Well, we don't know that much about supernovas. At least, I believe we don't. I've yet to venture into cosmology, but logic dictates that something we don't have any real experience with can not be accurately determined, only theorized.
As such, I'll simply theorize that the energy behind the supernova explosion continuously accelerates the shock wave. That doesn't make sense does it? The shock wave expands 'equally' in every direction (or so I believe). By that simple statement one can say that there's a point where there will be no shock wave left. Where that is, is debatable. That shock wave is stuffed with matter and radiation, so mass. Matter can't be destroyed or, well, created, so to speak. It's converted. And let's say we pick a certain part of the shock wave. Now each and every single matter has a mass. It's extremely condensed at the beginning. But by the time it reaches Earth, it should be noticeably expanded. Let's take it simple. E=(mv^2)/2. From that simple formula you can basically extract the energy behind the shock wave that reaches earth. And do remember that, no matter where in the universe you are, gravity will always bound you.
Now, one light year is many many many km away. (3x60x60x24x365)x10^8 km, if we roundabout light speed to 3x10^8 m/s.
.... if we are to believe, the shock wave that is 3000 ly away from Earth, it would simply mean that said shock wave required 3000 years traveling at the speed of light to reach earth.
So either the shock wave is made out of some kind of unknown particle that travels ~60 times faster than the speed of light, or the theory behind ultra super acceleration is true (which ain't).