United States Search and Rescue Task Force

Tsunamis


Tsunami, also known as seismic sea waves, are caused by sudden changes in the seafloor, generally earthquakes and more rarely large landslides.  Tsunami are sometimes mistakenly called "tidal waves", but they are not caused by tidal action.  Not all earthquakes are tsunami-genic (generate tsunami); to generate a tsunami, the earthquake must occur under or near the ocean, be large, and create vertical movements of the seafloor.  It is thought that tsunami-genic earthquakes release their energy over a couple of minutes, much more slowly than the sudden lurching earthquakes, which release their energy in seconds.  In fact, some tsunami-genic earthquakes can not be felt by people, so gradual is their energy release.  Much of the earthquake's energy, which can be equivalent to many atomic bombs, is transferred to the water column above it, producing a tsunami.  All oceanic regions of the world can experience tsunami, but the Pacific Ocean is especially vulnerable because of the many large earthquakes associated with the "Ring of Fire" along its margins.

In the deep ocean, tsunami have very small amplitudes (wave heights are only a few inches), wavelengths of up to 1000 kilometers, and speeds of more than 800 kilometers per hour (500 miles per hour), the speed of a jetliner.   The slope of a tsunami surface at sea is only about a centimeter per kilometer (an inch per mile).  A tsunami may take 4-6 hours to reach Hawaii from the Aleutian Islands, 7-8 hours from Japan, and 14-15 hours from Chile, but its energy will only dissipate slightly as it crosses the entire ocean.  In fact, once the tsunami reaches the other side of the ocean thousands of kilometers from its source, it can bounce off the land and return in the direction it came, although its energy will decrease from the reflection.  It is easy to see that at these scales the Pacific Ocean becomes like a pond to the tsunami.

A tsunami carries an enormous amount of energy that is spread over a large volume of water in the deep sea.  However, when a tsunami reaches shallow water, such as a coastline, the energy is concentrated into a smaller volume and the wave's power overwhelms whatever is in its path.  In shallow water, its speed decreases and its amplitude increases to dangerous heights, sometimes 50 feet or higher, and it spreads inland many hundreds of feet (in some cases a mile or more).  A tsunami is not a single wave, but a set that may last for several hours, and the first wave is not always the largest.

How do they form?

Tsunamis are formed as a result of earthquakes, volcanic eruptions, or landslides that occur under the sea.   When these events occur under the water, huge amounts of energy are released as a result of quick upward bottom movement.  For example, if a volcanic eruption occurs, the ocean floor may very quickly move upward several hundred feet.  When this happens, huge volumes of ocean water are pushed upward and a wave is formed.  A large earthquake can lift thousands of square kilometers of sea floor which will cause the formation of huge waves.  The Pacific Ocean is especially prone to tsunamis as a result of the large amount of undersea geological activity.

How big do they get?

In the open ocean tsunamis may appear very small with a height of less than 1 meter (3 feet). Tsunamis will sometimes go undetected until they approach shallow waters along a coast. These waves have a very large wavelength (up to several hundred miles) that is a function of the depth of the water where they were formed.  Although these waves have a small height, there is a tremendous amount of energy associated with them.  As a result of this huge amount of energy, these waves can become gigantic as they approach shallow water.  Their height, as they crash upon the shore, depends on the underwater surface features.   They can be as high as 30 m (100 feet) or more.  In 1737 , a huge wave estimated to be 64m (210 feet) in height hit Cape Lopatka, Kamchatka (NE Russia).   The largest Tsunami ever recorded occurred in July of 1958 in Lituya Bay, Alaska.   A huge rock and ice fall sent water surging up to a high water mark of 500m (1640 feet).  It's no wonder that these waves can cause such massive destruction and loss of life.

How fast do they move?

In the deep open sea, tsunamis move at speeds approaching a jet aircraft (500 mph or more).  As they approach the shore, they slow down.  When a tsunami arrives at the shore, it usually does so as a rapidly rising tide moving at about 70 km/hour (45 mph).


How much destruction do they cause?

Beyond the tremendous destruction of life that tsunamis cause, they have also caused massive physical damage.   They have entirely destroyed buildings and left towns looking like a nuclear war zone.  They have lifted boats high out of the water and violently hurled them against the shore, smashing them to pieces.  They have bent parking meters all the way down to the ground.  In one incredible story, during the huge tsunami in Lituya Bay, Alaska (mentioned above), a boat with two people in it was carried from the bay, over tree tops and over the land out into the ocean.  The people survived to tell the tale.

Can we detect them before they hit?

Yes.  About 35 years ago, 24 countries around the Pacific set up the Pacific Tsunami Warning System.  A group of seismic monitoring stations and a network of tide gauges are used for detection.   The biggest problem with this system is that it is difficult to predict how large and destructive the resulting waves will be.  Scientists are currently working on better predictive tools.

When you hear a tsunami warning, move at once to higher ground and stay there until local authorities say it is safe to return home.

BEFORE

Find out if your home is in a danger area.
Know the height of your street above sea level and the distance of your street from the coast. Evacuation orders may be based on these numbers.

Be familiar with the tsunami warning signs.
Because tsunamis can be caused by an underwater disturbance or an earthquake, people living along the coast should consider an earthquake or a sizable ground rumbling as a warning signal.  A noticeable rapid rise or fall in coastal waters is also a sign that a tsunami is approaching.

Make sure all family members know how to respond to a tsunami.

Make evacuation plans.
Pick an inland location that is elevated.  After an earthquake or other natural disaster, roads in and out of the vicinity may be blocked, so pick more than one evacuation route.

Teach family members how and when to turn off gas, electricity, and water.

Teach children how and when to call 9-1-1, police or fire department, and which radio station to listen for official information.

Have disaster supplies on hand.

Develop an emergency communication plan.
In case family members are separated from one another during a tsunami (a real possibility during the day when adults are at work and children are at school), have a plan for getting back together.

Ask an out-of-state relative or friend to serve as the "family contact."  After a disaster, often it's easier to call long distance. Make sure everyone knows the name, address, and phone number of the contact person.

DURING

Listen to a radio or television to get the latest emergency information, and be ready to evacuate if asked to do so.

If you hear an official tsunami warning or detect signs of a tsunami, evacuate at once. Climb to higher ground.  A tsunami warning is issued when authorities are certain that a tsunami threat exists.

Stay away from the beach.
Never go down to the beach to watch a tsunami come in.  If you can see the wave you are too close to escape it.

Return home only after authorities advise it is safe to do so.
A tsunami is a series of waves.  Do not assume that one wave means that the danger over. The next wave may be larger than the first one.  Stay out of the area.

AFTER

Stay tuned to a battery-operated radio for the latest emergency information.

Help injured or trapped persons.
Give first aid where appropriate.  Do not move seriously injured persons unless they are in immediate danger of further injury.  Call for help.

Remember to help your neighbors who may require special assistance--infants, elderly people, and people with disabilities.

Stay out of damaged buildings.  Return home only when authorities say it is safe.

Enter your home with caution.
Use a flashlight when entering damaged buildings. 

Open windows and doors to help dry the building.

Shovel mud while it is still moist to give walls and floors an opportunity to dry.

Check food supplies and test drinking water.

Fresh food that has come in contact with flood waters may be contaminated and should be thrown out.

INSPECTING UTILITIES IN A DAMAGED HOME

Check for gas leaks--If you smell gas or hear a blowing or hissing noise, open a window and quickly leave the building.  Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home.  If you turn off the gas for any reason, it must be turned back on by a professional.

Look for electrical system damage--If you see sparks or broken or frayed wires, or if you smell hot insulation, turn off the electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box or circuit breaker, don't do it!  Wait for professionals.

Check for sewage and water lines damage--If you suspect sewage lines are damaged, avoid using toilets. If water pipes are damaged, contact the water company and avoid the water from the tap.  You can obtain safe water by melting ice cubes.


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Tsunami Destruction

 

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Tsunami Destruction Which Carried This School 200 Feet Before It Locked Up On Surrounding Palm Trees

 


 

TEN DESTRUCTIVE TSUNAMIS