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The Richter magnitude scale is a mathematical technique used to quantify the size of earthquakes.

Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg, both of the California Institute of Technology, the Richter scale assigns a single number to quantify the size of an earthquake. It is more correctly called the ML Scale (for local magnitude). Richter's local magnitude scale was originally intended to be used only in a particular study area in California, and on seismograms recorded on a particular instrument, the Wood-Anderson torsion seismometer.

Richter magnitude is a logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. The diminution of amplitude due to distance between the earthquake epicenter and the seismometer is corrected for by subtracting the logarithm of the expected amplitude of a magnitude 0 event at that distance. This correction for distance is intended to make the local magnitude an absolute measure of earthquake size. Richter originally reported values to the nearest quarter of a unit but later, decimal numbers were used.

Richter's motivation for creating the local magnitude scale was to separate the vastly larger number of smaller earthquakes from the relatively fewer larger earthquakes observed in California at the time. His inspiration for the technique was the stellar 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 micron on a seismogram recorded using a Wood-Anderson torsion seismometer located 100 km from the earthquake epicenter. This choice was made to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit. Sensitive modern seismographs now routinely record quakes with negative magnitudes.

Because of the limitations of the Wood-Anderson torsion seismometer used to develop the scale, the original ML cannot be calculated for events larger than about 6.8. Many investigators have proposed extensions to the local magnitude scale, the most popular being the surface wave magnitude MS and the body wave magnitude Mb.

The major problem with Richter magnitude is that it is not easily related to physical characteristics of the earthquake source. Furthermore, there is a saturation effect at near 8.3-8.5, owing to the scaling law of earthquake spectra, that causes traditional magnitude methods (such as MS) to yield the same magnitude estimate for events that are clearly of different size. By the beginning of the 21st century, most seismologists considered the traditional magnitude scales to be largely obsolete, being replaced by a more physically meaningful measurement called the seismic moment which is more directly relatable to the physical parameters, such as the dimension of the earthquake rupture, and the energy released from the earthquake. In 1979 seismologist Hiroo Kanamori, also of the California Institute of Technology, proposed the Moment Magnitude Scale (MW), which provides a way of expressing seismic moments in a form that can be approximately related to traditional seismic magnitude measurements.

Magnitude must not be confused with intensity. Intensity scales, such as the Rossi-Forel and Modified Mercalli Intensity Scale, are used to describe relative earthquake effects. Intensity is sensitive to a host of local site conditions and is not an absolute measurements of earthquake size.

Events with magnitudes of about 4.5 or greater are strong enough to be recorded by seismographs all over the world.

DescriptorRichter Magnitudes Earthquake Effects Average Annually
MicroLess than 2.0 Microearthquakes, not felt. About 8,000 per day
Very minor2.0-2.9 Generally not felt, but recorded. About 1,000 per day
Minor3.0-3.9 Often felt, but rarely causes damage. 49,000 (estimated)
Light4.0-4.9 Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 (estimated)
Moderate5.0-5.9 Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800
Strong6.0-6.9 Can be destructive in areas up to about 100 miles across in populated areas. 120
Major7.0-7.9 Can cause serious damage over larger areas. 18
Great8.0 or greater Can cause serious damage in areas several hundred miles across. 1
(Adapted from U.S. Geological Survey documents.)

Great earthquakes occur once a year, on average. The largest recorded earthquake was Great Chilean Earthquake of May 22, 1960 which had a magnitude (MW) of 9.5 (Chile 1960). The largest earthquake to occur in the U.S. was the Good Friday Earthquake of South-central Alaska of March 28, 1964, moment magnitude 9.2 (Alaska 1964).