  Earthquake network captures the complexity of seismicity in a peculiar
manner. Given a seismic data, the procedure of constructing an earthquake
network proposed in [S. Abe, N, Suzuki, Europhys. Lett. 65 (2004) 581] contains
as a single parameter the size of the cells, into which a geographical region
under consideration is divided. Then, the characteristics of the network depend
on the cell size, in general. Here, the dependency of the clustering
coefficient, C, of network on the cell size is studied. Remarkably, C of the
earthquake networks constructed from the seismic data taken from California,
Japan and Iran well coincide for each value of the rescaled dimensionless cell
size. It is found that the networks in California and Japan are
three-dimensional, whereas the one in Iran is rather two-dimensional. In
addition, the values of C of all these three networks monotonically converge to
C~0.85 as the dimensionless cell size becomes larger than a common value,
highlighting a universal aspect of the concept of earthquake network. An
implication of the result to understanding the physics of seismicity is
discussed.
