Tsunamis

What Are They?

A tsunami is a series of ocean waves generated by a sudden displacement in the seafloor from events such as earthquakes, volcanic activity, or landslides (www.tsunami.noaa.gov). These seafloor disturbances cause vast amounts of water to be displaced. A tsunami is one of nature’s most dangerous natural hazards, having killed and injured thousands of people in the last decade alone and causing destruction in many coastal areas around the world.

Tsunamis have no “season,” and a tsunami can strike at any time of day or night, in good weather or bad. When a tsunami reaches shore, a series of waves can flood inland over a period of several hours. Tsunamis can be generated far away and take many hours to reach the shore, or they can be generated locally and reach the shore in minutes.

While most tsunamis occur in the Pacific because large earthquakes and geologic activity within the region are frequent, the Caribbean Islands, Southeast, and Gulf of Mexico coastlines have also experienced tsunamis in the past. View a map of tsunami-prone areas. Earthquake occurrences in these areas could generate local tsunamis, affecting adjacent coastlines, but could also generate distant tsunamis that travel across the ocean, Gulf of Mexico, or Caribbean Sea. Read this document for more information.

Tsunami waves may travel at the speed of a jet plane (over 500 miles per hour) in the deep ocean, but someone on a boat in deep water might not even notice tsunami waves passing under them.

As tsunami waves approach shore, they begin to slow down and build up in height. The shape of the wave is also changed by the depth of the water and the slope of the coast.

In some places the waves may come ashore as a huge flood going inland for hundreds of yards, or even miles, as was the case during the 2011 Japanese tsunami. In other places, there may be very little flooding at all. Bays, harbors, coves, and the mouths of rivers are more likely to have greater flooding than steep shoreline areas with rocky cliffs. Tsunami waves can also wrap completely around an island creating dangerous conditions even on the far side from which the tsunami is approaching.

Studying the history of past events helps us to know which areas are safe from tsunami waves and which areas are dangerous and should be evacuated. Tsunamis can be deadly; understanding them and learning what to do when one strikes will help communities be more resilient to their impacts.

Tsunami Evacuation Zones

Tsunami evacuation zones are created by evaluating how far tsunami waves have come inland in the past and how far they may come inland in the future using likely earthquake scenarios. Evacuations zones are also determined by using tsunami modeling. Tsunami evacuation zones often include rivers and streams because tsunami waves can be funneled up rivers and streams near the coast, making them unsafe.

Everyone who lives, works, attends school, or vacations in a tsunami evacuation zone should have a tsunami evacuation plan if a tsunami should approach. People should also be aware of tsunami evacuation plans for family members within the tsunami evacuation zone (e.g., school children).

What to Do Before, During, and After a Tsunami

Before a Tsunami

Tsunamis can happen at any time. It is important to have necessary supplies to last for several days after the event. Stores may not have food, and fresh water might be scarce. Keep emergency supplies in a backpack or container that can easily be carried. In a locally generated tsunami, you may have only minutes to get to high ground, so be prepared ahead of time.

During a Tsunami Threat

When a tsunami warning is issued, turn on local radio or television stations for more information on when to evacuate the tsunami evacuation zones. For locally generated tsunamis there may not be time for sirens or issued warnings. Warning signs will come from natural surroundings: an earthquake longer than 20 seconds, sea water suddenly retreating, or a roaring sound coming from the ocean. If any of those occur, evacuate immediately. Always evacuate inland or to higher ground. Avoid rivers and streams, since tsunami waters could funnel into the channels.

After the Danger Has Passed

A tsunami is never just one wave. There is always a series of waves with danger that can last for several hours. The first wave may not be the largest, so do not think it is safe to go back to the shore just because the first wave has passed. The first wave may clear the path for additional waves to move farther inland. Always wait for an official message stating that it is safe before returning to a coastal area. When out in the deep ocean in a boat, do not try to come to shore; instead, stay out at sea and wait for an official message before returning to port.

If surfing, get ashore as quickly as possible. Never try to surf a tsunami. Tsunami waves are not surfing waves, since there is no wave face to surf, and tsunami waves are full of debris and cause dangerous currents.

Resources

National Tsunami Hazard Mitigation Program

NOAA Center for Tsunami Research

NOAA National Weather Service’s Tsunami.gov website

Historic tsunami events in Hawaii

Surviving a Tsunami – Lessons from Chile, Hawaii, and Japan