We have constructed a model for the occurrence of aftershocks and aftershock sequences. The aftershock occurrence is assumed to be dependent on the pore fluid pressure p through the Coulomb fracture criterion coupled to the principle of effective stress. The fluid is assumed to flow out of a localized high-pressure fluid zone with the occurrence of the main shock. Our aftershock source model explains the following seismological observations of aftershocks in a unified manner; the Omori law of aftershock activity, the Gutenberg-Richter magnitude-frequency relation, the occurrence of secondary aftershock sequences, and the migration of aftershock activity. It is also an attractive feature of our model that the model can simulate not only aftershocks but also a variety of earthquake sequences by changing the values of poroelastic model parameters .