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Wednesday, July 10, 2013

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An earthquake is a shaking of the ground caused by the sudden dislocation of material within the earth's outer layer, or crust. When forces pushing on a mass of rock overcome the friction holding the rock in place and blocks of rock slip against each other an earthquake may occur. Some earthquakes are so slight, and some occur in such remote areas, that they are barely felt. Others are so violent that they cause extensive damage.
Earthquakes are caused by stresses below the earth's outer surface. These stresses usually build up until the rocks fracture along a "fault plane." This causes vibrations, also known as seismic waves. Seismic waves will then travel in all directions from the area of fracture. In large earthquakes seismic waves may be detected over the entire earth. 

Earthquakes can be caused by volcanoes in certain cases. Nuclear explosions under the ground can create waves that are very similar to natural seismic waves. The seismic energy created in an atomic bomb is one hundred-thousandth that of the largest earthquake.
Earthquakes take place or have taken place in all parts of the world. Frequent activity occurs along certain belts. 80% of all seismic energy is generated from a belt that is found at the border of the Pacific Ocean. A great deal of volcanoes is also found there, and volcanoes set off many earthquakes. Japan, the Philippine Islands, New Guinea, and New Zealand are all part of the Pacific belt.

A second seismic belt produces 15% of seismic activity. It goes through southern Asia to the region of the Mediterranean Sea. The final 5% of seismic energy comes from parts of the Arctic, Atlantic, and Indian Oceans. Antarctica and Australia experiencethe least amount of earthquake activity then any other areas of the world.
Earthquakes produce various damaging effects to the areas they act upon. This includes damage to buildings and in worst cases the loss of human life. The effects of the rumbling produced by earthquakes usually leads to the destruction of structures such as buildings, bridges, and dams. They can also trigger landslides. An example of how an earthquake can lead to even more destruction is the 1959 earthquake near Hebgen, Montana. It caused a land slide that killed several people and blocked the Madison River. Due to the fact that the Madison River was blocked, a lake was created which later flooded the nearby town of Ennis.
Besides producing floods and destroying buildings, earthquakes that take place under the ocean can sometimes cause tsunamis, or tidal waves. Tsunamis are high and long walls of water which travel at a very rapid rate. They are notorious for destroying entire populations and cities near coastlines. In 1896 Sanriku, Japan, with a population of 20,000, suffered such a fate.







Nguồn: Internet

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