Building Permits: Neighbor Excavated His Side of Property Line and Cosue Damages to My Property

My question involves a consumer law issue in the State of: California, Sacramento


Our neighbor excavated along the property line to depths ranging from about 15" to 20" (30 feet long) and then left it for a few years.
We have fence on this property line.
The fence felt of 3 weeks ago.
When I saw he removed all supporting land I was shocked.
fence is 6', posts 8', elevation difference almost 2'.
There was no way to replace posts.

He did not install a retaining wall so nothing was and is holding up our property so it caused concrete cracks, corner of the building cracked.

I asked him why he did no build any retaining wall at least one foot away from fence to support land?
He could not answer my question.

I proposed to build retaining wall on boundary line, but he said he doesn't wan to.
HE said it's my responsibility to make this wall.

My questions:
1. is he responsible to get permit before do such work?
2. is he responsible to build retaining wall to hold my property land protected?
3. How far from fence/boundary line he must stop his digging (i guess it call setback) so it allowes to maintain the fence stable.

What California laws apply in this case?
I could not find this on line

Thanks

Building Permits: Neighbor Excavated His Side of Property Line and Cosue Damages to My Property