What is meant by an earthquake's magnitude?

The magnitude scale was originally divised by Charles F. Richter in 1935 for local earthquakes in southern California. The Richter Magnitude Scale, as it has come to be known, is a measure of the amplitude of the seismic waves produced at the source of an earthquake. An increase of one unit on the Richter Scale, for example from magnitude 6.0 to 7.0, corresponds to a 10-fold increase in the amplitude of the seismic waves that shake the ground. Magnitude is related to the energy radiated from the earthquake source as seismic waves. An increase of one unit on the Richter Scale corresponds to approximately a 32-fold increase in the total energy released.

Today, seismologists use a number of different magnitude scales, such as surface wave magnitude (Ms), body wave magnitude (mb) and moment magnitude (Mw), which are extensions of the original Richter Scale. An earthquake's magnitude may be computed more than one way at each seismic station that records the event. These different estimates often vary by as much as half a magnitude unit, and the final magnitude reported is the average of many estimates.

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