The Role of a Rock Discontinuity’s Micro-structure and Aperture on the Detection of Seismic Precursors to Shear Failure

Whether shear failure will occur along a fracture or joint depends on the nature of the two fractures surfaces, matched or mismatched, the asperity distribution, and surficial geochemical properties. Here we examine if the nature of the discontinuity surfaces affects observations of geophysical precursors to failure, specifically the effect of aperture, asperity distributions, and surficial chemistry. Laboratory experiments were performed on specimens composed of two gypsum blocks in contact with each other that had either well-matched or mismatched surfaces. Direct shear experiments were conducted under normal stresses of 2 and 5 MPa, while compressional and shear wave ultrasonic signals were transmitted across the discontinuity to monitor slip along the discontinuity. Seismic precursors to shear failure were detected for all well-matched discontinuities for all normal stresses. However, seismic precursors appeared only under the higher normal stress for mismatched discontinuities. The interplay between wave transmission, the degree of mismatch, and discontinuity’s micro properties was assessed through 3D CT in-situ X-ray scans, micro-indentation testing, and scanning electron microscopy and electron X-ray diffraction. The data indicated that for the mismatched discontinuities used in this study: (1) the void aperture was large, so significant shear displacement was needed to interlock and damage the asperities; and (2) the micro-hardness of the asperities was larger than that of the well-matched discontinuity. The results suggest that monitoring changes in transmitted wave amplitude across a discontinuity is a promising method for predicting impending failure for rock discontinuities but works best when there is sufficient contact between the rock surfaces.