def _get_site_amplification(self, sites, rup, C):
        """
        Compute the third term of the equation described on p. 1144:

        ``(s11 + s12 * (M - 6)) * Sc + (s21 + s22 * (M - 6)) * Sd`
        """
        Sc, Sd = self._get_site_type_dummy_variables(sites)
        return (C["s11"] + C["s12"] * (rup.mag - 6.0)) * Sc +\
            (C["s21"] + C["s22"] * (rup.mag - 6.0)) * Sd