A ( ) is a series of
water waves (called
a
tsunami wave train) that is caused by the
displacement of a large
volume of a
body of water, such as an
ocean. The original Japanese term literally translates
as "
harbor wave." Tsunamis are a frequent
occurrence in Japan; approximately 195 events have been recorded.
Due to the immense volumes of water and energy involved, tsunamis
can devastate coastal regions. Casualties can be high because the
waves move faster than humans can run.
Earthquakes,
volcanic eruptions and other
underwater explosions (detonations of
nuclear devices at sea),
landslides and other
mass
movements,
bolide impacts, and
other disturbances above or below water all have the potential to
generate a tsunami.
The
Greek historian
Thucydides was the first to relate tsunami to
submarine earthquakes, but
understanding of tsunami's nature remained slim until the 20th
century and is the subject of ongoing research.
Many early
geological,
geographical, and
oceanographic texts refer to tsunamis as
"
seismic sea waves."
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.
Such a storm surge
inundated Burma
(Myanmar
) in May
2008.
Etymology
The term
tsunami comes from the Japanese,
meaning "
harbor"
(
tsu,
津) and
"
wave"
(
nami,
波). (For the plural,
one can either follow ordinary English practice and add an
s, or use an invariable plural as in the Japanese.)
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""tidal." The American Heritage® Stedman's
Medical Dictionary. Houghton Mifflin Company. 11 Nov. 2008.
/dictionary.reference.com/browse/tidal>. or "having the form or
character of" 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 a native word for
this disastrous wave. In the
Tamil
language, the word is
aazhi peralai. In the
Acehnese language, it is
ië beuna
or
alôn buluëk (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
semong, while in the Sigulai
language it is emong.
Causes
A tsunami can be generated when
convergent 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 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 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 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), and 2004 Indian
Ocean earthquake
(Mw 9.2) (00:58:53 UTC) 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.
In the
1950s, it was hypothesised that larger tsunamis than had previously
been believed possible may be caused by landslides, explosive volcanic eruptions (e.g., Santorini
and Krakatau
), and impact events
when they contact water. 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.
The media dub them
megatsunami.
Tsunamis caused by these mechanisms, unlike the
trans-oceanic tsunami, may dissipate quickly and
rarely affect distant coastlines due to the small sea area
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 . However, an extremely large landslide might
generate a
megatsunami that can travel
trans-oceanic distances, although there is no geological evidence
to support this hypothesis.
Earthquake-generated tsunami
An
earthquake may generate a tsunami if
the quake:
- occurs just below a body of water,
- is of moderate or high magnitude, and
- displaces a large-enough volume of water.
File:Eq-gen1.jpg|Drawing of
tectonic plate boundary before
earthquake.File:Eq-gen2.jpg|Overriding
plate bulges under strain, causing
tectonic uplift.File:Eq-gen3.jpg|Plate
slips, causing
subsidence and releasing
energy into water.File:Eq-gen4.jpg|The energy released produces
tsunami waves.
Characteristics

Chennai
While everyday
wind waves have a
wavelength (from crest to crest) of about and a
height of roughly , a tsunami in the deep ocean has a wavelength of
about . Such a wave travels at well over , 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 .
This makes tsunamis difficult to detect over deep water. Ships
rarely notice their passage.
As the tsunami approaches the coast and the waters become shallow,
wave shoaling compresses the wave and
its velocity slows below . Its wavelength diminishes to less than
and its amplitude grows enormously, producing a distinctly visible
wave. Since the wave still has such a long wavelength, the tsunami
may take minutes to reach full height. Except for the very largest
tsunamis, the approaching wave does not break (like a
surf break), but rather appears like a fast
moving
tidal bore. 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. 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.
About 80% of tsunamis occur in the Pacific Ocean, but are possible
wherever there are large bodies of water, including lakes. They may
be caused by landslides, volcanic explosions, bolides and seismic
activity.
Drawback
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 tectonic plate on one side of the
fault line sinks suddenly during the earthquake, causing the
overlaying water to propagate outwards with the trough of the wave
at its front. It is also for this reason that there would not be
any drawback when the tsunami travelling on the other side arrives
ashore, as the tectonic plate is "raised" on that side of the fault
line.
Drawback begins before the wave's arrival at an interval equal to
half of the wave's period. If the slope of the coastal seabed is
moderate, drawback can exceed hundreds of meters. People unaware of
the danger sometimes remain near the shore to satisfy their
curiosity or to collect fish from the exposed seabed. During the
Indian Ocean tsunami, the sea withdrew and many people went onto
the exposed sea bed to investigate. Pictures show people walking on
the normally submerged areas with the advancing wave in the
background. Few survived.
Tsunami intensity and magnitude scales
As with earthquakes, several attempts have been made to set up
scales of tsunami intensity or magnitude to allow comparison
between different events.
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. The latter scale was modified by Soloviev,
who calculated the Tsunami intensity
I according to the
formula
- \,\mathit{I} = \frac{1}{2} + \log_{2} \mathit{H}_{av}
where \mathit{H}_{av} 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. Difficulties in calculating the potential energy of the
tsunami mean that this scale is rarely used. Abe introduced the
tsunami magnitude scale \mathit{M}_{t}, calculated
from,
- \,\mathit{M}_{t} = {a} \log h + {b} \log R = \mathit{D}
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.
Warnings and predictions

240 px
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 eastern coasts 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 right magnitude
of an earthquake occurs in the right location.
Geologists,
oceanographers, and
seismologists 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 automated
systems can provide warnings immediately after an earthquake in
time to save lives. One of the most successful systems uses bottom
pressure sensors that are attached to buoys. The sensors constantly
monitor the pressure of the overlying water column. This is deduced
through the calculation:
- \,\! P = \rho gh
where
P = the overlying
pressure in Newtons per
metre square,
\rho = the
density of the
seawater= 1.1 x 10
3
kg/m
3,
g = the
acceleration due to gravity=
9.8 m/s
2 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
- \,\! P = \rho gh=(1.1 * 10^3 \frac{kg}{m^3})(9.8
\frac{m}{s^2})(5.0 * 10^3 m)=5.4*10^7 \frac{N}{m^2}=54 MPa
or about 5500 tonnes per metre square.
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.
The
Pacific Tsunami Warning
System is based in Honolulu
, Hawi
i
. 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.
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 currently being installed in
the Indian Ocean.
Computer models can predict tsunami
arrival—predicted arrival times are usually within minutes of the
actual time. Bottom pressure sensors relay information in real time
and based upon the pressure readings and other seismic information
and the seafloor's shape (
bathymetry) and
coastal
topography, the modesl 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
. 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. 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 built many
tsunami walls of up to 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 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.
Natural factors such as shoreline tree cover can mitigate tsunami
effects. Some locations in the path of the 2004 Indian Ocean
tsunami escaped almost unscathed because trees such as
coconut palms and
mangroves absorbed the tsunami's energy.
In one
striking example, the village of Naluvedapathy in India's Tamil Nadu
region suffered only minimal damage and few deaths
because the wave broke against a forest of 80,244 trees planted
along the shoreline in 2002 in a bid to enter the Guinness Book of Records.
Environmentalists have suggested tree planting along tsunami-prone
seacoasts. Trees require years to grow to a useful size, but such
plantations could offer a much cheaper and longer-lasting means of
tsunami mitigation than artificial barriers.
Tsunami in history
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.
2004 Indian Ocean tsunami
The
2004 Indian
Ocean 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.
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.
Tsunami in ancient 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.
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.
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
.
See also
Footnotes
- Fradin, Judith Bloom and Dennis Brindell Fradin. Witness to
Disaster: Tsunamis. Washington, D.C.: National Geographic Society,
2008.
- http://www.answers.com/topic/tsunami tsunami
- [a. Jap. tsunami, tunami, f. tsu harbour + nami
waves.—Oxford English Dictionary]
- -al. (n.d.). Dictionary.com Unabridged (v 1.1). Retrieved
November 11, 2008, from Dictionary.com website:
http://dictionary.reference.com/browse/-al
-
http://www.acehrecoveryforum.org/en/index.php?action=ARFNews&no=73
-
http://www.jtic.org/en/jtic/images/dlPDF/Lipi_CBDP/reports/SMGChapter3.pdf
-
http://earthsci.org/education/teacher/basicgeol/tsumami/tsunami.html
Tsunamis
- Thucydides:
“A History of the Peloponnesian War”,
3.89.1–4
- Thucydides:
“A History of the Peloponnesian War”,
3.89.5
- Stanley, Jean-Daniel & Jorstad, Thomas F. (2005), "
The 365 A.D. Tsunami Destruction of Alexandria,
Egypt: Erosion, Deformation of Strata and Introduction of
Allochthonous Material"
References
- IOC Tsunami Glossary by the Intergovernmental
Oceanographic Commission (IOC) at the International
Tsunami Information Centre (ITIC) of UNESCO

- Tsunami Terminology at NOAA
- abelard.org. tsunamis: tsunamis travel fast but
not at infinite speed. retrieved March 29, 2005.
- Dudley, Walter C. & Lee, Min (1988: 1st edition)
Tsunami! ISBN 0-8248-1125-9 website
- Iwan, W.D., editor, 2006, Summary report of the Great
Sumatra Earthquakes and Indian Ocean tsunamis of December 26, 2004
and March 28, 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. ISBN 1-932884-19-X website
- Kenneally, Christine (December 30, 2004). "Surviving the
Tsunami." Slate. website
- Lambourne, Helen (March 27, 2005). "Tsunami: Anatomy of a
disaster." BBC News. website
- 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.
- The NOAA's page on the 2004 Indian Ocean earthquake and
tsunami
- Tappin, D; 2001. Local tsunamis. Geoscientist. 11–8, 4–7.
-
http://www.telegraph.co.uk/news/1480192/Girl-10-used-geography-lesson-to-save-lives.html
Girl, 10, used geography lesson to save lives.
External links
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