Evolution of Fault-Interface Rayleigh Wave speed over simulated earthquake cycles in the lab: Observations, interpretations, and implications

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To image fault zone elastic and mechanical properties is of great importance for the understanding of fault mechanics and earthquake physics. In this study we show how a previously less-known wave—Fault-Interface Rayleigh Wave (FIRW), can be used to probe the feedback between fault zone properties and earthquake ruptures. We conducted meter-scale rock friction experiments to simulate earthquake cycles in the lab. Three tests were performed, under a fixed normal stress of −6.7 MPa (negative for compression), and a constant loading rate sequentially at 0.01, 0.1, and 1 mm/s. We installed a strain gauge array near the fault to monitor the local dynamics, focusing on mode-II rupture breaking the free edge of the fault and the breakout-induced FIRW propagating backward along the fault. By applying a waveform correlation method, we systematically measured the FIRW speed over an 800-mm-long interval in the central fault portion. The results show that a relative reduction of FIRW speed by 1.4% can be detected near the end of the test loaded at 1 mm/s. Such wave speed reduction can be explained by two mechanisms: rupture-induced brittle damage that softens fault zone rocks, and some dissipative processes (related to fault re-rupturing, viscous damping, etc.) that take energy away from FIRW. Both mechanisms are also supported by other independent observations (e.g. distribution of fault surface wear, behavior of macroscopic friction), implying that they may be coupled. These observations and interpretations reveal an application potential of FIRW for constraining the elastic and mechanical properties of fault zones. We suggest several directions for future research aimed at turning that application potential into reality.