  Forest fire models may be interpreted as a simple model for earthquake
occurrence by translating trees and fire into stressed segments of a fault and
their rupture, respectively. Here we adopt a twodimensional forest-fire model
in continuous time, and focus on the temporal changes of seismicity and the
b-value. We find the b-value change and seismic quiescence prior to large
earthquakes by stacking many sequences towards large earthquakes. As the
magnitude-frequency relation in this model is directly related to the
cluster-size distribution, decrease of the b-value can be explained in terms of
the change in the cluster-size distribution. Decrease of the b-value means that
small clusters of stressed sites aggregate into a larger cluster. Seismic
quiescence may be attributed to the decrease of stressed sites that do not
belong to percolated clusters.
