Seismicity migration from fluid injection: Laboratory experiments and numerical models illuminate volume-driven versus pressure-diffusion-driven migration
23 Fluid injection into the subsurface can induce seismicity by reactivating shear rupture, which 24 typically produces larger earthquake magnitudes than tensile rupture. In laboratory shear 25 rupture experiments, pressurization of the entire fault is often limited because large unconfined 26 samples allow fluid to leak at free surfaces. In this study, we investigated shear fault 27 reactivation by directly injecting fluid into a PMMA fault (760 mm long, 76 mm high) formed 28 as the interface between two separate PMMA blocks. To prevent leakage in the 76 mm 29 dimension, we made a low permeability barrier by coating the outer edges of the fault with 30 Teflon tape. Fluid pressure then extended along the 760 mm dimension, resulting in the 31 migration of seismicity away from the injection well. Changes in injection rate and fluid 32 viscosity revealed two mechanisms: (1) slow injection rate or low-viscosity fluid caused 33 seismicity migration governed by pressure diffusion, and (2) fast injection rate or high-34 viscosity fluid caused seismicity migration proportional to injected volume. Simulations with 35 a 2D poroelastic model showed that seismicity migrated with the fluid pressure front in the 36 volume-driven regime, whereas fluid pressure advanced well ahead of seismicity in the 37 pressure-diffusion-driven regime. These results highlight that Teflon tape effectively sealed 38 faults and controlled fluid flow, and that injection rate and fluid viscosity have a strong impact 39 on fault slip and induced seismicity. 40