The Richter magnitude scale, also known as the local magnitude (M_{L}) scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base10 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 M_{L} is about 6.8.
Though still widely used in the mass media, the Richter scale has been superseded by the moment magnitude scale, which is calibrated to give generally similar values for mediumsized earthquakes (magnitudes between 3 and 7). Unlike the Richter scale, however, the moment magnitude scale is built on more sound seismological ground, and does not saturate in the highmagnitude range.
The energy release of an earthquake, which closely correlates to its destructive power, scales with the ^{3}⁄_{2} power of the shaking amplitude. Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 ( = (10^{1.0})^{(3 / 2)}) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 ( = (10^{2.0})^{(3 / 2)} ) in the energy released.^{[1]}
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Developed in 1935 by Charles Richter in partnership with Beno Gutenberg, both of the California Institute of Technology, the scale was firstly intended to be used only in a particular study area in California, and on seismograms recorded on a particular instrument, the WoodAnderson torsion seismometer. Richter originally reported values to the nearest quarter of a unit, but decimal numbers were used later. His motivation for creating the local magnitude scale was to separate the vastly larger number of smaller earthquakes from the few larger earthquakes observed in California at the time.
His inspiration was the apparent magnitude scale used in astronomy to describe the brightness of stars and other celestial objects. Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 µm (0.00004in) on a seismograph recorded using a WoodAnderson torsion seismometer 100 km (62 mi) from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.
Because M_{L} is derived from measurements taken from a single, bandlimited seismograph, its values saturate when the earthquake is larger than 6.8.^{[2]} To overcome this shortcoming, Gutenberg and Richter later developed a magnitude scales based on surface waves, surface wave magnitude M_{S}, and another based on body waves, body wave magnitude m_{b}.^{[3]} M_{S} and m_{b} can still saturate when the earthquake is big enough.
These traditional magnitude scales have been superseded by the implementation of methods for estimating the seismic moment and its associated moment magnitude scale, although the former are still widely used because they can be calculated quickly.
The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). The original formula is:^{[4]}
where A is the maximum excursion of the WoodAnderson seismograph, the empirical function A_{0} depends only on the epicentral distance of the station, δ. In practice, readings from all observing stations are averaged after adjustment with stationspecific corrections to obtain the M_{L} value.
Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 31.6 times the amount of energy released, and each increase of 0.2 corresponds to a doubling of the energy released.
Events with magnitudes of about 4.6 or greater are strong enough to be recorded by any of the seismographs in the world, given that the seismograph's sensors are not located in an earthquake's shadow.
The following describes the typical effects of earthquakes of various magnitudes near the epicenter. This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, the depth of the earthquake's focus beneath the epicenter, and geological conditions (certain terrains can amplify seismic signals).
Richter magnitudes  Description  Earthquake effects  Frequency of occurrence 

Less than 2.0  Micro  Microearthquakes, not felt.  About 8,000 per day 
2.02.9  Minor  Generally not felt, but recorded.  About 1,000 per day 
3.03.9  Often felt, but rarely causes damage.  49,000 per year (est.)  
4.04.9  Light  Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.  6,200 per year (est.) 
5.05.9  Moderate  Can cause major damage to poorly constructed buildings over small regions. At most slight damage to welldesigned buildings.  800 per year 
6.06.9  Strong  Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas.  120 per year 
7.07.9  Major  Can cause serious damage over larger areas.  18 per year 
8.08.9  Great  Can cause serious damage in areas several hundred miles across.  1 per year 
9.09.9  Devastating in areas several thousand miles across. 
1 per 20 years  
10.0+  Epic  Never recorded; see below for equivalent seismic energy yield. 
Extremely rare (Unknown) 
(Based on U.S. Geological Survey documents.)^{[5]}
Great earthquakes occur once a year, on average. The largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960 which had a magnitude (M_{W}) of 9.5.^{[6]}
The following table lists the approximate energy equivalents in terms of TNT explosive force^{[7]}  though note that the energy here is that of the underground energy release (i.e. a small atomic bomb blast will not simply cause light shaking of indoor items) rather than the overground energy release. Most energy from an earthquake is not transmitted to and through the surface; instead, it dissipates into the crust and other subsurface structures.

