Tsunami Facts.....

For those who don't know.....

A tsunami (pronounced soo-NAH-mee) is a wave train, or series of waves, generated in a body of water by a pulsating or abrupt disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

Tsunamis are often called "tidal waves" as they may resemble a non-lunar-tidal rush of rapidly rising water, rather than big cresting waves reaching the shore. However, the term is discouraged by oceanographers since tsunamis are not related to tides.

The term "tsunami" comes from the Japanese tsu (harbor) and nami (wave).

Causes

Tsunamis are unlike wind-generated waves on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a surf beach, for example, spawned by a storm out in the pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wave length in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. 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/s/s.... hey, that's meters per second squared.... I bet Ms. Goth 'Genius' Daughter..knew that!) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.

Tsunamis 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 crust deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

 Schema of a Tsunami

Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, super-marine landslides and cosmic-body impacts 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 Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.

Physics

In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. A tsunami has a very long wave length (in the order of 100km), which makes it act as a shallow-water wave. Since the speed of a shallow-water wave is , where g is the gravitational acceleration and d is the water depth, a tsunami in the open ocean can obtain a speed of about 700 km/h. While in deep water a person at the surface of the water would probably not even notice, but the wave can increase to a height of 30m and more as it approaches the coastline and compresses. Tsunamis can cause severe destruction on coasts and islands, even at distances where the earthquake or other event that caused it is itself not even noticable without instruments.

Considering the fact that a tsunami can cause damage thousands of miles from its origin, there may be several hours between its creation and its impact on the coast, more than it takes for seismic waves to arrive.

Typically, tens of minutes before a tsunami, the sea will recede from the coast, exposing part of the seabed. If the slope is shallow, this recession can exceed 800 m. People unaware of the danger may stay at the shore, due to curiosity, but this may be a warning sign of a coming tsunami. There can be several waves, at two- to 45-minute intervals.

Megatsunamis

Evidence shows that megatsunamis(*1), which are caused by significant chunks of an island collapsing into the ocean, are also possible. Related to a tsunami is a seiche(*2). Often large earthquakes produce both tsunamis and seiches at the same time and there is evidence that some seiches have been caused by tsunamis.

Warning systems

Many cities around the Pacific, notably in Japan but also in Hawaii, have warning systems and evacuation procedures in the event of a serious tsunami. Tsunamis are predicted by various seismologic institutes around the world and their progress monitored by satellites.

Bottom pressure recorders with buoys as communication link are used to detect waves which would not be noticed by a human observer on deep water. The first rudimentary system to alert communities of an impending tsunami was attempted in Hawaii in the 1920s. More advanced systems were developed in the wake of the April 1, 1946 and may 23, 1960 tsunamis which caused massive devastation in Hilo, Hawaii. The United States created the Pacific Tsunami Warning Center (http://www.prh.noaa.gov/pr/ptwc/) in 1949, and linked it to an international data and warning network in 1965.

One system for providing tsunami warning is the CREST Project (Consolidated Reporting of Earthquakes and Tsunamis) implemented on the West coast (Cascadia), Alaska, and Hawaii of the United States by the USGS, NOAA, the Pacific Northwest Seismograph Network, and three other university seismic networks.

Tsunami prediction remains an imperfect science. Although the epicenter of a large underwater quake and the probable tsunami arrival times can be quickly calculated, it is almost always impossible to know whether massive underwater ground shifts have occurred, resulting in tsunami waves. As a result, false alarms are common.

No system can protect against a sudden tsunami. A devastating tsunami occurred off the coast of Hokkaido in Japan as a result of an earthquake on July 12, 1993. As a result, 202 people on the small island of Okushiri lost their lives, and hundreds more were missing or injured. This tsunami struck just three to five minutes after the quake and most victims were caught while fleeing for higher ground and secure places after surviving the earthquake.

While there remains the potential for sudden devastation from a tsunami, warning systems can be effective. For example if there were a very large subduction zone earthquake (magnitude 9.0) off the west coast of the United States, people in Japan, for example, would have up to 18 hours (and likely warnings from warning systems in Hawaii and elsewhere) before any tsunami arrived, giving them some time to evacuate areas likely to be affected.

History

Although tsunamis occur most frequently in the Pacific Ocean, they are known to occur anywhere. Many ancient descriptions of sudden and catastrophic waves exist, particularly in and around the Mediterranean. Thousands of Portuguese who survived the great 1755 Lisbon earthquake were killed by a tsunami which followed a few moments later. Before the great wave hit, the harbor waters retreated, revealing lost cargo and forgotten shipwrecks. In the North Atlantic, the Storegga Slide is a major incident.

The Krakatoa incident

The island volcano of Krakatoa in Indonesia, exploded with devastating fury in 1883. 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 wasnever resettled but went back to the jungle and is now the Ujung Kulon nature reserve.

22 May 1960 Chilean tsunami

The Grat Chilean Earthquake, the largest earthquake ever recorded, of the coast of Soutch 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.

26 Dec 2004 Indian Ocean tsunami The tsunami that struck Malé in the Maldives on December 26, 2004.

The most recent series of lethal tsunamis occurred on December 26, 2004, in the Indian Ocean, with fatalities last reported at 68,000, ranging from those in the immediate vicinity of the quake in Indonesia and Thailand to people thousands of kilometres away in Bangladesh, India, Sri lanka, the Maldives, and even Somalia 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 and an emphasis on developing a tropical cyclone warning system.

Tsunamis caused by a magnitude 9.0 earthquake under the sea west of Banda Aceh on the island of Sumatra in Indonesia caused severe devastation in the form of loss of property and lives on the Asian coasts of the Indian Ocean, particularly Sri Lanka, indonesia, India (most casualties have been reported in the state of Tamil Nadu and the Andaman and Nicobar Islands), Thailand (Esp. Phuket, where thousands of foreigners are missing), Maldives, Malaysia, Myanmar, and Bangladesh; and also in Somalia and Kenya in Africa. As of 10:14 pm EST on December 28, 2004, news reports estimate the total death toll at over 80,000, with Indonesia facing the brunt of the devastation with over 27,000 feared dead. The number of deaths caused by thistsunami is expected to double due to wide spread illness. Who fear that large number of corpses, if not handled properly, may cause contamination to the water, causing hasty dispersal of plague. Many foreign tourists are among the dead. 

  Animation of the 2004 Indonesian Tsunami

  Others

• One of the worst tsunami disasters engulfed whole villages along Sanriku, Japan, in 1896. A wave more than seven stories tall drowned some 26,000 people. More than 30,000 people died in Java from a 1883 tsunami cause by a volcanic eruption.

• 1946: An earthquake in the Aleutian Islands sent a tsunami to Hawaii, killing 159 people (only five died in Alaska).

• 1964: An Alaskan earthquake triggered a tsunami up to 20 feet tall that killed 11 people as far away as Crescent City, California and caused more than 120 deaths in all.

• 1983: 104 people in western Japan were killed by a tsunami spawned from a nearby earthquake.

• On July 17, 1998: A Papua New Guinea tsunami killed roughly 3,000 people. A 7.1 magnitude earthquake 15 miles offshore was followed within 10 minutes by a wave some 40 feet tall. The villages of Arop and Warapu were destroyed.

Future threats

In 2001, scientists predicted that a future eruption of the unstable Cumbre Vieja volcano in the Canary Islands could cause a supergiant undersea landslide which would cause a 100 m megatsunami to devastate the coast of northwest Africa, with a 30-50 m tsunami reaching the east coast of North America.

 

*1) A megatsunami is a rare tsunami more than 100 meters (325ft) high. Aside from some large tsunami in Alaska, including one 520 m high, the last megatsunami to hit a populated area is believed to have occurred 4,000 years ago. Geologists say it is usually caused by a very large landslide, such as a collapsing island, into a vast body of water such as an ocean or sea.   (This is oneof the dangers of Global Warming and the glaciers of the North Pole and Alaska melting.  If it warms up too fast, the ice will sheer off in massive arrow head form.)

Megatsunamis can rise to heights of hundreds of meters, travel at 890 km/h in mid-ocean and potentially reach 20 km inland in low-lying regions.

In deep ocean, a megatsunami is barely noticeable. It moves as a vertical shift of only a meter or so throughout the volume of water, with a crest to crest distance of hundreds of kilometers. However the huge amount of energy in the motion of this massive volume generates a much higher wave as it approaches shallow water.

Underwater earthquakes do not normally generate such large tsunamis unless they also trigger an underwater landslide — typically they have a height of less than ten meters.

Landslides that are large compared to the depth of water hit the water so fast that the displaced water cannot settle before the rocks hit the bottom. This means that the rocks displace the water at full speed all the way to the bottom. If the water is deep, the displaced volume is large and the lower parts are under high pressure. The resulting wave contains large amounts of energy.

Some have conjectured that historic megatsunamis underlie the deluge myths that are common to many cultures throughout the world. However this is unlikely, considering that megatsunamis usually occur without any warning, only hit coastal areas, and do not necessarily occur after a rain.

*2) A seiche (pronounced SAYSH) or an underwater wave is an underwater, irregular fluctuation or rhythmic rocking of the water level of a lake, first noted in Lake Geneva, Switzerland. Observers on the surface may not know a seiche is happening.

Scientifically, a seiche is a standing wave in an enclosed body of water and the definition does not include the causes.

Seiches are common between warm and cold-water layers, notably in Loch Ness, Scotland, and some evidence cited for the Loch Ness monster and the Lake Champlain monster may trace to seiches.

Small rhythmic seiches are almost always present on larger lakes, and the frequency of the oscillation is determined by the size of the body, its depth and contours, and the water temperature. On the North American Great Lakes seiche is often called slosh. It is always present but is usually unnoticeable except during periods of unusual calm.

Major seiches often occur during earthquakes and may be caused by wind or underwater landslides.  Lake Erie, because of its shallowness and elongation can occasionally have wind caused extreme seiches of up to 15 feet (5 meters) between the ends. The first appearance is similar to a storm surge like those caused by hurricanes along ocean coasts, but the seiche effect can cause oscillation back and forth across the lake for some time. Hurricane Hazel piled up water along the northwestern Lake Ontario shoreline near Toronto, causing extensive flooding, and established a seiche that subsequently caused flooding along the south shore. 

** All info and images from Wikipedia

So, now you know....

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