Effects of Bulk Poroelasticity on Repeating Earthquake Sequences: Insights From Numerical Modeling

Abstract

At seismogenic depths, faults and the surrounding rock are saturated with water. Fault slip induces volumetric strains in the bulk, causing compression or dilation of pore fluid, which alters pore pressure—a phenomenon known as the poroelastic effect. This pore pressure change can, in turn, influence fault slip behavior. However, most prior earthquake source models have neglected or oversimplified this effect. In this study, we simulate repeating earthquake sequences on a 1D fault embedded in a 2D poroelastic medium. The fault is modeled as a leaky shear zone with finite width and a linear pore-pressure distribution. Shear resistance is governed by a rate-and-state friction law, with effective normal stress accounting for pore-pressure changes. Our results show that poroelastic effects on fault slip strongly depend on assumptions about slip localization and can affect recurrence intervals and the ratio of seismic to total slip. Using the mean pore pressure over the shear-layer thickness in the effective normal stress leads to earthquake sequences comparable to the undrained limit for realistic bulk diffusivities, which can be mimicked using elastic models with undrained (larger) elastic moduli. These models are more stabilizing than those with drained moduli. Using the maximum pore pressure over the shear-layer thickness introduces further weakening, increasing seismic fault slip, while using pore pressure on one side of the shear layer leads to more complex rupture patterns and reduced seismic slip. Our findings emphasize the importance of incorporating poroelastic effects into earthquake source models and understanding how slip localizes within the shear zone.