Abstract
This contribution describes the possibility of extending to the elastic wave equ ation with the physical no stress boundary condition, the classical results on stable and unsta ble boundary observation. The proofs relies on one hand on the introduction of the equations satified by t he divergence and the curl of the solution and on the other hand on the analysis of the propagation of the wave front set done mostly by Taylor [T] and Yamamoto [Y]. With these ingredients the estimates for the observation are adapted from Bardos Lebeau and Rauch [BLR].
AMS Subject Classification: 58G17,73C02,93C20,93B07.