  We analyze low-frequency vibrational modes in a two-dimensional,
zero-temperature, quasistatically sheared model glass to identify a population
of structural "soft spots" where particle rearrangements are initiated. The
population of spots evolves slowly compared to the interval between particle
rearrangements and the soft spots are structurally different from the rest of
the system. Our results suggest that disordered solids flow via localized
rearrangements that tend to occur at soft spots, which are analogous to
dislocations in crystalline solids.
