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The wave, which carries the death (The Indian Ocean tsunami was 26.12.204)

Doctor of geological-mineralogical sciences N. Koronovskii 

The wave, which carries the death 

The earthquake that occurred December 26, 2004 off the coast of Indonesia caused a giant wave - the tsunami that struck the island of Sumatra, Sri Lanka, an island off the coast of Thailand, the east coast of India, the Maldives, and even in the coastal zone of Somalia in East Africa. The tsunami killed about 300,000 people and caused enormous material damage. According to UN estimates, the largest natural disaster that befell humanity in the last 100 years. What is a tsunami, how and where this phenomenon occurs, and whether it can be predicted?

The term "tsunami" comes from the Japanese language and literally means "big wave in the Gulf."Tsunamis occur during an earthquake in the sea or in the ocean when the conditions for the formation of the water column of powerful waves. These waves disperse in all directions from the epicenter, which is a projection of the hypocenter - the center of the conditional earthquake - on the surface of the seabed. The center is located in the interior of the Earth at a certain depth, usually within a few tens of kilometers. It was there, in the hearth, there are stresses and strains of rock, which lead to rupture and release the accumulated energy.

Not every earthquake that occurs in the ocean, causing a tsunami. The huge wave is formed when there is a sudden, very dramatic shift in the ocean floor, and very often the instantaneous vertical vzbrasyvanii (rise) of one of the wings of the tectonic rupture. Detailed analysis of tsunami excitation conditions showed that the maximum amplitude of tsunami waves occurs when the displacement of rocks occur at depths of about 10 km, and if the hypocenter is deeper, the amplitude decreases gradually.

 

The approximate scheme of formation of a tsunami
The approximate scheme of formation of a tsunami. Instantaneous displacement of the ocean floor in water causes the strain wave and long wave on the ocean surface. In the place where the ocean depth decreases to about half the wavelength, the latter begins to grow on the flat (shallow) side can reach a height of 20-40 m.
Above the tectonic displacement of the ocean floor in the surface layer of water there is a water hill, which, settling, forming waves, radiating as from a stone thrown into water, in all directions. In the open ocean, these waves have a very great length: the distance between two crests reaches 100-150 km. But the height of their small, only a few meters, it is very rare - tens of meters.

 

Thus, a sharp, almost instantaneous displacement of the bottom causes the simultaneous lifting of the entire thickness of the ocean water and waves on the surface, diverging at a rate of up to 600-800 km / h. The greater the depth of the ocean, the higher the velocity of the waves, which is roughly proportional to the square root of depth. Being in the open ocean on a ship or a yacht, a very long surface wave may not be noticed. But the situation changes when a wave approaches the shore shallows with a wide and gentle underwater slope.

The fact that the huge wave energy is redistributed as the friction of water on the bottom of the slow movement of the lower part of the water column, while its upper part is moving at a faster rate. This process starts to develop when the depth reaches about half the wavelength. When approaching the coast decreases as the speed of the wave and its length. For example, at a depth of about 1 km wave velocity of 350-360 km / h, and at a depth of 50 meters - less than 100 km / h

 

The scheme of the bottom topography north-eastern Indian Ocean
The scheme of the bottom topography north-eastern Indian Ocean. Well defined Trench. The yellow circle - the epicenter of the earthquake main shock December 26, 2004, circles of different color - magnitude earthquake at.
When the lower part of the wave begins to decelerate, a wave of "grow" by increasing its height, and all of its energy is concentrated in a relatively narrow front. On the crest of a growing wave of a white surf, and it takes an asymmetric shape: the inner side of the concave and steep, and external, facing the ocean - more than flat.

 

In the tsunami wave crest crowned with a gigantic breakers, and she, a height of 5, 10 or 30 meters, the whole mass of the giant wall of water hits the shore, and boiling water quickly rushes forward, sweeping away everything in its path. If the wave is a narrow bay, its height increases by several times, forming a water shaft (it is called "Forest"), which hit the shore like a volley of hundreds of guns. Gradually the power of the waves running out and the water begins its return run to the ocean, pulling any floating objects, cars, animals and people.

In the case of the recent disaster, which began the morning of December 26, 2004 in 7 hours 58 minutes 53 seconds local time in the Indian Ocean off the coast of Indonesia and Thailand, the epicenter of the first earthquake was near the northern tip of Sumatra Island, at coordinates 3 ° 30 'north latitude and 95 ° 87 'east longitude. In geological terms the area is the boundary between two lithospheric plates - large blocks of the crust.Thus there is a dive, subduction (subduction), the oceanic Indian plate beneath a continental plate east. Trench, stretching parallel to Sumatra, is a trace of such a dive.

 

Автор: admin от 05.03.2012





  

 

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