Fluid Pressure Diffusion and Elastic Stress Transfer From Seismic Slip Control Slip Front Expansion in Fluid‐Driven Swarm Seismicity

Abstract Observations of fluid‐driven swarm seismicity expanding with the same diffusive space‐time behavior as analytical solutions for aseismic slip have been interpreted as evidence that stress changes from aseismic slip trigger seismic slip. In some cases, aseismic slip is confirmed from crustal deformation measurements or sheared wellbore casing. Our work offers another interpretation of migrating seismicity that may be relevant when there is no independent evidence for aseismic slip. We present 2D earthquake sequence models that simulate constant‐pressure fluid injection into a velocity‐weakening rate‐and‐state fault, permeability enhancement with slip, and fluid transport. We also derive an analytical expression for a unilateral fluid‐driven aseismic shear crack with constant friction and permeability enhancement. Our numerical results show that pressure diffusion and elastic stress transfer from seismic slip drive seismicity fronts that expand diffusively outward from the injector, and that analytical solutions for aseismic slip match this seismicity pattern, even when almost all slip is seismic. Aseismic slip at the slip front increases permeability, allowing fluid influx and pressurization, weakening the fault prior to slipping seismically. Adding roughness to the fault introduces heterogeneous fault normal stress, resulting in a broader distribution of event sizes, distributed seismicity behind the slip front, and swarm‐like clusters of microseismicity. Even with this complexity, the nonplanar simulations produce a diffusively expanding slip front and mostly seismic slip. Our results suggest that aseismic slip solutions can be used to quantitatively interpret the space‐time behavior of migrating swarms, even in cases with negligible measured aseismic slip.