This paper introduces a model of rock deformation (anisotropic poro-elasticity or APE), where the response of fluid-saturated rock to changing conditions, prior to fracturing, can be calculated. The driving mechanism for deformation is fluid migration along pressure gradients between neighbouring intergranular microcracks and pores at different orientations to the stress field. The parameters that control changes to microcrack geometry also control the splitting (birefringence) of seismic shear-waves, so that changes in deformation can be directly monitored by analysing the shear-wave splitting which is observed in almost all rocks. Analysis of shear-wave splitting in the Earth's crust and APE-modelling show that distributions of, mostly intergranular, cracks in the crust are always geometrically close to fracturing with the implication that shear-wave splitting is sensitive to comparatively minor changes of stress and minor changes of in situ conditions. This has important implications for the state of criticality of the rockmass and, as a consequence, changes in shear-wave splitting have been observed before larger earthquakes on those few occasions when suitable source–receiver geometry coincides with appropriate seismic activity. APE also has implications for monitoring changing conditions in reservoirs during hydrocarbon recovery.