Universal clustering structure and C≈0.85C≈0.85 scaling in complex earthquake networks

• The clustering property of complex earthquake networks is discussed.
• The cell-size dependence of the clustering coefficient, CC, is analyzed.
• The universal C∼0.85C∼0.85 scaling with respect to dimensionless cell-size is discovered.
• The networks in California and Japan are three-dimensional.
• On the other hand, the network in Iran is rather two-dimensional.

Earthquake network captures the complexity of seismicity in a peculiar manner. Given a seismic data, the procedure of constructing an earthquake network proposed by Abe and Suzuki (2004) 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, CC, of network on the cell size is studied. Remarkably, CC 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 CC of all these three networks monotonically converge to C≈0.85C≈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.