  The isotope effects in Si$^-$ bound levels are studied using the
multi-configuration Hartree-Fock ab initio approach. Large scale calculations
are carried out for the $3p^3\ ^4S^o,\, ^2D^o$ and $^2P^o$ multiplets of Si$^-$
and the $3p^2\ ^3P$ multiplet of Si. We predict an anomalous isotope shift on
the electron affinity, dominated by the specific mass shift, with a value of
$IS(^e\!\!A)= -0.66(6)$ m$^{-1}$ for the ($30-28$) isotope pair. We also report
hyperfine structure parameters for the studied multiplets. Finally, we provide
the values of level electric field gradients at the nucleus that could be of
interest in a study of the metastable silicon isotopes. Relativistic
corrections are estimated using non-relativistic orbitals in the Breit-Pauli
and fully relativistic frameworks.
