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1. Rupture Propagation along Stepovers of Strike-Slip Faults: Effects of Initial Stress and Fault Geometry

   https://doi.org/10.1785/0120190233  

   Wang, Hui; Liu, Mian; Duan, Benchun; Cao, Jianling (April 2020, Bulletin of the Seismological Society of America)

   null (Ed.)

   ABSTRACT Large earthquakes on strike-slip faults often rupture multiple fault segments by jumping over stepovers. Previous studies, based on field observations or numerical modeling with a homogeneous initial stress field, have suggested that stepovers more than ∼5  km wide would stop the propagation of rupture, but many exceptions have been observed in recent years. Here, we integrate a dynamic rupture model with a long-term fault stress model to explore the effects of background stress perturbation on rupture propagation across stepovers along strike-slip faults. Our long-term fault models simulate steady-state stress perturbation around stepovers. Considering such stress perturbation in dynamic rupture models leads to prediction of larger distance a dynamic rupture can jump over stepovers: over 15 km for a releasing stepover or 7 km for a restraining stepover, comparing with the 5 km limit in models with the same fault geometry and frictional property but assuming a homogeneous initial stress. The effect of steady-state stress perturbations is stronger in an overlapping stepover than in an underlapping stepover. The maximum jumping distance can reach 20 km in an overlapping releasing stepover with low-static frictional coefficients. These results are useful for estimating the maximum length of potential fault ruptures and assessing seismic hazard. 

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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. The stability of frictional sliding on dip-slip and finite length faults

   https://doi.org/10.1093/gji/ggaf071  

   Skarbek, Rob_M (February 2025, Geophysical Journal International)

   SUMMARY This paper examines the linear stability of sliding on faults embedded in a 2-D elastic medium that obey rate and state friction and have a finite length and/or are near a traction-free surface. Results are obtained using a numerical technique that allows for analysis of systems with geometrical complexity and heterogeneous material properties; however only systems with homogeneous frictional and material properties are examined. Some analytical results are also obtained for the special case of a fault that is parallel to a traction-free surface. For velocity-weakening faults with finite length, there is a critical fault length $$L^{*}$$ for unstable sliding that is analogous to the critical wavelength $$h^{*}$$ that is usually derived from infinite fault systems. Faults longer than $$L^{*}$$ are linearly unstable to perturbations of any length. On vertical strike-slip faults or faults in a full-space $$L^{*} \approx h^{*}/e$$, where e is Euler’s number. For dip-slip faults near a traction-free surface $$L^{*} \le h^{*}/e$$ and is a function of dip angle $$\beta$$, burial depth d of the fault’s up-dip edge and friction coefficient. In particular, $$L^{*}$$ is at least an order of magnitude smaller than $$h^{*}$$ on shallowly dipping ($$\beta < 10^\circ$$) faults that intersect the traction-free surface. Additionally, $$L^{*} \approx h^{*}/e$$ on dip-slip faults with burial depths $$d \ge h^{*}$$. For sliding systems that can be treated as a thin layer, such as landslides, glaciers or ice streams, $$L^{*} = h^{*}/2$$. Finally, conditions are established for unstable sliding on infinitely-long, velocity-strengthening faults that are parallel to a traction-free surface. 

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4. 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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5. Deformation and Frictional Failure of Granular Media in 3D Analog and Numerical Experiments

   https://doi.org/10.1007/s00024-024-03464-6  

   Ioannidi, P_I; McLafferty, S.; Reber, J_E; Morra, G.; Weatherley, D. (April 2024, Pure and Applied Geophysics)

   Abstract Frictional sliding along grain boundaries in brittle shear zones can result in the fragmentation of individual grains, which ultimately can impact slip dynamics. During deformation at small scales, stick–slip motion can occur between grains when existing force chains break due to grain rearrangement or failure, resulting in frictional sliding of granular material. The rearrangement of the grains leads to dilation of the granular package, reducing the shear stress and subsequently leading to slip. Here, we conduct physical experiments employing HydroOrbs, an elasto-plastic material, to investigate grain comminution in granular media under simple shear conditions. Our findings demonstrate that the degree of grain comminution is dependent on both the normal force and the size of the grains. Using the experimental setup, we benchmark Discrete Element Method (DEM) numerical models, which are capable of simulating the movement, rotation, and fracturing of elasto-plastic grains subjected to simple shear. The DEM models successfully replicate both grain comminution patterns and horizontal force fluctuations observed in our physical experiments. They show that increasing normal forces correlate with higher horizontal forces and more fractured grains. The ability of our DEM models to accurately reproduce experimental results opens up new avenues for investigating various parameter spaces that may not be accessible through traditional laboratory experiments, for example, in assessing how internal friction or cohesion affect deformation in granular systems. 

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