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1. Along-Strike Variability of Surface Deformation on Thrust and Reverse Fault Ruptures: Insights from 3D Distinct Element Method Models

   https://doi.org/10.1785/0220250173  

   Chiama, Kristen; Plesch, Andreas; Shaw, John H (September 2025, Seismological Research Letters)

   Abstract This study examines how fault dip and sediment strength influence along-strike variability in patterns of ground surface deformation during thrust and reverse fault earthquakes. Expanding on the 2D distinct element method (DEM) analysis by Chiama et al. (2023) and Chiama, Bednarz, et al. (2025), we develop 3D DEM models to investigate the influence of along-strike variability of geological site parameters on resultant morphologies of coseismic ruptures. The main fault scarp types—monoclinal, pressure ridge, and simple—are successfully reproduced in these 3D models, aligning with surface rupture characteristics previously identified in 2D modeling. Uniform fault dips and homogeneous sediment properties produce symmetrical (or cylindrical) fault scarps with uniform scarp morphologies, whereas local variations in fault dip, sediment strengths, and sediment thickness above the fault tip form a range of scarp geometries, deformation zone widths, and patterns of secondary fracturing. These 3D DEM models reproduce patterns of surface fault ruptures observed in natural settings. Overall, the 3D models support the relationships of ground surface deformation characteristics (scarp class, width, and height) with source and sediment properties established in the 2D DEM results of Chiama, Bednarz, et al. (2025). In addition, they provide new insights into how fault dip and sediment strength govern along-strike transitions in fault scarp morphology. In combination, the results of the 2D and 3D DEM model results can be used to infer patterns of surface ruptures based on local geological site conditions and fault characteristics. 

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2. Identification and analysis of ground surface rupture patterns from thrust and reverse fault earthquakes using geomechanical models

   https://doi.org/10.3208/jgssp.v10.OS-15-06  

   Chiama, Kristen; Bednarz, William; Moss, Robb; H_Shaw, John (January 2024, Japanese Geotechnical Society Special Publication)

   We define the physical processes that control the style and distribution of ground surface ruptures on thrust and reverse faults during large magnitude earthquakes through an expansive suite of geomechanical models developed with the distinct element method (DEM). Our models are based on insights from analog sandbox fault experiments as well as coseismic ground surface ruptures in historic earthquakes. DEM effectively models the geologic processes of faulting at depth in cohesive rocks, as well as the granular mechanics of soil and sediment deformation in the shallow subsurface. We developed an initial suite of 45 2D DEM experiments on dense, 5.0 m thick sediment in a model 50 m wide with a fault positioned 20 m from the driving wall and slipped each model at a constant rate (0.3 m/s) from 0 to 5.0 m. We evaluated a range of homogeneous sediment mechanics (cohesion and tensile strength from 0.1 to 2.0 MPa) across a range of fault dip angles. In addition, we examined various depths of sediment above the fault tip. Based on these experiments, we developed a classification system of the observed fault scarp morphology including three main types (monoclinal, pressure ridge, and simple scarps), each of which can be subsequently modified by hanging wall collapse. After this initial suite of models, we generated an additional 2,981 experiments of homogeneous and heterogeneous sediment in dense, medium-dense, and loosely packed sediment across a wide range of sediment depths and mechanics, as well as a range of fault dips (20 – 70º). These models provide robust statistical relationships between model parameters such as the fault dip and sediment strength mechanics with the observed surface deformation characteristics, including scarp height, width, and dip as well as the tendency for secondary fault splays. These relationships are supported by natural rupture patterns from recent and paleo-earthquakes across a range of geologic settings. In conjunction with these natural examples, our models provide a basis to more accurately forecast ground surface deformation characteristics that will result from future earthquakes based on limited information about the earthquake source and local sediment properties. 

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3. Quantifying relationships between fault parameters and rupture characteristics associated with thrust and reverse fault earthquakes

   https://doi.org/10.1177/87552930251346434  

   Chiama, Kristen; Bednarz, William; Moss, Robb; Plesch, Andreas; Shaw, John H (July 2025, Earthquake Spectra)

   We investigate the influence of earthquake source characteristics and geological site parameters on fault scarp morphologies for thrust and reverse fault earthquakes using geomechanical models. A total of 3434 distinct element method (DEM) model experiments were performed to evaluate the impact of the sediment depth, density, homogeneous and heterogeneous sediment strengths, fault dip, and the thickness of unruptured sediment above the fault tip on the resultant coseismic ground surface deformation for a thrust or reverse fault earthquake. A machine learning model based on computer vision (CV) was applied to obtain measurements of ground surface deformation characteristics (scarp height, uplift, deformation zone width, and scarp dip) from a total of 346,834 DEM model stages taken every 0.05 m of slip. The DEM dataset exhibits a broad range of scarp behaviors, generating monoclinal, pressure ridge, and simple scarps—each of which can be modified by hanging wall collapse. The parameters that had the most influence on surface rupture patterns are fault displacement, fault dip, sediment depth, and sediment strength. The DEM results comprehensively describe the range of historic surface rupture observations in the Fault Displacement Hazards Initiative (FDHI) dataset with improved relationships obtained by incorporating additional information about the earthquake size, fault geometry, and surface deformation style. We suggest that this DEM dataset can be used to supplement field data and help forecast patterns of ground surface deformation in future earthquakes given specific anticipated source and site characteristics. 

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4. Dueling dynamics of low-angle normal fault rupture with splay faulting and off-fault damage

   https://doi.org/10.1038/s41467-023-37063-1  

   Biemiller, J.; Gabriel, A.-A.; Ulrich, T. (December 2023, Nature Communications)

   Abstract Despite a lack of modern large earthquakes on shallowly dipping normal faults, Holocene M w  > 7 low-angle normal fault (LANF; dip<30°) ruptures are preserved paleoseismically and inferred from historical earthquake and tsunami accounts. Even in well-recorded megathrust earthquakes, the effects of non-linear off-fault plasticity and dynamically reactivated splay faults on shallow deformation and surface displacements, and thus hazard, remain elusive. We develop data-constrained 3D dynamic rupture models of the active Mai’iu LANF that highlight how multiple dynamic shallow deformation mechanisms compete during large LANF earthquakes. We show that shallowly-dipping synthetic splays host more coseismic slip and limit shallow LANF rupture more than steeper antithetic splays. Inelastic hanging-wall yielding localizes into subplanar shear bands indicative of newly initiated splay faults, most prominently above LANFs with thick sedimentary basins. Dynamic splay faulting and sediment failure limit shallow LANF rupture, modulating coseismic subsidence patterns, near-shore slip velocities, and the seismic and tsunami hazards posed by LANF earthquakes. 

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5. Subduction Zone Geometry Modulates the Megathrust Earthquake Cycle: Magnitude, Recurrence, and Variability

   https://doi.org/10.1029/2024JB029191  

   Biemiller, J; Gabriel, A‐A; May, D A; Staisch, L (August 2024, Journal of Geophysical Research: Solid Earth)

   Abstract Megathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate codetandemto account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults. 

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