Wiki is not very detailed on this subject. I did some my own tests and I found following: Can somone confirm that I got this right ?
1) when I destroy gate it does not seems to cause splash (tested once)
2)fence that is outside of my claim can damage my own fence on the claim when destroyed. (tested) This is probably quite exploitable if its really true unless you dont claim 5 tiles away from yours fence.
3)There is some time splash damage wont work after fence gets constructed and its new. looks like its abouot 3 days.
3)splash will get applied only from higher tier fence or same tier. Plank fence cant damage brick.
4)there is diffrent damage % per fence type. looks like stone will take +- about 80% of the HP from another stone. What is this splash damage % for plank and brick fence ? Is it same ?
5)damage has range about 4 blocks and building some cheap fence in between 2 layers of stone fences wont help you to prevent it.