  In gamma-ray-bursts (GRB), ultra-relativistic blast waves are ejected into
the circumburst medium. We analyse in unprecedented detail the deceleration of
a self-similar Blandford-McKee blast wave from a Lorentz factor 25 to the
nonrelativistic Sedov phase. Our goal is to determine the stability properties
of its frontal shock. We carried out a grid-adaptive relativistic 2D
hydro-simulation at extreme resolving power, following the GRB jet during the
entire afterglow phase. We investigate the effect of the finite initial jet
opening angle on the deceleration of the blast wave, and identify the growth of
various instabilities throughout the coasting shock front. We find that during
the relativistic phase, the blast wave is subject to pressure-ram pressure
instabilities that ripple and fragment the frontal shock. These instabilities
manifest themselves in the ultra-relativistic phase alone, remain in full
agreement with causality arguments, and decay slowly to finally disappear in
the near-Newtonian phase as the shell Lorentz factor drops below 3. From then
on, the compression rate decreases to levels predicted to be stable by a linear
analysis of the Sedov phase. Our simulations confirm previous findings that the
shell also spreads laterally because a rarefaction wave slowly propagates to
the jet axis, inducing a clear shell deformation from its initial spherical
shape. The blast front becomes meridionally stratified, with decreasing speed
from axis to jet edge.
