  Brittle fractures of inhomogeneous materials like rocks, concrete, or
ceramics are of two types: Nominally brittle and driven by the propagation of a
single dominant crack or quasi-brittle and resulting from the accumulation of
many microcracks. The latter goes along with acoustic noise, whose analysis has
revealed that events form aftershock sequences obeying characteristic laws
reminiscent of those in seismology. Yet, their origin lacks explanation. Here
we show that such a statistical organization is not only specific to the
multi-cracking situations of quasi-brittle failure and seismology, but also
rules the acoustic events produced by a propagating crack. This simpler
situation has permitted us to relate these laws to the overall scale-free
distribution of inter-event time and energy and to uncover their selection by
the crack speed. These results provide a comprehensive picture of how acoustic
events are organized upon material failure in the most fundamental of fracture
states: single propagating cracks.
