Richter Scale vs Moment Magnitude Scale

Since the late 1930s it became commonplace to measure earthquakes by their magnitude, given the work done
by Gutenberg and Richter, and the publication of the logarithmic Richter Scale which related to a measure of the
energy radiated by the earthquake, using well-calibrated seismic stations. At the time, the general properties of
the radiated spectrum were not known and the concept of seismic moment and the moment tensor
had not yet been developed.

The Richter scale only describes the maximum wave amplitude, and does not give any indication of the total energy
that is released by the event. The moment magnitude scale measures the total energy released by an earthquake.
It now supersedes the Richter magnitude scale which measures the height of a seismic wave. The two scales will
indicate similar results if the earthquake magnitudes are between 3.0 and 7.0.

Seismologists studying larger earthquakes (greater than M3.5 - M4.0) generally report the size of the earthquake
using the moment magnitude scale. For smaller earthquakes, the published magnitude is usually given as MB,
MS, or ML. The concept of moment magnitude (MW) was introduced in 1979 by Hanks and Kanamori and has
since become the most commonly used method of describing the size of an event. Moment magnitude measures
the size of events in terms of how much total energy is released.


More Information about Earthquake Magnitude and Seismic Moment

Specifically, moment magnitude relates to the amount of movement by rock; i.e. the distance of movement
along a fault or fracture and the area of the fault or fracture surface. It is calculated as

MO = µAD

where µ is the shear modulus of the rocks included in the fault (dyne/cm2)
A is the area of the fault rupture in cm2
and D is the average fault displacement in cm

Thus, MO is shown in units of energy, dyne-cm

Both the earthquake magnitude and the seismic moment are related to the amount of energy that is radiated by an
earthquake. Richter, working with Dr. Beno Gutenberg, developed a relationship between magnitude and energy.
That relationship is:
logES = 11.8 + 1.5M

giving the energy ES in ergs from the magnitude M.

Note that ES is not the total "intrinsic'' energy of the earthquake, transferred from sources such as gravitational energy
or to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which, as was
said above, is in most cases only a small fraction of the total energy transferred during the earthquake process.