Seismicity migration from fluid injection: Laboratory experiments and numerical models illuminate volume-driven versus pressure-diffusion-driven migration

Fluid injection into the subsurface can induce seismicity by reactivating shear rupture, which typically produces larger earthquake magnitudes than tensile rupture. In laboratory shear rupture experiments, pressurization of the entire fault is often limited because large unconfined samples allow fluid to leak at free surfaces. In this study, we investigated shear fault reactivation by directly injecting fluid into a PMMA fault (760 mm long, 76 mm high) formed as the interface between two separate PMMA blocks. To prevent leakage in the 76 mm dimension, we made a low permeability barrier by coating the outer edges of the fault with Teflon tape. Fluid pressure then extended along the 760 mm dimension, resulting in seismicity migration away from the injection well. Changes in injection rate and fluid viscosity revealed two mechanisms: (1) Low injection rate or low-viscosity fluid caused seismicity migration governed by pressure diffusion, and (2) high injection rate or high-viscosity fluid caused seismicity migration proportional to injected volume. Simulations with a 2D poroelastic model showed that seismicity migrated with the fluid pressure front in the volume-driven regime, whereas fluid pressure advanced well ahead of seismicity in the pressure-diffusion-driven regime. These results highlight that Teflon tape effectively sealed faults and controlled fluid flow, and that injection rate and fluid viscosity have a strong impact on fault slip and induced seismicity.