Time-Dependent Deformation in the Damage Zone of the Chelungpu Fault System and Potential Stress Relaxation

High fracture density in fault damage zones not only reduces the elastic stiffness of rocks but may also promote time-dependent bulk deformation through the sliding of fracture and thus alter the stress in fault zones. On comparing the damage zones of the three faults in the Chelungpu fault system encountered in the Taiwan Chelungpu fault Drilling Project (TCDP), the youngest damage zone showed pronounced sonic velocity reduction even though fracture density is the same for all three fault zones, consistent with the shorter time for velocity recovery in the youngest fault. Caliper log data showed a time-dependent enlargement of the borehole wall at the damage zone. These damage zones record lower differential stress than the surrounding host rock, which cannot be explained by the reduced elastic stiffness in the damage zone. Stress relaxation caused by time-dependent bulk deformation in the damage zone may be responsible for the observed low differential stress. Earthquakes occur along large faults in the earth's crust. The region surrounding a fault hosts a dense network of fractures and slip surfaces, along which deformation occurs. In this study, we show evidence of velocity recovery and time-dependent deformation of off-fault rocks, which potentially modified stress on faults during the interseismic period and therefore has implications for seismic risk and hazard assessment. Mechanical waves passing through young fault damage zones are slower than waves passing through old damage zones in the Chelungpu fault system. This suggests a temporal recovery of elastic stiffness. We observe that the shear stress magnitude is lower in the fault damage zone than in the intact host rock. Since the contrast in elastic properties of damage zone rocks cannot explain the low shear stress, we suggest that other processes like time-dependent bulk deformation relax the stress in fault damage zones. The damage zones are characterized by sections of high fracture density resulting in borehole enlargement and low sonic velocityTime-dependent borehole enlargement and velocity recovery imply the occurrence of bulk viscous deformation in the damage zoneViscous stress relaxation may be responsible for the relatively isotropic stress state in the damage zone prior to the Chi-Chi earthquake