Relaxation of acoustic parameters in rocks after strong earthquakes

Numerous measurements show that after a strong earthquake the velocity of seismic waves changes and then slowly returns to its original value. The relaxation process can last from several months to several years, while the seismic wave velocity often changes logarithmically with time similar to what was observed in laboratory experiments. In this work, the relevant experimental results, including long-term observations at the Parkfield Seismic Observatory (California), are analyzed using the previously developed physical model that was successfully used to describe "slow time" effects in laboratory experiments. The model is based on an Arrhenius-type equation describing the evolution of contacts between grains and in cracks. The results of simulations of the recovery time are in satisfactory agreement with the experimental data. These results may be used for remote diagnostics of rocks' stress state.