Viscoplastic deformation of fault damage zones and its implications for fault stress accumulation.

Fault slips in the crust not only cause relative displacement between adjacent geological units but also create significant damage in the rock mass surrounding the fault plane. Stress concentrations caused by propagating rupture fronts and fault roughness can exceed rocks strengths, thereby causing brittle fracturing at various scales close to the fault plane. These fractures in the fault damage zone promote fluid transport as well as mechanical deformation which manifests itself geophysically as time-dependent healing of the fault zone. It is also important to note that the time-dependent mechanical deformation that promotes healing also cause stress changes, namely relaxation, as evidenced in some sedimentary basins.

We show laboratory experiments that demonstrate that damaged rocks exhibit time-dependent deformational behavior. Samples analogous to fault damage zone rocks are created by heating or impacting igneous and sedimentary rocks. Samples are subject to sustained anisotropic stress states to observe the creep strain behavior. Results show that damaged rocks can creep even in the absence of pore fluid at room temperature conditions and its mechanical behavior is best characterized as viscoplastic. Time-dependent deformation is pronounced in samples with greater amounts of damage, thus such deformation is promoted by the sliding and closure of microcracks. Under boundary conditions that involve strain boundary conditions, such mechanical properties lead to stress relaxation in the crust. We also show potential field evidences that demonstrate the occurrence of stress relaxation because of damage zone viscous deformation. Finite element modeling of the shearing of a viscous damage zone with variable damage zone thickness show that stress relaxation not only influence the magnitude of shear stress on fault planes, but also the along-fault distribution of fault shear stress.