Body wave magnitude
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Through Sedimentary rock
mB scale
The original "body-wave magnitude" – mB or mB (uppercase "B") – was developed by Gutenberg (1945b, 1945c) and Gutenberg & Richter (1956) to overcome the distance and magnitude limitations of the ML scale inherent in the use of surface waves. mB is based on the P- and S-waves, measured over a longer period, and does not saturate until around M 8. However, it is not sensitive to events smaller than about M 5.5. Use of mB as originally defined has been largely abandoned, now replaced by the standardized mBBB scale.
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mb scale
The mb or mb scale (lowercase "m" and "b") is similar to mB, but uses only P-waves measured in the first few seconds on a specific model of short-period seismograph. It was introduced in the 1960s with the establishment of the World-Wide Standardized Seismograph Network (WWSSN); the short period improves detection of smaller events, and better discriminates between tectonic earthquakes and underground nuclear explosions.
Measurement of mb has changed several times. As originally defined by Gutenberg (1945c) mb was based on the maximum amplitude of waves in the first 10 seconds or more. However, the length of the period influences the magnitude obtained. Early USGS/NEIC practice was to measure mb on the first second (just the first few P-waves), but since 1978 they measure the first twenty seconds. The modern practice is to measure short-period mb scale at less than three seconds, while the broadband mBBB scale is measured at periods of up to 30 seconds.
mbLg scale
The regional mbLg scale – also denoted mb_Lg, mbLg, MLg (USGS), Mn, and mN – was developed by Nuttli (1973) for a problem the original ML scale could not handle: all of North America east of the Rocky Mountains. The ML scale was developed in southern California, which lies on blocks of oceanic crust, typically basalt or sedimentary rock, which have been accreted to the continent. East of the Rockies the continent is a craton, a thick and largely stable mass of continental crust that is largely granite, a harder rock with different seismic characteristics. In this area the ML scale gives anomalous results for earthquakes which by other measures seemed equivalent to quakes in California.
Nuttli resolved this by measuring the amplitude of short-period (~1 sec.) Lg waves, a complex form of the Love wave which, although a surface wave, he found provided a result more closely related to the mb scale than the Ms scale. Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and MbLg is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions.