  The evolution of strange mode instabilities into the non linear regime has
been followed by numerical simulation for an envelope model of a massive star
having solar chemical composition, M=50M_sun, T_eff=10^4K and L=1.17*10^6
L_sun. Contrary to previously studied models, for these parameters shocks are
captured in the H-ionisation zone and perform rapid oscillations within the
latter. A linear stability analysis is performed to verify that this behaviour
is physical. The origin of an instability discovered in this way is identified
by construction of an analytical model. As a result, the stratification turns
out to be essential for instability. The difference to common stratification
instabilities, e.g., convective instabilities, is discussed.
