Retaining Wall: Who is responsible? (see picture)

[Luke Mike]

Hello,

Who should be responsible to maintain/replace this retaining wall, should it fail? The upper or lower owner?

I read that it typically is the lower owner, however the lower owner is at the street level.
The upper owner, as you can see is 6ft above the street. 

I would like to build on the empty lot, but I am afraid the cost of the retaining wall replacement could be prohibitive.