Scientists using advanced seismic imaging have completed the first comprehensive mapping of the Stad Slide, a buried submarine landslide that proves to be far larger than previously recognized. The massive underwater failure displaced approximately 4,300 cubic kilometers of sediment along Norway’s continental margin roughly 400,000 years ago, making it one of the largest known megaslides on Earth.
The volume of material moved by the Stad Slide equals the amount ejected by some of the largest volcanic eruptions ever recorded. To understand the scale, the displaced sediment would cover an area larger than Germany if spread evenly across the surface. Yet this enormous event remained poorly understood until researchers from Newcastle University, the Norwegian Geotechnical Institute, and Volcanic Basin Energy Research completed detailed mapping using 42,500 square kilometers of high-quality 3D seismic data combined with an extensive grid of 2D profiles.
The North Sea Fan, located between the Shetland Islands and western Norway, has experienced repeated massive underwater landslides throughout the Quaternary period spanning the last 2.6 million years. These submarine slope failures pose significant risks to coastal communities through tsunami generation and threaten underwater infrastructure including pipelines and communication cables. Understanding what causes these giant slides and how they develop remains essential for assessing future hazards in a warming world where submarine landslide frequency may increase.
The research team identified the Stad Slide as the largest megaslide by volume on the proximal North Sea Fan, surpassing even the famous Storegga Slide that occurred 8,200 years ago. The Storegga Slide displaced between 2,400 and 3,200 cubic kilometers of sediment and generated a tsunami with wave heights reaching 20 meters in the Shetland Islands. Tsunami deposits from that event have been mapped extensively across the North and Norwegian seas. The discovery that an even larger slide occurred earlier in the geological record carries important implications for tsunami hazard assessments that have traditionally treated the Storegga Slide as the maximum credible scenario for this region.
The Stad Slide affected a total area of 46,500 square kilometers with deposits reaching a maximum thickness of 360 meters. The remobilized sediments traveled approximately 250 kilometers downslope before coming to rest. Six major headwalls mark where the slope failed, ranging from 85 to 310 kilometers in length and standing 180 to 340 meters high. These headwalls reveal that the slide developed through multiple stages rather than as a single catastrophic failure.
The physical characteristics visible in seismic data indicate the Stad Slide developed retrogressively, meaning the failure began lower on the slope and progressively worked its way upward. Headwall A, located furthest downslope and lowest in the stratigraphy, likely failed first. The removal of this downslope material eliminated support for sediments above, causing them to lose strength and fail in turn. This retrogressive process propagated upslope both eastward and westward, creating additional headwalls at multiple stratigraphic levels.
The seismic data reveal acoustically chaotic deposits with steeply angled, discontinuous reflections interpreted as deformation structures including folds and faults that developed within the remobilized sediments during downslope movement. Lithological data from three wells that penetrate the slide deposits show the material consists of claystone, silty claystone, and silty sandstone. Large intact blocks and curvilinear ridges appear on the upper surface of the slide deposits, typically standing 30 to 60 meters higher than surrounding material with some reaching up to 150 meters high.
The sediments underlying the Stad Slide consist primarily of low-angle prograding packages of glacigenic debris flows formed by remobilization of glacial sediment delivered to the ancient shelf break. These glacigenic deposits are punctuated on the lower slope by high-amplitude seismic reflections interpreted as contourite lenses. Contourites are sediments deposited by ocean bottom currents and can reach thicknesses up to 100 meters in this region. When traced laterally, the base of the Stad Slide often corresponds with these high-amplitude reflections, suggesting the contourites formed weak layers along which the slope failed.
The timing of the Stad Slide aligns with a period of enhanced glacial sedimentation in this region. Although considerable uncertainties surround the precise age, stratigraphic correlation suggests the slide occurred during Marine Isotope Stage 12, approximately 429,000 to 477,000 years ago. This period probably marked the first time the Fennoscandian and British-Irish ice sheets became confluent across the central and southern North Sea. The merging of these ice sheets would have expanded the drainage basin area of the Norwegian Channel Ice Stream, dramatically increasing the rate of sediment delivery to the northern North Sea margin.
Rapid glacial sedimentation preconditions slopes for failure by increasing pore pressure in underlying sediments and decreasing their effective strength. Numerical modeling of the overlying Tampen Slide suggests that failure was preconditioned by glacial sediments deposited at rates of two to four meters per thousand years. The intercalation of glacigenic and contouritic sediments beneath the Stad Slide created numerous stacked units that facilitated formation of multiple glide planes and headwalls along weak layers of contrasting sediment properties.
Evidence for cyclical instability appears in the stratigraphic record above the Stad Slide. An approximately 200-meter-thick contourite drift fills the slide headwalls, indicating the slide was triggered toward the end of a glacial period or during an interglacial rather than during a full glacial phase. This timing proves consistent with previous models of slope failure on glaciated margins, where sliding typically occurs following a period of enhanced sediment loading. The top of this infilling contourite drift forms part of the glide plane for the younger Møre Slide, demonstrating how these sediments formed a weak layer for later failure.
While the final trigger for the Stad Slide remains uncertain, modeling of the overlying Tampen Slide suggests an earthquake of magnitude 6.9 or larger would have been required to initiate sliding. Such earthquake magnitudes fall within the range caused by post-glacial isostatic rebound offshore Norway. Combined with the inferred deglacial or interglacial timing based on contourite infilling of the slide scar, this suggests the Stad Slide may have been triggered by seismic shaking linked to isostatic rebound following ice sheet retreat.
The large volume of the Stad Slide carries significant implications for hazard assessments in countries surrounding the North Sea. The relationship between slide volume and tsunami generation remains complex, with many large slides producing either no tsunami or unexpectedly small ones, while much smaller landslides have generated devastating tsunamis. Factors including slide velocity, water depth of failure, and distance from shoreline all contribute to tsunami generation potential. Despite these uncertainties, the large volume of the Stad Slide, together with its locational and mechanistic similarities to the tsunamigenic Storegga Slide, warrant further investigation of a potential tsunami and resulting deposits.
The discovery adds to growing evidence that megaslides on glaciated margins are often linked to increased rates of glacial sedimentation. The repeated large-scale slope failures on the North Sea Fan appear to be encouraged by contourite deposition during times of reduced glacial input, combined with high rates of ice-stream-derived sedimentation during full-glacial periods. Understanding these cyclical processes proves essential for robust evaluations of future hazards, particularly as submarine landslide frequency is predicted to increase in a warming world.
The complete mapping of the Stad Slide provides unique insights into how the world’s largest submarine landslides develop and what volumes of sediment can be mobilized during single failure sequences. As one of the largest reported megaslides globally, it presents a larger maximum credible scenario than previously recognized for slope failure in this region and demonstrates the importance of comprehensive seafloor and subsurface imaging for understanding submarine landslide hazards.
Source:
Research published in the Journal of Quaternary Science by Tiller et al. reveals the Stad Slide as one of the world’s largest submarine landslides with a volume of approximately 4,300 cubic kilometers. https://doi.org/10.1002/jqs.70022
