In reality there aren't shockwaves in space... at least not the 2-d rings you are thinking of. Those are caused by a spherical pressure wave moving through the air from the explosion. (the 2-d part is where it interacts with the ground or cloud layer, whatever you happen to be looking at)
Although there are come cosmic-scale shockwaves, such as something I remember but cant find a picture of, where a star is ploughing through a gas cloud and basically making an interstellar "sonic boom".