced “soo-nah-mee”) is a series of waves generated by an undersea disturbance such as an earthquake. The term tsunami is Japanese in origin, represented by two characters: "tsu" (harbor) and "nami" (wave).  Tsunamis are often referred to, incorrectly, as "tidal waves."  In truth, tides result from the gravitational influences of the moon, sun, and planets, a phenomenon that has absolutely nothing to do with the generation of tsunamis (although the ultimate height of a tsunami striking a coastal area is determined by the tide level at the time of impact.) 

 

There are many events that result in the generation of a tsunami, but earthquakes are the most prevalent.  Other forces that generate the great waves include landslides, volcanic eruptions, explosions, and though extremely rare, the impact of extra-terrestrial objects, such as meteorites.

 

Tsunamis are generated when a large area of water is displaced, either by a shift in the sea floor as would occur following an earthquake, or by the introduction of mass, as described in the other generative forms listed above.  Waves are formed as the displaced water mass attempts to regain its equilibrium.  It is important to note that not all earthquakes generate tsunamis; to do so, earthquakes must occur underneath or near the ocean, be large in magnitude (studies have indicated a minimum 6.9 on the Richter Scale), and create movements in the sea floor. While all oceanic regions of the world can experience tsunamis, the countries lying in the Pacific Rim region face a much greater frequency of large, destructive tsunamis because of the presence of numerous large earthquakes in the seismically active ‘Ring of Fire’.

 

From the area of the disturbance, the resulting waves that are generated will travel outward in all directions, much like the ripples caused by a rock thrown into standing water.  The time between wave crests can range from as little as 5 to as many as 90 minutes, and the wave speed in the open ocean will average a staggering 450 to 600 miles per hour. 

 

Tsunamis reaching heights of more than 100 feet have been recorded. In the open ocean, tsunamis are virtually undetectable to most ships in their path.  As the waves approach the shallow coastal waters, they appear normal but their speed decreases significantly.  The compression of the wave resulting from the decrease in ocean depth causes the wave to rise in height and crash onto land – often with great destruction, injuries and death as the result. (NTHMP, 2003)

 

Tim Folger, in his article “Waves of Destruction”, described the generation of tsunamis.  He wrote, "As the tsunami wave reaches the shallower water above a continental shelf, friction with the shelf slows the front of the wave. As the tsunami approaches shore, the trailing waves pile onto the waves in front of them, like a rug crumpled against a wall creating a wave that may rise up to 30 feet before hitting the shore.  Although greatly slowed, a tsunami still bursts onto land at speeds of around 35 miles per hour, with enough momentum to flatten buildings and trees and to carry ships miles inland." (Folger, 1994)

 

The areas facing the greatest risk from the tsunami hazard are those populated centers that lie within one mile of the coastline and rise less than 50 feet above sea level.  It is in these areas that public education and planning for tsunamis has been focused.  Misinformation about tsunamis can be deadly, as has been exhibited when people have fled an initial tsunami wave of a series, only to be killed upon returning too soon by successive waves that followed.  Strange phenomena that precede a tsunami, such as the ocean receding for 100s of feet exposing the ocean floor, have resulted in the death of misinformed citizens who ventured out to explore, only to be drowned in a sudden return of water height.

 

The following list provides a small sample of the range of tsunami experiences that have occurred within the United States and Canada:

 

  • In 1964, an Alaskan earthquake generated a tsunami with waves between 10 and 20 feet high along parts of the California, Oregon, and Washington coasts. This tsunami caused more than $84 million in damage in Alaska and a total of 123 fatalities.
  • Although tsunamis are rare along the Atlantic coastline, a severe earthquake on November 18, 1929, in the Grand Banks of Newfoundland generated a tsunami that caused considerable damage and loss of life at Placentia Bay, Newfoundland.
  • In 1946, a tsunami with waves of 20 to 32 feet crashed into Hilo, Hawaii, flooding the downtown area and killing 159 people.

 

Most deaths during a tsunami are a result of drowning. Other risks associated with the tsunami hazard include flooding, polluted water supplies, destruction of crops, business interruption, loss of infrastructure (roads, electrical lines, etc.), and damaged gas lines.  Since 1945, more people have been killed as a result of tsunamis than as a direct result of an earthquake’s ground shaking.

 

Presently, the National Oceanic &Atmospheric Administration (NOAA) participates in the Tsunami Warning System, operating two Tsunami Warning Centers. The Alaska/West Coast Tsunami Warning Center (ATWC) in Palmer, Alaska, serves as the regional Tsunami Warning Center for Alaska, British Columbia, Washington, Oregon, and California. The Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawaii, serves as the regional Tsunami Warning Center for Hawaii and as a national/international warning center for tsunamis that pose a Pacific-wide threat. (NTHMP, 2003)

 

An important part of the effort to reduce the impacts of tsunamis in these high-risk areas has been public education and community preparedness.  Early efforts included the identification and marking of public evacuation routes, teaching supplies provided to schools, and literature distributed to the population at large.  However, a more comprehensive program was needed, and the NOAA National Weather Service (NWS) developed the TsunamiReady program to address this need.

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