  Opacity-driven shifts of the apparent VLBI core position with frequency (the
"core shift" effect) probe physical conditions in the innermost parts of jets
in active galactic nuclei. We present the first detailed investigation of this
effect in the brightest gamma-ray blazar 3C454.3 using direct measurements from
simultaneous 4.6-43 GHz VLBA observations, and a time lag analysis of 4.8-37
GHz lightcurves from the UMRAO, CrAO, and Metsahovi observations in 2007-2009.
The results support the standard Konigl model of jet physics in the VLBI core
region. The distance of the core from the jet origin r_c(nu), the core size
W(nu), and the lightcurve time lag DT(nu) all depend on the observing frequency
nu as r_c(nu)~W(nu)~ DT(nu)~nu^-1/k. The obtained range of k=0.6-0.8 is
consistent with the synchrotron self-absorption being the dominating opacity
mechanism in the jet. The similar frequency dependence of r_c(nu) and W(nu)
suggests that the external pressure gradient does not dictate the jet geometry
in the cm-band core region. Assuming equipartition, the magnetic field strength
scales with distance r as B = 0.4(r/1pc)^-0.8 G. The total kinetic power of
electron/positron jet is about 10^44 ergs/s.
