  OB associations are the prime star forming sites in galaxies. However the
detailed formation process of such stellar systems still remains a mystery. In
this context, identifying the presence of substructures may help tracing the
footprints of their formation process. Here, we present a kinematic study of
the two massive OB associations Cygnus OB2 and Carina OB1 using the precise
astrometry from the Gaia Data Release 2 and radial velocities. From the
parallaxes of stars, these OB associations are confirmed to be genuine stellar
systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters
and a halo which have different kinematic properties: the clusters occupy
regions of 5-8 parsecs in diameter and display small dispersions in proper
motion, while the halos spread over tens of parsecs with a 2-3 times larger
dispersions in proper motion. This is reminiscent of the so-called "line
width-size" relation of molecular clouds related to turbulence. Considering
that the kinematics and structural features were inherited from those of their
natal clouds would then imply that the formation of OB associations may result
from structure formation driven by supersonic turbulence, rather than from the
dynamical evolution of individual embedded clusters.
