  We measure the local yield stress, at the scale of small atomic regions, in a
deeply quenched two-dimensional glass model undergoing shear banding in
response to athermal quasistatic (AQS) deformation. We find that the occurrence
of essentially a single plastic event suffices to bring the local yield stress
distribution to a well-defined value for all strain orientations, thus
essentially erasing the memory of the initial structure. It follows that in a
well-relaxed sample, plastic events cause the abrupt (nucleation-like)
emergence of a local softness contrast and thus precipitate the formation of a
band, which, in its early stages, is measurably softer than the steady-state
flow. Moreover, this postevent yield stress ensemble presents a mean value
comparable to that of the inherent states of a supercooled liquid around the
mode-coupling temperature $T_{\rm MCT}$. This, we argue, explains that the
transition between brittle and ductile yielding in amorphous materials occurs
around a comparable parent temperature. Our data also permit to capture
quantitatively the contributions of pressure and density changes and
demonstrate unambiguously that they are negligible compared with the changes of
softness caused by structural rejuvenation.
