Factors leading to slope failure on a sediment-starved margin: The southwestern continental margin of the East Sea, Korea

Abstract

Submarine landslides are common geomorphological features of continental margins. Some of the largest submarine landslides occurred on low-angle (< 4°), sediment-starved margins, yet their preconditioning and trigger mechanisms are still largely unconstrained. The southwestern continental margin of the East Sea (between 37.5°N and 38.0°N), Korea, occupies a narrow shelf (< 10 km), is characterized by low sedimentation rates (~3–7 cm /ka) with an average gradient of less than 2°. Here, we investigate submarine landslides using newly collected datasets including multibeam echosounder (MBES), chirp sub-bottom profiler, multichannel seismic (MCS) data and ten piston cores. MBES data from the margin reveal at least four major submarine landslides initiated at depths of 400 m to over 600 m. These landslides left clear headwall scarps on the seafloor with reliefs reaching over ~130 m and extend for over 40 km. MCS data show that some of the failures have resulted in the complete disintegration of the failed mass, while others have resulted in the deposition of well-defined hummocky debris flows. Sediments recovered downslope of the headwall scarps contain slides and debris flow deposits and turbidites that are overlain by bioturbated hemipelagic layers. Radiocarbon dating from hemipelagic units overlying MTDs within the headwall scarps reveal that major failures occurred at ca. 11 to 19 ka, coinciding with the time of the Last Glacial Maximum (LGM) to early deglaciation. Since then, hemipelagic sedimentation has prevailed throughout the sediment starved slope. Slope stability analyses based on geotechnical properties of sediments indicate that all areas are stable under static, and even stable under loads derived from earthquakes in instrumental records, but there were probably earthquakes in pre-historical records (i.e., with a longer recurrence interval) of potentially significant larger magnitude. We suggest that the preferential occurrence of major failures adjacent to the major faults on the lower slope may ultimately be tectonic-controlled although other factors may have contributed as well. Our work shows that coarse-grained clastic sediments are abundant in the shallow subsurface and that these higher-permeability units, often identified as weak layers, would focus fluid flow and could act as slip planes for slope failure. Our data also indicate that tectonic steepening and gas charging are other key parameters for controlling instability in sediment-starved margins.