  We study the effect of radio-jet core shift, which is a dependence of the
position of the jet radio core on the observational frequency. We derive a new
method of measuring the jet magnetic field based on both the value of the shift
and the observed radio flux, which complements the standard method that assumes
equipartition. Using both methods, we re-analyse the blazar sample of
Zamaninasab et al. We find that equipartition is satisfied only if the jet
opening angle in the radio core region is close to the values found
observationally, $\simeq$0.1--0.2 divided by the bulk Lorentz factor,
$\Gamma_{\rm j}$. Larger values, e.g., $1/\Gamma_{\rm j}$, would imply magnetic
fields much above equipartition. A small jet opening angle implies in turn the
magnetization parameter of $\ll 1$. We determine the jet magnetic flux taking
into account this effect. We find that the transverse-averaged jet magnetic
flux is fully compatible with the model of jet formation due to BH spin energy
extraction and the accretion being a magnetically arrested disc (MAD). We
calculate the jet average mass-flow rate corresponding to this model and find
it consists of a substantial fraction of the mass accretion rate. This suggests
the jet composition with a large fraction of baryons. We also calculate the
average jet power, and find it moderately exceeds the accretion power, $\dot M
c^2$, reflecting BH spin energy extraction. We find our results for radio
galaxies at low Eddington ratios are compatible with MADs but require a low
radiative efficiency, as predicted by standard accretion models.
