A review of overpressure, flow focusing, and slope failure

Abstract

Submarine slope failures with low- Angle failure planes are commonly observed in the sedimentary record. These failures can be large (1,000s km 2) or small (1 km2), and can have long runouts (100s km) or short runouts (<1 km). While the size and runout are well documented, the initiation and evolution of these failures are insufficiently understood. I review the role that fluid overpressure and flow focusing play in preconditioning low- Angle slopes for failure by describing the stability of slopes in terms of overpressure, and reviewing the coupling of sedimentation, overpressure and fluid flow. I illustrate these processes with simple, one- And twodimensional examples. I then review three regions that have slope failures that have been attributed to overpressure: The Ursa region in the Gulf of Mexico; the Storegga slide offshore Norway; and the Cascadia margin offshore Canada. Each study confirms that elevated pore pressure was critical to failure, but only under very specific conditions were overpressures the sole cause of failure. All three environments invoked earthquake shaking as the ultimate trigger for failure; however the earthquake magnitude ranged from small- To-large depending on the environment and the pressure conditions. These studies thus show that pore fluids are a dominant factor for preconditioning slopes for failure; however earthquake triggering appears to be key to the initiation of failures, even along passive margins. Future research is needed to understand the dynamics of weak slopes during earthquakes and the linkages between preconditioning, triggers, and failure morphology. © Springer Science+Business Media B.V. 2012.