Tsunami

The tsunami that struck Thailand on December 26, 2004.

A tsunami (pronounced /tsuːˈnɑːmi/) is a series of waves created when a body of water, such as an ocean, is rapidly displaced. Earthquakes, mass movements above or below water, volcanic eruptions and other underwater explosions, landslides, underwater earthquakes, large meteoroid or asteroid impacts and testing with nuclear weapons at sea all have the potential to generate a tsunami. The effects of a tsunami are always devastating due to the immense volumes of water and energy involved.

The term tsunami comes from the Japanese meaning literally harbor wave (津波、つなみ).

The Greek historian Thucydides was the first to relate tsunamis to submarine quakes,^{[1] [2]} but understanding of the nature of tsunamis remained slim until the 20th century and is the subject of ongoing research.

Many early geological, geographic, oceanographic etc; texts refer to "Seismic sea waves" - these are now referred to as "tsunami."

Terminology

The term tsunami comes from the meaning harbor ("tsu", 津) and wave ("nami", 波). [a. Jap. tsunami, tunami, f. tsu harbour + nami waves. - Oxford English Dictionary]. For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in Japanese. Tsunamis are common throughout Japanese history; approximately 195 events in Japan have been recorded.

A tsunami has a much smaller amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 mm above the normal sea surface. Tsunamis have been historically labeled tidal waves because as they approach land, they take on the characteristics of a violent onrushing tide rather than the waves that are formed by wind action upon the ocean (with which people are more familiar). Since they are not actually related to tides the term is considered misleading and its usage is discouraged by geologists, oceanographers.

Causes

A tsunami can be generated when converging or destructive plate boundaries abruptly deform and vertically displace the overlying water. Such large vertical movements of the Earth's crust can occur at plate boundaries. Subduction zone related earthquakes are particularly effective in generating tsunami. A tsunami in the 1940's that inundated Hilo, Hawaii, was caused by an earthquake on one of the Aleutian Islands in Alaska. That earthquake was 7.8 on the Richter Scale. The area concerned is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.

Examples of tsunami being generated at locations away from convergent boundaries include - Storegga, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). In the case of the Grand Banks and Papua New Guinea tsunamis an earthquake caused sediments to become unstable and subsequently fail. These slumped and as they flowed down slope a tsunami was generated. These tsunami did not travel transoceanic distances.

It is not known what caused the Storegga sediments to fail. It may have been due to overloading of the sediments causing them to become unstable and they failed solely as a result of being overloaded. It is also possible that an earthquake caused the sediments to become unstable and then fail.

Tsunamis may be generated when an earthquake occurs causing the floor of the ocean to displace the water column - one part "rises" whilst the other part "sinks". This occurs in seconds and huge volumes of sea water have incredibly large levels of energy transferred into them. This energy radiates outwards in a series of low frequency and therefore long wavelength waves which generally have an amplitude of about 300 mm above the normal swell of the ocean and are rarely observed as a result. It should be noted that the displacement form of a trough and a peak transfer away from the zone of activation and will usually come ashore in the same manner. The trough causes "drawback" whilst the peak arrives as a sudden surge.

The Chile earthquake of May 1960 (9.5 Mw), Alaska 1964 (9.2 Mw), Sumatra 2004 (9.2 Mw), are recent examples of powerful earthquakes that generated a tsunami that crossed oceans. Smaller (4.2 Mw) earthquakes in Japan can trigger tsunami that can devastate nearby coasts within 15 minutes or less.

In the 1950s it was hypothesised that larger tsunamis than had previously been believed possible may be be caused by landslides, explosive volcanic action, and impact events when they contact water. These phenomena rapidly displace large volumes of water, as energy from falling debris or expansion is transferred to the water into which the debris falls at a rate faster than the ocean water can absorb it. They have been named by the media as "mega-tsunami."

Tsunami caused by these mechanisms, unlike the ocean-wide tsunami caused by some earthquakes, may dissipate quickly and rarely affect coastlines distant from the source due to the small area of sea affected. These events can give rise to much larger local shock waves (solitons), such as the landslide at the head of Lituya Bay 1958, which produced a wave with an initial surge estimated at 524m. However, an extremely large gravitational landslide might generate a so called "mega-tsunami" that may have the the ability to travel trans-oceanic distances. This though is strongly debated and there is no actual geological evidence to support this hypothesis.

Signs of an approaching tsunami

There is often no advance warning of an approaching tsunami. However, since earthquakes are often a cause of tsunami, an earthquake felt near a body of water may be considered an indication that a tsunami may occur. In Japan moderate - 4.2 Magnitude earthquakes can generate tsunami which can inundate the area within 15 minutes.

If the first part of a tsunami to reach land is a trough (draw back) rather than a crest of the wave, the water along the shoreline may recede dramatically, exposing areas that are normally always submerged. This can serve as an advance warning of the approaching tsunami which will rush in faster than it is possible to run. If a person is in a coastal area where the sea suddenly draws back - many survivors report a sucking sound, their only real chance of survival is to run for high ground or seek the high floors of high rise buildings.

In the 2004 tsunami that occurred in the Indian Ocean drawback was not reported on the African coast or any other western coasts it inundated, when the tsunami approached from the east. This was because of the nature of the wave - it moved downwards on the eastern side of the fault line and upwards on the western side. It was the western pulse that inundated coastal areas of Africa and other western areas.

Tsunamis occur most frequently in the Pacific Ocean, but are possible wherever large bodies of water are found, including inland lakes. They may be caused by landslides, volcanic explosions, bolides and seismic activity.

Warnings and prevention

Image:IMG 5028.jpg Tsunami hazard sign at The Wedge in Balboa Peninsula, Newport Beach, California.

Tsunami wall at Tsu, Japan

A tsunami cannot be prevented or precisely predicted, but there are some warning signs of an impending tsunami, and there are many systems being developed and in use to reduce the damage from tsunami.

In instances where the leading edge of the tsunami wave is the trough, the sea will recede from the coast half of the wave's period before the wave's arrival. If the slope of the coastal seabed is shallow, this recession can exceed many hundreds of meters. People unaware of the danger may remain at or near the shore out of curiosity, or for collecting fish from the exposed seabed. During the Indian Ocean tsunami of 26th December 2004, the sea withdrew and many people then went onto the exposed sea bed to investigate. Pictures taken show people on the normally submerged areas with the advancing wave in the background. Most people who were on the beach were unable to escape to high ground and died.

Tsunami warning sign on seawall in Kamakura, Japan, 2004. In the Muromachi period, a tsunami struck Kamakura, destroying the wooden building that housed the colossal statue of Amida Buddha at Kotokuin. Since that time, the statue has been outdoors.

Regions with a high risk of tsunami may use tsunami warning systems to detect tsunami and warn the general population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs advise people of evacuation routes.

The Pacific Tsunami Warning System is based in Honolulu. It monitors all sesimic activity that occurs anywhere within the Pacific. Based up the magnitude and other information a tsunami warning may be issued. It is important to note that the subduction zones around the Pacific are seismically active, but not all earthquakes generate tsunami and for this reason computers are used as a tool to assist in analysing the risk of tsunami generation of each and every earthquake that occurs in the Pacific Ocean and the adjoining land masses.

Computer models can predict tsunami arrival - observations have shown that predicted arrival times are usually within minutes of the predicted time. Bottom pressure sensors are able to relay information in real time and based upon the readings and other information about the seismic event that triggered it and the shape of the seafloor (bathymetry) and coastal land (topography), it is possible to estimate the amplitude and therefore the surge height, of the approaching tsunami. All the countries that border the Pacific Ocean collaborate in the Tsunami Warning System and most regularly practice evacuation and other procedures to prepare people for the inevitable tsunami. In Japan such preparation is a mandatory requirement of government, local authorities, emergency services and the population.

Animals may have the ability to detect an incoming tsunami as well as other natural phenomena. However, the evidence is controversial and has not been proven scientifically. There are some unsubstantiated claims that animals before the Lisbon quake were restless and moved away from low lying areas to higher ground. Yet many other animals in the same areas drowned. The phenomenon was also noted in Sri Lanka in the 2004 Indian Ocean earthquake. [1] Some scientists speculate that animals may have an ability to sense subsonic Rayleigh waves from an earthquake minutes or hours before a tsunami strikes shore (Kenneally, [2]). It is possible that the certain animals (e.g., elephants) heard the sounds of the tsunami as it approached the coast. The elephants reaction was to move away from the approaching noise - inland. Some humans, on the other hand, went onto the shore to investigate.

While it is not possible to prevent a tsunami, in some particularly tsunami-prone countries some measures have been taken to reduce the damage caused on shore. Japan has implemented an extensive programme of building tsunami walls of up to 4.5 m (13.5 ft) high in front of populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunami. However, their effectiveness has been questioned, as tsunami are often higher than the barriers. For instance, the Okushiri, Hokkaidō tsunami which struck Okushiri Island of Hokkaidō within two to five minutes of the earthquake on July 12, 1993 created waves as much as 30 m (100 ft) tall - as high as a 10-story building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami, but it did not prevent major destruction and loss of life. [3]

The effects of a tsunami may be mitigated by natural factors such as tree cover on the shoreline. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed as a result of the tsunami's energy being absorbed by trees such as coconut palms and mangroves. In one striking example, the village of Naluvedapathy in India's Tamil Nadu region suffered minimal damage and few deaths as the wave broke up on a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records. [4] Environmentalists have suggested tree planting along stretches of seacoast which are prone to tsunami risks. It would take some years for the trees to grow to a useful size, but such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than the construction of artificial barriers.

Tsunami in History

Main article: Historic tsunami

Historically speaking, tsunami are not rare, with at least 25 tsunami occurring in the last century. Of these, many were recorded in the Asia-Pacific region - particularly Japan. The Boxing Day Tsunami in 2004 caused approx. 350,000 deaths and many more injuries.

As early as 426 B.C. the Greek historian Thucydides inquired in his book History of the Peloponnesian War about the causes of tsunami, and argued rightly that it could only be explained as a consequence of ocean earthquakes.^[1] He was thus the first in the history of natural science to correlate quakes and waves in terms of cause and effect:^[2]

The cause, in my opinion, of this phenomenon must be sought in the earthquake. At the point where its shock has been the most violent the sea is driven back, and suddenly recoiling with redoubled force, causes the inundation. Without an earthquake I do not see how such an accident could happen.^[3]

The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15-19) describes the typical sequence of a tsunami including an incipient earthquake, the sudden retreat of the sea and a following gigantic wave on the occasion of the 365 A.D. tsunami devastating Alexandria.^{[4] [5]}

See also

Footnotes

References

• Iwan, W.D., editor, 2006, Summary report of the Great Sumatra Earthquakes and Indian Ocean tsunamis of 26 December 2004 and 28 March 2005: Earthquake Engineering Research Institute, EERI Publication #2006-06, 11 chapters, 100 page summary, plus CD-ROM with complete text and supplementary photographs, EERI Report 2006-06. [www.eeri.org] ISBN 1-932884-19-X
• Dudley, Walter C. & Lee, Min (1988: 1st edition) Tsunami! ISBN 0-8248-1125-9 link
• Kenneally, Christine (December 30 2004). "Surviving the Tsunami". Slate. link
• Macey, Richard (January 1 2005). "The Big Bang that Triggered A Tragedy", The Sydney Morning Herald, p 11 - quoting Dr Mark Leonard, seismologist at Geoscience Australia.
• Lambourne, Helen (March 27 2005). "Tsunami: Anatomy of a disaster". BBC News. link
• abelard.org. tsunamis: tsunamis travel fast but not at infinite speed. Website, retrieved March 29 2005. link
• The NOAA's page on the 2004 Indian Ocean earthquake and tsunami
• Tappin, D; 2001. Local tsunamis. Geoscientist. 11-8, 4-7.

External links

Wikimedia Commons has media related to:

Tsunami

Articles and websites

• How to survive a tsunami - Guide for children and youth
• USGS: Surviving a tsunami (United States)
• Tsunami database with detailed statistics
• NOVA: Wave That Shook The World — Site and special report shot within days of the 2004 Indian Ocean tsunami.
• NOAA Tsunami — General description of tsunamis and the United States agency NOAA's role
• Can HF Radar detect Tsunamis? — University of Hamburg HF-Radar.
• The International Centre for Geohazards (ICG)
• ITSU — Coordination Group for the Pacific Tsunami Warning System.
• Pacific Tsunami Museum
• Tsunamis and Earthquakes
• Tsunami Centers — United States National Weather Service.
• Tsunami Warning — Tsunami warnings via mobile phone.
• Science of Tsunami Hazards journal
• Envirtech Tsunami Warning System — Based on seabed seismics and sea level gauges.
• Scientific American Magazine (January 2006 Issue) Tsunami: Wave of Change What we can learn from the Indian Ocean tsunami of December 2004.
• Jakarta Tsunami Information Centre

Images and video

See also: Images and video, 2004 Indian Ocean earthquake

• [5] 5 Amateur Camcorder Video Streams of the December 26 2004 tsunami that hit Sri Lanka, Thailand and Indonesia (search on tsunamis).
• 2004 Asian Tsunami Satellite Images (Before and After)
• Satellite Images of Tsunami Affected Areas High resolution satellite images showing the effects of the 2004 tsunami on the affected areas in Indonesia, Thailand and Nicobar island of India.
• Computer-generated animation of a tsunami
• Animations of actual and simulated tsunami events from the NOAA Center for Tsunami Research
• Animation of 1960 tsunami originating outside coast of Chile
• The Survivors - A moving travelogue full of stunning images along the tsunami ravaged South-Western Coast of India [Unavailable]
• Origin of a Tsunami - animation showing how the shifting of continental plates in the Indian Ocean created the catastrophe of December 26th 2004.
• CBC Digital Archives – Canada's Earthquakes and Tsunamis
• Tsunami Aftermath in Penang and Kuala Muda, Kedah.
• [6] Amateur photo Thailand Tsunami 2004
• Photos and Videos of Humanitarian Assistance to Tsunami-hit areas by the Singapore Armed Forces

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