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A detailed facies analysis of long piston-core sediments from the Ulleung Basin reveals four mud facies: laminated mud, crudely laminated mud, bioturbated mud, and homogenous mud. The laminated and homogeneous muds were most likely deposited from turbidity currents, whereas the bioturbated and crudely laminated muds were formed by hemipelagic/pelagic sedimentation under well-oxygenated and poorly oxygenated bottom-water conditions, respectively. A detailed analysis of vertical distribution of the mud facies together with a chronostratigraphic framework derived from correlative tephra layers with known eruption ages reveals paleoenvironmental changes during the late Quaternary. During the period between 49 and 23 ka, eustatic lowering of sea level facilitated frequent slope failure, resulting in repeated deposition of fine-grained turbidites on the basin floor. Normal hemipelagic sediments were intensively bioturbated under well-oxygenated bottom-water conditions. During the period between 23 and ∼13 to 11 ka, deposition from turbidity currents continued, but hemipelagic sediments were non-bioturbated under poorly oxygenated bottom water conditions. The bottom-water deoxygenation is attributed to further lowering of sea level during the last glacial period and consequent intensified stratification in the water column by freshwater supply to the nearly isolated sea. Primary laminae preserved in the crudely laminated mud generally exhibit low compositional bimodality in backscattered electron images, which suggests low surface primary productivity and presence of superficial micro-bioturbation. After rapid rise of sea level at ∼13 to 11 ka, the slopes were stabilized and pelagic sedimentation has prevailed under well-oxygenated bottom-water conditions.

High-resolution (3.5 kHz) seismic profiles from the slope of the Korea Plateau, East Sea (Sea of Japan) show an extensive area of slide deposits with prominent slump scars. The sediment displaced from the steep slope (5–7°) is less expansive than massive failure deposits on gradual slopes (1–2°). Seaward of a large trough, thick wavy stratified sediments cover the gentle slope. Cyclic loading by earthquakes is the most probable triggering mechanism for the slope instability in this area. The topmost (2–3 m) sediment is chiefly composed of hemipelagic mud with interbeds of turbidites and volcanogenic material. These sediments are highly plastic and shear resistant (over 4 kPa at the surface) in both stable and failed areas. Grain size decreases seaward, from sandy mud on the shelf to silty clay on the lower slope. Total organic matter and water contents are closely associated with sediment texture, both increasing with clay amounts from 5–10% and from 50 up to 200%, respectively.