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Body wave magnitude

Summary

Body-waves consist of P-waves that are the first to arrive (see seismogram), or S-waves, or reflections of either. Body-waves travel through rock directly.[1]

mB scale edit

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

mb scale edit

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.[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) 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[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 mBBB  scale is measured at periods of up to 30 seconds.[11]

mbLg scale edit

 
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 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,[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 Ms  scale.[13] 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.[14]

Notes edit

  1. ^ Havskov & Ottemöller 2009, p. 17.
  2. ^ Bormann, Wendt & Di Giacomo 2013, p. 37; Havskov & Ottemöller 2009, §6.5. See also Abe 1981.
  3. ^ Havskov & Ottemöller 2009, p. 191.
  4. ^ Bormann & Saul 2009, p. 2482.
  5. ^ MNSOP-2/IASPEI IS 3.3 2014, §4.2, pp. 15–16.
  6. ^ Kanamori 1983, pp. 189, 196; Chung & Bernreuter 1980, p. 5.
  7. ^ Bormann, Wendt & Di Giacomo 2013, pp. 37, 39; Bolt (1993, pp. 88–93) examines this at length.
  8. ^ Bormann, Wendt & Di Giacomo 2013, p. 103.
  9. ^ IASPEI IS 3.3 2014, p. 18.
  10. ^ Nuttli 1983, p. 104; Bormann, Wendt & Di Giacomo 2013, p. 103.
  11. ^ IASPEI/NMSOP-2 IS 3.2 2013, p. 8.
  12. ^ Bormann, Wendt & Di Giacomo 2013, §3.2.4.4. The "g" subscript refers to the granitic layer through which Lg waves propagate. Chen & Pomeroy 1980, p. 4. See also J. R. Kayal, "Seismic Waves and Earthquake Location", here, page 5.
  13. ^ Nuttli 1973, p. 881.
  14. ^ Bormann, Wendt & Di Giacomo 2013, §3.2.4.4.

Sources edit

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