Earthquake

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What Are Earthquakes and Why Do They Occur?
An earthquake is ground shaking caused by a sudden movement of rock in the Earth’s crust. Such movements occur along faults, which are thin zones of crushed rock separating blocks of crust. When one block suddenly slips and moves relative to the other along a fault, the energy released creates vibrations called seismic waves that radiate up through the crust to the Earth’s surface, causing the ground to shake.

Earthquakes may last only a few seconds or may continue for up to several minutes. They can occur at any time of the day or night and at any time of the year. They are caused by stress that builds up over time as blocks of crust attempt to move but are held in place by friction along a fault. (The Earth’s crust is divided into large plates that continually move over, under, alongside, or apart from one another atop the partly molten outer layer of the Earth’s core.) When the pressure to move becomes stronger than the friction holding them together, adjoining blocks of crust can suddenly slip, rupturing the fault and creating an earthquake. Back To Top

How Do Earthquakes Affect People?
Although thousands of earthquakes occur in the United States each year, most are too small to affect us. Earthquakes of larger magnitude, however, which release more energy during fault ruptures, can be hazardous, exposing us to the risk of harm or loss.

The stronger ground shaking generated in such events is unlikely to affect people directly (other than by startling or frightening them). It is what these ground motions can do to the natural and man-made environments around us that can significantly affect us by endangering our lives, property, and livelihoods.

Intense ground shaking can generate many sources of potential harm or loss. In the natural environment, such hazards include the following:

Landslides or avalanches.
Surface faulting, in which the surface of the ground along one side of a fault is displaced...
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