Dear Worried in Chino Hills,
Retaining walls or free standing walls are normally constructed with reference to a property line. In general, engineers define this line as the "layout line" it may be defined in the center of the wall or outer exterior edge of the wall and this layout line runs parallel to the property line or falls right on the property line.
Without looking at the wall construction drawings it’s not possible to determine whether the wall was built with the layout line defined as the center of the wall or by an exterior edge of the wall. In plain English without looking at the plans you won’t know for certain whether the wall was built on your property line or your neighbor’s property line. The wall could even be in the center, meaning that half the wall is on your side and the other side on your neighbors. A survey staking to mark the property corners would easily illustrate where the wall is located in reference to both properties.
The scenarios in my opinion are as follows: 1) if the wall were to fail and the entire wall falls in your neighbor’s property they would be responsible for the entire repair and any additional damage that may happen as a result of their negligence in maintaining the wall.2) If the wall is entirely within your property, then you are responsible for maintaining the wall, unless the neighbor is imposing a direct additional load that the wall was not design to take, in this case the neighbor would be liable for repairs if there is evidence that this additional load occurred as a result of their actions, if this cannot be demonstrated then you would be responsible for not maintaining the wall 3) If the wall falls in both your properties you and your neighbor are responsible for maintaining the wall, again if your neighbor is imposing an additional lateral load then it would be the responsibility of your neighbor. In either case it would probably be a good idea to speak with your neighbors and propose a retrofit to reinforce the wall and avoid any future problems. I always recommend to my clients that communication avoids litigation. Call me with any additional questions I would be happy to help (909) 881-6542 Hector Moreno P.E.