Multiscale Numerical Modeling of Ultrasound-Induced Granular Avalanches

Understanding the mechanisms of seismic-wave-induced triggering of landslides and earthquakes at micro-strain amplitudes is crucial for quantifying seismic hazards. Granular materials, as an out-of-equilibrium and metastable model system, offer insights into landslides and fault dynamics within the unjamming transition framework from solid to liquid states. Recent experiments suggest that ultrasound-induced granular avalanches result from reduced interparticle friction via shear acoustic lubrication. However, investigating crack growth or slip at the grain contact scale in optically opaque granular media remains challenging. We present a new multiscale numerical modeling of 2D dense granular flows triggered by basal acoustic vibrations of an inclined plane. We introduce a time-scale separation method, addressing the characteristic scales of grain motion on one hand and the propagation of acoustic vibrations on the other. Our approach results from the coupling between the Convex Optimization Contact Dynamics model (COCD) and the computation of vibration modes. Numerical simulations of ultrasonic vibrations in the millisecond range and flow onset in the second range reveal a correlation between local rearrangements at the grain scale and continuous flows at the macroscopic scale. Ultrasounds primarily propagate through strong-force chains, while a decrease in interparticle friction occurs in weak contact forces perpendicular to these chains. This friction reduction initiates local rearrangements leading to continuous flows through a percolation process with a delay dependent on proximity to failure. Ultrasound-induced flow, compared to gravity-driven flow, appears more spatially uniform, suggesting the role of effective temperature induced by ultrasonic vibration. The simulations align well with experimental observations of granular flows triggered by ultrasound below avalanche angles, supporting the validity of our numerical method.