I don’t think there’s a universally accepted solution but in California it would be up to the state to figure it out. It would be a great time to be a Real Estate lawyer after a quake there.

California has the Cullen Earthquake Act.

Essentially one affected party comes up with a proposed solution, files paperwork with the court, and then all the rest of the affected parties get together (under court supervision) to make whatever changes are necessary until the solution is fair. If the court agrees that it is a fair solution, it becomes final.

https://law.justia.com/codes/california/code-ccp/part-2/titl...