Recurrent submarine landslides preconditioned by internal waves impacting gentle continental slopes

Recurrent submarine landslides and associated tsunamis are common on gentle continental slopes, where weak layers precondition long-term slope instabilities. However, the exact influence of oceanographic processes, such as internal waves, on the formation of these weak layers remains poorly understood. Here, for the first time, we utilize high-resolution 3D seismic data from the northern continental slope of the South China Sea to image a series of previously unrecognized submarine landslides. These slides are located on a gentle continental slope (∼1°), landward of the well-known Baiyun Slide Complex, expanding its unstable area by approximately 700 km 2 . Importantly, these newly identified submarine landslides are shown to have repeatedly failed along a specific interval characterized by sediment waves. The acoustic and geometric characteristics of these sediment waves, combined with the known post-Pliocene oceanographic setting of the South China Sea, suggest that this interval consists of relatively coarse-grained sediments that were induced by large-scale internal waves. This specific coarse-grained interval is susceptible to liquefaction and weakening by earthquakes and localized fluid overpressure, which we infer to be a significant contributing factor for the long-term slope instability. Our findings thus not only demonstrate that the giant Baiyun Slide Complex is larger than previously considered, but also suggest that internal-wave-related deposition can create candidate weak-layer intervals on open continental slopes where slope morphology, relative sea level and water-column stratification favour internal-waves’ dissipation.