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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. 

Abstract With the rise of data volume and computing power, seismological research requires more advanced skills in data processing, numerical methods, and parallel computing. We present the experience of conducting training workshops in various forms of delivery to support the adoption of large-scale high-performance computing (HPC) and cloud computing, advancing seismological research. The seismological foci were on earthquake source parameter estimation in catalogs, forward and adjoint wavefield simulations in 2D and 3D at local, regional, and global scales, earthquake dynamics, ambient noise seismology, and machine learning. This contribution describes the series of workshops delivered as part of research projects, the learning outcomes for participants, and lessons learned by the instructors. Our curriculum was grounded on open and reproducible science, large-scale scientific computing and data mining, and computing infrastructure (access and usage) for HPC and the cloud. We also describe the types of teaching materials that have proven beneficial to the instruction and the sustainability of the program. We propose guidelines to deliver future workshops on these topics. 

Abstract The Turkana Depression, located between the Ethiopian and East African plateaus, displays an anomalous rift architecture. It is missing the narrow, magma‐rich morphology observed in the Main Ethiopian Rift that cuts through the Ethiopian Plateau. Instead, diffuse faulting and isolated volcanic centers are widespread over several hundred kilometers. Turkana has also experienced less magmatism over the last 30 Myr than adjacent plateaus, despite having a thin crust and residing above a mantle that is inferred to be hot and partially molten. We hypothesize that lithospheric weakening has been the key control on magma transport across the lithosphere in the Turkana Depression and subsequent rift development. Using poro‐viscoelastic–viscoplastic models of melt transport, we show that magma extraction across a thin, weakened lithosphere is slower than across a thick, elastic lithosphere. Our results suggest that pre‐rift lithospheric strength can explain the magma‐poor character of Turkana for most of its tectonic history. 

Abstract Reconstructing fault surfaces from volumetric data is a longstanding challenge in geosciences. We present a novel 3D method based on the medial axis to transform a volumetric strain‐rate invariant field from long‐term geodynamic simulations into fault surfaces. In these geodynamic models, faults correspond to regions of locally high values of the second invariant of the strain‐rate commonly referred to as shear zones. The proposed workflow begins by normalizing the strain‐rate to define fault indicator field . An iso‐surface of a chosen value is then extracted to form an envelope around the shear zones. Using the shrinking ball algorithm (Ma et al., 2012,https://doi.org/10.1007/s00371‐011‐0594‐7), we compute the medial axis of this 3D envelope to generate a point cloud representing the geometric skeleton of the shear zones. We reconstruct fault surfaces by applying Delaunay triangulation followed by Laplacian smoothing. For models involving multiple intersecting faults, we perform a local principal component analysis (PCA) of the coordinates defining the medial axis and use the resulting eigenvectors to detect first‐order orientation variations, enabling the separation and individualization of faults. We demonstrate the generality and robustness of the method by applying it several diverse 3D geodynamic scenarios: A single strike‐slip fault, a branching strike‐slip fault in a restraining bend, a dense strike‐slip fault network, a rift system, and a subduction zone with a megathrust and a conjugate thrust fault. 

Abstract Deep earthquakes at depths below 500 km are under prohibitive pressure and temperature conditions for brittle failure. Individual events show diverse rupture behaviors and a coherent mechanism to explain their rupture nucleation, propagation, and characteristics has yet to be established. We systematically resolve the rupture processes of 40 large deep earthquakes from 1990 to 2023 and compare the rupture details to their local metastable olivine wedge (MOW) structures informed from thermo‐mechanical simulations in seven subduction zones. Our results suggest that these events likely initiate from metastable olivine transformations within the cold slab core and rupture beyond the MOW due to sustained weakening from molten rock at the rupture tip. Over half of the earthquakes likely rupture beyond the MOW boundary and are controlled by both mechanisms. Rupturing outside the MOW boundary leads to greater moment release, increased geometric complexity, and a reduction in rupture length, causing greater stress drops. 

Abstract Megathrust earthquakes are the largest on Earth, capable of causing strong ground shaking and generating tsunamis. Physical models used to understand megathrust earthquake hazard are limited by existing uncertainties about material properties and governing processes in subduction zones. A key quantity in megathrust hazard assessment is the distance between the updip and downdip rupture limits. The thermal structure of a subduction zone exerts a first‐order control on the extent of rupture. We simulate temperature for profiles of the Cascadia, Nankai and Hikurangi subduction zones using a 2D coupled kinematic‐dynamic thermal model. We then build reduced‐order models (ROMs) for temperature using the interpolated Proper Orthogonal Decomposition (iPOD). The resulting ROMs are data‐driven, model agnostic, and computationally cheap to evaluate. Using the ROMs, we can efficiently investigate the sensitivity of temperature to input parameters, physical processes, and modeling choices. We find that temperature, and by extension the potential rupture extent, is most sensitive to variability in parameters that describe shear heating on the slab interface, followed by parameters controlling the thermal structure of the incoming lithosphere and coupling between the slab and the mantle. We quantify the effect of using steady‐state versus time‐dependent models, and of uncertainty in the choice of isotherm representing the downdip rupture limit. We show that variability in input parameters translates to significant differences in estimated moment magnitude. Our analysis highlights the strong effect of variability in the apparent coefficient of friction, with previously published ranges resulting in pronounced variability in estimated rupture limit depths. 

Abstract Seismic faults are surrounded by damaged rocks with reduced rigidity and enhanced attenuation. These damaged fault zone structures can amplify seismic waves and affect earthquake dynamics, yet they are typically omitted in physics‐based regional ground motion simulations. We report on the significant effects of a shallow, flower‐shaped fault zone in foreshock‐mainshock 3D dynamic rupture models of the 2019 Ridgecrest earthquake sequence. We find that the fault zone structure both amplifies and reduces ground motions not only locally but at distances exceeding 100 km. This impact on ground motions is frequency‐ and magnitude‐dependent, particularly affecting higher frequency ground motions from the foreshock because its corner frequency is closer to the fault zone's fundamental eigenfrequency. Within the fault zone, the shallow transition to a velocity‐strengthening frictional regime leads to a depth‐dependent peak slip rate increase of up to 70% and confines fault zone‐induced supershear transitions mostly to the fault zone's velocity‐weakening roots. However, the interplay of fault zone waves, free surface reflections, and rupture directivity can generate localized supershear rupture, even in narrow velocity‐strengthening regions, which are typically thought to inhibit supershear rupture. This study demonstrates that shallow fault zone structures may significantly affect intermediate‐ and far‐field ground motions and cause localized supershear rupture penetrating into velocity‐strengthening regions, with important implications for seismic hazard assessment. 

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. 

Abstract The nonlinear mechanical responses of rocks and soils to seismic waves play an important role in earthquake physics, influencing ground motion from source to site. Continuous geophysical monitoring, such as ambient noise interferometry, has revealed co‐seismic wave speed reductions extending tens of kilometers from earthquake sources. However, the mechanisms governing these changes remain challenging to model, especially at regional scales. Using a nonlinear damage model constrained by laboratory experiments, we develop and apply an open‐source 3D discontinuous Galerkin method to simulate regional co‐seismic wave speed changes during the 2015 M_w7.8 Gorkha earthquake. We find pronounced spatial variations of co‐seismic wave speed reduction, ranging from <0.01% to >50%, particularly close to the source and within the Kathmandu Basin, while disagreement with observations remains. The most significant reduction occurs within the sedimentary basin and varies with basin depths, whereas wave speed reductions correlate with the fault slip distribution near the source. By comparing ground motions from simulations with elastic, viscoelastic, elastoplastic, and nonlinear damage rheologies, we demonstrate that the nonlinear damage model effectively captures low‐frequency ground motion amplification due to strain‐dependent wave speed reductions in soft sediments. We verify the accuracy of our approach through comparisons with analytical solutions and assess its scalability on high‐performance computing systems. The model shows near‐linear strong and weak scaling up to 2,048 nodes, enabling efficient large‐scale simulations. Our findings provide a physics‐based framework to quantify nonlinear earthquake effects and emphasize the importance of damage‐induced wave speed variations for seismic hazard assessment and ground motion predictions. 

Abstract Understanding the dynamics of microearthquakes is a timely challenge with the potential to address current paradoxes in earthquake mechanics, and to better understand earthquake ruptures induced by fluid injection. We perform fully 3D dynamic rupture simulations caused by fluid injection on a target fault for Fault Activation and Earthquake Ruptures experiments generatingM_w ≤ 1 earthquakes. We investigate the dynamics of rupture propagation with spatially variable stress drop caused by pore pressure changes and assuming different slip‐weakening constitutive parameters. We show that the spontaneous arrest of propagating ruptures is possible by assuming a high fault strength parameter S, that is, a high ratio between strength excess and dynamic stress drop. In faults with high S values (low rupturing potential), even minor variations inD_c(from 0.45 to 0.6 mm) have a substantial effect on the rupture propagation and the ultimate earthquake size. Modest spatial variations of dynamic stress drop determine the rupture mode, distinguishing self‐arresting from run‐away ruptures. Our results suggest that several characteristics inferred for accelerating dynamic ruptures differ from those observed during rupture deceleration of a self‐arresting earthquake. During deceleration, a decrease of peak slip velocity is associated with a nearly constant cohesive zone size. Moreover, the residual slip velocity value (asymptotic value for a crack‐like rupture) decreases to nearly zero. This means that an initially crack‐like rupture becomes a pulse‐like rupture during spontaneous arrest. These findings highlight the complex dynamics of small induced earthquakes, which differ from solutions obtained from conventional crack‐like models of earthquake rupture.