FedEx is an excellent company, and I am sure they have serious business continuity plans. But that is not really the point. If there was an earthquake like there was in 1811 and 1812, Memphis would suffer seriously. To some extent, the impact on FedEx probably would depend a lot on how many of the aircraft in their fleet would be on the ground at the time.
Indianapolis is the "national hub" and Memphis is the "SuperHub". There are also hubs in Oakland, Newark, Fort Worth, Anchorage, and other cities in the US. I have no numbers, but when I look at the live flights in a mobile app when FedEx planes are most active, I would guestimate that Memphis gets 3X or 4X the number of flights as Indy, and Indy gets more flights than the other smaller hubs. 100% anecdotal. Also, Memphis is active all day while the other hubs are not.