  Spatiotemporal correlations of the two-dimensional spring-block
(Burridge-Knopoff) model of earthquakes are extensively studied by means of
numerical computer simulations. The model is found to exhibit either
``subcritical'' or ``supercritical'' behavior, depending on the values of the
model parameters. Transition between these regimes is either continuous or
discontinuous. Seismic events in the ``subcritical'' regime and those in the
``supercritical'' regime at larger magnitudes exhibit universal scaling
properties. In the ``supercritical'' regime, eminent spatiotemporal
correlations, {\it e.g.}, remarkable growth of seismic activity preceding the
mainshock, arise in earthquake occurrence, whereas such spatiotemporal
correlations are significantly suppressed in the ``subcritical'' regime.
Seismic activity is generically suppressed just before the mainshock in a close
vicinity of the epicenter of the upcoming event while it remains to be active
in the surroundings (the Mogi doughnut). It is also observed that, before and
after the mainshock, the apparent $B$-value of the magnitude distribution
decreases or increases in the ``supercritical'' or ``subcritical'' regimes,
respectively. Such distinct precursory phenomena may open a way to the
prediction of the upcoming large event.
