Coulombic faulting from the grain scale to the geophysical scale: lessons from ice

Coulombic faulting, a concept formulated more than two centuries ago, still remains pertinent in describing the brittle compressive failure of various materials, including rocks and ice. Many questions remain, however, about the physical processes underlying this macroscopic phenomenology. This paper reviews the progress made in these directions during the past few years through the study of ice and its mechanical behaviour in both the laboratory and the field. Fault triggering is associated with the formation of specific features called comb-cracks and involves frictional sliding at the micro(grain)-scale. Similar mechanisms are observed at geophysical scales within the sea ice cover. This scale-independent physics is expressed by the same Coulombic phenomenology from laboratory to geophysical scales, with a very similar internal friction coefficient (mu approximate to 0.8). On the other hand, the cohesion strongly decreases with increasing spatial scale, reflecting the role of stress concentrators on fault initiation. Strong similarities also exist between ice and other brittle materials such as rocks and minerals and between faulting of the sea ice cover and Earth's crust, arguing for the ubiquitous nature of the underlying physics.