Temporal evolution of a shear-type rock fracture process zone (FPZ) along continuous, sequential and spontaneously well-separated laboratory instabilities—from intact rock to thick gouged fault

Abstract

SUMMARY The development of shear-type fault analogues from intact rock at the laboratory scale provides a unique opportunity for investigating tectonic-scale phenomena through the lens of geophysics. The transition from rock fracture creation to laboratory fault slip must exist. We observe three spontaneously temporally well-separated mechanical instabilities attributed to the continuous evolution of a shear-type rock fracture between two artificial flaws. Their separation is validated with rapid mechanical stress drops and stabilizations, periodical acoustic emission (AE) behaviours (AE event number and AE moment release rate) and b-value drops. One instability occurs near the stress peak and corresponds to fracture incipience where fault development is mostly identified via optical observations; the other two instabilities are in the post-stress-peak domain and correspond to the fault nucleation and slip stages, respectively, with distinguishable AE releases from the fault region. The macroscale fracture has been created at the moment of rapid-stress drop for the second instability; off-fault damage, increasing gouge powder generation and slip acceleration can be identified within the fault slip stage. AE behaviour throughout fault nucleation shows a reversal of the Omori–Utsu (O–U) law. AEs attributed to the fault slip display regular O–U law decay and the distinction between the AE behaviour for fault nucleation and fault slip is pronounced. These observations and analyses can provide further understanding on the analogue relationship between a laboratory loading-induced fault and a natural fault.