Suyehiro, Kiyoshi1,2, Sacks, I. Selwyn2, Rydelek, Paul 2, Smith, Deborah 2, and Takanami, Tetsuo21: Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan2: Carnegie Institution of Washington, Washington, DC, USAAbstract : A simple cellular automaton model with Coulomb’s failure criterion is used to explain observed seismicity changes before large earthquakes. Specifically, we try to explain the decrease of the b -value of the Gutenberg-Richter’s Law towards large earthquakes and the seismic quiescence of larger magnitude earthquakes recognized from a few years prior to large earthquakes. Model results show a gradual decrease of the b -value in time as the tectonic stress is increased. This seems to be an inherent feature of the model corresponding to the reordering of the stress distribution as earthquakes are generated in the model space. The magnitude dependent quiescence can be reproduced by strengthening, at some threshold level, only a small percentage of the cells that are loaded with higher stresses. In nature, such spatially heterogeneous strengthening of the crust may be created by the opening of micro-fractures, which acts to reduce the pore pressure as observed in the lab. Furthermore, we infer from observations of anomalies prior to large earthquakes at shorter time scale of ~days that these strengthened patches undergo selective weakening via fluid migration, which depends on both the spatial distribution of permeability and the location of fluid sources. Geologically mature faults are known to possess high permeability that is orders of magnitude greater than the surrounding rock, which may eventually become the main rupture zone of a major earthquake.