Field and Experimental Observations Show Elevated Shear Strength of Active Margin Sediments Requires Processes Beyond Normal Consolidation

Abstract

Near‐seafloor marine sediments on active margins have been shown to be stronger than those on passive margins. However, the reasons for this are not fully understood. To investigate the role of intrinsic properties, we performed resedimentation and oedometer tests and measured undrained shear strength, grain size, mineralogy, and plasticity on sediments from three active margins (Nankai, Cascadia, Surveyor Fan) and three passive margins (Amazon Fan, Carolina, and New Jersey) and compared them to their in situ field measurements. We find that all shear strengths on resedimented samples fall within the expected range for normal consolidation. However, Nankai and Cascadia exhibit anomalously high undrained shear strength and low porosity in situ, which cannot be reproduced with 1‐D consolidation experiments. These results suggest that other factors occurring on active margins contribute to strengthening near‐seafloor sediments such as the repeated exposure to earthquakes and lateral shear strain induced by non‐uniaxial stress paths.