Ground motion from two-dimensional steady state dynamic ruptures is examined for both subshear and supershear rupture velocities. Synthetic seismograms demonstrate that coherent high-frequency information about the source process rapidly attenuates with distance from the fault for subshear ruptures. Such records provide almost no resolution of the spatial extent of the stress breakdown zone. At supershear speeds, S waves radiate away from the fault, preserving the full source spectrum and carrying an exact history of the slip velocity on both the fault-parallel and fault-normal components of motion, whose amplitudes are given by a function of rupture speed that vanishes at $sqrt{2}$ times the S-wave speed. The energy liberated from the strain field by the passage of a supershear rupture is partitioned into fracture energy dissipated within the fault zone and far-field S-wave radiation. The partition depends on both the rupture velocity and the size of the breakdown zone.