It's a file called desc_rebel_factions.txt
The thing is it's not by region its by rebel faction and another thing is that the chance causes a associted amount of unrest in any region that rebel is in.

For instance:

rebel_type Saxones //Germans
category peasant_revolt
chance 3
description Saxones
unit barb infantry slave
unit barb infantry slave
unit barb peasant slave
unit barb naked fanatics slave
unit barbarian rebel general

his entry will cause all the provinces with Saxones to have a 15% unrest penalty.

To find out which rebel a province has look in desc_regions.txt

Tribus_Saxones
Bordesholm
germans
Saxones
232 41 55
germans, brits, timber, amber, dogs, pigs
5
3

This province has Saxones and will cause a 15% unrest penalty. I think the penalty doesn't apply to the province creator (in this case the germans) but I'd have to check that.

Hope this helps.