Stad Slide: extent, morphology, and drivers of one of the world’s largest submarine megaslides

Underwater landslides are associated with multiple geohazards, including tsunamis and damage to underwater infrastructure, but a lack of real-time observations of these events hinders our understanding of their development mechanisms. Analysis of ancient deposits from underwater landslides has the potential to address this by providing insights into landslide preconditioning and failure. Here, we map a hitherto understudied megaslide—the Stad Slide (~0.4 Ma)—within the North Sea Fan on the northern North Sea margin. The aims are to determine its distribution and thickness, analyse its morphological characteristics and contextual stratigraphy, and identify the factors that are likely to have preconditioned and triggered failure. A database comprising 42 500 km² of high-resolution 3D seismic reflection data and a grid of 2D seismic-reflection profiles covering 150 000 km² was used to map the Stad Slide in full for the first time. With a volume of ~4300 m³, the Stad Slide is revealed to be the largest megaslide by volume in the region and one of the largest slides by volume in the world. The broad timing of the Stad Slide (~ 0.4 Ma) aligns with enhanced glacial sedimentation in this region, which may have preconditioned failure by increasing overpressure in underlying sediments. The slide’s multiple headwalls suggest that its large volume was facilitated by multiple stages of failure along layers of glacimarine and contouritic sediment.  Whilst the relationship between large slides and tsunamis is complex, the large volume of the Stad Slide suggests that it could have triggered a tsunami that affected the North Sea region. A ~200 m-thick contourite drift infills the slide headwalls, which potentially formed a weak layer for subsequent sliding in the North Sea Fan. As the Stad Slide marks the onset of repeated Quaternary megasliding in this region, this research advances our understanding of what causes and preconditions large-scale sediment failures on glaciated margins.