A tsunami,
also known as a seismic sea wave or as a tidal
wave, is a series of waves in a body of water
caused by the displacement of a large volume of water, generally in 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. In
being
generated by the displacement of
water, a tsunami contrasts both with a normal
ocean wave
generated by wind and with
tides, which are generated
by the gravitational pull of the moon and the sun on bodies of
water.
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".
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 at least
290,000 people killed or missing in 14 countries bordering the
Indian Ocean.
The Greek
historian Thucydides
suggested in his late-5th century BC History of the Peloponnesian War,
that tsunamis were related to submarine earthquakes,
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.
Terminology
Various terms are used in
English-speaking
countries to describe waves created in a body of water by the displacement of
water. None of the terms in common use are entirely accurate.
Tsunami
The term
tsunami, meaning "harbor wave" in literal
translation, comes from the Japanese
津波, composed of the two
kanji 津 (
tsu)
meaning "
harbour"
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.)
There are only a few other languages that have an equivalent native word. In
Acehnese language, the words are
ië beuna
or
alôn buluëk (depending on the dialect). In
Tamil
language, it is
aazhi peralai. On
Simeulue
island, off the western coast of
Sumatra in
Indonesia, in
Devayan
language the word is
smong, while in
Sigulai
language it is
emong.
In
Singkil (in Aceh province) and surrounding,
the people use the word
gloro/galoro for tsunami. In
Nias
language, it is called
oloro/galoro and in
Ende
it is called
ae mesi nuka tana lala
Tidal wave
Tsunami are sometimes referred to as
tidal waves.
This once-popular term derives from the most common appearance of tsunami,
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 may be much greater, giving the impression of an incredibly
high and forceful tide. In recent years, the term "tidal wave" has
fallen out of favor, especially in the scientific community, because tsunami
actually have nothing to do with
tides, which are produced by the gravitational pull of the moon
and sun rather than the displacement of water. Although the meanings of
"tidal" include "resembling" or
"having the form or character of" the
tides, use of the term
tidal wave is discouraged by geologists and
oceanographers.
Seismic sea wave
The term
seismic sea wave also is used to refer to the
phenomenon, because the waves most often are generated by
seismic activity
such as
earthquakes.
Prior to the rise of the use of the term "tsunami" in
English-speaking countries, scientists generally encouraged the use of the term
"seismic sea wave" rather than the inaccurate term "tidal
wave." However, like "tsunami," "seismic sea wave" is
not a completely accurate term, as forces other than
earthquakes
– including underwater
landslides, volcanic eruptions, underwater explosions, land
or ice slumping into the ocean,
meteorite impacts, or even the weather when the atmospheric
pressure changes very rapidly – can generate such waves by displacing water.
History
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.
Tsunamis are an often underestimated hazard in the
Mediterranean
Sea region and Europe in general. Of historical and current (with regard to
risk assumptions) importance are e.g. the
1755 Lisbon earthquake and tsunami (which
was caused by the
Azores–Gibraltar Transform Fault),
the
1783 Calabrian earthquakes, each causing
several ten thousand deaths and the
1908 Messina earthquake and tsunami. The
latter took
more than 123,000 lives in
Sicily and Calabria and is among the most deadly natural disasters in modern
Europe. The
Storegga Slide in the Norwegian sea and some
examples of
Tsunamis affecting the British
Isles refer to landslide and meteotsunamis predominatly and less to earth
quake induced waves.
As early as 426 BC 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 AD tsunami devastated
Alexandria.
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. This
displacement of water is usually attributed to either earthquakes, landslides,
volcanic eruptions, glacier calvings or more rarely by meteorites and nuclear
tests. The
waves formed in this way are then sustained by gravity.
Tides do not play any
part in the generation of tsunamis.
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. 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.
Tsunamis have a small
amplitude (wave height) offshore, and a very long
wavelength
(often hundreds of kilometres long, whereas normal ocean waves have a
wavelength of only 30 or 40 metres),
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-metre high (46 ft) 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 destabilised 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),
1964 Alaska earthquake (
Mw
9.2),
2004 Indian Ocean earthquake (
Mw
9.2), 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.
Landslides
In the 1950s, it was discovered that larger tsunamis than had previously
been believed possible could be caused by giant
submarine landslides. These rapidly displace
large water volumes, as energy 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.
Another landslide-tsunami event occurred in 1963 when a massive landslide
from
Monte
Toc went into the
Vajont Dam in Italy. The resulting wave overtopped the
262 m (860 ft) high dam by 250 metres (820 ft) and destroyed
several towns. Around 2,000 people died.
Scientists named these waves
megatsunami. Scientists discovered that extremely large
landslides from volcanic island collapses may be able to generate megatsunamis
that can cross oceans.
In general, landslides generate displacements mainly in the shallower parts
of the coastline, and there is conjecture about the nature of truly large
landslides that end in water. This is proven to lead to huge effect in closed
bays and lakes, but an open oceanic landslide big enough to cause a tsunami
across an ocean has not yet happened since before seismology has been a major
area of scientific study, and only very rarely in human history. Susceptible
areas focus for now on the islands of
Hawaii and
Las Palmas
in the
Canary Islands, where large masses of relatively
unconsolidated volcanic shield on slopes occur. Considerable doubt exists about
how loosely linked these slopes actually are.
Meteotsunamis
Some
meteorological conditions, especially deep
depressions such as
tropical
cyclones, can generate a type of
storm surge
called a
meteotsunami which raises water heights above normal levels,
often suddenly at the shoreline.
In the case of deep tropical cyclones, this is due to very low
atmospheric pressure and inward swirling winds
causing an uplifted dome of water to form under and travel in tandem with the
storm. When these water domes reach shore, they rear up in shallows and surge
laterally like earthquake-generated tsunamis, typically arriving shortly after
landfall of the storm's
eye.
Man-made or triggered tsunamis
There have been studies and at least one attempt to create tsunami waves as
a
tectonic
weapon or whether human behavior may trigger tsunamis, e.g. in the
(debunked)
Clathrate gun hypothesis.
In World War II, the
New Zealand Military Forces initiated
Project
Seal, which attempted to create small tsunamis with explosives in the area
of today's
Shakespear Regional Park; the attempt
failed.
There has been considerable speculation on the possibility of using
nuclear
weapons to cause tsunamis near to an enemy coastline. Even during
World War
II consideration of the idea using conventional explosives was explored.
Nuclear testing in the
Pacific Proving Ground by the United States
seemed to generate poor results.
Operation Crossroads fired two 20
kilotonnes of TNT (84 TJ) bombs, one in the air and one underwater, above
and below the shallow (50 m (160 ft)) waters of the
Bikini
Atoll lagoon. Fired about 6 km (3.7 mi) from the nearest island,
the waves there were no higher than 3–4 m (9.8–13.1 ft) upon reaching
the shoreline. Other underwater tests, mainly
Hardtack I/Wahoo (deep water) and
Hardtack
I/Umbrella (shallow water) confirmed the results. Analysis of the effects
of
shallow and
deep underwater explosions indicate that the
energy of the explosions doesn't easily generate the kind of deep, all-ocean
waveforms which are tsunamis; most of the energy creates steam, causes vertical
fountains above the water, and creates compressional waveforms.
Tsunamis are hallmarked by permanent large vertical displacements of very large
volumes of water which don't occur in explosions.
Characteristics
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 a large amount of debris with it, even
with waves that do not appear to be 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 much larger
wavelength of up to 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). This
makes tsunamis difficult to detect over deep water, where ships are unable to
feel their passage.
The reason for the Japanese name "harbour wave" is that 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.
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 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
All
waves have a
positive and negative peak, i.e. a ridge and a trough. In the case of a
propagating wave like a tsunami, either may be the first to arrive. If the
first part to arrive at shore is the ridge, a massive breaking wave or sudden
flooding will be the first effect noticed on land. However if the first part to
arrive is a trough, a
drawback will occur as the shoreline recedes
dramatically, exposing normally submerged areas. 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.
A typical wave period for a damaging tsunami is about 12 minutes. This means
that if the drawback phase is the first part of the wave to arrive, the sea
will recede, with areas well below sea level exposed after 3 minutes. During
the next 6 minutes the tsunami wave trough builds into a ridge, and during this
time the sea is filled in and destruction occurs on land. During the next 6
minutes, the tsunami wave changes from a ridge to a trough, causing flood
waters to drain and drawback to occur again. This may sweep victims and debris
some distance from land. The process repeats as the next wave arrives.
Warnings and predictions
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.
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.
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 can 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. 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 and 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.
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 countermeasure
s and response plans. That
country has built many tsunami walls of up to 12 metres (39 ft) high to
protect populated coastal areas. Other localities have built
floodgates of
up to 15.5 metres (51 ft) high and channels to redirect the water from
incoming tsunami. However, their effectiveness has been questioned, as tsunami
often overtop the barriers.
The Fukushima Daiichi nuclear disaster
was directly triggered by the
2011 Tōhoku earthquake and tsunami,
when waves that exceeded the height of the plant's sea wall.
Iwate
Prefecture, which is an area at high risk from tsunami, had tsunami
barriers walls totalling 25 kilometres (16 mi) long at coastal towns. The
2011 tsunami toppled more than 50% of the walls and caused catastrophic damage.
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.
7 INTERESTING TSUNAMI FACTS
1.
Highest
Amount of Energy Release in Past 25 Years
The first fact in these interesting tsunami facts is about the
2004
tsunami in Indian Ocean. In 2004 the energy release in 9.0
earthquake
of Indonesia was
more than
the combined energy release of the
earthquakes of
past 25 years on our earth. An area of seafloor more than the total area of
California State got dislocated and moved about 30 feet upward. A
huge
amount of
water got displaced and created
approximately 25 meter high waves.
2. Tsunami Facts: Area of
Destruction
Most of the causalities occur around the 250 miles radius of the tsunami
centre and usually within 30 minutes. At the coastal areas if people feel
earthquake
they should consider it a warning for potential tsunami waves and get moved
towards some higher
region.
3. Tsunami That Wiped Out
All Life from the Earth
This is one of the most interesting tsunami facts about the tsunami due to
meteorite showers. There has never been a tsunami due to meteorite strike in
recent history. But according to some scientists, almost 3.5 billion years ago
there was a huge meteorite strike which created a tsunami so big that it wiped
out all the life from the earth. There is another theory about tsunami caused
by an asteroid 4800 years ago in
Indian Ocean which raised huge 180
meters high tsunami waves.
4. Tsunami Facts: The
Largest Earthquake in the History of World
The largest earthquake recorded in the
history
of world took place in 1960. Its centre was 100 miles off coast
of Chile. Hardly15 minutes had passed when the 80 feet high waves hit the
coast. It hit Hawaii 15 hours later. And 22 hours later the waves reached Japan
after covering a distance of 10,000 miles.
5. Tsunami Waves Can Travel
With the Speed of A Jet Plane
In this interesting list of tsunami facts, this fact is about the
unbelievable speed of tsunami waves. The tsunami waves can travel with a speed
of 600
miles per hour which
is equivalent to the speed of a jet plane. The normal
water
waves usually travel only about 2 to 60 miles per hour.
6. Tsunami Facts: About 9000
Tourists Were Killed in 2004 Tsunami
In 2004 the Indian Ocean tsunami killed about 283,000 people. More than
9,000 tourists from all over the world were also among this
large number of casualities. Large number of tourists
from countries like U.S, U.K, Australia, France and Germany were there to spend
Christmas vacations at the
beaches of Southeast Asian countries Indonesia, Malaysia, and Sri Lanka.
7. The Intensity of Tsunami
Waves is usually Low In Deep Ocean
The last one of these tsunami facts is about the
power of tsunami waves in deep Ocean. In very deep
ocean areas tsunami waves occur to be only 1-3 feet tall. Sailors sometimes
don’t even know that these waves are passing under their boats.
Source :
1.
http://en.wikipedia.org/wiki/Tsunami
2.
http://ohmygodfacts.com/7-interesting-tsunami-facts/
3.
http://www.australiangeographic.com.au/topics/science-environment/2011/03/tsunamis-how-they-form/
4.
http://www.wallpaper2020.com/deep-sea-hd-wallpaper/
