  Discoveries of the scale-free and small-world features are reported on a
network constructed from the seismic data. It is shown that the connectivity
distribution decays as a power law, and the value of the degrees of separation,
i.e., the characteristic path length or the diameter, between two earthquakes
(as the vertices) chosen at random takes a small value between 2 and 3. The
clustering coefficient is also calculated and is found to be about 10 times
larger than that in the case of the completely random network. These features
highlight a novel aspect of seismicity as a complex phenomenon.
