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An elementary calculation shows that the actual volume change
of an expanding spherical source desribed by the moment tensor
(1) is given by eq. (3) while eq. (2) defines the transformational
volume change, which is larger. It is instructive to look at
the moments required for a given expansion .
The
total moment
results from
a moment
that
expands the interior of the sphere, and a moment
that pushes the surrounding medium
away. The outer displacement field is divergence-free both in
liquids and in solids, so the volume change of the spherical
source can be measured at any distance and over any
closed surface.

*Erhard Wielandt *

2003-05-30