  The viscoelastic material functions for the Becker and the Lomnitz
rheological models, sometimes employed to describe the transient flow of rocks,
are studied and compared. Their creep functions, which are known in a closed
form, share a similar time dependence and asymptotic behavior. This is also
found for the relaxation functions, obtained by solving numerically a Volterra
equation of the second kind. We show that the two rheologies constitute a clear
example of broadly similar creep and relaxation patterns associated with neatly
distinct retardation spectra, for which analytical expressions are available.
