Animated textures
I don't know of any good decompilers. What do you use?
It seems that is uses a prop_dynamic. It has a player clip under it that initializes the whole thing. Interesting, yet complicated.
Lost In Transition has a really nice conveyer belt.
PM the author and ask him how he did it.
The Valve maps don't employ a surface with a scrolling texture, they use a bunch of props parented to a movement entity.
The easiest way to scroll an arbitrary texture is with material proxies. I used TextureScroll to scroll the walls up when falling down a shaft in one of my previous maps.
Which map was that?
And can you apply the effect gradually or did you just suddenly have a brush with the "scrolling" texture to simulate falling faster and faster?
Look, I am a relatively new mapper so, I do not know how to do any of that.
That's why you're here to learn 
"Such monstrous sucking-constructions aren't built within one afternoon"