  We present a numerical analysis for SiO_2 of the fraction of diffusive
direction f_diff for temperatures T on both sides of the fragile-to-strong
crossover. The T-dependence of f_diff clearly reveals this change in dynamical
behavior. We find that for T above the crossover (fragile region) the system is
always close to ridges of the potential energy surface (PES), while below the
crossover (strong region), the system mostly explores the PES local minima.
Despite this difference, the power law dependence of f_diff on the diffusion
constant, as well as the power law dependence of f_diff on the configurational
entropy, shows no change at the fragile to strong crossover.
