The role of poroelasticity and permeability barriers in governing the interplay between precursory slow slip and foreshocks

Ample observations attest to the importance of pore pressure dynamics and fault-zone fluid flow in producing earthquake swarms, foreshocks, and aftershocks. However, poroelasticity effects incorporating the two-way coupling of solid and fluid phases where pore-pressure evolves due to e.g. slip, dilatancy, and compaction, are rarely considered in simulations of fault slip. Here we study how coupled dynamics of frictional slip, pore pressure, permeability and porosity evolution control the occurrence of precursory slow-slip and foreshocks leading to the mainshock. We model fully dynamic seismic cycles with a newly-developed Hydro-Mechanical Earthquake Cycles (H-MECs) code where uniform velocity-weakening rate-and-state friction and a constant and far-field loading rate are applied on a 2-D anti-plane fault model embedded in a poro-visco-elasto-plastic medium. We also include the effect of permeability barriers, represented by regions of low permeability and high pore-fluid pressure with wavelengths similar to the nucleation length. Despite the relatively simple model setup, a complex fault behavior arises from the numerical experiments, including small seismic events, complete ruptures, as well as aseismic slip transients. Our results indicate that permeability barriers – where pore-fluid pressure is high – lead to fault creep, whereas foreshocks and mainshocks unzip from locked asperities with relatively lower pore-fluid pressure. We find that the ratio between the size of locked asperity and the wavelength of permeability barriers controls both the nucleation and propagation of aseismic creep, slow-slip transients, cascade of foreshocks, and full seismic rupture. Further numerical experiments accounting for both permeability enhancement due to fault slip and permeability reduction due to healing and sealing show that, once the permeability seals break, fluid is injected and redistributed through the fault zone, which diffuses primarily on-fault, thus leading to the nucleation of a complete rupture. As a result, permeability evolution and pore-fluid pressure variations modulate the ratio of seismic to aseismic moment release of seismic swarms. Our results, compared to observations, demonstrate that pore-fluid pressure evolution and poroelastic effects on- and off-fault play a critical role in the dynamism of earthquake swarms, as they control the stability of faults and whether slip is seismic or aseismic.