Decoding Granular States: Using granular mechanics to link observations to internal dynamics of geophysical systems

Granular materials compose some of the most complex natural systems on Earth, governing the dynamics of hillslope creep, rock avalanches, landslides, debris flows, and concentrated pyroclastic density currents. Despite their prevalence, the mechanics of these granular systems remain highly enigmatic. To link external observations to internal states, we utilize particle-resolved numerical methods. By simulating both gravity-driven and fluid-destabilized granular beds, we track the fundamental grain-scale processes that govern macroscopic failure. We find that the entire lifespan of a granular flow—from the static state through yielding, fluidization, and final arrest—can be continuously mapped using two distinct probes: one isolating boundary stress fluctuations, and another measuring internal kinematic and structural evolution. We then validate this internal volumetric probe using optical Particle Image Velocimetry (PIV) data from scaled laboratory experiments, demonstrating how these numerical insights may be directly applied to remote geophysical monitoring.

Decoding Granular States: Using granular mechanics to link observations to internal dynamics of geophysical systems, Zrelak (2026), USGS Landslide Hazards Seminar, 11 March 2026.