  Spatio-temporal correlations of earthquakes are studied numerically on the
basis of the one-dimensional spring-block (Burridge-Knopoff) model. As large
events approach, the frequency of smaller events gradually increases, while,
just before the mainshock, it is dramatically suppressed in a close vicinity of
the epicenter of the upcoming mainshock, a phenomenon closely resembling the
``Mogi doughnut''
