Regularity and complexity of aftershock occurrence due to mechanical interactions between fault slip and fluid flow

Abstract

SUMMARY A sequence of aftershocks 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 pore fluid. The fluid is assumed to flow out of localized high-pressure fluid compartments in the fault zone at the instant of the main shock occurrence. We successfully simulate both regularity and complexity observed for aftershocks in a unified way. For example, we can simulate both the Omori law and the Gutenberg‐Richter relation, which are the most outstanding regularity observed seismologically. A large majority of simulated aftershocks are shown to consist of repeated slips, that is, slips on fault segments that have experienced slips earlier in the aftershock sequence. Our calculations show that the emergence of the Gutenberg‐ Richter relation is closely related to the occurrence of these repeated slips. Complexity is also a striking feature of aftershocks. One of the examples is the occurrence of secondary aftershocks, which can also be simulated successfully if we assume several high-pressure fluid compartments formed in a fault zone or a significant change in the permeability caused by the rupture occurrence.