  The origin of the radio emission in radio-quiet quasars (RQQ) is not
established yet. We present new VLBA observations at 1.6 and 4.9 GHz of ten RQQ
(nine detected), which together with published earlier observations of eight
RQQ (five detected), forms a representative sample of 18 RQQ drawn from the
Palomar-Green sample of low z (< 0.5) AGN. The spectral slope of the integrated
emission extends from very steep (alpha < -1.98) to strongly inverted (alpha =
+2.18), and the slopes of nine of the 14 objects are flat (alpha > -0.5). Most
objects have an unresolved flat-spectrum core, which coincides with the optical
Gaia position. The extended emission is generally steep-spectrum, has a low
brightness temperature (< 10^7 K), and is displaced from the optical core (the
Gaia position) by ~ 5-100 pc. The VLBA core flux is tightly correlated with the
X-ray flux, and follows a radio to X-ray luminosity relation of log L_R/L_X =
-6, for all objects with a black hole mass log M_BH/M_Sun < 8.5. The flatness
of the core emission implies a compact source size (< 0.1 pc), which likely
originates from the accretion disk corona. The mas-scale extended emission is
optically thin and of clumpy structure, and is likely produced by an outflow
from the center. Radio observations at higher frequencies can further test the
accretion disk coronal emission interpretation for the core emission in RQQ.
