Shear Strength Development During Early Burial on Seismically Active Margins: A Geotechnical Investigation into Seismic Strengthening

Recent work has demonstrated elevated shear strength in the uppermost 100 meters below seafloor (mbsf) on seismically active margins. This observation is consistent with the seismic strengthening hypothesis that repeated exposure to earthquake shaking progressively dewaters and densifies sediment, which leads to increased shear strength and slope stability.  However, the relative contribution of seismic strengthening versus intrinsic properties on shear strength remain largely unknown. Here, we compare sediments from seismically active and passive margins from scientific ocean drilling sites that exhibit significant shear strength differences. Active margin sites are Nankai (Site C0001), Cascadia (Site 1054), and Southern Alaska (Site U1418), and passive margin sites are Amazon Fan (Site 942), North Carolina Slope (Site 1054), and New Jersey (Site 1073). From each site, we sampled 500 g of sediment equally distributed throughout the top 100 mbsf. We combined samples to create a representative bulk sample per continental margin and reconstituted them with saltwater that matched field-measured salinity. We measured particle size (hydrometer), plasticity states (Atterberg limits), mineralogy (powder X-ray diffraction), compression behavior and permeability (1-D resedimentation experiments), and undrained shear strength (fall cone device). All samples are siliciclastic marine mud that classify as silty clay or clayey silt. Despite the apparent similarity in lithology, sand fraction varies from 0.8 wt. % (Amazon) to 10.3 wt. % (N. Carolina) and clay fraction (<2 mm) varies from 37.7 wt. % (N. Carolina) to 56.0 wt. % (Amazon). Void ratios, measured in resedimentation experiments range from 1.6 (porosity = 62%) (Nankai) to 1.0 (porosity = 50%) (S. Alaska) at a vertical effective stress of 100 kPa. Resedimentation experiments are followed by consolidation to 1 MPa (equivalent to 100 meters of burial depth) and undrained shear strength measurements, which are compared with field-measured shear strengths. We find the previously observed strengthening effect observed in the active margin field- strength is no longer present in the lab-strengths. This suggests that the exposure to seismicity in the field is potentially leading to enhanced shear strength during early burial.