  We have investigated a frequency-dependent shift in the absolute position of
the optically thick apparent origin of parsec-scale jets ("core shift" effect)
to probe physical conditions in ultra-compact relativistic outflows in AGN. We
used multi-frequency Very Long Baseline Array (VLBA) observations of 191
sources carried out in 12 epochs in 2006 within the MOJAVE program. The
observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We implemented a
method of determining the core shift vector based on (i) image registration by
two-dimensional normalized cross-correlation and (ii) model-fitting the source
brightness distribution to take into account a non-zero core component offset
from the phase center. The 15.4-8.1, 15.4-8.4, and 15.4-12.1 GHz core shift
vectors are derived for 163 sources, and have median values of 0.128, 0.125,
and 0.088 mas, respectively, compared to the typical measured errors of 0.050,
0.051, 0.035 mas. The effect occurs predominantly along the jet direction, with
departures smaller than 45 deg from the median jet position angle in over 80%
of the cases. Despite the moderate ratio of the observed frequencies (<2), core
shifts significantly different from zero (>2sigma) are detected for about 55%
of the sources. These shifts are even better aligned with the jet direction,
deviating from the latter by less than 30 deg in over 90% of the cases. There
is an indication that the core shift decreases with increasing redshift.
Magnetic fields in the jet at a distance of 1 parsec from the central black
hole, calculated from the obtained core shifts, are found to be systematically
stronger in quasars (median B1~0.9 G) than those in BL Lacs (median B1~0.4 G).
We also constrained the absolute distance of the core from the apex of the jet
at 15 GHz as well as the magnetic field strength in the 15 GHz core region.
