Well, there are four files I see:

1. descr_disasters in the data file: Just a few entries
2. descr_disasters in the world/maps/base file: lots of entries that seem to make them random
3. descr_events: allows plagues and volcanoes to be placed according to when they historically happened
4. disasters.tga: for some reason in the imperial_campaign folder. Has dots of different colors in places on the map that would seem to indicate where they should take place.

#2 seems to be the one to work on. Here is the example with their info:

Code:
; event			event_type
; frequency		in years
; winter		only occurs in winter 
; summer		only occurs in summer
; warning		provides a one year warning
; position		position event can occur in strat coords [can specify multiple positions on consecutive lines]
; region		region event can occur by region name [can specify multiple regions on consecutive lines]
; climate		climate event can occur by climate name [can specify multiple climates on consecutive lines]
; min_scale		minimum size for event
; max_scale		maximum size for event

event		earthquake
frequency	20
winter		false
summer		false
warning		false
climate		rocky_desert
min_scale	2
max_scale	5
Now I will say that disasters.tga seems to match exactly the size of regions.tga, so there may be a way to get it to work using that. But I really don't know. I see that EB has storms set at very frequent in the text file we use but they never occur. Would really wish someone who has had some success could enlighten us.