Generation. Tsunamis are most commonly generated by earthquakes in marine and coastal regions. Major tsunamis are produced by large (greater than 7 on the Richer scale), shallow focus (< 30km depth in the earth) earthquakes associated with the movement of oceanic and continental plates. They frequently occur in the Pacific, where dense oceanic plates slide under the lighter continental plates. When these plates fracture they provide a vertical movement of the seafloor that allows a quick and efficient transfer of energy from the solid earth to the ocean. When a powerful earthquake (magnitude 9.3) struck the coastal region of Indonesia in 2004, the movement of the seafloor produced a tsunami in excess of 30 metres (100 feet) along the adjacent coastline killing more than 240,000 people. From this source the tsunami radiated outward and within 2 hours had claimed 58,000 lives in Thailand, Sri Lanka, and India.

Underwater landslides associated with smaller earthquakes are also capable of generating destructive tsunamis. The tsunami that devastated the northwestern coast of Papua New Guinea on July 17, 1998, was generated by an earthquake that registered 7.0 on the Richter scale that apparently triggered a large underwater landslide. Three waves measuring more than 7 metres high struck a 10-kilometer stretch of coastline within ten minutes of the earthquake/slump. Three coastal villages were swept completely clean by the deadly attack leaving nothing but sand and 2,200 people dead. Other large-scale disturbances of the sea -surface that can generate tsunamis are explosive volcanoes and asteroid impacts. The eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883 produced a 30-metre tsunami that killed over 36,000 people. In 1997, scientists discovered evidence of a 4km diameter asteroid that landed offshore of Chile approximately 2 million years ago that produced a huge tsunami that swept over portions of South America and Antarctica.

Tsunami ('soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. Tsunamis can travel up to 800-900 kilometres per hour. Tsunami waves move outwards, away from their source. One or more waves can be created per event. Successive peaks can be anywhere from five to ninety minutes apart. The wave train that reaches the coast can range from 30 metre high breakers to barely noticed ripples - but in the open ocean, tsunami's have relatively small heights

Q. How do Tsunami's appear

Tsunami are actually waves caused by sudden movement of the ocean due to earthquakes, landslides on the sea floor, land slumping into the ocean, major volcanic eruptions or large meteorite impacts.

Most tsunami are caused by large earthquakes at the seafloor, when large slabs of rock are forced to move past each other suddenly causing the overlying water to move. The resulting wave moves outwards and away from this event.

Underwater landslides can cause tsunami, and so can land which slumps into the ocean. Landslides happen when slopes become too steep to withstand gravity.

Less common are tsunami initiated by volcanic eruptions. These occur in several ways:

• if an underwater volcano erupts, the hot lava may heat the surrounding water quickly and explosively.
• massive flows of volcanic debris such as ash can travel down the side of a volcano and into the ocean, pushing water outwards.
• the top of an underwater volcano may collapse downwards, so that the overlying water also drops.

Large meteorite impacts that occur at sea can also trigger tsunami.

Q. How is a tsunami wave different from a normal wave?

The waves you see at the beach are generated by wind blowing over the sea surface.  The size of these waves depends on the strength of the wind creating them and the distance over which it blows.  Generally the distance between these waves, known as the wavelength, ranges from under a metre (a couple of feet) to perhaps 300 metres (1,000 ft.).   The speed of these waves as they travel across the ocean ranges from a few kilometres an hour up to 100 kilometres an hour (60 miles an hour) in some instances.
Tsunami waves resulting from physical mechanisms behave much differently than wind generated waves.  The magnitude of the disturbance causing the tsunami is the primary factor influencing the size and strength of the waves.   The height of the wave when it is generated is very small, usually less than a a metre or two (a few feet).  The distance between successive wave crests or the wavelength however, is much larger than that of a normal wave and may be hundreds of kilometres apart.  Depending on the depth of the water in which the tsunami is traveling, it may attain speeds of up to 800 kilometres per hour (500 miles per hour).

Q. How does a Tsunami behave as it approaches land?

When the waves of a tsunami approach land, their appearance and behavior become dependent on several local factors. Two of the most important factors are the topography of the sea floor (bathymetry) and the actual shape of the shoreline. As a tsunami encounters shallow waters surrounding the shoreline, its height can increase from a metre (3 ft.) or less to over 15 metres (50 ft.). Wave heights can also be increased when concentrated on headlands or when traveling into bays having wide entrances that become progressively more narrow. The presence of an offshore coral reef can dissipate the energy of a tsunami, decreasing the impact on the shoreline. Normal wind swell may ride atop of a tsunami wave thereby increasing wave height.
The image most people have of a tsunami is a large, steep wave breaking on the shore. This image is seldom the case. Most tsunamis appear as an advancing tide without having a developed wave face, resulting in rapid flooding of low-lying coastal areas. Sometimes, a bore can form during which an abrupt front of whitewater will rapidly advance inland much similar to the tidal bore formed at the mouth of large rivers.
Another event that may result from a tsunami is a standing wave or seiche. A seiche occurs in bodies of water that are partially or completely enclosed, creating a standing wave that continually sloshes back and forth. The appearance of a seiche would be very similar to what happens when you place a glass of water on the table; the water rocks back and forth before settling. When a seiche is generated by a tsunami, subsequent tsunami waves may arrive in unison with a seiche resulting in an increase in the drawdown in sea level and a dramatic increase in wave height. Seiche waves may continue several days after a tsunami.

Q. What is "Run up" and inundation

When a tsunami approaches a coastline, the wave begins to slow down and increase in height, depending on the topography of the sea floor.  Often the first signs of a tsunami are a receding water level caused by the trough of the wave.  In some instances though, a small rise in the water level just before the recession, has been observed.  Regardless, the incoming wave approaches much like the incoming tide though on a much faster scale.  The maximum vertical height to which the water is observed with reference to sea level is referred to as run-up.  The maximum horizontal distance that is reached by a tsunami is referred to as inundation.

Q. What is the "Wrap-Around" effect?

Whether a tsunami is generated in the North or South Pacific, it has the potential to affect all shores of Pacific Islands.  As large tsunami waves approach the islands, they may refract or bend around the islands and diffract through the channels between the islands as well.  The ability of a tsunami wave to bend around and through the islands is called the wrap-around effect.  During the wrap-around effect, the energy of the tsunami often decreases resulting in smaller wave heights.  Sometimes tsunami waves will reflect off of a land mass instead of bending around, thereby increasing wave height of the approaching wave.  Therefore, whether a tsunami warning is issued from an earthquake in Chile, Alaska, or Japan, inhabitants along all shores of the Islands should take the necessary precautions..

Q. How are Tsunami wave heights measured.

The wave height of a tsunami can be highly variable in a local area depending on the underwater topography, orientation to the oncoming wave, the tidal level and the magnitude of the tsunami.  Because direct physical measurement of a tsunami wave would be a life threatening event, a common method for determining tsunami wave height is by measuring the runup, the highest vertical point reached by the wave.  Runup heights are measured by looking at the distance and extent of salt-killed vegetation, and the debris left once the wave has receded.  This distance is referenced to a datum level, usually being the mean sea level or mean lower low water level.  The reference to mean lower low water is more significant in areas with greater tidal ranges such as in Alaska where a smaller tsunami wave can be more devastating during a high tide than a larger wave at low tide. Tide gauges, found in most harbours, are the other tool for measuring tsunami wave height and period (wavelength).

Q. Do all oceans have Tsunami's, and what is the Ring of Fire.

Yes.  Tsunamis have been recorded in all the major oceans of the world. However, this phenomenon is mainly seen in the Pacific and the Indian oceans. The Pacific basin is an area surrounded by volcanic island arcs, mountain chains and subduction zones earning the nickname the "ring of fire" and is the most geologically active area on the planet.  The amount of activity in this region makes it much more susceptible to submarine faulting and subsequent tsunami events. The east side of the Indian ocean is also a very active seismic region. The Atlantic and other oceans and seas are far less geologically active, with some exceptions and therefore the occurrence of tsunamis is rare.

What to do when a Tsunami WARNING is issued

• Listen to radio or television, Coast Guard emergency frequency station, Civil Defence, 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.

What to do after a Tsunami

• Continue listening to the radio, Coast Guard emergency frequency station, or other reliable source for emergency information. The tsunami may have damaged roads, bridges, or other places that may be unsafe.
• Help injured or trapped persons. Give first aid where appropriate. Call for help. Do not move seriously injured persons unless they are in immediate danger of further injury.
• Help a neighbor who may require special assistance--infants, elderly people, and people with disabilities. Elderly people and people with disabilities may require additional assistance. People who care for them or who have large families may need additional assistance in emergency situations.
• Use the telephone only for emergency calls. Telephone lines are frequently overwhelmed in disaster situations. They need to be clear for emergency calls to get through.
• Stay out of buildings if waters remain around them. Tsunami waters, like flood waters, can undermine foundations, causing buildings to sink, floors to crack, or walls to collapse.
• When re-entering buildings or homes, use extreme caution. Tsunami-driven flood waters may have damaged buildings where you least expect it. Carefully watch every step you take.
• Wear sturdy shoes. The most common injury following a disaster is cut feet.
• Use battery-powered lanterns or flashlights when examining buildings. Battery-powered lighting is the safest and easiest, preventing fire hazard for the user, occupants, and building.
• Examine walls, floors, doors, staircases, and windows to make sure that the building is not in danger of collapsing.
• Inspect foundations for cracks or other damage. Cracks and damage to a foundation can render a building uninhabitable.
• Look for fire hazards. There may be broken or leaking gas lines, flooded electrical circuits, or submerged furnaces or electrical appliances. Flammable or explosive materials may come from upstream. Fire is the most frequent hazard following floods.
• 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 using 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 burning 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, call an electrician first for advice. Electrical equipment should be checked and dried before being returned to service.
• Check for sewage and water line damage. If you suspect sewage lines are damaged, avoid using the toilets and call a plumber. If water pipes are damaged, contact the water company and avoid using water from the tap. You can obtain safe water from undamaged water heaters or by melting ice cubes.
• Use tap water if local health officials advise it is safe.
• Watch for loose plaster, drywall, and ceilings that could fall.
• Take pictures of the damage, both of the building and its contents, for insurance claims.
• Open the 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. Any food that has come in contact with flood waters may be contaminated and should be thrown out.

Produced by the National Disaster Education Coalition: American Red Cross, FEMA, IAEM, IBHS, NFPA, NWS, USDA/CSREES, and USGSThis information is in the public domain and is intended to be used and shared without copyright restrictions.From: Talking About Disaster: Guide for Standard Messages. Produced by the National Disaster Education Coalition, Washington, D.C., 1999.