IASPEI formula
Ms = log (A/T) + 1.66 log D + 3.3
where
-
A
-
is the maximum ground amplitude in micrometers (microns) of
the vertical component of the surface wave within the
period range 18 <= T <= 22.
-
T
-
is the period in seconds.
-
D
-
is the distance in geocentric degrees (station to epicenter) and
20° <= D <= 160°.
No depth corrections are applied, and Ms magnitudes are not
generally computed for
depths greater than 50 kilometers.
The Ms value published is the average of the individual station
magnitudes from reported T and A data.
If the uncertainty of the computed depth is considered great enough
that the depth could be less than 50 kilometers,
an Ms value may still be published, computed by the
IASPEI formula and NOT corrected for depth.
In general, the Ms magnitude is more reliable than the mb magnitude
as a means of yielding the
relative "size" of a shallow-focus earthquake.
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