mb and Ms

The body-wave magnitude, mb, and the surface-wave magnitude, Ms, measure the size of ground motions at very different periods of vibration [the period is the time it takes to complete one oscillation] and using different waves. As a result, these magnitudes may not always provide an identical estimation of an earthquake's size.

mb is measured using P waves at a period of about 1 second, Ms is measured using Rayleigh waves at a period of about 20 seconds. Although P-wave amplitudes are not sensitive to the earthquake's depth, the Rayleigh waves amplitudes are (deep earthquakes do not excite large Rayleigh waves at intermediate periods such as 20 seconds).

Even more interesting is the fact that the nature of earthquake ruptures causes mb and Ms to differ for large earthquakes, even the shallow ones. Above about a magnitude in the range of 6.3-6.5, the value of Ms is a better representation of the earthquake size.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.

 

mbMSChart

Contour chart of mb versus Ms illustrating the relationship between the two magnitude scales for shallow earthquakes (less than 50 km depth) occurring between 1990 and 2010. The dashed line shows the one-to-one relationship, which is well matched for magnitudes between 5.0 and about 6.3. The saturation of the mb scale is apparent starting for magnitudes of roughly 6.5 and larger. Data at the larger magnitudes are sparse, since there are many fewer events.