
Earthquake Magnitudes
Earthquakes are measured in a variety of ways. Just as inches, feet, and yards and miles (or millimeters, centimeters, meters, and kilometers) are different ways of measuring distance, there are different ways measuring earthquakes. Below is an excellent summary adapted from the US Geological Survey:
Seismologists indicate the size of an earthquake in units of magnitude. There are many different ways that magnitude is measured from seismograms because each method only works over a limited range of magnitudes and with different types of seismometers. Some methods are based on body waves (which travel deep within the structure of the earth), some based on surface waves (which primarily travel along the uppermost layers of the earth), and some based on completely different methodologies. However, all of the methods are designed to agree well over the range of magnitudes where they are reliable.
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:
Magnitude type

Applicable magnitude range

Distance range

Comments

Duration (Md)

<4

0400 km

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. 
Local (Ml)

26

0400 km

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. 
Surface wave (Ms)

58

20180 degrees

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

>3.5

all

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. 
Body (Mb)

47

16100 degrees (only deep earthquakes)

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