13/06/2024

Dr. Mayukh Talukdar

4:00 pm

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, causing brittle fracturing at various scales close to the fault plane. These fractures in the fault damage zone promote fluid transport and mechanical deformation, which manifests as time-dependent healing of the fault zone. It is also important to note that the time-dependent mechanical deformation that promotes healing also causes 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 without pore fluid at room temperature, and their 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 evidence demonstrating stress relaxation because of damage zone viscous deformation. Comparison with examples of post-earthquake velocity recovery in the field shows that the magnitude of velocity recovery observed in the lab is comparable to or smaller than that observed in the field. Differences may be explained by enhanced mechanical healing at saturated and in-situ temperature conditions and/or by the chemical sealing of fracture volumes that were not reproduced in the lab.