The deformation of the medium at that point can be described by the strain tensor formula_7, the 3×3 matrix whose elements are  where formula_9 denotes partial derivative with respect to position coordinate formula_10. The strain tensor is related to the 3×3 stress tensor formula_11 by the equation  Here formula_13 is the Kronecker delta (1 if formula_14, 0 otherwise) and formula_15 and formula_16 are the Lamé parameters (formula_16 being the material's shear modulus). It follows that  From Newton's law of inertia, one also gets  where formula_20 is the density (mass per unit volume) of the medium at that point, and formula_21 denotes partial derivative with respect to time. Combining the last two equations one gets the "seismic wave equation in homogeneous media"  Using the nabla operator notation of vector calculus, formula_23, with some approximations, this equation can be written as  Taking the curl of this equation and applying vector identities, one gets  This formula is the wave equation applied to the vector quantity formula_26, which is the material's shear strain. Its solutions, the S-waves, are linear combinations of sinusoidal plane waves of various wavelengths and directions of propagation, but all with the same speed formula_27  Taking the divergence of seismic wave equation in homogeneous media, instead of the curl, yields a wave equation describing propagation of the quantity formula_28, which is the material's compression strain. The solutions of this equation, the P-waves, travel at the speed formula_29 that is more than twice the speed formula_30 of S-waves. The steady-state SH waves are defined by the Helmholtz equation where k is the wave number.