D. Landau and E. M. Lifshitz, Theory of Elasticity Pergamon, 1986.

G. Hill and L. Knopoff, 12 D. F. Strenzwilk, J. Geophys. Res. J. Appl. Phys. J. Geophys. Res, vol.85, issue.71, 1979.

G. J. Walton and . Astron, Soc. 43, 293 1975. 25 We do not display the relevant cumbersome expression, which may be found with a formal calculus software such as MATH- EMATICA