Active geophysical monitoring of potential failure along mechanical discontinuities in rock requires identification of precursory signatures to failure in geophysical signals. Active ultrasonicmonitoring of shear failure along frictional discontinuities was performed to determine the signatures of potential failure. An instrumented direct shear apparatus was used to apply a constant shearing rate to a discontinuity thatwas held under a constant normal stress. Transmitted and reflected compressional and shear waves were recorded during the shearing process. Ultrasonic precursors were identified as distinct maxima in the amplitude of transmitted shear waves as well as minima in the amplitude of reflected shear waves that occurred well before the peak shear strength of a frictional discontinuity. The precursors are linked to changes in the local shear specific stiffness along the discontinuity, while the discontinuity's macroscopic shear strength continues to increase prior to failure.
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