california
The relevant parties may go to court for an equitable solution.
See the 2010 California Code of Civil Procedure
Chapter 3.6. Cullen Earthquake Act:
751.50. If the boundaries of land owned either by public or by private entities have been disturbed by earth movements such as, but not limited to, slides, subsidence, lateral or vertical displacements or similar disasters caused by man, or by earthquake or other acts of God, so that such lands are in a location different from that at which they were located prior to the disaster, an action in rem may be brought to equitably reestablish boundaries and to quiet title to land within the boundaries so reestablished.**
(my emboldenment and embedded link)