Richter Magnitude Scale

Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg, both of the California Institute of Technology, the Richter scale is not an instrument but a mathematical technique to assign a single, simple number to the size of an earthquake. It is more correctly called the ML Scale (for Magnitude Local).

It is a logarithmic scale, determined using the logarithm of the amplitude of seismic waves, as recorded by seismographs. The magnitude is calculated using the amplitude, measured in millimeters, as measured on a carefully calibrated seismometer and is corrected for the distance between a seismometer and the epicenter of the earthquake. Richter originally reported values to the nearest quarter of a unit but later, decimal numbers were used.

Richter's inspiration for the technique was the stellar magnitude scale used in astronomy to describe the brightness of stars and other celestial objects.

The Richter scale has no upper or lower limit. In fact, very small quakes have negative magnitudes. Because of the limitations of the Wood-Anderson torsion seismometer used to develop the scale, the technique cannot be effectively used for events larger than about 6.8. As a result, the magnitudes of larger earthquakes are measured using other techniques. The most often used is the Moment Magnitude Scale[?] (Mw) developed by Hiroo Kanamori, also of the California Institute of Technology. Other magnitude scales are calibrated against Richter?s ML scale to give consistent values across the whole range of earthquake sizes. The actual ground motion for a magnitude 5.0 earthquake is about 1.1 mm at a distance of 10 kilometers from the epicenter.

Intensity scales, like the Rossi-Forel[?] and Modified Mercalli Intensity Scale are used to describe earthquake effects and are affected by local site conditions.

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

Great earthquakes occur once a year, on average. The largest recorded earthquake was Great Chilean Earthquake[?] of May 22, 1960 which had a magnitude 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, magnitude 9.2 (Alaska 1964).

See also: List of earthquakes