  We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the
Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their
structure and proper motions on pc scales and to add some constraints on the
many possible causes of the radio-quietness of Seyferts. The component
configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas
the configurations in NGC 5506 and Mrk 1210 have multiple components with no
clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact
symmetric object. Comparing the images at different epochs, we find a proper
motion in NGC 7674 of (0.92+-0.07) c between the two central components
separated by 282 pc and, in NGC 5506, we find a 3 sigma upper limit of 0.50 c
for the components separated by 3.8 pc. Our results confirm and extend earlier
work showing that the outward motion of radio components in Seyfert galaxies is
non-relativistic on pc scales. We briefly discuss whether this non-relativistic
motion is intrinsic to the jet-formation process or results from deceleration
of an initially relativistic jet by interaction with the pc or sub-pc scale
interstellar medium. We combined our sample with a list compiled from the
literature of VLBI observations made of Seyfert galaxies, and found that most
Seyfert nuclei have at least one flat-spectrum component on the VLBI scale,
which was not seen in the spectral indices measured at arcsec resolution. We
found also that the bimodal alignment of pc and kpc radio structures displayed
by radio galaxies and quasars is not displayed by this sample of Seyferts,
which shows a uniform distribution of misalignment between 0 deg and 90 deg.
The frequent misalignment could result from jet precession or from deflection
of the jet by interaction with gas in the interstellar medium.
