How do the impact magnitude and duration affect the recovery process in rock?

This Letter addresses several unresolved issues regarding the non-classical acoustic features of rocks and other structured materials. These issues refer to the impact (excitation) stage and the effect of excitation amplitude on relaxation (slow time) process. The Letter describes material softening during a prolonged impact, including the oscillating excitation, using a modification of the thermodynamic (Arrhenius-type) theory that has earlier been successfully applied to the long-time relaxation of acoustic waves in rock. Then, using a simple statistic, the dependence of the wave velocity perturbation on the initial strain, in the laboratory and after an earthquake, is considered. The theoretical results are compared with experimental data.