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1. Fluid-induced aseismic fault slip outpaces pore-fluid migration

   https://doi.org/10.1126/science.aaw7354  

   Bhattacharya, Pathikrit; Viesca, Robert C. (May 2019, Science)

   Earthquake swarms attributed to subsurface fluid injection are usually assumed to occur on faults destabilized by increased pore-fluid pressures. However, fluid injection could also activate aseismic slip, which might outpace pore-fluid migration and transmit earthquake-triggering stress changes beyond the fluid-pressurized region. We tested this theoretical prediction against data derived from fluid-injection experiments that activated and measured slow, aseismic slip on preexisting, shallow faults. We found that the pore pressure and slip history imply a fault whose strength is the product of a slip-weakening friction coefficient and the local effective normal stress. Using a coupled shear-rupture model, we derived constraints on the hydromechanical parameters of the actively deforming fault. The inferred aseismic rupture front propagates faster and to larger distances than the diffusion of pressurized pore fluid. 

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2. Self-similar fault slip in response to fluid injection

   https://doi.org/10.1017/jfm.2021.825  

   Viesca, Robert C. (December 2021, Journal of Fluid Mechanics)

   There is scientific and industrial interest in understanding how geologic faults respond to transient sources of fluid. Natural and artificial sources can elevate pore fluid pressure on the fault frictional interface, which may induce slip. We consider a simple boundary value problem to provide an elementary model of the physical process and to provide a benchmark for numerical solution procedures. We examine the slip of a fault that is an interface of two elastic half-spaces. Injection is modelled as a line source at constant pressure and fluid pressure is assumed to diffuse along the interface. The resulting problem is an integro-differential equation governing fault slip, which has a single dimensionless parameter. The expansion of slip is self-similar and the rupture front propagates at a factor $$\lambda$$ of the diffusive length scale $$\sqrt {\alpha t}$$ . We identify two asymptotic regimes corresponding to $$\lambda$$ being small or large and perform a perturbation expansion in each limit. For large $$\lambda$$ , in the regime of a so-called critically stressed fault, a boundary layer emerges on the diffusive length scale, which lags far behind the rupture front. We demonstrate higher-order matched asymptotics for the integro-differential equation, and in doing so, we derive a multipole expansion to capture successive orders of influence on the outer problem for fault slip for a driving force that is small relative to the crack dimensions. Asymptotic expansions are compared with accurate numerical solutions to the full problem, which are tabulated to high precision. 

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3. Fluid-driven aseismic fault slip with permeability enhancement and dilatancy

   https://doi.org/10.1098/rsta.2023.0255  

   Dunham, Eric M (August 2024, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   Injection-induced seismicity and aseismic slip often involve the reactivation of long-dormant faults, which may have extremely low permeability prior to slip. In contrast, most previous models of fluid-driven aseismic slip have assumed linear pressure diffusion in a fault zone of constant permeability and porosity. Slip occurs within a frictional shear crack whose edge can either lag or lead pressure diffusion, depending on the dimensionless stress-injection parameter that quantifies the prestress and injection conditions. Here, we extend this foundational work by accounting for permeability enhancement and dilatancy, assumed to occur instantaneously upon the onset of slip. The fault zone ahead of the crack is assumed to be impermeable, so fluid flow and pressure diffusion are confined to the interior, slipped part of the crack. The confinement of flow increases the pressurization rate and reduction of fault strength, facilitating crack growth even for severely understressed faults. Suctions from dilatancy slow crack growth, preventing propagation beyond the hydraulic diffusion length. Our new two-dimensional and three-dimensional solutions can facilitate the interpretation of induced seismicity data sets. They are especially relevant for faults in initially low permeability formations, such as shale layers serving as caprock seals for geologic carbon storage, or for hydraulic stimulation of geothermal reservoirs. This article is part of the theme issue ‘Induced seismicity in coupled subsurface systems’. 

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4. Quartz fluid inclusion abundance and off-fault damage in a deeply exhumed, strike-slip, seismogenic fault

   https://doi.org/10.1016/j.jsg.2020.104118  

   Song, Won Joon; Johnson, Scott E.; Gerbi, Christopher C. (June 2020, Journal of Structural Geology)
5. Asymptotic solutions for self-similarly expanding fault slip induced by fluid injection at constant rate

   https://doi.org/10.1017/jfm.2025.10593  

   Viesca, Robert C (September 2025, Journal of Fluid Mechanics)

   We examine the circular, self-similar expansion of frictional rupture due to fluid injected at a constant rate. Fluid migrates within a thin permeable layer parallel to and containing the fault plane. When the Poisson ratio$$\nu =0$$, self-similarity of the fluid pressure implies fault slip also evolves in an axisymmetric, self-similar manner, reducing the three-dimensional problem for the evolution of fault slip to a single self-similar dimension. The rupture radius grows as$$\lambda \sqrt {4\alpha _{hy} t}$$, where$$t$$is time since the start of injection and$$\alpha _{hy}$$is the hydraulic diffusivity of the pore fluid pressure. The prefactor$$\lambda$$is determined by a single parameter,$$T$$, which depends on the pre-injection stress state and injection conditions. The prefactor has the range$$0\lt \lambda \lt \infty$$, the lower and upper limits of which correspond to marginal pressurisation of the fault and critically stressed conditions, in which the fault-resolved shear stress is close to the pre-injection fault strength. In both limits, we derive solutions for slip by perturbation expansion, to arbitrary order. In the marginally pressurised limit ($$\lambda \rightarrow 0$$), the perturbation is regular and the series expansion is convergent. For the critically stressed limit ($$\lambda \rightarrow \infty$$), the perturbation is singular, contains a boundary layer and an outer solution, and the series is divergent. In this case, we provide a composite solution with uniform convergence over the entire rupture using a matched asymptotic expansion. We provide error estimates of the asymptotic expansions in both limits and demonstrate optimal truncation of the singular perturbation in the critically stressed limit. 

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