  The statistical properties of the seismic time series data in southern
California are studies. In particular, the calm time intervals, which are the
time intervals between successive significant earthquakes above the fixed
threshold value of magnitude, are analyzed. It is found that the system
undergoes a series of transition between stationary states, whose cumulative
waiting time distributions all obey the Zipf-Mandelbrot law, exhibiting a new
scale free nature of earthquakes. Dependencies of the two parameters
characterizing the Zipf-Mandelbrot distribution on the threshold are also
discussed.
