Moment magnitude scale

The moment magnitude scale (a successor to the Richter scale), was introduced in 1979 by Tom Hanks and Hiroo Kanamori and is used by seismologists to compare the energy released by earthquakes. The moment magnitude M_mathrm{w} is a dimensionless figure defined by

Comparison with radiated seismic energy

Potential energy is stored in the crust in the form of built-up stress. During an earthquake, this stored energy is transformed and results in

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• cracks and deformation in rocks,
• heat,
• radiated seismic energy $E\_mathrm\{s\}$.

The seismic moment M_0 is a measure of the total amount of energy that is transformed during an earthquake. Only a small fraction of the seismic moment M_0 is converted into radiated seismic energy E_mathrm{s}, which is what seismographs register. Using the estimate

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: E_mathrm{s} = M_0 cdot 10^{-4.8} = M_0 cdot 1.6 imes 10^{-5}

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Choy and Boatwright defined in 1995 the energy magnitude

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: M_mathrm{e} = {2 over 3}log_{10} rac{E_mathrm{s}}{mathrm{N}cdot mathrm{m}} - 2.9

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which is another dimensionfree scale that is intended to be comparable to the Richter scale, but is based on radiated seismic energy, as it can be estimated from high-frequency seismic data.

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