TL;DR It’s complicated, but my interpretation of all of that is that the default is that what land is “yours” does in fact change when there’s slow movement of the land, but you can also fight it in court and have property lines redrawn to be roughly the same relative to the buildings when all of the buildings have moved.
https://www.reddit.com/r/Surveying/comments/1bmasw8/what_happens_to_property_lines_in_large_landslide/
TL;DR It’s complicated, but my interpretation of all of that is that the default is that what land is “yours” does in fact change when there’s slow movement of the land, but you can also fight it in court and have property lines redrawn to be roughly the same relative to the buildings when all of the buildings have moved.