  We study plastic strain during individual avalanches in overdamped
particle-scale molecular dynamics (MD) and meso-scale elasto-plastic models
(EPM) for amorphous solids sheared in the athermal quasi-static limit. We show
that the spatial correlations in plastic activity exhibit a short lengthscale
that grows as $t^{3/4}$ in MD and ballistically in EPM, and is generated by
mechanical excitation of nearby sites not necessarily close to their stability
thresholds, and a longer lengthscale that grows diffusively for both models and
is associated with remote marginally stable sites. These similarities in
spatial correlations explain why simple EPMs accurately capture the size
distribution of avalanches observed in MD, though the temporal profiles and
dynamical critical exponents are quite different.
