I can actually kinda answer, but for an entirely different legislation (I'm from Chile): If earth moves because of a natural disaster or such, moving from one persons property to another's, it still belongs to the original owner, but only for the effect of moving it (and everything in it that still belongs to the original owner, such as trees, fences etc) back across, and if they're not moved back in a year, ownership defaults to whoever owns the propierty the land moved into (652nd article of the Civil Code).

This of course refers to the physical earth (and everything on it). Property lines themselves are a fiction, not a literal line, and they can be proved in different ways (demarcations, records, different points of reference, etc) so it can't really be moved by an earthquake AFAIK (otherwise the law I just cited wouldn't really make sense).

I remember this rule from law school because it seems so unlikely yet must still be learned, that we used to laugh about it all the time.