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.