  The star-forming region W5 is a major part of the Cassiopeia OB6 association.
Its internal structure and kinematics may provide hints of the star formation
process in this region. Here, we present a kinematic study of young stars in W5
using the Gaia data and our radial velocity data. A total 490 out of 2,000
young stars are confirmed as members. Their spatial distribution shows that W5
is highly substructured. We identify a total of eight groups using the k-means
clustering algorithm. There are three dense groups in the cavities of H II
bubbles, and the other five sparse groups are distributed at the ridge of the
bubbles. The three dense groups have almost the same ages (5 Myr) and show a
pattern of expansion. The scale of their expansion is not large enough to
account for the overall structure of W5. The three northern groups are, in
fact, 3 Myr younger than the dense groups, which indicates the independent star
formation events. Only one group of them shows the signature of feedback-driven
star formation as its members move away from the eastern dense group. The other
two groups might have formed in a spontaneous way. On the other hand, the
properties of two southern groups are not understood as those of a coeval
population. Their origins can be explained by dynamical ejection of stars and
multiple star formation. Our results suggest that the substructures in W5
formed through multiple star-forming events in a giant molecular cloud.
