Richard,

See the IASPEI Manual of Seismological Observatory Practice for details on
computing magnitudes.  This covers Local (ML), Surface (MS) and Body Wave
(mb) magnitudes.

The 1979 version:
http://www.seismo.com/msop/msop79/par/par.html
may be easier to follow than the "new" version:

http://www.seismo.com/msop/nmsop/03%20source/source4/source4.html

John
John C. Lahr
1925 Foothills Road
Golden, CO 80402
(303) 215-9913
john@........
http://lahr.org/john-jan/science.html
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