  The propagation of an interfacial crack front through a weak plane of a
transparent Plexiglas block has been studied experimentally. A stable crack in
mode I was generated by loading the system by an imposed displacement. The
local velocities of the fracture front line have been measured by using an high
speed CCD camera. The distribution of the velocities exhibits a power law
behavior for velocities larger than the average front velocity <v> with a
crossover to a slowly increasing function for velocities lower than <v>. The
fluctuations in the velocities reflect an underlying irregular bursts activity
with a power law distribution of the bursts. We further found that the size of
the local bursts scales differently in the direction parallel to and
perpendicular to the fracture front.
