We experimentally address the importance of tuning in athermal phase transitions, which are triggered only by a slowly varying external field acting as tuning parameter. Using higher order statistics of fluctuations, evidence consistent with the existence a singular critical instability is detected, in spite of an apparent universal self-similar kinetics over a broad range of driving force. The results as well as the experimental technique are likely to be of significance to many slowly driven nonequilibrium systems from geophysics to material science which display avalanche dynamics.

• Received 24 October 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.025701