The mechanical and hydraulic properties of rocks are strongly influenced by the presence and properties of discontinuities, or fractures. The ability to locate and characterize natural as well as induced discontinues in rock is of paramount importance to many engineering problems such as slope stability, rock bridge integrity, hydraulic fracturing, geothermal energy and CO2 sequestration, to name a few. Although fracturing in rock has been much studied, the current state of knowledge, both theoretical and empirical, is largely based on observations on the surface of the specimens where direct inspection of the existing or induced fractures can be made. The fundamental reason for this is the limitations of our techniques to illuminate damage in the interior of rock. Experiments on rock and rock-model materials show that active seismic monitoring can be used to detect the onset of slip along a frictional discontinuity, as well as the initiation of damage inside rock in the form of tensile or shear cracks. Precursors to failure along a frictional discontinuity undergoing shear were identified as the maximum in transmitted wave amplitude across the discontinuity or the minimum in the amplitude of the wave reflected from the discontinuity. Ultrasonic precursors were observed well before slip or failure occurred along the discontinuity and were attributed to a reduction in the discontinuity local shear stiffness. In rock specimens subjected to uniaxial compression, tensile and shear crack initiation were identified as a distinct decrease in the amplitude of transmitted waves, which occurred prior to the detection of the crack on the specimen surface. In contrast, the amplitude of the transmitted waves did not change during shear crack initiation. However, seismic wave conversions (P-to-S or S-to-P wave) were found to be effective in identifying the initiation of shear cracks in rock.

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