Tsunami..some facts and important education..
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.
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; practically unnoticeable to people on ships. The wave travels across the ocean at speeds from 500 to 1000 km/h, so 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.
Education and signs
The tsunami that struck Malé in the Maldives on December 26, 2004.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.
Past tsunamis
See also List of historic tsunamis by death toll.
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:
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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/)
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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.
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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.
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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.
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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.
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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, 1607: Bristol Channel thousands of people were drowned, houses and villages swept away, farmland inundated and flocks destroyed. It was commemorated in a contemporary pamphlet "God's warning to the people of England by the great overflowing of the waters or floods."
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 (only 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.
Other tsunamis in South Asia
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
Future threats
In 2001, scientists predicted that a future eruption of the unstable Cumbre Vieja volcano in La Palma (an island of the Canary Islands) could cause a giant undersea landslide. Later research showed that the threat was less than had originally been theorized. The next volcanic eruption is expected in the second half of the 21st century, but it is not necessarily the eruption that causes an immediate landslide. In the worst case scenario, the western half of the island (weighing perhaps 500 billion tonnes) would catastrophically slide into the ocean. Such a landslide could cause a 100 m wave to devastate the coast of northwest Africa, with a 10-25 m tsunami reaching the east coast of North America 7-8 hours later causing massive coastal devastation and the deaths of perhaps thousands of people, threatening Miami, suburbs of New York, parts of Boston, and all coastal cities in between. ([3] (http://en.wikipedia.org/wiki/Tsunami |
