Lawrence Berkeley National Laboratory

Abstract Snow dampens sounds, but anecdotal reports concisely describe audible propagating collapse events—firnquakes—in Antarctic and Arctic snowfields. We propose combining granular and continuum mechanics to form a testable theory for conditioning, triggering, and propagation of firnquakes consistent with scarce data. A central condition for collapse events is unconsolidated firn at depth. As firn grains compact, stresses are transmitted along force chains which carry the overburden and transition into a continuous medium by pressure sintering. This granular legacy creates solid‐like supports of denser layers that keep the material below unconsolidated. Dynamic amplification triggers local brittle failure of the supports, which induces a cascade of collapse propagation. Using bulk density from ice cores as proxy for stiffness, we find the flexural wave speed by collapsing supports matches the recorded firnquake velocities on the order of 100 m/s. Our theory is to be tested in firn sheets and other compacting granular materials. Plain Language Summary Firnquakes are loud train‐like collapse events reported in old snow (firn) deposits in the Antarctic and Arctic. These poorly understood rupture events occur at depths of about 10 m and propagate at speeds of approximately 100 m/s, making them audible. The propagation speed, depth, and pathways differs from other known shallow, radial snow collapse events such as whumpfs. We propose a model combining granular and solid mechanics to explain how appropriate collapse conditions arise and suggest a failure mechanism capable of propagating the collapse over large distances. We theorize that firn, as a granular material, develops solid‐ice‐like internal support structures as it compacts, while the surrounding snow remains loose, even at 10 m depth. The collapse and propagation of these structures are modeled using principles of solid mechanics: when a support structure breaks, the load is transferred to the next, causing a domino‐like chain reaction. These structures, heterogeneous densities, and more events might thus be found in seismic records. We provide mechanical and structural reasoning for loud collapse events in firn, and a conceptual framework for similar mechanics in compacting granular systems with potentially hazardous consequences like landslides or avalanches. Key Points Compacting homogeneous old snow grains create solid legacy support structures in deeper firn layers which produces heterogeneous stiffness Structural and dynamic amplification allows to trigger firnquakes at 10 m depth The propagation speed of firnquakes is controlled by the progressive brittle failure of pre‐stressed supports within a radius of influence