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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. 

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. 

Each DEM experiment is sorted into directories by model parameters: D, M, L = Dense, Medium, and Loose sediment; 3, 5, and 10 m sediment depths; cohesion and tensile strength in Pa; fault dip in degrees; FS = fault seed height as a product of the total sediment depth. 

Each DEM experiment is sorted into directories by model parameters: D, M, L = Dense, Medium, and Loose sediment; 3, 5, and 10 m sediment depths; cohesion and tensile strength in Pa; fault dip in degrees; FS = fault seed height as a product of the total sediment depth. 

ABSTRACT We seek to improve our understanding of the physical processes that control the style, distribution, and intensity of ground surface ruptures on thrust and reverse faults during large earthquakes. Our study combines insights from coseismic ground surface ruptures in historic earthquakes and patterns of deformation in analog sandbox fault experiments to inform the development of a suite of geomechanical models based on the distinct element method (DEM). We explore how model parameters related to fault geometry and sediment properties control ground deformation characteristics such as scarp height, width, dip, and patterns of secondary folding and fracturing. DEM is well suited to this investigation because it can effectively model 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. Our results show that localized fault scarps are most prominent in cases with strong sediment on steeply dipping faults, whereas broader deformation is prominent in weaker sediment on shallowly dipping faults. Based on insights from 45 experiments, the key parameters that influence scarp morphology include the amount of accumulated slip on a fault, the fault dip, and the sediment strength. We propose a fault scarp classification system that describes the general patterns of surface deformation observed in natural settings and reproduced in our models, including monoclinal, pressure ridge, and simple scarps. Each fault scarp type is often modified by hanging-wall collapse. These results can help to guide both deterministic and probabilistic assessment in fault displacement hazard analysis.