Variability of the core shift in 3C 454.3. (a) Core shift between the X (8 GHz) and U (15 GHz) bands vs. time. The purple triangles represent the core shifts derived by Pushkarev et al. (2012). (b) Core shift between the C (5 GHz) and Q (43 GHz) bands vs. time. In both panels the mean value is represented by the blue line and ±σ is displayed by the shaded cyan background. The epoch 2008 January 03, which had three missing antennas, was dropped from this plot as an outlier with a CQ core shift of 1.64 ± 0.04 mas. Zero core shift physically means that the cores at two frequencies coincide at the same location.