How earthquakes are measured

Earthquake magnitude is measured on a scale created by Charles F. Richter in 1934. The Richter scale is a numerical calculation with the number rising with the severity of the tremors. It has no upper limit and is based on a logarithmic scale (base 10,) which means intensity - and damage - increases tenfold with each whole number. A seismograph, pictured here, measures the intensity of earthquakes.

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Magniture 2.5 o r less

These quakes - 2.5 or less - are usually not felt but are indentified by seismographs. There are believed to be some 900,000 of these a year worldwide as the plates beneath the earth's surface shift.

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Minor :3 -3.9

A minor earthquake is considered one that falls between 3 and 3.9 on the Richter scale. There are tens of thousands of these worldwide annually and, while they may be felt, they cause little or no damage. One such quake was measured last month off the coast of Gulf Shores, Ala., pictured here in 2010.

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Light: 4-4.9

A light earthquake is measured at between 4 and 4.9 on the Richter scale. Like minor quakes, they occur often worldwide, can be felt but generally cause no damage. A 4.5-magnitude earthquake struck Hawaii on March 11, 2011, as residents braced for a tsunami after a massive earthquake in Japan. Here, waves hit the deserted Waikiki shoreline on March 11, 2011.

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Moderate: 5 - 5.9

A moderate earthquake registers between 5 and 5.9 on the Richter scale and causes slight damage to buildings and other structures. There are about 500 of these around the globe every year. An earthquake of magnitude 5.5 struck the border between Quebec, seen here, and Ontario in June 2010. It caused very little damage but was felt across a broad swath of Canada and in several U.S. states including Michigan, Vermont and New York.

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Strong: 6 - 6.9

A strong earthquake is one that registers between 6 and 6.0 on the Richter scale. There are about 100 of these around the world every year and they usually cause some damage. In populated areas, the damage may be severe. A magnitude 6.5 quake struck southeastern Iran Dec. 20, 2010. Here, Iranian Red Crescent rescue workers search through rubble in the Iranian village of Chah Ghanbar.

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Major: 7 - 7.9

A major earthquake registers between 7 and 7.9 on the Richter scale. There are about 20 a year worldwide and they cause very serious damage. Indonesia was hit by a magnitude 7.5 earthquake which triggered a tsunami in November of 2010. More than 500 people died and many buildings were destroyed. Here, tsunami victims rush to get relief goods from a Indonesian military helicopter in South Pagai island, on Nov. 2, 2010.

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Great: 8 or more

A great earthquake registers 8 or more on the Richter scale and occurs only once every five or 10 years. These temblors cause widespread death and damage and can destroy whole communities near the epicenter. Here, a tsunami, triggered by a massive 8.9-magnitude earthquake, smashes vehicles and houses at Kesennuma city in Miyagi prefecture, northern Japan, on March 11, 2011.

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