233-5 Controls on Normalized Undrained Shear Strength Across Active and Passive Margins: A Linear Mixed-Effects Model

Abstract:

Studies have shown that active continental margins possess normalized undrained shear strengths (Sᵤ/σ′ᵥ) that are two to three times greater than those on passive margins within the upper 100 meters below the seafloor (mbsf). This disparity is frequently attributed to seismic strengthening. However, this hypothesis has not been thoroughly investigated at the field scale against alternative explanations, including inherent sediment properties, such as void ratio and clay mineralogy, as well as depositional controls, such as sedimentation rate and depositional setting. Thus, the relative contributions of seismic strengthening and these other factors to the contrast in undrained shear strength remain unquantified.

Here, we use a linear mixed-effects model (LMM) at eight Integrated Ocean Drilling Program (IODP)/Ocean Drilling Program (ODP) sites, comprising four active- and four passive-margin locations. The model evaluates the impact of intrinsic and depositional covariates on normalized undrained shear strength, incorporating site as a random intercept to account for baseline variability. The analysis focuses predominantly on fine-grained siliciclastic sediments within 100 mbsf. Predictor variables for the model are obtained from core-based measurements. These include undrained shear strength, predominantly from shipboard automated vane shear (AVS) measurements with handheld Torvane data used where AVS coverage is limited, and void ratio from the moisture and density (MAD) dataset. We estimate vertical effective stress assuming hydrostatic conditions. This assumption is validated at each site with Gibson’s (1958) time factor using continuous 1D consolidation indicating negligible excess pore pressures[IK1] . Bulk and clay mineralogy are assessed using full-pattern Rietveld refinement (Profex–BGMN) of bulk X-ray diffraction (XRD) analyses including total clay, the illite–smectite (I/S) group, kaolinite, chlorite, and framework minerals such as quartz and feldspar, expressed as isometric log-ratio (ilr) balances, with published oriented <2 µm analyses serving as a cross-check.

This study elucidates the extent to which sediment properties and depositional processes account for the strength contrast between active and passive margins. Additionally, it investigates whether a residual margin effect remains significant beyond these factors, thereby offering a field-scale framework for evaluating the susceptibility of submarine slope failures across margins.

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