  We report a similarity of fluctuations in equilibrium critical phenomena and
non-equilibrium systems, which is based on the concept of natural time. The
world-wide seismicity as well as that of San Andreas fault system and Japan are
analyzed. An order parameter is chosen and its fluctuations relative to the
standard deviation of the distribution are studied. We find that the scaled
distributions fall on the same curve, which interestingly exhibits, over four
orders of magnitude, features similar to those in several equilibrium critical
phenomena (e.g., 2D Ising model) as well as in non-equilibrium systems (e.g.,
3D turbulent flow).
