This paper reviews a new understanding of shear-wave splitting (seismic-birefringence) that is a fundamental revision of conventional fluid-rock deformation.It is a New Geophysics with implications for almost all solid-earth geosciences,including hydrocarbon exploration and production,and earthquake forecasting.Widespread observations of shear-wave splitting show that deformation in in situ rocks is controlled by stress-aligned fluid-saturated grain-boundary cracks and preferentially orientated pores and pore-throats pervasive in almost all igneous,metamorphic,and sedimentary rocks in the Earth’s crust. These fluid-saturated microcracks are the most compliant elements of the rock-mass and control rock deformation.The degree of splitting shows that the microcracks in almost all rocks are so closely spaced that they verge on fracture-criticality and failure by fracturing,and are critical systems with the“butterfly wing’s”sensitivity of all critical systems.As a result of this crack-criticality,evolution of fluid-saturated stress-aligned microcracked rock under changing conditions can be modelled with anisotropic poro- elasticity (APE).Consequently,low-level deformation can be:monitored with shear-wave splitting;future behaviour calculated with APE;future behaviour predicted with APE,if the change in conditions can be quantified;and in principle,future behaviour controlled by feed-back.This paper reviews our current understanding of the New Geophysics of low-level pre-fracturing deformation. This paper reviews a new understanding of shear-wave splitting (seismic-birefringence) that is a fundamental revision of conventional fluid-rock deformation. It is a New Geophysics with implications for almost all solid-earth geosciences, including hydrocarbon exploration and production, and earthquake forecasting. Wide spread observations of shear-wave splitting show that deformation in in situ rocks is controlled by stress-aligned fluid-saturated grain-boundary cracks and preferentially orientated pores and pore-throats pervasive in almost all igneous, metamorphic, and sedimentary rocks in the Earth’s crust. These fluid-saturated microcracks are the most compliant elements of the rock-mass and control rock deformation. The degree of splitting shows that the microcracks in almost all rocks are so closely spaced that they verge on fracture-criticality and failure by fracturing, and are critical systems with the “butterfly wing’s ” sensitivity of all critical systems. As a result of this crack-cri ticality, evolution of fluid-saturated stress-aligned microcracked rock under changing conditions can be modeled with anisotropic poro- elasticity (APE). Conferred, low-level deformation can be: monitored with shear-wave splitting; future behavior calculated with APE; future behavioour predicted with APE, if the change in conditions can be quantified; and in principle, future behavior controlled by feed-back. This paper reviews our current understanding of the New Geophysics of low-level pre-fracturing deformation.