Strain evolution, internal deformation & morphology of landslides as a function of margin architecture
Monika Wiebe | Marum
Research in submarine landslides is driven by three main questions: why do they happen, how do they happen, and what are their impacts. The SLATE network aims to increase the understanding in all these areas. My personal project targets the why-question, or more specifically the factors that weaken the slope and initiate slope failure.
The physics behind slope failure can be described in a simple way. The sediment in a slope has a certain strength, the shear strength. A number of forces act on the slope, e.g. gravity or stress through seismic activity. If this driving force exceeds the shear strength or resisting force of the slope, it breaks. Consequently, for a slope to turn from an equilibrium state into a state of failure, either the shear strength must be reduced, or the driving force increased.
One way to reduce the shear strength is to increase the pore pressure. This can happen when gas hydrate dissociates. Gas hydrates are solid gas and water mixtures that form in low temperature and high pressure conditions. If these conditions change, due to e.g. a temperature rise, the gas hydrate dissociates, and water and gas is released. The presence of free gas increases the pore pressure, which reduces the shear strength, as stated above.
Now the question remains, what exactly happens when gas is released from gas hydrate? Where does the gas migrate to? Can it actually generate enough pore pressure rise that failure occurs, and how much gas is needed? Furthermore, which influence does the slope architecture have on the triggering of landslides? My project aims to find answers to these questions by using numerical modelling as a method. A modelling software (e.g. COMSOL Multiphysics) allows me to set up the geometry of a slope, relevant physical equations, and physical properties of sediment, water and gas. With this set of of known variables I can simulate a number of scenarios with unknown variables, e.g. the strength and location of a gas source, or location of high- and low-permeable layers. The results illustrate where and under which conditions submarine landslides occur.
One example of slope failure and a resulting submarine landslide is the Tuaheni landslide complex offshore New Zealand. It serves as an ideal case study to set up a model as described above. Gas hydrate has been found there, and the landslide has been thoroughly investigated in recent years, which provides a good data base. The first step of my project is to investigate the effects of gas and gas hydrate on this particular slope. Subsequently I will assess how slopes that differ in slope angle, slope material, location of faults etc. are affected by gas and gas hydrates in different ways. The aim of my project is to develop new conceptual models to evaluate the influence of gas hydrate as well as slope architecture on slope failure in general.