Velocity Recovery of Damaged Sedimentary Rocks during Creep Experiments and Comparison with Field Scale Observations

Fault damage zones surrounding the primary slip plane is characterized by the presence of fractures, which are either open, healed or sealed. Open fractures seal or heal by chemical precipitation or mechanical compaction of the fracture volume respectively. Signatures of fracture closure has been observed by repeated seismic surveys after large earthquakes, which show progressive seismic velocity increase (or recovery) after the earthquake. However, the relative contribution of the mechanical and chemical processes on this recovery is unclear. In this study, we conduct creep experiments on mechanically damaged rocks to study the time-dependent velocity increase associated with the compaction of the fracture volume and to evaluate the potential magnitude of seismic velocity recovery that could be caused in the field by mechanical healing. To study the time-dependent evolution of fracture volume, we used cylindrical sedimentary rock specimens impacted in a Split Hopkinson Pressure Bar to create a range of fracture volumes (resembling fracture intensities at various distances from the fault core). The damaged rocks were then subject to pressure hold stages in a triaxial apparatus. We observed that higher damaged specimens show more creep deformation. Micro-CT scans conducted before and after the creep tests indicate that samples with more fracture volume undergo more successive healing. This observation is consistent with field observations which show pronounced seismic velocity recovery in high fracture density regions of natural fault damage zones (Vidale and Li, 2003). Analysis of ultrasonic velocity measurements acquired during the creep tests will provide insights on whether mechanical healing can explain velocity recovery observed after major earthquakes and help delineate the relative contribution of chemical and mechanical processes on fracture volume closure in fault damage zones.