Tsunamis Their Causes and Damage Term Paper

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Tsunamis

A succinct definition of a tsunami is " ... A natural phenomenon consisting of a series of waves generated when water in a lake or the sea is rapidly displaced on a massive scale. ( Wikipedia: Tsunami) A more explicit and technical definition is, " A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water. Tsunamis are sometimes called "seismic sea waves." ( Nelson A.)

The essential characteristic of a tsunami is therefore a radical displacement of water. This displacement of water can be caused by various events, such as, earthquakes, ocean landslides, volcanic eruptions and large meteorite impacts as well as large explosions. The effect of a Tsunami depends on the severity and size of the initial causative factors and the original displacement of water mass. The effects can range from a minor and hardly noticeable ocean surge to complete devastation. Tsunamis are often not even noticed out at sea. This is due to another tsunami characteristic, namely that they "have a much smaller amplitude offshore, and a very long wavelength -often hundreds of kilometers long," (Wikipedia: Tsunami) A tsunami can reach speeds of over 500 mph or 800 kph. The average heights of a tsunami wave are 30 feet or nine meters. However tsunamis of have been reordered of over 100 feet to 30 meters. (What are tsunamis)

The main areas that are threatened by Tsunamis are coasts throughout the Pacific Ocean, where earthquakes are a frequent occurrence. At risk are the islands of Japan and Hawaii and the Alaskan and northern South American coasts. Tsunamis also occur in the Atlantic Ocean, and Mediterranean Sea

In the last 200 years tsunamis have affected the parts of the United States.

'Since 1946, six tsunamis have killed more than 350 people and caused significant property damage in Hawaii, Alaska, and along the West Coast. Tsunamis have also occurred in Puerto Rico and the Virgin Islands."

(Rosenberg, M.)

2. Causes

Simply stated, tsunamis are phenomenon created by the displacement of water mass. As this displaced mass moves according to gravitational forces, it radiates across the waters with a ripple effect. As they approach the land, tsunamis take on the characteristics of tide surge. The resemblance to a sudden tide surge has resulted in the name "tidal wave" being applied to them. However, they are not related to tides or tidal actions in any sense.

The most common cause of a tsunami is the displacement of the sea bed structure. This is usually a result of intense earthquake action. "Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water." Such large vertical movements of the earth's crust can occur at plate boundaries and subduction zones. Subduction zones " ... exist at convergent plate boundaries where Oceanic lithosphere collides with either continental lithosphere or oceanic lithosphere and sinks below the latter plate into the mantle." ( Wikipedia: subduction) Subduction earthquakes are one of the main generators of tsunamis. They occur when there is movement of the denser oceanic plates which move beneath the continental plates.

The sense of displacement of the ocean floor is an important factor in wave generation. Tsunamis are formed when there is a vertical displacement of the ocean floor. Therefore, it is not the case that even large earthquakes will necessarily produce large tsunami. For example, the 1906 earthquake near San Francisco California had a Richter Magnitude of about 7.1, yet no tsunami was generated as a result of this event " because the motion on the fault was strike-slip motion with no vertical displacement. " ( Nelson A.)

The reason why so many tsunamis are generated in the Pacific Ocean is mainly due to the fact that the Pacific is surrounded by plate boundaries and most of these events are the result of earthquakes which occur along the subduction boundaries.

The process of the tsunamis aproach to the shore is described as follows.

As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami increases. The change of total energy of the tsunami, however, remains constant.

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Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase. Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters

( Nelson A.)

Tsunamis can reach incredible heights such as the Alaskan Lituya Bay tsunami which produced a water wave estimated at 50 -- 150 metes. There are numerous example of tsunami generated an earthquake. In 1948 a magnitude 7.3 earthquake took place near Unimak Island in the Aleutian Islands west of Alaska, near the Alaska Trench generating a large tsunami and in 1960 the coast of Chile was hit by a tsunami, when an earthquake occurred along the subduction zone near South America. In this case the population of Chile, who were familiar with earthquakes and tsunamis, had already moved to higher ground before the large tsunami reached the coast. ( ibid)

This was not however not the case in the Sumatran tsunami of Dec. 26, 2004. This was the largest and most devastating tsunami in recorded history. The force of the tsunami killed approximately 200,000 people and the region is still reeling from the aftermath and devastation of the phenomenon. This tsunami was generated by an undersea earthquake measuring at 9.3 in magnitutrude. (Wikipedia: Ocean Earthquake)

Another albeit less common cause of tsunami occurrences is volcanic eruptions. Volcanoes that are situated coastal areas, such as Japan, can create tsunamis. The massive explosion can displace water with the same effect as a tsunami created by an earthquake. "Explosive eruptions can rapidly emplace pyroclastic flows into the water, landslides and debris avalanches produced by eruptions can rapidly move into water, and collapse of volcanoes to form calderas can suddenly displace the water. "( Nelson A.) An example of tsunamis caused in this way was the eruption of Krakatau in the Straights of Sunda, near Sumatra in 1883. A number of tsunamis were created in this explosion and 36,417 people were killed. ( ibid)

Tsunami can also be caused underwater explosions and nuclear testing by the Unites States in the 1940's near the Marshal Islands generated tsunamis. It is also conjectured that tsunami can be caused by the impact of meteors. There is no historical proof of this, but scientists claim that the impact of a meteor during the Cretaceous Period, approximately 65 million years ago, caused a large tsunami along the Gulf coast of Mexico

3. Description and effects

The appearance of a tsunami is closer to an oncoming tide rather than the popular image of a large 'tidal wave'. The force of a tsunami moves or overruns any obstacle in its path. In actuality the central force and most destructive part of a tsunami lies behind the initial wave front. The central and most destructive force is located in the large water mass that lies behind the first waves. The power and weight of this larger water mass is capable of destroying anything in its path; " ... often reducing buildings to their foundations and scouring exposed ground to the bedrock. Large objects such as ships and boulders can be carried several miles inland before the tsunami subsides." ( Wikipedia. Tsunami)

Tsunamis contain immense energy. They are also capable of traveling for large distances across the ocean with very little loss of energy. Therefore a tsunami is capable of creating destruction and havoc hundreds and even thousands of kilometers from its point of origin. This also means that there can be large expanses of time between the initial cause of the tsunami, such as an earthquake, and the actual impact on the shore. "When they do, they can carry the largest ocean vessels miles (kilometers) inland, inundate coasts with flood water, and drag entire communities out to sea as they recede. (What are tsunamis?)

Most of the damage caused by a tsunami is a result of the destructive power of the wave and water. There are secondary effects from the debris that is carried in the powerful water surge. Erosion from the water can also undermine the foundations of buildings and cause extensive damage and danger to those in and around them.

However it is often in the aftermath of the tsunami that much of the damage is done. "Tertiary effects include loss of crops and water and.....

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