  The afterglows to gamma-ray bursts (GRBs) are due to synchrotron emission
from shocks generated as an ultra-relativistic outflow decelerates. A forward
and a reverse shock will form, however, where emission from the forward shock
is well studied as a potential counterpart to gravitational wave-detected
neutron star mergers the reverse shock has been neglected. Here, we show how
the reverse shock contributes to the afterglow from an off-axis and structured
outflow. The off-axis reverse shock will appear as a brightening feature in the
rising afterglow at radio frequencies. For bursts at $\sim100$ Mpc, the system
should be inclined $\lesssim20^\circ$ for the reverse shock to be observable at
$\sim0.1-10$ days post-merger. For structured outflows, enhancement of the
reverse shock emission by a strong magnetic field within the outflow is
required for the emission to dominate the afterglow at early times. Early radio
photometry of the afterglow could reveal the presence of a strong magnetic
field associated with the central engine.
