SLOPE STABILITY AND GEOTECHNICAL PROPERTIES OF SEDIMENT OF THE SOUTHERN MARGIN OF ULLEUNG BASIN, EAST SEA (SEA OF JAPAN)

Ulleung Basin, a deep back-arc basin in the Sea of Japan, is tectonically active in relation to the subduction zone fronting the Japanese islands. Cores and high-resolution (3.5 kHz) seismic profiles were obtained from the southern margin of the basin Based on the results of geotechnical tests, pseudo-static, infinite slope stability analyses were carried out for the surficial slope sediments. The shelf sediments are mostly massive, medium to fine sand, whereas the slope sediments are strongly bioturbated, water-rich (170-200% water content), and highly plastic (125-135% liquid limit, 70-80% plasticity index) silty clays. These hemipelagic muds are slightly overconsolidated and highly compressible. Most geotechnical properties show distinctive downslope variations. The amount of clay fraction, water content, and Atterberg limits gradually increase downslope, whereas calcium carbonate content decreases from 5% on the upper slope to nearly nil on the lower slope (> 1500 m). Total organic matter is somewhat consistent (8-14%) throughout the slope. Shear strength displays no systematic variations downslope but gradually increases downcore in most places. Seismic data indicate that the slope surface is heavily moulded by mass movements. A complex system of gullies and slump scars begins at the upper slope, extending to the lower slope as slide/slump and debris-flow deposits. The surficial sediments on the entire slope are quite stable statically under both undrained and drained conditions. At present, they appear to be least susceptible to slope failure by any probable earthquake loading, associated with the regionally active tectonic regime.