  In analyzing synthetic earthquake catalogs created by a two-dimensional
Burridge-Knopoff model, we have found that a probability distribution of the
interoccurrence times, the time intervals between successive events, can be
described clearly by the superposition of the Weibull distribution and the
log-Weibull distribution. In addition, the interoccurrence time statistics
depend on frictional properties and stiffness of a fault and exhibit the
Weibull - log Weibull transition, which states that the distribution function
changes from the log-Weibull regime to the Weibull regime when the threshold of
magnitude is increased. We reinforce a new insight into this model; the model
can be recognized as a mechanical model providing a framework of the Weibull -
log Weibull transition.
