We model the evolution of regional stress following the 1992 Landers earthquake in order to test the importance of pore fluid flow in producing aftershocks. Rising fluid pressure due to pore fluid flow and the resulting Coulomb stress changes were found to be strongly correlated with the time and location of aftershock events. Regional aftershock frequencies computed by integrating pore pressure decay rates also agreed quite well with aftershock data. Calculations show that regions of rising postseismic poroelastic Coulomb stress overlap considerably with regions of positive coseismic Coulomb stress. Thus pore fluid flow, which affects pore pressure within faults and causes regional poroelastic stress evolution following earthquakes, gradually evolves the initial coseismic stress changes. Together these changes provide a reasonable physical mechanism for aftershock triggering which agrees with data for the 1992 Landers earthquake.