  Earthquake network is known to be complex in the sense that it is scale-free,
small-world, hierarchically organized and assortatively mixed. Here, the time
evolution of earthquake network is analyzed around main shocks in the context
of the community structure. It is found that the maximum of the modularity
measure quantifying existence of communities exhibits a peculiar behavior: its
maximum value stays at a large value before a main shock, suddenly drops to a
small value at the main shock, and then increases to relax to a large value
again relatively slowly. Thus, a main shock absorbs and merges communities to
create a larger community, showing how a main shock can be characterized in the
complex-network representation of seismicity.
