Aftershocks are the response of a damaged rock surrounding large earthquake ruptures to the stress perturbations produced by the large events. Lyakhovsky et al. (1997a) developed a damage rheology model that provides a quantitative treatment for macroscopic effects of evolving distributed fracturing with local crack density represented by a damage state variable. A thermodynamically-based equation for damage evolution accounts for both degradation and healing of rock as a function of on-going deformation. The governing material properties are constrained by analyses of stress-strain and acoustic emission laboratory data during deformation leading to brittle failure of rocks. For analysis of aftershocks, we consider the relaxation process of a material following the application of a strain step associated with the occurrence of a main shock. The coupled differential equations governing the damage evolution and stress relaxation processes are written in non-dimensional form by scaling the elastic stress to its initial value and the time to characteristic time of damage evolution τ d . With this, the system behavior is controlled by the single non-dimensional ratio R representing the ratio between damage time scale to the Maxwell relaxation time (R = τ d /τ M ). For very small R there is no relaxation and the response consists of constant elastic strain leading to constant rate of damage increase until failure. For very large R there is rapid relaxation without significant change to the level of damage. For intermediate cases the equations are strongly coupled and nonlinear. The analytical solution for the damage evolution can be fitted well for various values of R with a power law similar to the modified Omori law for aftershocks. This also holds for 3-D numerical simulations of aftershock sequences in a multi-layered lithosphere model. Analytical and numerical results suggest that high values of R, corresponding to low viscosity material, produce diffuse aftershock sequences, while low values of R, corresponding to more brittle material, produce clear aftershock sequences.
INTRODUCTIONRocks exhibit a wide variety of rheological behaviors ranging from ductile plastic flow and viscoelastic deformation in the earth mantle and lower crust, to fracture processes controlling the mechanical response and stability of rock mass in the seismogenic zone. A great challenge of theoretical geodynamic studies is to incorporate the interaction between mantle and lower crust into models that simulate deformational processes in the upper crust. Fundamental nonlinear aspects of rock deformation, such as microcrack and flaw nucleation and development of process zones at rupture tips do not have at present accepted quantitative theoretical framework. These aspects are of crucial importance for evolutionary self-organization of faults at various spatio-temporal domains. Lyakhovsky et al. (1997a) developed a thermodynamically-based version of damage rheology, which holds a potential for providing a framework for understanding r...