  We study slip-avalanches in two-dimensional soft athermal disks by
quasi-static simulations of simple shear deformations. Sharp drops in shear
stress, or slip-avalanches, are observed intermittently during steady state.
Such the stress drop is caused by restructuring of the contact networks,
accompanied by drastic changes of the interaction forces. The changes of the
forces happen heterogeneously in space, indicating that collective non-affine
motions of the disks are most pronounced when slip-avalanches occur. We analyze
and predict statistics for the force changes, by transition rates of the force
and contact angle, where slip-avalanches are characterized by their wide
power-law tails. We find that the transition rates are described as a
q-Gaussian distribution regardless of the area fraction of the disks. Because
the transition rates quantify structural changes of the force-chains, our
findings are an important step towards a microscopic theory of slip-avalanches
in the experimentally accessible quasi-static regime.
