An earthquake is a violent movement of the rocks in the Earth's crust which create waves of energy travelling through the Earth. Earthquakes are usually quite brief (and even somewhat repeated itself, known as an aftershock), but may repeat over a period of time. The study of earthquakes is called seismology.
Occurrence and Cause
Earthquakes usually occur where tectonic plates are in constant motion, in which these areas of constant motion (or violent motion) are usually at the edges of the plates. For example, several and several earthquakes have been recorded in the Western part of South America due to the constant motion between the South American plate and the Nazca plate. Earthquakes usually occur on faults, which are breaks in the Earth's crust formed by the sliding, pushing, or pulling of tectonic plates.
What causes earthquakes is elastic deformation, which is when rocks change in shape when pressure/forces is applied on them, but then return to its normal state after the pressure/force has gone. This is what causes earthquakes, and not plastic deformation, when permanent deformation occurs on a rock.
This term is used to describe the abrupt return of elastically deformed rock to its original shape. Elastic Rebound is somewhat similar to a rubber band being stretched and then ripped. After the band was ripped into one whole string, the shape of the string came back to being its original shape. This occurs when stress and stress is applied to a rock. So much stress, to the point the rock cannot withstand it! During the process of elastic rebound, energy is released. Some of this energy travels in what is known as "seismic waves", and these waves are the originators of some earthquakes.
Faults and Earthquake Zones
When the earth moves offshore in the ocean, it can cause a tsunami. A tsunami can cause just as much death and destruction as an earthquake. Landslides can happen, too. This is an important part of the Earth's cycle.
Earthquakes are measured using observations from seismometers. The magnitude of an earthquake and the intensity of shaking are measured on a numerical scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over wide areas.
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- Earth Science. Austin, Texas 78746-6487: Holt, Rinehart Winston. 2001. ISBN 0-03-055667-8.