Nonlinear slow dynamics and memory in rocks

Abstract

Longitudinal resonance experiments on thin bars of sandstone reveal that their nonlinear behavior is richly diverse; they exhibit nonlinear mesoscopic elasticity. Resonance experiments show that, as a function of increasing drive level, resonance frequencies shift downward, curve-shape changes, and harmonics and slow dynamics appear. Slow dynamics [Geophys Res. Lett. 23, 3019–3022 (1996)] refers to the following: a large strain applied to a rock (‘‘conditioning’’) reduces its modulus (i.e., it behaves like a softer spring); if the strain is applied for several minutes, the rock will remain ‘‘soft’’ even after the strain is removed. The rock eventually recovers to its initial stiffness although the recovery may take minutes to hours. In order to explore the mechanisms of nonlinear response, including slow dynamics, experiments were performed to study the processes of conditioning and recovery. It was found that the two processes appear to be asymmetric. Conditioning takes place quickly; recovery takes place in two stages. After the conditioning strain is removed, the rock’s modulus quickly recovers to nearly the same stiffness. However, the final stage of recovery (i.e., completely back to its original state) takes minutes to hours, depending on the length of time the conditioning strain was applied.