Slow Dynamics of Earth Materials: An Experimental Overview

In 1996 Johnson et al. were the first to identify peculiar rate effects in resonant bar experiments on various earth materials. The effects were evident on time scales of minutes to hours. They were also seen in both sedimentary and crystalline rocks, and have since been seen in geomaterials like concrete. Although these effects resemble some aspects of creep and creep recovery, they can be induced by a sinusoidal acoustic drive at strains three orders of magnitude below typical creep experiments. These strains are only a few tenths of a microstrain. Moreover, unlike most creep behavior, the effects have been shown to be macroscopically reversible and repeatable, over hundreds of experiments spanning nearly a year. The unique excitation and character of these rate effects cause them to be called slow dynamics. A review and discussion of slow dynamics is presented, pointing out similarities and differences with ordinary creep and focusing on laboratory experiments. A brief description of some possible mechanisms is included, and a new experiment on a sample of Berea sandstone in ultra high vacuum is shown to point out new research that hopes to help ascertain the role of water as a potential mechanism.