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The horizontal mantle current eventually meets an opposing current, and they both turn downward into the mantle again at the site of one of the trenches . Enormous pressures are produced in this region, for the continental crust and the oceanic crust are moving toward each other. (The continental crust is being carried, so to speak, on the back of the mantle current. On the other hand, the oceanic crust is believed by some geophysicists to be identical with the mantle current--it is simply the solid and somewhat altered top layer of the mantle current.) The descending mantle current tends to drag the crust down with it, forming a deep trench or piling up young mountains. At the same time, the continental crust tends to ride over the oceanic crust, for it is the lighter of the two. The trenches may be filled as the advancing edge of the continental crust is thrust up to form mountains, and numerous earthquakes originate from the plane along which the oceanic crust is forced to slip under the continental crust.

The horizontal mantle current eventually meets an opposing current, and they both turn downward into the mantle again at the site of one of the trenches . Enormous pressures are produced in this region, for the continental crust and the oceanic crust are moving toward each other. (The continental crust is being carried, so to speak, on the back of the mantle current. On the other hand, the oceanic crust is believed by some geophysicists to be identical with the mantle current--it is simply the solid and somewhat altered top layer of the mantle current.) The descending mantle current tends to drag the crust down with it, forming a deep trench or piling up young mountains. At the same time, the continental crust tends to ride over the oceanic crust, for it is the lighter of the two. The trenches may be filled as the advancing edge of the continental crust is thrust up to form mountains, and numerous earthquakes originate from the plane along which the oceanic crust is forced to slip under the continental crust.

When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. Even though the oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the deepest part of the subducting plate breaks into smaller pieces. These smaller pieces become locked in place for long periods of time before moving suddenly and generating large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.

An earthquake is caused by a sudden slip on a fault. A fault is a fracture in the crust of the earth along which rocks on one side have movedrelative to those on the other side. Stresses in the earth’s outer layer push the sides of the fault together, pressure builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the the shaking that we feel during an earthquake.

Stresses in the earth’s outer layer cause a pushing effect against the sides of the fault, because of this motion, rocks slip or collide against each other releasing energy. This energy travels in waves through the earth's crust and causes the shaking that we feel during an earthquake. Under the surface of the earth, the two sides of a fault are constantly moving, relative to one another. The movement of these two sides is not smooth and is accompanied by a gradual build-up of elastic strain energy within the rocks along the fault. Eventually, the strain along the fault becomes too much. The fault then ruptures with a sudden movement releasing all the energy it has built up. This energy is released in the form of vibrations called 'seismic waves'. These waves travel along the surface and through the earth at varying speeds depending on the material through which they move. It is actually these seismic waves that cause most of the destructive effects, which we associate with earthquakes.. When these seismic waves reach the surface of the earth, they give rise to strong ground motion causing building and other man-made structures to shake or collapse or develop cracks and fissures. Earthquakes can also cause landslides,sudden eruptions as in the case of a hot lava flow from a volcano or giant waves called tsunamis. Sometimes new land mass are also formed.

This is the Great Alaskan Earthquake of 1964 was an earthquake so bad it triggered a tsunami. The 9.2 earthquake released underwater landslides that created devastating tsunamis all along the North American coastline from San Francisco to British Columbia. The earthquake North America’s largest ever and the second largest in recorded human history.

Earthquake of May 12, 2008 called the Great Sichuan Earthquake. Had its epicenter in Sichuan province but could be felt over 1,000 miles away in Shanghai and over 900 miles away in Beijing.

 

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