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]