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1. 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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2. Propagation of Slow Slip Events on Rough Faults: Clustering, Back Propagation, and Re‐Rupturing

   https://doi.org/10.1029/2024JB029384  

   Sun, Yudong; Cattania, Camilla (February 2025, Journal of Geophysical Research: Solid Earth)

   Seismic and geodetic observations show that slow slip events (SSEs) in subduction zones can happen at all temporal and spatial scales and propagate at various velocities. Observation of rapid tremor reversals indicates back‐propagating fronts traveling much faster than the main rupture front. Heterogeneity of fault properties, such as fault roughness, is a ubiquitous feature often invoked to explain this complex behavior, but how roughness affects SSEs is poorly understood. Here we use quasi‐dynamic seismic cycle simulations to model SSEs on a rough fault, using normal stress perturbations as a proxy for roughness and assuming rate‐and‐state friction, with velocity‐weakening friction at low slip rate and velocity‐strengthening at high slip rate. SSEs exhibit temporal clustering, large variations in rupture length and propagation speed, and back‐propagating fronts at different scales. We identify a mechanism for back propagation: as ruptures propagate through low‐normal stress regions, a rapid increase in slip velocity combined with rate‐strengthening friction induces stress oscillations at the rupture tip, and the subsequent “delayed stress drop” induces secondary back‐propagating fronts. Moreover, on rough faults with fractal elevation profiles, the transition from pulse to crack can also lead to the re‐rupture of SSEs due to local variations in the level of heterogeneity. Our study provides a possible mechanism for the complex evolution of SSEs inferred from geophysical observations and its link to fault roughness. 

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3. Imminent seismic velocity changes in earthquake cycle simulations

   Thakur, Prithvi; Huang, Yihe (October 2021, AGU Fall Meeting 2021)

   null (Ed.)

   Earthquake prediction is the holy grail of seismology. Many previous studies have searched for robust precursory signals to inform us of imminent earthquakes, the most significant of which are seen in laboratory experiments as temporal changes in pressure and shear wave velocities during the seismic cycle. Similar changes are seen in natural faults and the surrounding structurally complex network of fractures with nested hierarchy of localized deformation, referred to as fault damage zone. However, little is known whether such temporal changes in material properties contains any precursory signals for imminent earthquakes.Conversely, the effect of precursory velocity changes on the seismic cycle is not well understood. By imposing shear wave velocity changes in fault damage zones, we investigate the effects of these precursors on multiple stages of the seismic cycle, including nucleation, coseismic, postseismic, and interseismic stages. We perform 2D fully dynamic earthquake cycle simulations with a fault-parallel damage zone for strike-slip fault systems with antiplane geometry. The fault is governed by rate-state-dependent friction laws, and the fault damage zone material is considered elastic. Our preliminary results show that the temporal onset of shear wave velocity drop causes a reduction in earthquake recurrence intervals over the seismic cycle. Furthermore, a dynamic earthquake rupture within the seismic cycle terminates much faster and abruptly in models with precursory velocity changes. We will also discuss how the precursory velocity changes affect the fault-slip behavior, including fast-slip, slow-slip, and aseismic creep, for different amplitudes of shear wave velocity changes at different compliance contrast of the fault damage zones. Our results highlight the importance of short and long-term monitoring of fault zone structures for better assessment of regional seismic hazard. 

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4. A Diffuse Interface Method for Earthquake Rupture Dynamics Based on a Phase‐Field Model

   https://doi.org/10.1029/2023JB027143  

   Hayek, Jorge N.; May, Dave A.; Pranger, Casper; Gabriel, Alice‐Agnes (December 2023, Journal of Geophysical Research: Solid Earth)

   Abstract In traditional modeling approaches, earthquakes are often depicted as displacement discontinuities across zero‐thickness surfaces embedded within a linear elastodynamic continuum. This simplification, however, overlooks the intricate nature of natural fault zones and may fail to capture key physical phenomena integral to fault processes. Here, we propose a diffuse interface description for dynamic earthquake rupture modeling to address these limitations and gain deeper insight into fault zones' multifaceted volumetric failure patterns, mechanics, and seismicity. Our model leverages a steady‐state phase‐field, implying time‐independent fault zone geometry, which is defined by the contours of a signed distance function relative to a virtual fault plane. Our approach extends the classical stress glut method, adept at approximating fault‐jump conditions through inelastic alterations to stress components. We remove the sharp discontinuities typically introduced by the stress glut approach via our spatially smooth, mesh‐independent fault representation while maintaining the method's inherent logical simplicity within the well‐established spectral element method framework. We verify our approach using 2D numerical experiments in an open‐source spectral element implementation, examining both a kinematically driven Kostrov‐like crack and spontaneous dynamic rupture in diffuse fault zones. The capabilities of our methodology are showcased through mesh‐independent planar and curved fault zone geometries. Moreover, we highlight that our phase‐field‐based diffuse rupture dynamics models contain fundamental variations within the fault zone. Dynamic stresses intertwined with a volumetrically applied friction law give rise to oblique plastic shear and fault reactivation, markedly impacting rupture front dynamics and seismic wave radiation. Our results encourage future applications of phase‐field‐based earthquake modeling. 

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5. Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths

   https://doi.org/10.1029/2021JB023821  

   Song, Bo Ra; Song, Won Joon; Johnson, Scott E.; Gerbi, Christopher C.; Vel, Senthil S. (July 2022, Journal of Geophysical Research: Solid Earth)

   Abstract Mature faults with large cumulative slip often separate rocks with dissimilar elastic properties and show asymmetric damage distribution. Elastic contrast across such bimaterial faults can significantly modify various aspects of earthquake rupture dynamics, including normal stress variations, rupture propagation direction, distribution of ground motions, and evolution of off‐fault damage. Thus, analyzing elastic contrasts of bimaterial faults is important for understanding earthquake physics and related hazard potential. The effect of elastic contrast between isotropic materials on rupture dynamics is relatively well studied. However, most fault rocks are elastically anisotropic, and little is known about how the anisotropy affects rupture dynamics. We examine microstructures of the Sandhill Corner shear zone, which separates quartzofeldspathic rock and micaceous schist with wider and narrower damage zones, respectively. This shear zone is part of the Norumbega fault system, a Paleozoic, large‐displacement, seismogenic, strike‐slip fault system exhumed from middle crustal depths. We calculate elastic properties and seismic wave speeds of elastically anisotropic rocks from each unit having different proportions of mica grains aligned sub‐parallel to the fault. Our findings show that the horizontally polarized shear wave propagating parallel to the bimaterial fault (with fault‐normal particle motion) is the slowest owing to the fault‐normal compliance and therefore may be important in determining the elastic contrast that affects rupture dynamics in anisotropic media. Following results from subshear rupture propagation models in isotropic media, our results are consistent with ruptures preferentially propagated in the slip direction of the schist, which has the slower horizontal shear wave and larger fault‐normal compliance. 

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