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High-resolution radio observations are ideal for constraining the value ofphysical parameters in the inner regions of active-galactic-nucleus jets andcomplement results on multiwavelength (MWL) observations. This study is part ofa wider multifrequency campaign targeting the nearby TeV blazar Markarian 421(z=0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray(Swift, RXTE, MAXI), and gamma rays (Fermi-LAT, MAGIC, VERITAS). We investigatethe jet's morphology and any proper motions, and the time evolution of physicalparameters such as flux densities and spectral index. The aim of our widermultifrequency campaign is to try to shed light on questions such as the natureof the radiating particles, the connection between the radio and gamma-rayemission, the location of the emitting regions and the origin of the fluxvariability. We consider data obtained with the Very Long Baseline Array (VLBA)over twelve epochs (one observation per month from January to December 2011) at15 GHz and 24 GHz. We investigate the inner jet structure on parsec scalesthrough the study of model-fit components for each epoch. The structure of Mrk421 is dominated by a compact (~0.13 mas) and bright component, with aone-sided jet detected out to ~10 mas. We identify 5-6 components in the jetthat are consistent with being stationary during the 12-month period studiedhere. Measurements of the spectral index agree with those of other works: theyare fairly flat in the core region and steepen along the jet length.Significant flux-density variations are detected for the core component. Fromour results, we draw an overall scenario in which we estimate a viewing angle2{\deg} < theta < 5{\deg} and a different jet velocity for the radio and thehigh-energy emission regions, such that the respective Doppler factors are{\delta}r ~3 and {\delta}h.e. ~14.

VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011