Elasticity in the simple theory is described by 2 parameters, chosen from the five common ones. Young s modulus describes resistance to stretching a bar, bulk modulus describes resistance to volume change, shear modulus gives resistance to shear, Poisson s ratio describes ratio of transverse to longitudinal strain in a stretched bar, and the lame parameter is one of convenience. Additionally, Q is used to describe damping of waves, high Q means low attenuation. Q has no dimensions, and is typically in the range 10-1000 for rocks. Finally, we may wonder how to compute the properties of rocks or fluids composed of several constituents or mineral grains.
Rocks are more complicated with various degrees of cementation, etc.
These, plus actual lab measurements by rock squeezers are important in exploration and whole earth studies. Then in place of force we note that force depends on the spatial rate of change of stress, since if stress is uniform there is no force. Also note that since the Earth is a finite size body, another description of the waves is possible in terms of free oscillations or standing waves, in place of travelling waves. Generally we go to the free oscillation description only when wavelengths become comparable to the size of the earth. For smaller wavelengths, the travelling wave description, including rays, is most useful 3.