The Effects of Weak Dynamic Pulses on the Slip Dynamics of a Laboratory Fault

摘要

Investigated are the regularities of fault slip under weak impulsive disturbances in laboratory experiments. The fault was simulated by an interface between two granite blocks loaded with normal and shear stresses, and the impulsive disturbances were excited by impacts of steel balls. We found that the passage of an elastic wave does not produce any residual displacement in the absence of shear load. But the process of inelastic fault deformation manifests quite vividly even under a slight constant shear stress. The sign of residual deformation does not depend on the direction of pulse propagation, but it does coincide with the direction of the applied shear load. Depending on the stress-strain state of the fault and the parameters of the impact, there are two scenarios of fault evolution: (1) production of residual deformations with decaying amplitudes under repeated cycles or (2) initiation of slow deformation process and accumulation of displacements up to the occurrence of slip instability. In the case of a continuously increased shear load, the effects of weak periodic impacts essentially depend on the initial slip behavior. The weakest effect is observed in the case of high-amplitude stick-slip. In contrast, in the case of slow, irregular quasi-dynamic slip, tapping leads to an almost complete transformation of the potential energy of deformation into aseismic creep. Thus, a short dynamic disturbance propagating within a stressed blocky medium can trigger a slow deformation process whose contribution to the cumulative deformation may be quite appreciable. Because postdynamic movements can contribute substantially, a delay in the manifestation of dynamic events with respect to the moment of the initial disturbance may be observed. In turn, periodic dynamic disturbances of the stress-strain state can essentially change the mode of background seismicity and the proportion of radiation efficiency of events.