Moment tensor, fault damage and stiffness variation during fluid injection in laboratory faults

Fluid injection induced seismicity is thought to be associated with substantial aseismic deformation which is more difficult to detect than micro-seismic events. We investigate the transition between slow, stable slip and abrupt stick-slip on laboratory faults at varying pore-pressures. We fracture cylindrical Westerly granite samples at high differential stress and control fracture orientation by introducing saw-cut notches at 30 degrees to the loading axis. Micro-seismic event migration reveals a slow, initial fracture process that starts from the end of the notches and propagates into the intact, asperity-like region until dynamic failure occurs. The fracture process shows clear evidence for precursory dilatational behavior expressed by (i) an increase in fluid volume in the sample, (ii) a pronounced decrease in seismic velocities and (iii) accelerating AE rates and moment release. Seismic activity during slip on the incipient fault zones indicates a spatial correlation between fault roughness and seismicity clusters. We test the influence of varying pore pressure and rock damage on slip behavior. The latter is accomplished by preheating a subset of samples to 700 degree C, resulting in pervasive micro-cracking and significantly lower seismic velocities. Dry and saturated samples show stick-slip behavior in the absence of thermally induced damage. Thermally treated samples show amplified dilation before fracture and stable sliding at pore pressures between 0.1 to 15 MPa. The transition to stable sliding can be explained by a stiffening effect of fluid-saturated damage zones which prevent abrupt earthquake-like slip. Associated moment tensor solutions show more dominant double-couple components rather than compressional events within the fault damage zone. These results highlight that fault damage and fluid pressure evolution play a key role in controlling micro- and macro slip behavior in lab and nature.