Local magnitude

The Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the size of an earthquake. The Richter scale, developed in the 1930s, is a base-10 logarithmic scale, which defines magnitude as the logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude, as recorded on a standardized seismograph at a standard distance.

As measured with a seismometer, an earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times greater than an earthquake that registered 4.0 at the same distance. As energy release is generally proportional to the shaking amplitude raised to the 3/2 power, an increase of 1 magnitude corresponds to a release of energy 31.6 times that released by the lesser earthquake. This means that, for instance, an earthquake of magnitude 5 releases 31.6 times as much energy as an earthquake of magnitude 4.

The Richter scale built on the previous, more subjective Mercalli scale by offering a quantifiable measure of an earthquake's size.

In the United States, the Richter scale was succeeded in the 1970s by the moment magnitude scale. The moment magnitude scale is now the scale used by the United States Geological Survey to estimate magnitudes for all modern large earthquakes.

In 1935, seismologists Charles Francis Richter and Beno Gutenberg of the California Institute of Technology developed a scale, later dubbed the Richter magnitude scale, for computing the magnitude of earthquakes, specifically those recorded and measured with the Wood-Anderson torsion seismograph in a particular area of California. Originally, Richter reported mathematical values to the nearest quarter of a unit, but the values later were reported with one decimal place; the local magnitude scale compared the magnitudes of different earthquakes. Richter derived his earthquake-magnitude scale from the apparent magnitude scale used to measure the brightness of stars.

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