Earthquake magnitude is a logarithmic measure of earthquake size. In simple terms, this means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake. The total amount of energy released by the earthquake, however, goes up by a factor of 32.
Magnitudes commonly used by seismic networks include:







Based on the duration of shaking as measured by the time decay of the amplitude of the seismogram. Often used to compute magnitude from seismograms with "clipped" waveforms due to limited dynamic recording range of analog instrumentation, which makes it impossible to measure peak amplitudes. 



The original magnitude relationship defined by Richter and Gutenberg for local earthquakes in 1935. It is based on the maximum amplitude of a seismogram recorded on a WoodAnderson torsion seismograph. Although these instruments are no longer widely in use, Ml values are calculated using modern instrumentation with appropriate adjustments. 



A magnitude for distant earthquakes based on the amplitude of Rayleigh surface waves measured at a period near 20 sec. 



Based on the moment of the earthquake, which is equal to the rigidity of the earth times the average amount of slip on the fault times the amount of fault area that slipped. 



Based on the amplitude of P bodywaves. This scale is most appropriate for deepfocus earthquakes. 