For an isotropic material which obeys Hooke’s law, where strain is linearly proportional to stress, there are only two independent elastic constants, those of stress and strain. The elastic properties of such materials are defined by elastic moduli, notably the bulk modulus, shear modulus, Young’s modulus, Lamé’s constant, and Poisson’s ratio. Seismic wave velocities are governed by the elastic moduli and the densities of the media through which they travel.