  We propose a microscopic model to study the avalanche problem of insulating
glass deformed by external static uniform strain below $T=60$K. We use
three-dimensional real-space renormalization procedure to carry out the glass
mechanical susceptibility at macroscopic length scale. We prove the existence
of irreversible stress drops in amorphous materials, corresponding to the steep
positive-negative transitions in glass mechanical susceptibility. We also
obtain the strain directions in which the glass system is brittle. The
irreversible stress drops in glass essentially come from non-elastic
stress-stress interaction which is generated by virtual phonon exchange
process.
