The evolution of regional seismicity between large earthquakes

[1] We describe a simple model that links static stress (Coulomb) modeling to the regional seismicity around a major fault. Unlike conventional Coulomb stress techniques, which calculate stress changes, we model the evolution of the stress field relative to the failure stress. Background seismicity is attributed to inhomogeneities in the stress field which are created by adding a random field that creates local regions above the failure stress. The inhomogeneous field is chosen such that when these patches fail, the resulting earthquake size distribution follows a Gutenburg-Richter law. Immediately following a large event, the model produces regions of increased seismicity (aftershocks) where the overall stress field has been elevated and regions of reduced seismicity where the stress field has been reduced (stress shadows). The high stress levels in the aftershock regions decrease due to loading following the main event. Combined with the stress shadow from the main event, this results in a broad seismically quiet region of lowered stress around the epicenter. Pre-event seismicity appears as the original stress shadows finally fill as a result of loading. The increase in seismicity initially occurs several fault lengths away from the main fault and moves inward as the event approaches. As a result of this effect, the seismic moment release in the region around the future epicenter increases as the event approaches. Synthetic catalogues generated by this model are virtually indistinguishable from real earthquake sequences in California and Washington.