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1. Effects of Dilatant Hardening on Fault Stabilization and Structural Development

   https://doi.org/10.1029/2024GL108840  

   Williams, S_A; French, M_E (May 2024, Geophysical Research Letters)

   Abstract Dilatant hardening is one proposed mechanism that causes slow earthquakes along faults. Previous experiments and models show that dilatant hardening can stabilize fault rupture and slip in several lithologies. However, few studies have systematically measured the mechanical behavior across the transition from dynamic to slow rupture or considered how the associated damage varies. To constrain the processes and scales of dilatant hardening, we conducted triaxial compression experiments on cores of Crab Orchard sandstone and structural analyses using micro‐computed tomography imaging and petrographic analysis. Experiments were conducted at an effective confining pressure of ∼10 MPa, while varying confining pressure (10–130 MPa) and pore fluid pressure (1–120 MPa). Above 15 MPa pore fluid pressure, dilatant hardening slows the rate of fault rupture and slip and deformation becomes more distributed amongst multiple faults as microfracturing increases. The resulting increase in fracture energy has the potential to control fault slip behavior. 

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2. Ground-Motion Characteristics of Cascading Earthquakes in a Multiscale Fracture Network

   https://doi.org/10.1785/0120240281  

   Palgunadi, Kadek Hendrawan; Gabriel, Alice-Agnes; Garagash, Dmitry Igor; Ulrich, Thomas; Schliwa, Nico; Mai, P Martin (June 2025, Bulletin of the Seismological Society of America)

   ABSTRACT Fault zones exhibit geometrical complexity and are often surrounded by multiscale fracture networks within their damage zones, potentially influencing rupture dynamics and near-field ground motions. In this study, we investigate the ground-motion characteristics of cascading ruptures across damage zone fracture networks of moderate-size earthquakes (Mw 5.5–6.0) using high-resolution 3D dynamic rupture simulations. Our models feature a listric normal fault surrounded by more than 800 fractures, emulating a major fault and its associated damage zone. We analyze three cases: a cascading rupture propagating within the fracture network (Mw 5.5), a non-cascading main-fault rupture with off-fault fracture slip (Mw 6.0), and a main-fault rupture without a fracture network (Mw 6.0). Cascading ruptures within the fracture network produce distinct ground-motion signatures with enriched high-frequency content, arising from simultaneous slip of multiple fractures and parts of the main fault, resembling source coda-wave-like signatures. This case shows elevated near-field characteristic frequency (fc) and stress drop, approximately an order of magnitude higher than the estimation directly on the fault of the dynamic rupture simulation. The inferred fc of the modeled vertical ground-motion components reflects the complexity of the radiation pattern and rupture directivity of fracture-network cascading earthquakes. We show that this is consistent with observations of strong azimuthal dependence of corner frequency in the 2009–2016 central Apennines, Italy, earthquake, sequence. Simulated ground motions from fracture-network cascading ruptures also show pronounced azimuthal variations in peak ground acceleration (PGA), peak ground velocity, and pseudospectral acceleration, with average PGA nearly double that of the non-cascading cases. Cascading ruptures radiate high-frequency seismic energy, yield nontypical ground-motion characteristics including coda-wave-like signatures, and may result in a significantly higher seismologically inferred stress drop and PGA. Such outcomes emphasize the critical role of fault-zone complexity in affecting rupture dynamics and seismic radiation and have important implications for physics-based seismic hazard assessment. 

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3. The Role of Background Stress State in Fluid‐Induced Aseismic Slip and Dynamic Rupture on a 3‐m Laboratory Fault

   https://doi.org/10.1029/2022JB024371  

   Cebry, S. B. L.; Ke, C. ‐Y.; McLaskey, G. C. (August 2022, Journal of Geophysical Research: Solid Earth)

   Abstract Fluid injection stimulates seismicity far from active tectonic regions. However, the details of how fluids modify on‐fault stresses and initiate seismic events remain poorly understood. We conducted laboratory experiments using a biaxial loading apparatus with a 3 m saw‐cut granite fault and compared events induced at different levels of background shear stress. Water was injected at 10 mL/min and normal stress was constant at 4 MPa. In all experiments, aseismic slip initiated on the fault near the location of fluid injection and dynamic rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MPa‐level injection pressures were reached and the dynamic rupture propagated beyond the fluid pressure perturbed region. At lower stress levels, dynamic rupture initiated hundreds of seconds later and was limited to regions where aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We found that the initiation of slow slip was broadly consistent with a Coulomb failure stress, but that initiation of dynamic rupture required additional criteria to be met. Even high background stress levels required aseismic slip to modify on‐fault stress to meet initiation criteria. We also observed slow slip events prior to dynamic rupture. Overall, our experiments suggest that initial fault stress, relative to fault strength, is a critical factor in determining whether a fluid‐induced rupture will “runaway” or whether a fluid‐induced rupture will remain localized to the fluid pressurized region. 

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4. Three-dimensional wave propagation and earthquake dynamic rupture simulations in complex poroelastic media

   https://doi.org/10.1093/gji/ggaf184  

   Wolf, Sebastian; Gabriel, Alice-Agnes; Galis, Martin; Moczo, Peter; Gregor, David; Bader, Michael (May 2025, Geophysical Journal International)

   SUMMARY Numerical simulations of earthquakes and seismic wave propagation require accurate material models of the solid Earth. In contrast to purely elastic rheology, poroelasticity accounts for pore fluid pressure and fluid flow in porous media. Poroelastic effects can alter both the seismic wave field and the dynamic rupture characteristics of earthquakes. For example, the presence of fluids may affect cascading multifault ruptures, potentially leading to larger-than-expected earthquakes. However, incorporating poroelastic coupling into the elastodynamic wave equations increases the computational complexity of numerical simulations compared to elastic or viscoelastic material models, as the underlying partial differential equations become stiff. In this study, we use a Discontinuous Galerkin solver with Arbitrary High-Order DERivative time stepping of the poroelastic wave equations implemented in the open-source software SeisSol to simulate 3-D complex seismic wave propagation and 3-D dynamic rupture in poroelastic media. We verify our approach for double-couple point sources using independent methods including a semi-analytical solution and a finite-difference scheme and a homogeneous full-space and a poroelastic layer-over-half-space model, respectively. In a realistic carbon capture and storage reservoir scenario at the Sleipner site in the Utsira Formation, Norway, we model 3-D wave propagation through poroelastic sandstone layers separated by impermeable shale. Our results show a sudden change in the pressure field across material interfaces, which manifests as a discontinuity when viewed at the length scale of the dominant wavelengths of S or fast P waves. Accurately resolving the resulting steep pressure gradient dramatically increases the computational demands, requiring high-resolution modelling. We show that the Gassmann elastic equivalent model yields almost identical results to the fully poroelastic model when focusing solely on solid particle velocities. We extend this approach using suitable numerical fluxes to 3-D dynamic rupture simulations in complex fault systems, presenting the first 3-D scenarios that combine poroelastic media with geometrically complex, multifault rupture dynamics and tetrahedral meshes. Our findings reveal that, in contrast to modelling wave propagation only, poroelastic materials significantly alter rupture characteristics compared to using elastic equivalent media since the elastic equivalent fails to capture the evolution of pore pressure. Particularly in fault branching scenarios, the Biot coefficient plays a key role in either promoting or inhibiting fault activation. In some cases, ruptures are diverted to secondary faults, while in others, poroelastic effects induce rupture arrest. In a fault zone dynamic rupture model, we find poroelasticity aiding pulse-like rupture. A healing front is induced by the reduced pore pressure due to reflected waves from the boundaries of the poroelastic damage zone. Our results highlight that poroelastic effects are important for realistic simulations of seismic waves and earthquake rupture dynamics. In particular, our poroelastic simulations may offer new insights on the complexity of multifault rupture dynamics, fault-to-fault interaction and seismic wave propagation in realistic models of the Earth’s subsurface. 

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5. Delayed Dynamic Triggering and Enhanced High‐Frequency Seismic Radiation From Brittle Rock Damage in 3D Dynamic Rupture Simulations

   https://doi.org/10.1029/2025JB031632  

   Niu, Zihua; Gabriel, Alice‐Agnes; Ben‐Zion, Yehuda (September 2025, Journal of Geophysical Research: Solid Earth)

   Abstract Using a novel high‐performance computing implementation of a nonlinear continuum damage‐breakage model, we explore interactions between 3D co‐seismic off‐fault damage, seismic radiation, and rupture dynamics. Our simulations demonstrate that off‐fault damage enhances high‐frequency wave radiation above 1 Hz, reduces rupture speed and alters the total kinetic energy. We identify distinct damage regimes separated by solid‐granular transition, with smooth distributions under low damage conditions transitioning to localized, mesh‐independent shear bands upon reaching brittle failure. The shear band orientations depend systematically on the background stress and agree with analytical predictions. The brittle damage inhibits transitions to supershear rupture propagation and the rupture front strain field results in locally reduced damage accumulation during supershear transition. The dynamically generated damage yields uniform and isotropic ratios of fault‐normal to fault‐parallel high‐frequency ground motions. Co‐seismic damage zones exhibit depth‐dependent width variations, becoming broader near the Earth's surface consistent with field observations, even under uniform stress conditions. We discover a new delayed dynamic triggering mechanism in multi‐fault systems, driven by reductions in elastic moduli and the ensuing stress heterogeneities in 3D tensile fault step‐overs. This mechanism affects the static and dynamic stress fields and includes the formation of high shear‐traction fronts around localized damage zones. The brittle damage facilitates rupture cascading across faults, linking delay times directly to damage rheology and fault zone evolution. Our results help explain near‐fault high‐frequency isotropic radiation and delayed rupture triggering, improving our understanding of earthquake processes, seismic wavefields and fault system interactions. 

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