I haven't loked it up, but I do not remember the word "earthquake" appearing in the decisons book.
This situation is not contemplated in the rules and therefore would have to be decided in equity (Rule 1-4).
There are two possible outcomes, and I will not speculate as to how the rules commitee would decide.
1) A one shot penalty, and the ball must be replaced. The rational is that an earthquake is a natural event and therefore, like wind and water, is not an outside agency. The principal of like situations being treated alike applies.
2) No penalty. Although earthquakes are not unusual the situation as described certainly is, so the player is absolved of the rules breech and the ball must be replaced.
You decide, but I favor #2.
JVB?