  We compute the memory effect due to a gravitational wave striking a Kerr
black hole as seen by an observer at null infinity. This is done by working in
Bondi--Sachs coordinates. It was shown by Hawking, Perry, and Strominger (HPS)
that the memory effect due to a gravitational shockwave is seen as a pure BMS
supertranslation from null infinity. Hence, it is of interest to compute the
supertranslated Kerr solution in Bondi--Sachs coordinates. Finally, the
gravitational wave is said to implant soft supertranslation hair on the event
horizon of the black hole which carries superrotation charge. We will
explicitly calculate the change in superrotation charge on the event horizon
due to the supertranslation hair.
