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 nuclear detonations

In the United States, it is customary that journalists compare major earthquakes with large underground nuclear detonations. The energy released by nuclear weapons is traditionally expressed in terms of the energy stored in a kiloton or megaton of the conventional explosive trinitrotoluene (TNT). The often quoted rule of thumb that a 1 kt TNT explosion is roughly equivalent to a magnitude 4 earthquake leads to the equation

Related Topics:
United States - Kiloton - Megaton - Trinitrotoluene - Rule of thumb

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:M_mathrm{n} = {2 over 3}log_{10} rac{m_{mathrm{TNT}}}{mbox{kg}} = {2 over 3}log_{10} rac{m_{mathrm{TNT}}}{mbox{kt}} + 4 = {2 over 3}log_{10} rac{m_{mathrm{TNT}}}{mbox{Mt}} + 6.

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where m_{mathrm{TNT}} is the mass of the explosive TNT that is quoted for comparison.

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Such comparison figures are not very meaningful. Like with earthquakes, during an underground explosion of a nuclear weapon, only a small fraction of the total amount of energy transformed ends up being radiated as seismic waves. Therefore a seismic efficiency has to be chosen for a bomb that is quoted as a comparison. Using the conventional specific energy of TNT (4.184 MJ/kg), the above formula implies the assumption that about 0.5% of the bomb's energy is converted into radiated seismic energy E_mathrm{s}. For real underground nuclear tests, the actual seismic efficiency achieved varies significantly and depends on the site and design parameters of the test.

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