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The Richter magnitude scale, also known as the local magnitude ([i]ML[/i]) scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a Wood-Anderson torsion seismometer output. So, for example, an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. The effective limit of measurement for local magnitude ML is about 6.8.

Though still widely used, the Richter scale has been superseded by the moment magnitude scale, which gives generally similar values.

The energy release of an earthquake, which closely correlates to its destructive power, scales with the 3â