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. 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
American Red Cross chapter. If you are in a tsunami risk area, learn how to protect yourself, your family, and your property.
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:
• Drop, cover, and hold on. You should first protect yourself from the earthquake.
• When the shaking stops, gather your family members and evacuate quickly. Leave everything else behind. A tsunami may be coming
within minutes. Move quickly to higher ground away from the coast.
• Be careful to avoid downed power lines and stay away from buildings and bridges from which heavy objects might fall during an aftershock.
What to Do When a Tsunami WATCH Is Issued
• Listen to a NOAA Weather Radio, Coast Guard emergency frequency station, or other reliable source for updated emergency
information. As the energy of a tsunami is transferred through open water, it is not detectable. Seismic action may be the only advance
warning before the tsunami approaches the coastline.
• Check your Disaster Supplies Kit. Some supplies may need to be replaced or restocked.
• Locate family members and review evacuation plans. Make sure everyone knows there is a potential threat and the best way to
• If you have special evacuation needs (small children, elderly people, or persons with disabilities) consider early evacuation. Evacuation may take longer, allow extra time.
• If time permits, secure unanchored objects around your home or business. Tsunami waves can sweep away loose objects. Securing these items or moving them inside will reduce potential loss or damage.
• Be ready to evacuate. Being prepared will help you to move more quickly if a tsunami warning is issued.
What to Do When a Tsunami WARNING Is Issued
• Listen to a NOAA Weather Radio, Coast Guard emergency frequency station, or other reliable source for updated emergency information. Authorities will issue a warning only if they believe there is a real threat from tsunami.
• Follow instructions issued by local authorities. Recommended evacuation routes may be different from the one you use, or you may be
advised to climb higher.
If you are in a tsunami risk area, do the following:
• If you hear an official tsunami warning or detect signs of a tsunami, evacuate at once. A tsunami warning is issued when authorities are certain that a tsunami threat exists, and there may be little time to get out.
• Take your Disaster Supplies Kit. Having supplies will make you more comfortable during the evacuation.
• Get to higher ground as far inland as possible. Officials cannot reliably predict either the height or local effects of tsunamis. Watching a tsunami from the beach or cliffs could put you in grave danger. If you can see the wave, you are too close to escape it. 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.