Tsunami
tsunami (pronounced /(t)suːˈnɑːmi/) 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, some volcanic eruptions and other underwater explosions, landslides, underwater earthquakes, large asteroid impacts and detonation of nuclear weapons at sea all have the potential to generate a tsunami. Due to the immense volumes of water and energy involved, the effects of a tsunami can be devastating. Since meteorites are small, they will not generate a tsunami.
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".
Some meteorological storm conditions—deep depressions causing cyclones, hurricanes—can generate a storm surge which can be several metres above normal tide levels. This is due to the low atmospheric pressure within the centre of the depression. As these storm surges come ashore the surge can resemble a tsunami, inundating vast areas of land. These are not tsunami. Such a storm surge inundated Burma (Myanmar) in May 2008.
Contents[hide] |
Terminology
The term tsunami comes from the Japanese 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.
Tsunamis are sometimes referred to as tidal waves, a term that has fallen out of favor, especially in the scientific community, in recent years because tsunamis actually have nothing to do with tides. The once popular term derives from their most common appearance, which is that of an extraordinarily high incoming tide. Tsunamis and tides both produce waves of water that move inland, but in the case of tsunamis the inland movement of water is much greater and lasts for a longer period, giving the impression of an incredibly high tide. Although the meanings of "tidal" include "resembling"[3] or "having the form or character of"[4] the tides, and the term tsunami is no more accurate because tsumanis are not limited to harbours, use of the term tidal wave is discouraged by geologists and oceanographers.
The only other language than Japanese that has a word for this disastrous wave is Tamil language[dubious ] and the word is "Aazhi Peralai". South Eastern coasts of India have experienced these waves some 700 years before and was a regular event by that time as per the stone carvings (scriptures in stone) read.
Causes
A tsunami can be generated when converging or destructive plate boundaries abruptly move and vertically displace the overlying water. It is very unlikely that they can form at divergent (constructive) or conservative plate boundaries. This is because constructive or conservative boundaries do not generally disturb the vertical displacement of the water column. Subduction zone related earthquakes generate the majority of all tsunamis.
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. A tsunami can occur at any state of the tide and even at low tide will still inundate coastal areas if the incoming waves surge high enough.
On April 1, 1946 a Magnitude 7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 m high surge. The area where the earthquake occurred 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 during the Neolithic era, 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 then failed solely as a result of being overloaded. It is also possible that an earthquake caused the sediments to become unstable and then fail. Another theory is that a release of gas hydrates (methane etc.,) caused the slump.
The "Great Chilean earthquake" (19:11 hrs UTC) May 22, 1960 (9.5 Mw), the March 27, 1964 "Good Friday earthquake" Alaska 1964 (9.2 Mw), and the "Great Sumatra-Andaman earthquake" (00:58:53 UTC) December 26, 2004 (9.2 Mw), are recent examples of powerful megathrust earthquakes that generated a tsunami that was able to cross 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 caused by landslides, explosive volcanic action e.g., Santorini, Krakatau, 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 trans-oceanic 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 524 m. However, an extremely large gravitational landslide might generate a so called "mega-tsunami" that may have the ability to travel trans-oceanic distances. This though is strongly debated and there is no actual geological evidence to support this hypothesis.
Characteristics
While everyday wind waves have a wavelength (from crest to crest) of about 100 m (300 ft) and a height of roughly 2 m (7 ft), a tsunami in the deep ocean has a wavelength of about 200 km (120 miles). This wave travels at well over 800 km/h (500 mph), but due to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 m (3 ft). This makes tsunamis difficult to detect over deep water. Their passage usually goes unnoticed by ships.
As the tsunami approaches the coast and the waters become shallow, the wave is compressed due to wave shoaling and its forward travel slows below 80 km/h (50 mph). Its wavelength diminishes to less than 20 km (12 miles) and its amplitude grows enormously, producing a distinctly visible wave. Since the wave still has a wavelength on the order of several km (a few miles), the tsunami may take minutes to ramp up to full height, with victims seeing a massive deluge of rising ocean rather than a cataclysmic wall of water. Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep breaking front.
Signs of an approaching tsunami
There is often no advance warning of an approaching tsunami. However, since earthquakes are often a cause of tsunami, any earthquake occurring near a body of water may generate a tsunami if it occurs at shallow depth, is of moderate or high magnitude, and the water volume and depth is sufficient.
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 an accompanying 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 eastern 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.
About 80% of all tsunamis occur 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.
Indian Ocean Tsunami According to an article in "Geographical" magazine (April 2008), the Indian Ocean tsunami of December 26, 2004 was not the worst that the region could expect. Professor Costas Synolakis of the Tsunami Research Center at the University of Southern California co-authored a paper in "Geophysical Journal International" which suggests that a future tsunami in the Indian Ocean basin could affect locations such as Madagascar, Singapore, Somalia, Western Australia and many others. The Boxing Day tsunami killed over 300,000 people with many bodies either being lost to the sea or unidentified. Some unofficial estimates have claimed that approximately 1 million people may have died directly or indirectly solely as a result of the tsunami.
Warnings and prevention
A tsunami cannot be prevented or precisely predicted—even if the right magnitude of an earthquake occurs in the right location. Geologists, Oceanographers and Seismologist analyse each earthquake and based upon many factors may or may not issue a tsunami warning. However, 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. One of the most important systems that is used and constantly monitored are bottom pressure sensors. These are anchored and attached to buoys. Sensors on the equipment constantly monitor the pressure of the overlying water column—this can be deduced by the simple calculation of:
where
P = the overlying pressure in Newtons per metre square,
ρ = the density of the seawater= 1.1 x 103 kg/m3,
g = the acceleration due to gravity= 9.8 m/s2 and
h = the height of the water column in metres.
Hence for a water column of 5,000 m depth the overlying pressure is equal to
or about 5.7 Million tonnes per metre square.
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 December 26, 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.
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.
As a direct result of the Indian Ocean tsunami, a re-appraisal of the tsunami threat of all coastal areas is being undertaken by national governments and the United Nations Disaster Mitigation Committee. A tsunami warning system is currently being installed in the Indian Ocean.
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.
Some zoologists hypothesise that animals may have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. Some animals seem to have the ability to detect natural phenomena and if correct, careful observation and monitoring could possibly provide advance warning of earthquakes, tsunami etc. 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 by media sources in Sri Lanka in the 2004 Indian Ocean earthquake.[5][6] It is possible that certain animals (e.g., elephants) may have 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 to the shore to investigate and many drowned as a result.
It is not possible to prevent a tsunami. However, in some tsunami-prone countries some earthquake engineering 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 often surge 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.[7]
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.[8] 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.
