Natural-time analysis of critical phenomena: The case of seismicity P. A. Varotsos, N. V. Sarlis, E. S. Skordas, S. Uyeda and M. Kamogawa EPL, 92 2 (2010) 29002 Published online: 17 November 2010 DOI: 10.1209/0295-5075/92/29002