  We show that the Weibel-mediated collisionless shocks are driven at
non-relativistic propagation speed (0.1c < V < 0.45c) in unmagnetized
electron-ion plasmas by performing two-dimensional particle-in-cell
simulations. It is shown that the profiles of the number density and the mean
velocity in the vicinity of the shock transition region, which are normalized
by the respective upstream values, are almost independent of the upstream bulk
velocity, i.e., the shock velocity. In particular, the width of the shock
transition region is ~100 ion inertial length independent of the shock
velocity. For these shocks the energy density of the magnetic field generated
by the Weibel-type instability within the shock transition region reaches
typically 1-2% of the upstream bulk kinetic energy density. This mechanism
probably explains the robust formation of collisionless shocks, for example,
driven by young supernova remnants, with no assumption of external magnetic
field in the universe.
