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Tsunami
Story
From Wikipedia, the free encyclopedia.The term "tsunami"
comes from the Japanese language meaning harbour ("tsu") and wave
("nami"). The term was created by fishermen who returned to port to find the
area surrounding the harbour devastated, although they had not been aware of any wave in
the open water. A tsunami is not a sub-surface event in the deep ocean; it simply has a
much smaller amplitude (wave heights) offshore, and a very long wavelength (often hundreds
of kilometers long), which is why they generally pass unnoticed at sea, forming only a
passing "hump" in the ocean.
Tsunamis were historically referred to as tidal waves because as they approach land they
take on the characteristics of a violent onrushing tide, rather than the sort of cresting
waves that are formed by wind action upon the ocean (with which people are more familiar).
However, as they are not actually related to tides, the term is considered misleading, and
its use is deprecated by oceanographers.
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| Causes |
 A tsunami can be
generated by any disturbance that displaces a large mass of water, such as an earthquake,
landslide or meteor impact. However, the most common cause is an undersea earthquake. An
earthquake which is too small to create a tsunami by itself may trigger an undersea
landslide quite capable of generating a tsunami.
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the
overlying water. Large vertical movements of the earth's crust can occur at plate
boundaries. Denser oceanic plates slip under continental plates in a process known as
subduction, and subduction earthquakes are particularly effective in generating tsunamis.
Submarine landslides (which sometimes accompany large earthquakes) as well as collapses of
volcanic edifices, can also disturb the overlying water column as sediment and rocks slump
downslope and are redistributed across the sea floor. Similarly, a violent submarine
volcanic eruption can uplift a water column and generate a tsunami.
Waves are formed as the displaced water mass moves under the influence of gravity to
regain its equilibrium and radiates across the ocean like ripples on a pond.
Large landslides and cosmic-body impacts can disturb the water from above, as momentum
from falling debris is transferred to the water into which the debris falls. Generally
speaking, tsunamis generated from these mechanisms - unlike the ocean-wide tsunamis caused
by some earthquakes - dissipate quickly and rarely affect coastlines distant from the
source area, due to the small area of sea affected.
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Characteristics
Tsunamis act very differently from typical surf swells; they propagate at high speeds and
can travel great transoceanic distances with little energy loss. A tsunami can cause
damage thousands of kilometres from its origin, so there may be several hours between its
creation and its impact on a coast, arriving long after seismic wave generated by the
originating event arrives.
In open water, tsunamis have extremely long periods (the time for the next wave top to
pass a point after the previous one), from minutes to hours, and long wavelengths of up to
several hundred kilometres. (Compare to the typical wind-generated swell one sees at a
beach, which might be spawned by a faraway storm and rhythmically roll in, one wave after
another, with a period of about 10 seconds and a wavelength of 150 m). The actual height
of a tsunami wave in open water is often less than one metre. This is often practically
unnoticeable to people on ships. The energy of a tsunami passes through the entire water
column to depths of 4,000 mt. or more, unlike surface waves, which typically reach only 10
mt. or so.
The wave travels across the ocean at speeds from 500 to 1000 km/h, so the second or third
waves may not arrive until more than an hour later. As the wave approaches land, the sea
shallows and the wave no longer travels as quickly, so it begins to 'pile-up': the wave
becomes steeper and taller, and there is less distance between crests. While a person at
the surface of deep water would probably not even notice the tsunami, the wave can
increase to a height of 30 m or more as it approaches the coastline and compresses.
A wave becomes a 'shallow-water wave' when the ratio between the water depth and its
wavelength gets very small, and since a tsunami has an extremely large wavelength
(hundreds of kilometres), tsunamis act as a shallow-water wave even in deep oceanic water.
Shallow-water waves move at a speed that is equal to the square root of the product of the
acceleration of gravity (9.8 m/s2) and the water depth. For example, in the Pacific Ocean,
where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (720
km/h or 442 mi/h) with little energy loss, even over long distances. At a water depth of
40 m, the speed would be 20 m/s (about 72 km/h or 44 mi/h), which is much slower than the
speed in the open ocean but the wave would still be difficult to outrun.
Tsunamis propagate outward from their source, so coasts in the "shadow" of
affected land masses are usually fairly safe. However, tsunami waves can diffract around
land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern
Sri Lanka and India). They also need not be symmetrical; tsunami waves may be much
stronger in one direction than another, depending on the nature of the source and the
surrounding geography.
Local geographic peculiarities can lead to seiche or standing waves forming, which can
amplify the onshore damage. For instance, the tsunami that hit Hawaii on April 1, 1946 had
a fifteen-minute interval between wave fronts. The natural resonant period of Hilo Bay is
about thirty minutes. That meant that every second wave was in phase with the motion of
Hilo Bay, creating a seiche in the bay. As a result, Hilo suffered worse damage than any
other place in Hawaii, with the tsunami/seiche reaching a height of 14 m and killing 159
inhabitants.
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Education
and signs
In instances where the leading edge of the tsunami wave is its trough, the sea will recede
from the coast half of the wave's period before the wave's arrival. If the slope is
shallow, this recession can exceed 800 m. People unaware of the danger may remain at the
shore due to curiosity, or for collecting fish from the dry sea bottom.
In instances where the leading edge of the tsunami is its first peak, low-lying coastal
areas are flooded before following waves reach them. Again, being educated about a tsunami
is important, to realize that when the water level drops the first time, the danger is not
yet over.
Regions with a high risk of tsunamis use tsunami warning systems to forecast tsunamis and
warn the general population
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Past
tsunamis
Although tsunamis occur most frequently in the Pacific Ocean, they are known to occur
anywhere.
Each of the following tsunamis is also described, sometimes at much greater length, in its
own article:
6100 B.C. and before
In the North Atlantic Ocean, the Storegga Slides were a major series of sudden underwater
land movements over the course of tens of thousands of years.
1650 B.C. - Santorini
At some time between 1650 BC and 1600 BC (still debated), the volcanic Greek island
Santorini erupted, causing a 100 m to 150 m high tsunami that devastated the north coast
of Crete, 70 km (45 miles) away, and would certainly have eliminated every timber of the
Minoan fleet along Crete's northern shore. Santorini is regarded as the most likely source
for Plato's literary parable of Atlantis, and is believed by some scientists to have
informed Great Flood accounts which were eventually recorded in Jewish, Christian, and
Islamic texts.
1755 - Lisbon, Portugal
Tens of thousands of Portuguese who survived the great 1755 Lisbon earthquake were killed
by a tsunami which followed minutes later. Many townspeople fled to the waterfront,
believing the area safe from fires and from falling debris from aftershocks. Before the
great wall of water hit the harbor, waters retreated, revealing lost cargo and forgotten
shipwrecks.
The earthquake, tsunami, and subsequent fires killed more than a third of Lisbon's
pre-quake population of 275,000. Historical records of explorations by Vasco da Gama and
Christopher Columbus were lost, and countless buildings were destroyed (including most
examples of Portugal's Manueline architecture). The destruction of Lisbon sharply checked
the colonial ambitions of the Portuguese Empire. Europeans of the 18th century struggled
to understand the disaster within religious and rational belief systems. Philosophers of
the Enlightenment, notably Voltaire, wrote about the event. The philosophical concept of
the sublime, as described by philosopher Immanuel Kant in the Observations on the Feeling
of the Beautiful and Sublime, took inspiration in part from attempts to comprehend the
enormity of the Lisbon quake and tsunami.
Many animals sensed danger and fled to higher ground before the water arrived. The Lisbon
quake is the first documented case of such a phenomenon in Europe. The phenomenon was also
noted in Sri Lanka in the 2004 Indian Ocean earthquake. Some scientists speculate that
animals may have an ability to sense subsonic Rayleigh waves from an earthquake minutes or
hours before a tsunami strikes shore.[1] (http://www.slate.com/id/2111608/)
1883 - Krakatoa explosive eruption
The island volcano of Krakatoa in Indonesia exploded with devastating fury in 1883,
blowing its underground magma chamber partly empty so that much overlying land and seabed
collapsed into it. A series of large tsunami waves was generated from the explosion, some
reaching a height of over 40 meters above sea level. Tsunami waves were observed
throughout the Indian Ocean, the Pacific Ocean, the American West Coast, South America,
and even as far away as the English Channel. On the facing coasts of Java and Sumatra the
sea flood went many miles inland and caused such vast loss of life that one area was never
resettled but went back to the jungle and is now the Ujung Kulon nature reserve.
1946 - Pacific Tsunami
The Aleutian Island earthquake tsunami that killed 165 people on Hawaii and Alaska
resulted in creation of tsunami warning system established in 1949 for Pacific Ocean area
countries.
1960 - Chilean tsunami
The Great Chilean Earthquake, at magnitude 9.5 the largest earthquake ever recorded, off
the coast of South Central Chile, generated one of the most destructive tsunamis of the
20th century. It spread across the entire Pacific Ocean, with waves measuring up to 25
meters high. When the tsunami hit Onagawa, Japan, almost 22 hours after the quake, a tide
gauge recorded a wave height of 10 feet above high tide. The number of people killed by
the earthquake and subsequent tsunami is estimated to be between 490 to 2,290.
1964 - Good Friday tsunami
After the magnitude 9.2 Good Friday Earthquake, tsunamis struck Alaska, British Columbia,
California and coastal Pacific Northwest towns, killing 122 people. The tsunamis were up
to 6 m tall, and killed 11 people as far away as Crescent City, California.
2004 - Indian Ocean tsunami
Main article: 2004 Indian Ocean Earthquake
Animation of the 2004 Indonesian TsunamiThe magnitude 9.0 2004 Indian Ocean Earthquake
triggered a series of lethal tsunamis on December 26, 2004 that killed more than 160,000
people, making it the deadliest tsunami in recorded history. The tsunami killed people
over an area ranging from the immediate vicinity of the quake in Indonesia, Thailand and
the north-western coast of Malaysia to thousands of kilometres away in Bangladesh, India,
Sri Lanka, the Maldives, and even as far as Somalia and Kenya in eastern Africa.
Unlike the Pacific Ocean, there is no organised alert service covering the Indian Ocean.
This is in part due to the absence of major tsunami events since 1883 (the Krakatoa
eruption).
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Other
historical tsunamis
Other tsunamis that have occurred include the following:
January 20, 1606/1607: around the coast of the Bristol Channel (main
article) thousands of people were drowned, houses and villages swept away, farmland was
inundated and flocks were destroyed by a flood that might have been a tsunami. The cause
of the flood remains disputed, it is quite possible that it was caused by a combination of
meteorological extremes and tidal peaks (discussion
(http://www.severnsolutions.co.uk/twblog/archive/2005/01/06/greatflood1606)).
One of the worst tsunami disasters engulfed whole villages along Sanriku, Japan, in 1896.
A wave more than seven stories tall (about 20 m) drowned some 26,000 people. 1946: An
earthquake in the Aleutian Islands sent a tsunami to Hawaii, killing 159 people (five died
in Alaska).
1958:
A very localized tsunami in Lituya Bay, Alaska was the highest ever recorded: more than
500 m (1500 ft) above sea level. It did not extend much beyond the outlet of the fjord in
which it occurred but did kill two people in a fishing vessel.
1976:
August 16 (midnight) a tsunami killed more than 5000 people in the Moro Gulf region
(Cotabato city) of the Philippines.
1983:
104 people in western Japan were killed by a tsunami spawned from a nearby earthquake.
July 17,
1998: A Papua New Guinea tsunami killed roughly 2200 people. A 7.1 magnitude
earthquake 15 miles offshore was followed within 10 minutes by a tsunami about 12 m tall.
While the magnitude of the quake was not large enough to create these waves directly, it
is believed the earthquake generated an undersea landslide, which in turn caused the
tsunami. The villages of Arop and Warapu were destroyed.
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Other
tsunamis in South Asia
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Tsunamis in South Asia
(Source: Amateur Seismic Centre, India)[2] (http://asc-india.org/menu/waves.htm) |
| Date |
Location |
| 1524 |
Near Dabhol,
Maharashtra |
| 02 April 1762 |
Arakan Coast,
Myanmar |
| 16 June 1819 |
Rann of Kachchh,
Gujarat |
| 31 October 1847 |
Great Nicobar
Island |
| 31 December 1881 |
Car Nicobar Island |
| 26 August 1883 |
Krakatoa volcanic
eruption |
| 28 November 1945 |
Mekran coast,
Balochistan |
| 26 December 2004 |
Sumatra,Indonesia |
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