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The Richter scale
The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the mathematical basis of the scale, each whole number increase in magnitude represents a quake that is 10 times greater than the previous decimal. Each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.
United States Geological Survey
Earthquake Magnitude Classes
|Class||Magnitude||Estimated number each year (globally)||
|Great||greater than 8.0||one every 5-10 years||A great earthquake that can totally destroy communities near the epicenter|
|Major||7.0-7.9||20||A major earthquake that causes serious damage|
|Strong||6.0-6.9||100||May cause a lot of damage in very populated areas|
|Moderate||5.5-5.9||500||Slight damage to buildings and other structures|
|Minor to Moderate||2.5-5.4||30,000||Often felt, but causes only minor damage|
|Very Minor||less than 2.5||900,000||Usually not felt, but can be recorded by a seismograph|
Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.
Source: United States Geological Survey