Are black hole event horizons filled with black holes? An observer hovering close to an event horizon will observe huge energies, like blue shifted radiation falling in, or Hawking radiation going out. So does the observer observe that black holes are created, when high energy particles collide, and these black holes then absorb energy at fast rate?
 A: First of all, Hawking radiation is a bold speculation. It has never been observed and its theoretical background is discutable as so far we do not know quantum theory of gravity which appears to be needed to speculate precisely about such phenomena. However, most phisicists agree that its existence is well possible. 
Even if it existed, it is predicted to be really, really weak for any black holes of reasonable size. So, there will be no new black holes formed near the horizon because of that.
When it comes to very small black holes (e.g. size of proton) current theroies are helpless and we can not make any predictions. Therefore the answer is: they may or they may not. 
Now, any other known radiation is too weak to produce black holes near the horizon for practical astrophysical situations. This is because energy concentration required to produce a horizon is immensly immense :) . Remember $E=mc^2$ - you need a lot of matter to form a black hole and still $c^2$ is sooo huge. I am working on black hole formation in my thesis and radiation can of course produce it - but simulations suggest that energy concentration has to be unreasonably strong. 
