An Earthquake is in fact the shaking of the ground caused by sudden movements in the earth's crust. The biggest earthquakes are set off by the movement of tectonic plates. Some plates slide past each other gently, but others can cause a heavy pressure on the rocks, so they finally crack and slide past each other. By this, vibrations or shock waves are caused, which go through the ground. It is these vibrations or seismic waves which cause an earthquake. The closer to the source of the earthquake (the focus or hypocenter), the more damage occurs. Earthquakes are classified according to the depth of the focus.
|0-43 miles (0-70 km) below ground:||shallow earthquakes|
|43-186 miles (70-300 km) below ground:||intermediate earthquakes|
|deeper than 186 miles (300 km) below ground:||deep earthquakes|
The closer the focus to the surface, the heavier the earthquake. The earthquake is always the most intense on the surface directly above the focus (Epicenter). In general big earthquakes begin with light vibrations (foreshocks). These are the initial fractures in the rocks. After the main shock, there may be minor aftershocks, most of the time for months. This occurs as the rocks settle down.
|In general people are not so glad with earthquakes because they cause a lot of damage. On the other hand, earthquakes have been very helpful to get information about the inside of the earth. Vibrations (seismic waves) not only move to the earth's surface, but go to all sides. Big earthquakes can therefore be measured at places all over the world. All these vibrations from the earth are broken or diffracted by the different layers of the earth. The way which seismic waves are diffracted is helpful for scientists to understand the structure of the earth. By measuring many earthquakes and by data processing in computers, three-dimensional images can be made of the variations in the density and the temperature of the mantle.|
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