  We measure the horizontal subsurface flow in a fast emerging active region
(NOAA 11158) using the ring-diagram technique and the HMI high-spatial
resolution Dopplergrams. This active region had a complex magnetic structure
and displayed significant changes in the morphology during its disk passage.
Over the period of six days from 2011 February 11 to 16, the temporal variation
in the magnitude of total velocity is found to follow the trend of magnetic
field strength. We further analyze regions of individual magnetic polarity
within AR 11158 and find that the horizontal velocity components in these
sub-regions have significant variation with time and depth. The leading and
trailing polarity regions move faster than the mixed-polarity region. Further,
both zonal and meridional components have opposite signs for trailing and
leading polarity regions at all depths showing divergent flows within the
active region. We also find a sharp decrease in the magnitude of total
horizontal velocity in deeper layer around major flares. It is suggested that
the re-organization of magnetic fields during flares combined with the sunspot
rotation decreases the magnitude of horizontal flows or that the flow kinetic
energy has been converted into the energy released by flares. After the decline
in flare activity and the sunspot rotation, the flows tend to follow the
pattern of the magnetic activity.We also observe less variation in the velocity
components near the surface but these tend to increase with depth, further
demonstrating that the deeper layers are more affected by the topology of
active regions.
