In the story in question, an extremely powerful earthquake decimates the Earth (but does not destroy it) due to Unforeseen Consequences of Our Actions. Ideally some humans need to survive, so accounting for damages to infrastructure, farming capability, etc., what’s the biggest earthquake that could happen on an Earth-like planet without destroying the planet or wiping out all life? It would have to be strong enough to create huge chasms, level mountains, and destroy most of a civilization, but at least some of humanity has to survive.
The reason for the earthquake can be arbitrary or nonexistent, so long as it causes as much damage as possible without destroying all life along.
