Scale-invariant statistics of period in directed earthquake network
  A new law regarding structure of the earthquake networks is found. The seismic data taken in California is mapped to a growing directed network. Then, statistics of period in the network, which implies that after how many earthquakes an earthquake returns to the initial location, is studied. It is found that the period distribution obeys a power law, showing the fundamental difficulty of statistical estimate of period. 
Sumiyoshi Abe (1) and Norikazu Suzuki (2)((1)Institute of Physics,   University of Tsukuba, Ibaraki, Japan,(2)College of Sci...