THE WORST EVER TSUNAMI IN ASIAN HISTORY

INDONESIAN TSUNAMI / 2004 INDIAN OCEAN TSUNAMI AND EARTHQUAKE

The 2004 Indian Ocean earthquake was an undersea megathrust earthquake that occurred at 00:58:53 UTC on Sunday, December 26, 2004, with an epicentre off the west coast of Sumatra, Indonesia. The quake itself is known by the scientific community as the Sumatra-Andaman earthquake.^[3][4] The resulting tsunami is given various names, including the 2004 Indian Ocean tsunami, South Asian Tsunami, Indonesian Tsunami, and Boxing Day Tsunami.

Whatever name they call it, the Tsunami had left a very big impact to the affected areas in Asian region which changed the lives of the survivors and the landscape forever.  

The earthquake was caused by subduction and triggered a series of devastating tsunamis along the coasts of most landmasses bordering the Indian Ocean, killing over 230,000 people in fourteen countries, and inundating coastal communities with waves up to 30 meters (100 feet) high.^[5] It was one of the deadliest natural disasters in recorded history. Indonesia was the hardest hit, followed by Sri Lanka, India, and Thailand.

With a magnitude of between 9.1 and 9.3, it is the third largest earthquake ever recorded on a seismograph. This earthquake had the longest duration of faulting ever observed, between 8.3 and 10 minutes. It caused the entire planet to vibrate as much as 1 centimetre (0.4 inches)^[6] and triggered other earthquakes as far away as Alaska.^[7] Its hypocentre was between Simeulue and mainland Indonesia.^[8]

The plight of the many affected people and countries prompted a worldwide humanitarian response. In all, the worldwide community donated more than $14 billion (2004 U.S. dollars) in humanitarian aid.^[9]

Perhaps it was the worst disaster ever suffered by the mankind in the modern world.






The disaster not only hit the countries in Asia but also countries in Africa. The tsunami hit the Indian Ocean and its huge waves destroyed any thing on its way up to the shore of South Africa.

According to the U.S. Geological Survey a total of 227,898 people died. Measured in lives lost, this is one of the ten worst earthquakes in recorded history, as well as the single worst tsunami in history. Indonesia was the worst affected area, with most death toll estimates at around 170,000.   However, another report by health minister of Indonesia, Fadilah Supari has estimated the death total to be as high as 220,000 in Indonesia alone, giving a total of 280,000 casualties.

TSUNAMIS

MEGATSUNAMIS

What is Megatsunami? Megatsunami (also known as iminami or "purification wave") ^[1] is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis. Unlike usual tsunamis, which originate from tectonic activity and the raising or lowering of the sea floor, known megatsunamis have originated from large scale landslides or impact events.

Concept

A megatsunami is meant to refer to a tsunami with an initial wave amplitude (height) measured in several tens, hundreds, or possibly thousands of meters.

Normal tsunamis generated at sea result from movement of the sea floor. They have a small wave height offshore, and a very long wavelength (often hundreds of kilometers long), and generally pass unnoticed at sea, forming only a slight swell usually of the order of 30 cm (12 in) above the normal sea surface. When they reach land the wave height increases dramatically as the base of the wave pushes the water column above it upwards.

By contrast, megatsunamis are caused by giant landslides and other impact events. Underwater earthquakes or volcanic eruptions do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes can, since they cause a massive amount of displacement. If the landslide or impact occurs in a limited body of water, as happened at the Vajont Dam (1963) and Lituya Bay (1958) then the water may be unable to disperse and one or more exceedingly large waves may result.

Two heights are sometimes quoted for megatsunamis - the height of the wave itself (in water), and the height to which it washes when it reaches land, which depending upon the locale, can be several times larger.

History of the hypothesis

Main article: 1958 Lituya Bay megatsunami

Geologists searching for oil in Alaska in 1953 observed that in Lituya Bay, mature tree growth did not extend to the shoreline as it did in many other bays in the region. Rather, there was a band of younger trees closer to the shore. Forestry workers, glaciologists, and geographers call the boundary between these bands a trim line. Trees just above the trim line showed severe scarring on their seaward side, whilst those from below the trim line did not. The scientists hypothesized that there had been an unusually large wave or waves in the deep inlet. Because this is a recently deglaciated fjord with steep slopes and crossed by a major fault, one possibility was a landslide-generated tsunami.^[2]

On 9 July 1958, an earthquake of magnitude 7.7 (on the Richter scale), caused 90 million tonnes of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force to create a wave approximately 524 metres (1,719 ft) high. Howard Ulrich and his son, Howard Jr., were in the bay in their fishing boat when they saw the wave. They both survived and reported that the wave carried their boat "over the trees" on one of the initial waves which washed them back into the bay, though the larger wave did not harm them a great lot. A similar tsunami out at sea could come tens of kilometers inland.

This event and evidence of a potentially similar past event at the same location inspired the term megatsunami.

List of Megatsunami

Prehistoric

1-The asteroid which created the Chicxulub crater in Yucatan approximately 65 million years BP would have generated some of the largest megatsunami in Earth's history.

2-A series of megatsunami were generated by the bolide impact that created the Chesapeake Bay impact crater, about 35.5 million years BP.^[3]

3-At Seton Portage, British Columbia, Canada, a freshwater megatsunami may have occurred approximately 10,000 BP.^[4] A huge block of the Cayoosh Range suddenly slid northwards into what had been a large lake spanning the area from Lillooet, British Columbia to near Birken, in the Gates Valley or Pemberton Pass to the southwest. The event has not been studied in detail, but the proto-lake (freshwater fjord) may have been at least as deep as the two present-day halves, Seton and Anderson Lakes, on either side of the Portage, suggesting that the surge generated by the giant landslide in the narrow mountain confines of the fjord valley may have been comparable in scale to Lituya Bay. Another more recent landslide on the south shore of Anderson Lake dropped a large portion of high mountainside down a debris chute, creating a rockwall "fan" which must have made a megatsunami-type wave, though not as large as the main one at the Portage.

4-Approximately 8,000 BP, a massive volcanic landslide off of Mt. Etna, Sicily caused a megatsunami which devastated the eastern Mediterranean coastline on three continents.^[5] 

5-In the Norwegian Sea, the Storegga Slide caused a megatsunami approximately 7,000 years BP.^[6] 

6-Approximately 6000 years ago, a landslide on Réunion island, to the east of Madagascar, may have caused a megatsunami.^[7] 

7-The recently discovered undersea Burckle Crater located at the bottom of the Indian Ocean would have caused a megatsunami at the time of impact estimated to be c. 3000–2800 BCE. It is unknown whether the Burckle Crater is connected to the Fenambosy Chevron which provides evidence of another megatsunami.

8-Evidence for large landslides has been found in the form of extensive underwater debris aprons around many volcanic ocean islands which are composed of the material which has slid into the ocean. The island of Molokai had a catastrophic collapse over a million years ago; this underwater landslide likely caused large tsunamis. In recent years, five such debris aprons have been located around the Hawaiian Islands. The Canary Islands have at least 14 such debris aprons associated with the archipelago.

Modern

1792: Mount Unzen, Japan

Main article: Mount Unzen

In 1792, Mount Unzen in Japan erupted, causing part of the volcano to collapse into the sea. The landslide caused a megatsunami that reached 100 meters (328 ft) high and killed 15,000 people in the local fishing villages.

1958: Lituya Bay, Alaska, USA

 

Main articles: 1958 Lituya Bay megatsunami and Lituya Bay

 

Damage from the 1958 Lituya Bay megatsunami can be seen in this oblique aerial photograph of Lituya Bay, Alaska as the lighter areas at the shore where trees have been stripped away.

On 9 July 1958, a giant landslide at the head of Lituya Bay in Alaska, caused by an earthquake, generated a wave with an initial amplitude of 524 meters (1,719 ft). This is the highest wave ever recorded, and surged over the headland opposite, stripping trees and soil down to bedrock, and surged along the fjord which forms Lituya Bay, destroying a fishing boat anchored there and killing two people. Howard Ulrich and his son managed to ride the wave in their boat, and both survived.

1963: Vajont Dam, Italy

Main article: Vajont Dam

On 9 October 1963, a landslide above Vajont Dam in Italy produced a 250 m (820 ft) surge that overtopped the dam and destroyed the villages of Longarone, Pirago, Rivalta, Villanova and Faè, killing nearly 2,000 people.

1980: Spirit Lake, Washington, USA

Main articles: Spirit Lake (Washington), 1980 eruption of Mount St. Helens, and Mount St. Helens

On May 18, 1980, the upper 460 meters of Mount St. Helens failed and detached in a massive landslide. This released the pressure on the magma trapped beneath the summit bulge which exploded as a lateral blast, which then released the over-pressure on the magma chamber and resulted in a plinian eruption. One lobe of the avalanche surged onto Spirit Lake, causing a megatsunami which pushed the lake waters in a series of surges, which reached a maximum height of 260 metres^[8] above the pre-eruption water level (~975 m asl). Above the upper limit of the tsunami, trees lie where they were knocked down by the pyroclastic surge; below the limit, the fallen trees and the surge deposits were removed by the megatsunami and deposited in Spirit Lake.^[9]

Potential Future Megatsunami

Experts interviewed by the BBC think that a massive landslide on a volcanic ocean island is the most likely future cause of a megatsunami.^[10] The size and power of a wave generated by such means could produce devastating effects, travelling across oceans and inundating up to 25 kilometres (16 mi) inland from the coast.

British Columbia

Some geologists consider an unstable rock face at Mount Breakenridge, above the north end of the giant fresh-water fjord of Harrison Lake in the Fraser Valley of southwestern British Columbia, Canada, to be unstable enough to collapse into the lake, generating a megatsunami that might destroy the town of Harrison Hot Springs (located at its south end).^[11]

Canary Islands

Geologists S. Day and S. Ward consider that a megatsunami could be generated during a future eruption involving the Cumbre Vieja on the volcanic ocean island of La Palma, in the Canary Islands.^[12][13] In 1949, the Cumbre Vieja volcano erupted at its Duraznero, Hoyo Negro and San Juan vents. During this eruption, an earthquake with an epicentre near the village of Jedy occurred. The following day Rubio Bonelli, a local geologist, visited the summit area and discovered that a fissure about 2.5 kilometers (2 mi) long had opened on the eastern side of the summit. As a result, the western half of the Cumbre Vieja (which is the volcanically active arm of a triple-armed rift) had slipped about 2 meters (7 ft) downwards and 1 meters (3 ft) westwards towards the Atlantic Ocean.

The Cumbre Vieja volcano is currently in a dormant stage, but will almost certainly erupt again in the future. Day and Ward hypothesize^[12][13] that if such an eruption causes the western flank to fail, a megatsunami will be generated.

La Palma is currently the most volcanically active island in the Canary Islands Archipelago. It is likely that several eruptions would be required before failure would occur on Cumbre Vieja.^[12][13] However, the western half of the volcano has an approximate volume of 500 cubic kilometres (120 cu mi) and an estimated mass of 1,500,000,000,000 metric tons (1.7×10¹² short tons) If it were to catastrophically slide into the ocean, it could generate a wave with an initial height of about 1,000 metres (3,300 ft) at the island, and a likely height of around 50 metres (164 ft) at the Caribbean and the Eastern North American seaboard when it runs ashore eight or more hours later. Tens of millions of lives would be lost as New York, Boston, Baltimore, Washington, D.C., Miami, Havana, and many other cities near the Atlantic coast are leveled. The likelihood of this happening is a matter of vigorous debate.^[14]

The last Cumbre Vieja eruption occurred in 1971 at the southern end of the sub-aerial section without any movement. The section affected by the 1949 eruption is currently stationary and does not appear to have moved since the initial rupture.^[15]

Geologists and volcanologists also disagree about whether an eruption on the Cumbre Vieja would cause a single large gravitational landslide or a series of smaller landslides.

Hawaii

Prehistoric sedimentary deposits on the Kohala Volcano, Lanai and Molokai controversially indicates that landslides from the flank of the Kilauea and Mauna Loa volcanoes in Hawaii may have triggered past megatsunamis, most recently at 120,000 BP.^[16][17][18] A future tsunami event is also possible, with the tsunami potentially reaching up to about 1 kilometre (3,300 ft) in height.^[19][20] According to a documentary called National Geographic's Ultimate Disaster: Tsunami, if a big landslide occurred at Mauna Loa, a 30 metres (98 ft) tsunami would take only thirty minutes to reach Honolulu, Hawaii. There, hundreds of thousands of people would be killed as the tsunami leveled Honolulu and traveled 25 kilometres (16 mi) inland.

HISTORIC TSUNAMIS

Recorded Historic Tsunamis - Part 1

This article is telling about the lists of notable historic tsunamis, which are sorted by the date and location that the tsunami occurred, the earthquake that generated it, or both.

Because of seismic and volcanic activities at tectonic plate boundaries along the Pacific Ring of Fire, tsunamis occur most frequently in the Pacific Ocean, but are also worldwide natural phenomena. Tsunamis are possible wherever large bodies of water are found, including inland lakes, where they can be caused by landslides and glacier calving. Very small tsunamis, non-destructive and undetectable without specialized equipment, occur frequently as a result of minor earthquakes and other events.

As early as 426 BC, the Greek historian Thucydides inquired in his book History of the Peloponnesian War (3.89.1-6) about the causes of tsunamis. He argued rightly that it could only be explained as a consequence of ocean earthquakes, and could see no other possible causes for the phenomenon.^[1]

Crete and the Argolid and other locations were destroyed by a tsunami caused by the eruption of Thira, which destroyed Minoan civilization on Crete and related cultures in the Cyclades and in areas facing the eruption on the Greek mainland such as the Argolid.

During the Persian siege of the sea town Potidaea, Greece, in 479 BC,^[2] the Greek historian Herodotus reports how the Persian attackers who tried to exploit an unusual retreat of the water were suddenly surprised by "a great flood-tide, higher, as the people of the place say, than any one of the many that had been before". Herodotus attributes the cause of the sudden flood to the wrath of Poseidon.^[3]

Here are some facts provided by the history about the tsunamis:

Tsunamis - Before 1000 AD

 

1) 6100 BC: Norwegian Sea

Main article: Storegga Slide

The Storegga Slides occurred 100 km north-west of the Møre coast in the Norwegian Sea, causing a very large tsunami in the North Atlantic Ocean. This collapse involved an estimated 290 km length of coastal shelf, with a total volume of 3,500 km³ of debris.^[4] Based on carbon dating of plant material recovered from sediment deposited by the tsunami, the latest incident occurred around 6100 BC.^[5] 

In Scotland, traces of the subsequent tsunami have been recorded, with deposited sediment being discovered in Montrose Basin, the Firth of Forth, up to 80 km inland and 4 metres above current normal tide levels.

 

2) 1600 BC: Santorini, Greece

Main article: Minoan eruption

The volcanic eruption on Santorini, Greece is assumed to have caused severe damage to cities around it, most notably the Minoan civilization on Crete. A tsunami is assumed to be the factor that caused the most damage.

 

3) 426 BC: Maliakos Gulf, Greece

Main article: 426 BC Maliakos Gulf tsunami

In the summer of 426 BC, a tsunami hit hard the Maliakos bay in Eastern Greece.^[6] The Greek historian Thucydides (3.89.1-6) described how the tsunami and a series of earthquakes intervened with the events of the raging Peloponnesian War (431-404 BC) and correlated for the first time in the history of natural science quakes and waves in terms of cause and effect.^[7]

 

4) 373 BC: Helike, Greece

An earthquake and a tsunami destroyed the prosperous Greek city Helike, lying 2 km away from the sea. The fate of the city, which remained permanently submerged, was often commented upon by ancient writers^[8] and may have inspired the contemporary Plato to the myth of Atlantis.

 

5) 365 AD: Alexandria, Eastern Mediterranean

Main article: 365 Crete earthquake

In the morning of July 21, 365 AD, an earthquake of great magnitude caused a huge tsunami more than 100 feet high. It devastated Alexandria and the eastern shores of the Mediterranean, killing thousands and hurling ships nearly two miles inland.^[9][10] The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15-19) describes in his vivid account the typical sequence of the tsunami including an incipient earthquake, the sudden retreat of the sea and a following gigantic wave:

Slightly after daybreak, and heralded by a thick succession of fiercely shaken thunderbolts, the solidity of the whole earth was made to shake and shudder, and the sea was driven away, its waves were rolled back, and it disappeared, so that the abyss of the depths was uncovered and many-shaped varieties of sea-creatures were seen stuck in the slime; the great wastes of those valleys and mountains, which the very creation had dismissed beneath the vast whirlpools, at that moment, as it was given to be believed, looked up at the sun's rays. Many ships, then, were stranded as if on dry land, and people wandered at will about the paltry remains of the waters to collect fish and the like in their hands; then the roaring sea as if insulted by its repulse rises back in turn, and through the teeming shoals dashed itself violently on islands and extensive tracts of the mainland, and flattened innumerable buildings in towns or wherever they were found. Thus in the raging conflict of the elements, the face of the earth was changed to reveal wondrous sights. For the mass of waters returning when least expected killed many thousands by drowning, and with the tides whipped up to a height as they rushed back, some ships, after the anger of the watery element had grown old, were seen to have sunk, and the bodies of people killed in shipwrecks lay there, faces up or down. Other huge ships, thrust out by the mad blasts, perched on the roofs of houses, as happened at Alexandria, and others were hurled nearly two miles from the shore, like the Laconian vessel near the town of Methone which I saw when I passed by, yawning apart from long decay.^[9]

The tsunami in 365 AD was so devastating that the anniversary of the disaster was still commemorated annually at the end of the 6th century in Alexandria as a "day of horror."^[11]

Researchers at the University of Cambridge recently carbon dated corals on the coast of Crete which were lifted 10 metres and clear of the water in one massive push. This indicates that the tsunami of 365 AD was generated by an earthquake in a steep fault in the Hellenic trench near Crete. The scientists estimate that such a large uplift is only likely to occur once in 5,000 years, however the other segments of the fault could slip on a similar scale - and could happen every 800 years or so. It is unsure whether "one of the contiguous patches might slip in the future."^[12]

 

6) 684 AD: Hakuho, Japan (白鳳大地震)

Japan is the nation with the most recorded tsunamis in the world. The number of tsunamis in Japan totals 195 over a 1,313 year period (thru 1997), averaging one event every 6.73 years, the highest rate of occurrence in the world.

The Great Hakuho Earthquake was the first recorded tsunami in Japan. It hit in Japan on November 29, 684. It occurred off the shore of the Kii Peninsula, Nankaido, Shikoku, Kii, and Awaji region. It has been estimated to be a magnitude 8.4 ^[13] It was followed by a huge tsunami, but no estimates on how many deaths.^[14]

 

7) 869 AD: Sendai, Japan

Main article: 869 Sanriku earthquake and tsunami

The Sendai region was struck by a major tsunami that caused flooding extending 4 km inland from the coast. The town of Tagajō was destroyed, with an estimated 1,000 casualties.

 

8) 887 AD: Ninna Nankai, Japan (仁和南海地震)

On August 26 of the Ninna era, there was a strong shock in the Kyoto region, causing great destruction and some victims. At the same time, there was a strong earthquake in Osaka, Shiga, Gifu, and Nagano prefectures. A tsunami flooded the coastal locality, and some people died. The coast of Osaka and primarily Osaka Bay suffered especially heavily from the tsunami. The tsunami was also observed on the coast of Hyuga-Nada.^[13]

TSUNAMI - Part 5

Warnings and Predictions

See also: Tsunami warning system

 

Tsunami warning sign

 

 

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 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 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 that are attached to buoys. The sensors constantly monitor the pressure of the overlying water column. This is deduced through the calculation:

where
P = the overlying pressure in newtons per metre square,
ρ = the density of the seawater= 1.1 x 10³ kg/m³,
g = the acceleration due to gravity= 9.8 m/s² 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

or about 5500 tonnes-force per square metre.

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.^[27]

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.

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  Tsunami hazard sign at Bamfield, British Columbia
  
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  A tsunami warning sign on a seawall in Kamakura, Japan, 2004
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  The monument to the victims of tsunami at Laupahoehoe, Hawaii
  
  
  

  

  Tsunami memorial in Kanyakumari beach
  

A seawall at Tsu, Japan

 

 

Tsunami Evacuation Route signage along U.S. Route 101, in Washington

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.^[28][29] 

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.

Mitigation

See also: Tsunami barrier

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.^[30] 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.^[31]

As a weapon

There have been studies and at least one attempt to create tsunami waves as a weapon. 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.^[32]

TSUNAMI -Part 4

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 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).^[22] 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 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.^[23] 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.^[23] 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.^[24]

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.

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.

 

 

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 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.

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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.^[25]

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 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.^[25] Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely used. Abe introduced the tsunami magnitude scale M_t, 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.^[26]

 

TSUNAMI - Part 3

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.^[17] This displacement of water is usually attributed to either earthquakes, landslides, volcanic eruptions,glacier calvings or more rarely by meteorites and nuclear tests.^[18][19] 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.^[20] 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.

                                              Drawing of tectonic plate boundary before earthquake

                                             Overriding plate bulges under strain, causing tectonic uplift.

                                             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),^[21] 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 (M_w 9.5) (19:11 hrs UTC), 1964 Alaska earthquake (M_w 9.2), 2004 Indian Ocean earthquake (M_w 9.2) (00:58:53 UTC) and 2011 Tōhoku earthquake (M_w9.0) are recent examples of powerful megathrust earthquakes that generated tsunamis (known as teletsunamis) that can cross entire oceans. Smaller (M_w 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 discovered that larger tsunamis than had previously been believed possible could be caused by giant landslides. 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.