On the dynamics of subaqueous clay rich gravity mass flows-the giant Storegga slide, Norway

The Storegga slide, released about 8200 years ago off the western coast of Norway, is a complex system of debris flows which occurred on the remains of previous Pleistocenic slides materials. Following geological mapping and interpretation of data gathered with geological and geophysical methods, the Storegga slide has been interpreted as consisting of a giant slide (volume ≈3100 km3 and runout of about 400 km) followed by a swarm of progressively smaller events. The flow of the giant slide, object of the present, study, represents a dynamical enigma. Indeed, if one simulates the flow of the giant slide with simple non-Newtonian soil properties constant along the path, extremely small yield strength must be attributed to the material. This finding is incompatible with observations of the smaller slides, where the simplest non-Newtonian soil properties models work well with a yield strength more than one order of magnitude larger. A possible conclusion is that the material has charged its properties during the flow. Numerical simulations are conducted with a newly introduced model where in analogy with soil mechanical models, the material progressively loses strength during the flow. The model gives results comparable to field data if the shear strength decreases by at least a factor of 30 during flow.