A
tsunami (plural: tsunamis or tsunami; from
Japanese:
津波, lit. "harbor wave";
[1] English pronunciation:
/suːˈnɑːmi/ soo-NAH-mee or
/tsuːˈnɑːmi/ tsoo-NAH-mee[2]) is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a
large lake.
Earthquakes,
volcanic eruptions and other
underwater explosions (including detonations of underwater
nuclear devices), landslides,
glacier calvings,
meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami.
[3]
Tsunami waves do not resemble normal
sea waves,
because their wavelength is far longer. Rather than appearing as a
breaking wave, a tsunami may instead initially resemble a rapidly rising
tide, and for this reason they are often referred to as
tidal waves. Tsunamis generally consist of a series of waves with
periods ranging from minutes to hours, arriving in a so-called "wave train".
[4]
Wave heights of tens of metres can be generated by large events.
Although the impact of tsunamis is limited to coastal areas, their
destructive power can be enormous and they can affect entire ocean
basins; the
2004 Indian Ocean tsunami was among the deadliest natural disasters in human history with over 230,000 people killed in 14 countries bordering the
Indian Ocean.
The
Greek historian
Thucydides suggested in 426 B.C. that tsunamis were related to
submarine earthquakes,
[5][6]
but the understanding of a tsunami's nature remained slim until the
20th century and much remains unknown. Major areas of current research
include trying to determine why some large earthquakes do not generate
tsunamis while other smaller ones do; trying to accurately forecast the
passage of tsunamis across the oceans; and also to forecast how tsunami
waves would interact with specific shorelines.
Etymology
The term
tsunami comes from the Japanese 津波, composed of the two
kanji 津 (
tsu) meaning "
harbor" and
波 (
nami), meaning "
wave". (For the plural, one can either follow ordinary English practice and add an
s, or use an invariable plural as in the Japanese.
[7])
Tsunami are sometimes referred to as
tidal waves. In recent
years, this term has fallen out of favor, especially in the scientific
community, 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
tidal bore.
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"
[8] or "having the form or character of"
[9] 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.
There are only a few other languages that have an equivalent native word. In the
Tamil language, the word is
aazhi peralai. In the
Acehnese language, it is
ië beuna or
alôn buluëk[10] (Depending on the dialect. Note that in the fellow
Austronesian language of
Tagalog, a major language in the
Philippines,
alon means "wave".) On
Simeulue island, off the western coast of Sumatra in Indonesia, in the
Defayan language the word is
smong, while in the
Sigulai language it is
emong.
[11]
History
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 was the first to argue that ocean earthquakes must be the cause.
[5][6]
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.[12]
The
Roman historian
Ammianus Marcellinus (
Res Gestae
26.10.15-19) described the typical sequence of a tsunami, including an
incipient earthquake, the sudden retreat of the sea and a following
gigantic wave, after the
365 A.D. tsunami devastated
Alexandria.
[13][14]
While Japan may have the longest recorded history of tsunamis, the sheer destruction caused by the
2004 Indian Ocean earthquake and tsunami
event mark it as the most devastating of its kind in modern times,
killing around 230,000 people. The Sumatran region is not unused to
tsunamis either, with earthquakes of varying magnitudes regularly
occurring off the coast of the island.
[15]
Generation mechanisms
The principal generation mechanism (or cause) of a tsunami is the
displacement of a substantial volume of water or perturbation of the
sea.
[16]
This displacement of water is usually attributed to either earthquakes,
landslides, volcanic eruptions,glacier calvings or more rarely by
meteorites and nuclear tests.
[17][18] The waves formed in this way are then sustained by gravity.
Tides do not play any part in the generation of tsunamis.
Tsunami generated by seismicity
Tsunami can be generated when the sea floor abruptly deforms and
vertically displaces the overlying water. Tectonic earthquakes are a
particular kind of earthquake that are associated with the Earth's
crustal deformation; when these earthquakes occur beneath the sea, the
water above the deformed area is displaced from its equilibrium
position.
[19] More specifically, a tsunami can be generated when
thrust faults associated with
convergent or destructive
plate boundaries
move abruptly, resulting in water displacement, owing to the vertical
component of movement involved. Movement on normal faults will also
cause displacement of the seabed, but the size of the largest of such
events is normally too small to give rise to a significant tsunami.
-
-
-
Plate slips, causing
subsidence and releasing energy into water.
-
The energy released produces tsunami waves.
Tsunamis have a small
amplitude (wave height) offshore, and a very long
wavelength (often hundreds of kilometers long, whereas normal ocean waves have a wavelength of only 30 or 40 metres),
[20]
which is why they generally pass unnoticed at sea, forming only a
slight swell usually about 300 millimetres (12 in) above the normal sea
surface. They grow in height when they reach shallower water, in a
wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.
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 metres (46 ft) 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 originating at locations away from
convergent boundaries include
Storegga about 8,000 years ago,
Grand Banks 1929,
Papua New Guinea
1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis came
from earthquakes which destabilized sediments, causing them to flow
into the ocean and generate a tsunami. They dissipated before traveling
transoceanic distances.
The cause of the Storegga sediment failure is unknown. Possibilities
include an overloading of the sediments, an earthquake or a release of
gas hydrates (methane etc.)
The
1960 Valdivia earthquake (
Mw 9.5) (19:11 hrs UTC),
1964 Alaska earthquake (
Mw 9.2),
2004 Indian Ocean earthquake (
Mw 9.2) (00:58:53 UTC) and
2011 Tōhoku earthquake (
Mw9.0) are recent examples of powerful
megathrust earthquakes that generated tsunamis (known as
teletsunamis) that can cross entire oceans. Smaller (
Mw 4.2) earthquakes in Japan can trigger tsunamis (called
local and
regional tsunamis) that can only devastate nearby coasts, but can do so in only a few minutes.
Tsunami generated by landslides
In the 1950s, it was discovered that larger tsunamis than had previously been believed possible could be caused by giant
landslides. Underwater landslides that generate tsunamis are called
sciorrucks.
[21]
These phenomena rapidly displace large water volumes, as energy from
falling debris or expansion transfers to the water at a rate faster than
the water can absorb. Their existence was confirmed in 1958, when a
giant landslide in
Lituya Bay,
Alaska,
caused the highest wave ever recorded, which had a height of 524 metres
(over 1700 feet). The wave didn't travel far, as it struck land almost
immediately. Two people fishing in the bay were killed, but another boat
amazingly managed to ride the wave. Scientists named these waves
megatsunami.
Scientists discovered that extremely large landslides from volcanic
island collapses can generate megatsunamis that can cross oceans.
Meteotsunamis
Some
meteorological conditions, such as deep
depressions that cause
tropical cyclones, can generate a
storm surge, called a
meteotsunami, which can raise
tides several metres above normal levels. The displacement comes from low
atmospheric pressure within the centre of the depression. As these
storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land.
[22]
Characteristics
When the wave enters shallow water, it slows down and its amplitude (height) increases.
The wave further slows and amplifies as it hits land. Only the largest waves crest.
Tsunamis cause damage by two mechanisms: the smashing force of a wall
of water travelling at high speed, and the destructive power of a large
volume of water draining off the land and carrying all with it, even if
the wave did not look large.
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). Such a wave travels at well over 800
kilometres per hour (500 mph), but owing 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).
[23] This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.
This is the reason for the Japanese name "harbor wave": sometimes a village's
fishermen
would sail out, and encounter no unusual waves while out at sea
fishing, and come back to land to find their village devastated by a
huge wave.
As the tsunami approaches the coast and the waters become shallow,
wave shoaling
compresses the wave and its speed decreases below 80 kilometres per
hour (50 mph). Its wavelength diminishes to less than 20 kilometres
(12 mi) and its amplitude grows enormously. Since the wave still has the
same very long
period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not
break, but rather appears like a fast-moving
tidal bore.
[24]
Open bays and coastlines adjacent to very deep water may shape the
tsunami further into a step-like wave with a steep-breaking front.
When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed
run up. Run up is measured in metres above a reference sea level.
[24]
A large tsunami may feature multiple waves arriving over a period of
hours, with significant time between the wave crests. The first wave to
reach the shore may not have the highest run up.
[25]
About 80% of tsunamis occur in the Pacific Ocean, but they are
possible wherever there are large bodies of water, including lakes. They
are caused by earthquakes, landslides, volcanic explosions glacier
calvings, and
bolides.
Drawback
Wave animation showing the initial "drawback" of surface water
If the first part of a tsunami to reach land is a trough—called a
drawback—rather than a wave crest, the water along the shoreline recedes dramatically, exposing normally submerged areas.
A drawback occurs because the water propagates outwards with the
trough of the wave at its front. Drawback begins before the wave arrives
at an interval equal to half of the wave's period. Drawback can exceed
hundreds of metres, and people unaware of the danger sometimes remain
near the shore to satisfy their curiosity or to collect fish from the
exposed seabed.
Scales of intensity and magnitude
As with earthquakes, several attempts have been made to set up scales
of tsunami intensity or magnitude to allow comparison between different
events.
[26]
Intensity scales
The first scales used routinely to measure the intensity of tsunami were the
Sieberg-Ambraseys scale, used in the
Mediterranean Sea and the
Imamura-Iida intensity scale, used in the
Pacific Ocean. The latter scale was modified by Soloviev, who calculated the Tsunami intensity
I according to the formula
where
Hav is the average wave height along the nearest coast. This scale, known as the
Soloviev-Imamura tsunami intensity scale, is used in the global tsunami catalogues compiled by the
NGDC/
NOAA and the Novosibirsk Tsunami Laboratory as the main parameter for the size of the tsunami.
Magnitude scales
The first scale that genuinely calculated a magnitude for a tsunami,
rather than an intensity at a particular location was the ML scale
proposed by Murty & Loomis based on the potential energy.
[26] Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely used. Abe introduced the
tsunami magnitude scale Mt, calculated from,
where
h is the maximum tsunami-wave amplitude (in m) measured by a tide gauge at a distance
R from the epicenter,
a,
b &
D are constants used to make the M
t scale match as closely as possible with the moment magnitude scale.
[27]
Warnings and predictions
One of the deep water
buoys used in the
DART tsunami warning system
Drawbacks can serve as a brief warning. People who observe drawback
(many survivors report an accompanying sucking sound), can survive only
if they immediately run for high ground or seek the upper floors of
nearby buildings. In 2004, ten-year old
Tilly Smith of
Surrey,
England, was on
Maikhao beach in
Phuket,
Thailand
with her parents and sister, and having learned about tsunamis recently
in school, told her family that a tsunami might be imminent. Her
parents warned others minutes before the wave arrived, saving dozens of
lives. She credited her geography teacher, Andrew Kearney.
In the
2004 Indian Ocean tsunami
drawback was not reported on the African coast or any other east-facing
coasts that it reached. This was because the wave moved downwards on
the eastern side of the fault line and upwards on the western side. The
western pulse hit coastal Africa and other western areas.
A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known.
Geologists,
oceanographers, and
seismologists
analyse each earthquake and based on many factors may or may not issue a
tsunami warning. However, there are some warning signs of an impending
tsunami, and automated systems can provide warnings immediately after an
earthquake in time to save lives. One of the most successful systems
uses bottom pressure sensors, attached to buoys, which constantly
monitor the pressure of the overlying water column.
Regions with a high tsunami risk typically use
tsunami warning systems
to warn the population before the wave reaches land. On the west coast
of the United States, which is prone to Pacific Ocean tsunami, warning
signs indicate evacuation routes. In Japan, the community is
well-educated about earthquakes and tsunamis, and along the Japanese
shorelines the tsunami warning signs are reminders of the natural
hazards together with a network of warning sirens, typically at the top
of the cliff of surroundings hills.
[28]
The
Pacific Tsunami Warning System is based in
Honolulu,
Hawaiʻi.
It monitors Pacific Ocean seismic activity. A sufficiently large
earthquake magnitude and other information triggers a tsunami warning.
While the subduction zones around the Pacific are seismically active,
not all earthquakes generate tsunami. Computers assist in analysing the
tsunami risk of every earthquake that occurs in the Pacific Ocean and
the adjoining land masses.
-
-
-
-
-
A Tsunami-hazard-zone sign (in Spanish and English) in
Iquique,
Chile
As a direct result of the Indian Ocean tsunami, a re-appraisal of the
tsunami threat for all coastal areas is being undertaken by national
governments and the United Nations Disaster Mitigation Committee. A
tsunami warning system is being installed in the Indian Ocean.
Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors relay information in
real time. Based on these pressure readings and other seismic information and the seafloor's shape (
bathymetry) and coastal
topography, the models estimate the amplitude and surge height of the approaching tsunami. All
Pacific Rim
countries collaborate in the Tsunami Warning System and most regularly
practice evacuation and other procedures. In Japan, such preparation is
mandatory for government, local authorities, emergency services and the
population.
Some zoologists hypothesise that some animal species have an ability to sense subsonic
Rayleigh waves
from an earthquake or a tsunami. If correct, monitoring their behavior
could provide advance warning of earthquakes, tsunami etc. However, the
evidence is controversial and is not widely accepted. There are
unsubstantiated claims about the Lisbon quake that some animals escaped
to higher ground, while 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.
[29][30] 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. By
contrast, some humans went to the shore to investigate and many drowned
as a result.
Along the United States west coast, in addition to sirens, warnings are sent on television & radio via the
National Weather Service, using the
Emergency Alert System.
Forecast of tsunami attack probability
Kunihiko Shimazaki (
University of Tokyo),
a member of Earthquake Research committee of The Headquarters for
Earthquake Research Promotion of Japanese government, mentioned the plan
to public announcement of tsunami attack probability forecast at
Japan National Press Club
on 12 May 2011. The forecast includes tsunami height, attack area and
occurrence probability within 100 years ahead. The forecast would
integrate the scientific knowledge of recent
interdisciplinarity and
aftermath of the 2011 Tōhoku earthquake and tsunami. As the plan, announcement will be available from 2014.
[31][32][33]
Mitigation
In some tsunami-prone countries
earthquake engineering measures have been taken to reduce the damage caused onshore.
Japan, where tsunami science and response measures first began following a
disaster in 1896, has produced ever-more elaborate countermeasures and response plans.
[34]
That country has built many tsunami walls of up to 4.5 metres (15 ft)
to protect 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 overtop 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 metres (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.
[35]
SUMBER:http://en.wikipedia.org/wiki/Tsunami
