Tsunami
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tsu·na·mi
a great sea wave produced especially by submarine earth movement or volcanic eruption
"Tsunami" is a series of traveling ocean waves of extremely long length generated by disturbances associated primarily with earthquakes occurring below or near the ocean floor. Underwater volcanic eruptions and landslides can also generate tsunamis. The length from wave crest to wave crest can be hundred of miles in deep oceans but as they near land they become closer have a wave height of only a few feet or less. Normally, they can't be felt aboard ships or can they be seen from the air in the open ocean. In deep water, the waves may reach speeds exceeding 500 miles per hour.
Tsunamis are a threat to life and property to anyone living near the ocean. Large tsunamis have been known to rise over a 100 feet but even 10 to 20 feet ones can be very destructive. Many deaths and widespread damage and destruction have been caused by tsunamis.
The Tsunami Warning System (TWS), consist of 26 participating international Member States that monitor seismological and tidal stations throughout the Pacific Basin. The System evaluates potentially tsunamigenic earthquakes and disseminates tsunami warning information. The Pacific Tsunami Warning Center (PTWC) is the operational center of the Pacific TWS. Located in Honolulu, Hawaii, PTWC provides tsunami warning information to national authorities in the Pacific Basin.
NOAA has primary responsibility for providing tsunami warnings to the Nation, and a leadership role in tsunami observations, research. Blue boxes in the diagram indicate NOAA responsibilities.
The first part of a tsunami's to reach the shore is called the trough. The trough is the low point beneath the wave's crest. It produces a vacuum effect that sucks coastal water seaward and exposes the sea floor. The retreating of sea water is an important warning sign of a tsunami because the wave's crest is only minutes away. A tsunami is usually composed of a series of waves called a wave train. If you experience a tsunami, you should remember that the danger has not passed with the first wave. You should wait for official word that it is safe to return to vulnerable locations.
Some tsunamis do not present as massive wave but instead they resemble a quickly surging tide that inundates the coastal area. The best defence is an early warning that allows people to seek higher ground.
Where do tsunami occur?
Most tsunami occur in the Pacific and Indian Oceans. The Ring of Fire is the boundary of the Pacific Ocean where frequent earthquakes occur. The Indian Ocean also has subduction zones that can generate tsunamis. Subduction zone earthquakes are the most common source of destructive tsunami. These earthquakes are generated when to tectonic plates meet and one goes under the other. The sinking plate drags against the upper plate causing flexing. The stress on the plate boundary results in the upper plate rebounding to its original position, thus displacing sea water above.
How big is a tsunami?
In the open ocean it can be difficult to notice a tsunami wave but when the wave approaches land and movies into shallow water. This causes the leading edge of the wave to slow down but the back of the wave is still travelling at its original speed. This causes the water to bunch up and increases the wave height. When it reaches land, it behaves like a series of breaking wave or one large powerful wave.
The energy of the wave can be so tremendous that it causes great quantities of water to surge inland, far beyond where even the highest of tides would commonly reach.
Some of the largest tsunami waves have been generated by the volcanic eruption of Krakatoa in 1883. This tsunami reached a height of 37 m. In 1737, a tsunami was estimated to be 64 m high as it struck Cape Lopatka in northeast Russia.
Tsunami waves are different from normal waves
Normal Waves are generated by the wind and the only water near the surface is moving. Tsunami are different from normal waves. With a tsunami all the water from the surface to the sea bed is moving and it is generated by something that has displaced the water like an earthquake. In the open ocean, a tsunami creates little movement and little threat to shipping.
When a tsunami reaches the shore, its wavelength might be more than 100 km and it can last for several hours or even days depending on the location.
The energy behind a tsunami can strip sand off beaches, undermine trees, and crush buildings. People and boats are powerless against the force of a tsunami. The amount of water carried inland is capable of inundating vast areas of normally dry land.
Know Your Tsunami Terms
Familiarize yourself with these terms to help identify a tsunami hazard:
Advisory
An earthquake occurred in the Pacific basin, which might generate a tsunami and produce strong currents or waves dangerous to those in or near the water. Coastal regions historically prone to damage due to strong currents induced by tsunamis are at the greatest risk. The threat may continue for several hours after the arrival of the initial wave, but significant widespread inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.
Information Statement
An earthquake occurred or a tsunami watch, advisory, or warning was issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory, or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.
Warning
A potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.
Watch
A tsunami was or may have been generated, but is at least two hours travel time to the area in watch status. The watch area may be upgraded to an advisory or warning or canceled based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.
Tsunami Myths and Facts
SOME FACTS
- Tsunamis that strike coastal locations in the Pacific Ocean Basin are almost always caused by earthquakes. These earthquakes might occur far away or near where you live.
- Some tsunamis can be very large. In coastal areas their height can be as great as 10 m or more (30 m in extreme cases), and they can move inland several hundred meters.
- All low lying coastal areas can be struck by tsunamis.
- A tsunami consists of a series of waves with crests arriving every 10 to 60 minutes. Often the first wave may not be the largest. The danger from a tsunami can last for several hours after the arrival of the first wave. Tsunami waves typically do not curl and break, so do not try to surf a tsunami!
- Tsunamis can move faster than a person can run.
- Sometimes a tsunami initially causes the water near shore to recede, exposing the ocean floor.
- The force of some tsunamis is enormous. Large rocks weighing several tons, along with boats and other debris, can be moved inland hundreds of meters by tsunami wave activity, and homes and buildings destroyed. All this material and water move with great force, and can kill or injure people.
- Tsunamis can occur at any time, day or night.
- Tsunamis can travel up rivers and streams from the ocean.
- Tsunami can easily wrap around islands and be just as dangerous on coasts not facing the source of the tsunami.