  To characterize the dynamical features of seismicity as a complex phenomenon,
the seismic data is mapped to a growing random graph, which is a small-world
scale-free network. Here, hierarchical and mixing properties of such a network
are studied. The clustering coefficient is found to exhibit asymptotic
power-law decay with respect to connectivity, showing hierarchical
organization. This structure is supported by not only main shocks but also
small shocks, and may have its origin in the combined effect of vertex fitness
and deactivation by stress release at faults. The nearest-neighbor average
connectivity and the Pearson correlation coefficient are also calculated. It is
found that the earthquake network has assortative mixing. This is a main
difference of the earthquake network from the Internet with disassortative
mixing. Physical implications of these results are discussed.
