Post-seismic fluid flow and Coulomb stress changes in a poroelastic medium

It is generally agreed that the occurrence of seismic sequences implies a kind of interaction between different fault segments. The coseismic stress transfer produced by each dislocation is the most obvious component of such an interaction. However the time intervals elapsing between subsequent events in a sequence indicate that the coseismic stress is not sufficient to trigger other seismic events by itself. We investigate the possibility that the coseismic stress field may induce flow of pore fluids, altering the pore pressure distribution in the region. Since the crust is a fluid-saturated medium at many locations, we consider the crust as a poroelastic solid. Because poroelastic materials exhibit time-dependent stress fields, we examine if this behavior can explain the triggering of aftershocks. We consider some available analytical solutions for a semi-infinite plane fault. Permeable and impermeable dislocation planes are considered. We compare the solutions for a poroelastic medium with those for a porous medium, and evaluate the effect of the coupling between deformation and fluid diffusion. We find that the Coulomb stress changes due to the mainshock may be initially negative at some locations, but become positive as pore fluids are redistributed. These changes are significantly large. If the crust were to behave as an isotropic, fluid filled, poro-elastic medium, as we assume here, Coulomb stress triggering by means of pore fluid diffusion is likely an important mechanism for aftershock generation over distances and widths of about 2.5 and 0.5 fault lengths, respectively. These distance ranges are smaller than those predicted by previous models which disregarded the mechanical interactions between elastic deformation and pore fluid diffusion. For typical porosities, the stress changes due to fluid flow are diminished greatly after about one year after the mainshock.