A Tsunami (津波 )(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 tsunami can be
devastating.
The Greek historian Thucydides was the first to relate tsunami to submarine quakes,[1] [2] but understanding of the nature of tsunami 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(s)".
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.
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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. Tsunami are common throughout Japanese history; approximately 195 events in Japan have been recorded.
Tsunami are sometimes referred to as tidal waves,
a term that has fallen out of favor, especially in the scientific
community, in recent years because tsunami 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.
Tsunami and tides both produce waves of water that move inland, but in
the case of tsunami 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 tsunami are not limited to harbours, use of the term tidal wave is discouraged by geologists and oceanographers.
The only language other than Japanese that has a word for this disastrous wave is the Tamil language[dubious – discuss];
the word is "Aazhi Peralai". The South Eastern coasts of India had
experienced these waves some 700 years before, and they were a regular
event by that time, as the stone carvings (scriptures in stone) read.
The Acehnese language word for tsunami is ië beuna or alôn buluëk[5](depending on the dialect), whereas in the Defayan language of Simeuluesemong. The Sigulai language of Simeulue also has a word for tsunami: emong [6]. Regency, Indonesia, the word for tsunami is
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.
Tsunamis have a small 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. They grow in height when they reach
shallower water, in a "shoaling" process described below. 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 earthquakePacific Ocean floor is subducting (or being pushed downwards) under Alaska. occurred is where the
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
A devastated Marina beachChennai after the Indian Ocean Tsunami in
While everyday wind waves have a wavelength (from crest to crest) of
about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a
tsunami in the deep ocean has a wavelength of about 200 kilometres
(120 mi). This wave travels at well over 800 kilometres per hour
(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 metre (3.3 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 kilometres per hour (50 mph). Its
wavelength diminishes to less than 20 kilometres (12 mi) 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
The monument to the victims of tsunami at Laupahoehoe, Hawaii.
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.
This occurred in Phuket Thailand, at Maikhao beach. Ten-year old Tilly Smith
of Surrey, England, was on the beach with her parents and sister, and
having learned about tsunamis recently in school, was able to warn her
family that a tsunami might be imminent. Her parents warned others on
the beach and the hotel staff minutes before the tsunami hit. Ms. Smith
is credited with saving dozens of lives as a result of her recent
geography lesson. She gave credit to her geography teacher, Mr. Andrew
Kearney.
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.
