Properties of Rocks and Minerals: Physical Origins of Anelasticity and Attenuation in Rock

At high temperature and long periods, and in the presence of fluids, viscoelastic relaxation can lead to substantial departures from the ideal elastic behavior of geologic materials. The nonelastic behavior is associated with the stress-induced perturbation of an internal variable that yields an extra, delayed contribution to the total strain. This additional, indirect, coupling between stress and strain is manifest in reduced and frequency-dependent effective elastic moduli and wave speeds and associated strain energy dissipation. Such internal variables parameterize the operation of microscopic agents of deformation. These range widely from point defects and dislocations, to subgrain, twin-domain, grain, and phase boundaries, to the distribution of a dispersed intergranular fluid phase. This chapter seeks to integrate theoretical descriptions of the various relaxation mechanisms with the results emerging from a new class of seismic-frequency laboratory experiments in a synthesis that allows a critical assessment of the geophysical relevance of the various relaxation mechanisms.