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Eq. 4 decomposes the wavefield into a near-field term decaying with r-2 and a far-field term decaying with r-1:
un and uf determine each other:
In case of a harmonic time dependence
is the wavelength. The far-field term thus dominates at radii greater than
wavelengths. When the deformation is quasistatic (i.e. sufficiently slow), only the near-field term is relevant.