Tectonic faults fail in a continuum of modes from slow earthquakes to elastodynamic rupture. Precursory variations in elastic wavespeed and amplitude, interpreted as indicators of imminent failure, have been observed in limited experimental and natural settings for this spectrum of slip modes. Such variations are thought to arise from microcracking within and around the fault zone. However, the physical mechanisms and connections to fault creep are not well understood. Here, we vary loading stiffness to generate a range of slip modes and measure fault zone properties using elastic waves transmitted through the fault. We find that elastic wave amplitudes show clear changes before failure. The temporal onset of amplitude reduction scales with lab earthquake magnitude and the magnitude of this reduction varies with fault slip. Our data suggest that continuous seismic monitoring in proximity to natural faults could be useful for assessing fault state and seismic hazard potential.
