Slow Dynamics and Strength Recovery in Unconsolidated Granular Earth Materials: A Mechanistic Theory

Rock materials often display long-time relaxation, commonly termed aging or slow dynamics, after the cessation of acoustic perturbations. In this paper, we focus on unconsolidated rock materials and propose to explain such nonlinear relaxation through the shear-transformation-zone theory of granular media, adapted for small stresses and strains. The theory attributes the observed relaxation to the slow, irreversible change of positions of constituent grains and posits that the aging process can be described in three stages: fast recovery before some characteristic time associated with the subset of local plastic events or grain rearrangements with a short time scale, log linear recovery of the elastic modulus at intermediate times, and gradual turnover to equilibrium steady state behavior at long times. We demonstrate good agreement with experiments on aging in granular materials such as simulated fault gouge after an external disturbance. These results may provide insights into observed modulus recovery after strong shaking in the near surface region of earthquake zones. Plain Language Summary This paper presents a theory of slow relaxation and aging in granular earth materials and demonstrates good agreement between the theory and experimental measurements in laboratory fault materials composed of glass beads. Understanding slow relaxation in earth materials helps us gain deeper understanding of the response of the Earth to disturbances such as seismic waves.