  The tidal effects generated by a nonlinear gravitational wave are
investigated in double-null v - u coordinates, as an exact solution of
Einstein's field equations. The components $\xi^{v}$ and $\xi^{u}$ of the
separation vector behave as in flat space but the transversal components
$\xi^{x}$ and $\xi^{y}$ depend nonlinearly on $v$ through the Bessel and
Neumann functions, far from the null surface $v = 0$. We show that the same
results are obtained by means of the tetrad formalism.
