  The spatial distribution of structural relaxation in a supercooled liquid is
studied using molecular dynamics simulations of a 2D binary mixture. It is
shown that the spatial heterogeneity of the relaxation along with the time
scale of the relaxation is determined, not by the frequency with which
particles move a distance pi/2kBragg, but by the frequency with which particles
can achieve persistent displacements. We show that these persistent
displacements are achieved through the coupled action of local reorganizations
and unrecoverable thermal strains.
