Simulation of seismicity due to fluid migration in a fault zone

Spatio-temporal variation of rupture activity is modelled assuming fluid migration in a narrow, porous fault zone formed along a vertical strike-slip fault in a semi-infinite elastic medium. The principle of the effective stress coupled to the Coulomb failure criterion introduces mechanical coupling between fault slip and the pore fluid. The fluid is assumed to flow out of a localized high-pressure fluid compartment in the fault at the onset of earthquake rupture. The duration of the earthquake sequence is assumed to be much shorter than the recurrence period of characteristic events on the fault. Both an earthquake swarm and a foreshock–main-shock sequence can be simulated by changing the relative magnitudes of the initial tectonic stress, pore fluid pressure, fracture strength and so on. When an inhomogeneity is introduced into the spatial distribution of fracture strength, high complexity is observed in the spatio-temporal variation of rupture activity. For example, the time interval between two successive events is highly irregular, and a relatively long quiescence of activity is sometimes observed in a foreshock–main-shock sequence. The quiescence is caused by the temporary arresting of rupture extension, due to an encounter with fault segments having locally high strengths. The frequency–magnitude statistics of intermediate-size events obey the Gutenberg–Richter relation. The calculations show the temporal variation of the b value during some foreshock sequences, and the degree of the change seems to depend on the statistical distribution of the fracture strength.