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When a large earthquake happens beneath an ocean floor, it can change the level of the floor suddenly, raising and lowering it substantially. This produces a large disturbance in the sea. The size and energy of disturbance depends on the magnitude of the earth quake.
Most severe earthquakes occur near the subduction zone of the tectonic plates. A wave starts spreading out. The height of the wave might be only a few meters, but this wave is very different from the normal oceanic waves produced by the action of the wind on the surface.
This wave invokes up and down movement of the whole column of the ocean above the affected zone that might be hundreds of kilometres in length. The speed of the wave in the deep ocean is nearly the same as the cruising speed of a jet liner, namely hundred kilometres per hour.
In the middle of the ocean surface, this wave might be seen as a gentle swell and fall of the ocean surface and does not represent a major hazard to boats and ships.
But it becomes dangerously high and devastating when it approaches the coast. This is called the much feared tsunami. Tsu means harbour and nami stands for wave. Tsunamis are large waves that are generated when the sea floor is deformed by seismic activity, vertically displacing the overlying water in the ocean.
An earthquake occurred with its epicentre km south-southwest of Sumatra in December The magnitude of the earthquake was 8. That is why, it was most powerful in the world in the past 40 years. Most of the destruction was caused by seismic waves or tsunami that hit India, Sri Lanka, Malaysia and Thailand within two hours of the first impact of earthquake.
It has been observed that the Sumatra quake occurred at a place where several massive geological plates push against each other with a strong force.
The survey indicates that km section along the boundary of the plates shifted motion that triggered the sudden displacement, causing the huge tsunamis. It may be several meters high when it hits the sea shore.
Tsunami may not be one giant wave but a series of waves that come to the coast in a short interval.
An earthquake occurred on 8th May, at Siachuan in China, which caused widespread destruction. However, this time earthquake has not caused widespread destruction and loss of lives, because the strength of tsunami was very low.
The principal generation mechanism or cause of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to earthquakes, landslides, volcanic eruptions and glacier calvings or more rarely by meteorites and nuclear test.
The waves formed in this way are then sustained by gravity. Tides do not play any part in the generation of tsunamis. Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water.
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.
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 meters which is why they generally pass unnoticed at sea, forming only a slight swell usually about millimeters 12 above the normal sea surface.
They grow in height when they reach shallower water, in a wave shoaling process described below.This disaster is estimated to have cost Indonesia IDR trillion (approx. USD $16 billion or percent of the country's gross domestic product) and it released some million tons of carbon each day (a figure that exceeds the million tons of daily carbon emissions in the European Union), thus being one of the worst ever natural disasters in human history.
Introduction A natural disaster is a major adverse event resulting from natural processes of the Earth; examples include floods, volcanic eruptions, earthquakes, tsunamis, and other geologic processes. - A natural disaster is an event caused by natural destructive factors, which can be further divided into either climatic disasters such as floods and tornados, or geological disasters such as earthquakes, which consequently lead to great physical damage or life loss (Bankoff, , 56).
A natural disaster is unforeseen, severe and immediate. Pollution, ozone depletion in the stratosphere and global warming come in this category. Natural disasters include cyclones, earthquakes, floods, drought (though these two are now being increasingly considered ‘man- made’ disasters) heat and cold waves, landslides, avalanches, flash floods, severe thunderstorms, hail, low level wind shears, and .
A natural disaster is unforeseen, severe and immediate.
Pollution, ozone depletion in the stratosphere and global warming come in this category. Natural disasters include cyclones, earthquakes, floods, drought (though these two are now being increasingly considered ‘man- made’ disasters) heat and cold waves, landslides, avalanches, flash floods, severe thunderstorms, hail, low level wind shears, and .
In the latest tsunami on 26th December, in Indian Ocean caused because of an undersea disturbance which was the result of an magnitude earthquake, just off the northern tip of Indonesia's Sumatra Islands. This created a havoc in Indonesia, Hong Kong, Sri Lanka and India.