Curiouser and curiouser:

1) There are three "descr_disasters.txt" files in the game

2) They can be found in the following folders:
"Rome - Total War\Data"
"Rome - Total War\Data\world\maps\base"
"Rome - Total War\Data\world\maps\campaign\sons_of_mars"

3) The format of these files is significantly different. The one in the "Data" folder looks like this:

disaster Brighton
{
type storm
year 0
position 272 227
size 10
}
The files in the other folders look like this:

event storm
frequency 4
winter true
summer false
warning false
region the sea
min_scale 2
max_scale 5
4) Maybe what we're seeing is the difference between scripted events and random ones, but it seems odd that the formats would look so different.

5) Oh, and to answer my own question: No, there's no difference between the sons_of_mars file and the one in the "base" folder.