Probably a 4-pt. strike that radiates from the hex designated.
Hex hit: It will 'pillage' 4 times. Since every district can have at most 3 buildings, this will wreck the district hit.
1st ring around hex hit: It will 'pillage' 3 times. This will wreck all districts except those that have all of their buildings, and even then the buildings will be toast.
2nd ring: 'Pillage' 2 times.
3rd ring: 'Pillage' 1 time.
Of course, they could have both the target hex and the 1st ring get hit with a 4-pt. strike and 'go down' from there.
But given the nature of pillaging in CiVI, and they apparently have already said that it will have diminishing impact away from Ground Zero, this is probably how they'll handle it.