Why firn (old snow) quakes - a continuum mechanics theory with granular legacy

Granular materials compact, increase in density, and degas as they accumulate. This changes the material properties, storage capacities, and fracture mechanics.  We developed a mechanical model for compacting granular old snow.  Based on minimal assumptions and data, we address a general phenomenon in compacting granular medium: propagating ruptures or “firnquakes”.  

Compacting snow becomes firn then ice. As the snowpack consolidates, it transitions from a non-homogeneous granular material to a more elastic continuum material. We propose that the granular legacy produces spatial variations in density, stiffness, and pre-stress. This creates an internal structure of supports in unconsolidated snow at depth. Firn can quake when these supports collapse. By combining granular with brittle fracture mechanics and making use of statistical percolation theory, we can explain the conditioning, triggering, and progression of firnquakes in a bulk homogeneous material, with near constant boundary conditions.

Our model provides means to assess ruptures in granular materials, which unlike firnquakes, can have hazardous consequences, like landslides, avalanches, powder tailing failure. It also provides mechanistic explanations and statistical approaches to assess storage structure and capacity, which, in the case of Antarctic’s firn, has been linked to icesheet disintegration.