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Because
earthquakes excite both body waves,
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which travel
into and through the Earth, and surface
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waves, which
are constrained to follow the Earth's
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uppermost
layers, two magnitude scales evolved -
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the mb
and MS
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The standard
body-wave magnitude formula is
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mb
= log10(A/T) + Q(D,h)
,
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where A
is the amplitude of ground motion; T is the
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corresponding
period; and Q(D,h) is an
empirical function of
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distance, D,
between epicenter and station and focal depth, h.
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The standard
surface-wave formula is
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MS
= log10 (A/T) + 1.66 log10
(D) + 3.30 .
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