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What is the "magnitude" of an earthquake?
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.
Magnitude Type 
Applicable Magnitude Range 
Distance Range 
Comments 
Md
Duration

<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. 
ML
Local

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. 
Ms
Surface Wave

58

20180 degrees

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

>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. 
Mb
Body

47

16100 degrees
(only deep earthquakes)

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