Submarine landslides on the Eastern South Korea Plateau (ESKP)—Do pumice-rich tephra layers control slope stability?

Abstract

Tephra layers embedded in marine sediments can be significant in controlling submarine landslide dynamics and seafloor morphology, but their preconditioning effects on slope failure remain uncertain. Here, we study the morphology and preconditioning factors of submarine landslides from a volcanically active region, the Eastern South Korea Plateau (ESKP) using a recently acquired multibeam echosounder (MBES), high-resolution sub-bottom chirp profiler, and piston core data, together with previous geophysical data. At least 50 translational landslides were identified on the southern margin of the ESKP in an area of ∼470 km2, with a relatively small volume of remobilized sediment (∼10 km3). The landslide headscarps are arcuate, up to 400 m in height, and lie at water depths of 980 to 2300 m. Landslides on the upper ESKP margin are more disintegrative (debris flow type) than those on the lower margin which consists largely of blocky type failures with small runouts. Landslide deposits identified in the cores consist of debris flows and slides/slumps while background deposits contain hemipelagic mud interbedded with ca. 10-cm-thick coarse-grained, poorly-sorted pumiceous tephra. Seismic and core integration indicate that the glide planes of the observed slides correspond to the pumice-rich tephra layers. These tephras are predominantly composed of fresh volcanic glass devoid of clay, indicating that the composition of the tephra did not control the formation of weak layers or slide planes. We infer therefore that the weakness of the glide planes resulted from high hydraulic (pore) pressure at the interface between some of the porous tephra layers and their overlying, relatively impermeable, hemipelagic mud. In addition to burial compaction and build-up of pore pressure in the tephra layers, fluid overpressure could have been facilitated by earthquakes causing granular convection within, and compaction of, the poorly sorted tephra. The landslide predominantly formed > ca. 84 ka ago, which suggests time-restricted triggering associated with a temporary increase in seismicity and/or sediments reaching their overpressure threshold during burial compaction. Our study suggests that regional occurrences of discrete layers of porous tephra deposits within finer grained hemipelagic sediments can potentially control the generation of landslides in subaqueous environments affected by explosive volcanism.