The familiar Richter scale (which is not a physical device but rather a mathematical formula) is no longer widely used by scientists or the media to report an earthquake's size.

Today, an earthquake's size is typically reported simply by its magnitude, which is a measure of the size of the earthquake's source, where the ground began shaking.

While there are many modern scales used to calculate the magnitude, the most common is the moment magnitude, which allows for more precise measurements of large earthquakes than the Richter scale.

In the news, however, when an earthquake's magnitude is given , the scale used to calculate the magnitude is not usually specified since the modern scales are all very similar.

A network of geological monitoring stations, each with instruments that measure how much the ground shakes over time called seismographs allow scientists to calculate an earthquake's time, location and magnitude.

Seismographs record a zigzag trace that shows how the ground shakes beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

From these measurements, a quake's magnitude is usually reported as, for example, a magnitude-7.0 in the case of the earthquake that struck Haiti on Jan. 12.

Based on their magnitude, quakes are assigned to a class, according to the U.S. Geological Survey. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. The classes are as follows:

• Great: Magnitude is greater than or equal to 8.0. A magnitude-8.0 earthquake is capable of tremendous damage.
• Major: Magnitude in the rage of 7.0 to 7.9. A magnitude-7.0 earthquake is a major earthquake that is capable of widespread, heavy damage.
• Strong: Magnitude in the rage of 6.0 to 6.9. A magnitude-6.0 quake can cause severe damage.
• Moderate: Magnitude in the rage of 5.0 to 5.9. A magnitude-5.0 quake can cause considerable damage.
• Light: Magnitude in the rage of 4.0 to 4.9. A magnitude-4.0 quake is capable of moderate damage.
• Minor: Magnitude in the rage of 3.0 to 3.9.
• Micro: Magnitude less than-3.0. Quakes between 2.5 and 3.0 are the smallest generally felt by people.

After an earthquake strikes, its magnitude is continuously revised as time passes and more stations report their seismic readings. Several days can pass before a final number is agreed upon.

Brett Israel is a staff writer for OurAmazingPlanet, a sister site to Life's Little Mysteries.

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