Body wave magnitude

The original "body-wave magnitude" – mB or m_B (uppercase "B") – was developed by Gutenberg (1945b, 1945c) and Gutenberg & Richter (1956)[2] to overcome the distance and magnitude limitations of the M_L  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.[3] Use of mB  as originally defined has been largely abandoned,[4] now replaced by the standardized mB_BB  scale.[5]

The mb or m_b 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.[6] 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.[7]

Measurement of mb  has changed several times.[8] As originally defined by Gutenberg (1945c) m_b 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[9]), but since 1978 they measure the first twenty seconds.[10] The modern practice is to measure short-period mb  scale at less than three seconds, while the broadband mB_BB  scale is measured at periods of up to 30 seconds.[11]

Differences in the crust underlying North America east of the Rocky Mountains makes that area more sensitive to earthquakes. Shown here: the 1895 New Madrid earthquake, M ~6, was felt through most of the central U.S., while the 1994 Northridge quake, though almost ten times stronger at M 6.7, was felt only in southern California. From USGS Fact Sheet 017–03.

The regional mb_Lg scale – also denoted mb_Lg, mbLg, MLg (USGS), Mn, and m_N – was developed by Nuttli (1973) for a problem the original M_L scale could not handle: all of North America east of the Rocky Mountains. The M_L 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 M_L 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,[12] 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 M_s  scale.[13] Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and Mb_Lg is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions.[14]