Moment magnitude scale
The moment magnitude scale, introduced by Hiroo Kanamori, is used by seismologists to compare the size of earthquakes. The moment magnitude <math>M_W<math> is defined by the equation
<math>M_W = {2 \over 3}\log_{10} M_0 - 10.7<math> ,
where <math>M_0<math> is the seismic moment measured in dyne-centimeters (dyn·cm = 10-7N·m). In SI-Units the formula is approximately
<math>M_W = {2 \over 3}\log_{10} M_{0,\mathrm{SI}} - 6<math> .
From this formula, one can determine that for each step on the magnitude scale, approximately 32 times more energy is required.
The constants in the equation are chosen so that estimates of moment magnitude roughly agree with estimates using other scales such as the Richter scale. One advantage of the moment magnitude scale is that, unlike other magnitude scales, it does not saturate at the upper end. That is, there is no particular value beyond which all large earthquakes have about the same magnitude. For this reason, moment magnitude is now the most-often used estimate of earthquake magnitude.
- See also: Geophysics, List of earthquakes
