Scale-invariant statistics of the degrees of separation in directed earthquake network

Discovery of a new scale-free law of earthquake phenomenon is reported. It is relevant to the structural and dynamical properties of the earthquake network proposed in a recent work [S. Abe and N. Suzuki, Europhys. Lett. 65, 581 (2004)]. The seismic data taken in southern California are mapped to an evolving directed network. It is found that statistics of the degrees of separation between two vertices is characterized by the Zipf-Mandelbrot distribution. This shows how a given earthquake can influence a large number of subsequent earthquakes and describes spatio-temporal criticality/complexity of seismicity in a novel manner.