Precursors to failure in rocks while undergoing anelastic deformations

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Abstract
Modeling of geological processes in the laboratory is often quite difficult. Stress accumulation prior to an earthquake occurs over a long time and is associated with large anelastic deformation. We have chosen pyrophyllite as a model material because of its large anelastic deformation (several percent) before brittle failure. Samples of pyrophyllite were stressed to failure in a biaxial press. Photography through transparent pistons, optical holography, microscopy, and sound velocity measurements were used as tools to study the processes leading to brittle failure. At about 50% of ultimate failure strength a zone of preferential deformation forms around the incipient fault, and the seismic velocities decrease perpendicular to the axis of maximum principal stress. At higher stress levels the intense deformation zone becomes very narrow and the velocities increase again before failure. En echelon cracks form in the most narrow part of the intense deformation zone and precede the formation of a macrocrack and a stress drop. The intense deformation zone remains very narrow in front of the macrocrack. Further en echelon crack formation within the intense deformation zone appears before a further advance of the macrocrack. The above observations, especially the velocity reversal before failure, are rate and moisture dependent. They were clearly detectable at a strain rate of 3 × 10−8/s but barely noticeable at 5 × 10−7/s.

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