Study of Recent Small-Scale Landslides in Geologically Active Marine Areas Through Repeated Multibeam Surveys: Examples from the Southern Italy

Repeat multibeam surveys are used to investigate recent submarine landslides in three different areas on southern Italy, i.e. Stromboli, Southern Messina Strait and Puma Alice. High-resolution Digital Terrain Models (DTMs) depict the morphology of mass-wasting events, while residual maps of the difference between successive bathymetric surveys indicate failure volumes ranging from tens of thousands to millions of cubic meters. A tentative estimate of recurrence time for the slope failures was based on chronological constraints and historical reports, and gave values ranging from tens to hundreds of years. Characterizing mass-wasting events and their recurrence time is a necessary step for geo-hazard assessment of densely populated coastal areas

Applied geomorphology and geohazard assessment for deepwater development

Development of offshore hydrocarbon resources has in recent decades advanced into frontier deepwater regions around the world posing significant technical challenges for the design and installation of oil and gas wells and facilities. Development sites are typically remote and inaccessible and little is known about the seabed geomorphology and ground conditions to be encountered. Potential geohazards are at a larger scale than found onshore and include deep canyons and terrain highs, landslides and turbidity flows, faults, salt diapirism, gas/fluid expulsion, sedimentary bedforms and adverse soil conditions. Triggering events may include seismicity, volcanism, deep ocean currents and construction activity. Early acquisition and calibration of field-wide Autonomous Underwater Vehicle (AUV) high-resolution data is essential to ensure that development plans are not exposed to avoidable geohazard risks. A key element of the approach is the application of integrated geophysical, geomorphological and geotechnical methods that make best use of high-resolution data. This paper presents an illustrated approach for applied geomorphology and geohazard assessment for deepwater development that has been adopted by leading offshore oil and gas companies. Experience from major projects around the world demonstrates considerable value in the avoidance and de-risking of geohazards through comprehensive geomorphological assessment

How do ~2° slopes fail in areas of slow sedimentation? A sensitivity study on the influence of accumulation rate and permeability on submarine slope stability

Overpressure generation due to rapid sediment deposition can result in low effective stresses within the sediment column. It has been proposed that these large overpressures are the main preconditioning factor for causing large-scale submarine slope failure on passive continental margins, such as those in the Gulf of Mexico and offshore Norway. The rate of overpressure generation depends on the sedimentation rate, sediment compressibility and permeability. The Gulf of Mexico and the Norwegian continental slope have experienced comparatively high sediment input, but large-scale slope failure also occurs in locations with very low sedimentation rates such as the Northwest African continental margin. Here we show results from 2D numerical modelling of a 2° continental slope subjected to deposition rates of 0.15 m/ka. These results do not indicate any evidence for significant overpressure or slope instability. We conclude that factors other than overpressure must be fundamental for initiating slope failure, at least in locations with low sedimentation rate

Large landslides on passive continental margins: processes, hypotheses and outstanding questions

The volume, area affected, and runout of submarine landslides can exceed those of terrestrial events by two orders of magnitude. The Storegga Slide off Norway affected an area the size of Scotland and moved enough sediment to bury the entire country to a depth of 80 m. Modern geophysics provides a clear picture of large landslides and what their source and depositional areas look like. From this, we can deduce the processes that operated during downslope transport. However, our understanding of many aspects of landslide processes is based on hypotheses that are difficult to test. Elevated pore pressures are essential for landslide initiation on low continental margin slopes, yet understanding of how high pressures are generated or how fluid migration affects slope stability is limited. Sediments may be pre-conditioned for failure by the processes that originally deposited them, e.g., through creation of weak layers, but the processes and parameters that might control this are largely unknown. <br/