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 a 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 a atomic bomb is one hundred-thousandth that of the largest earthquake.
Kinds of Waves
Some seismic waves travel through the earth's interior while others travel only over the earth's surface. There are two types of body waves. The primary, or P wave, is the faster of the two. P waves always travel at higher speeds than S waves, so whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide. These waves are transmitted through all parts of the earth. S waves, which are slower than P waves, are known as the shear waves. This type of wave causes lateral vibrations. S waves are not capable of travelling through liquids. S waves have not been detected in any part of the earth deeper than about 1,800 mi, which indicates that below this depth the earth is molten.
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.
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