For a shear stress, Sigma.S , the constant of proportionality between shear stress and shear strain (gamma) is called the shear modulus, G, where: Sigma.S = G.(chi). It is clear from the figure that the ratio of F/A (shear stress) to X/h which is the shear strain is the shear modulus. The amount of shear, all other variables being constant depends on the rheology. For steel, this value is 84 billion pascals. The values for most rocks will be of much lower order.