  In a recent paper [S. Abe and N. Suzuki, Europhys. Lett., 65 (2004) 581], the
concept of earthquake network has been introduced in order to describe
complexity of seismicity. There, the cell size, which is the scale of coarse
graining needed for constructing an earthquake network, has remained as a free
parameter. Here, a method is presented for determining it based on the scaling
behavior of the network. Quite remarkably, both the exponent of the power-law
connectivity distribution and the clustering coefficient are found to approach
the respective universal values and remain invariant as the cell size becomes
larger than a certain value, $l_*$, which depends on the number of events
contained in the analysis, in general. This $l_*$ fixes the scale of coarse
graining. Universality of the result is demonstrated for all of the networks
constructed from the data independently taken from California, Japan and Iran.
