Brittle failure in rocks is generally preceded by a localization of deformation in the region of the final macrofracture. This process is associated with a non-uniform distribution of subcritical microcracks which should be reflected in the distribution of acoustic velocities and surface deformation. The temporal and spatial development of such deformation zones has been studied by monitoring these quantities during the deformation of Westerly granite samples.

Holographic interferometry revealed localization of surface deformation above about 70% of the tangential strain at failure. As loading proceeded, acoustic velocities became highly anisotropic, finally varying by a factor of 2 in different directions. They were smoothly dependent on path orientation, but not on path position within the sample. Thus, surprisingly, they did not reflect the localized deformation observed with holography.