alt Hacker NewsThis is almost certainly what it was. Wiki even has a photo of the USS Gerald Ford undergoing blast tests off of Ponce Inlet, and mentions that it registered as a M3.9 quake: https://en.wikipedia.org/wiki/USS_Gerald_R._Ford#Operational...
If I had to guess, this is probably the USS John F. Kennedy: https://en.wikipedia.org/wiki/USS_John_F._Kennedy_(CVN-79)
I dont understand the purpose of using 40,000 lbs of TNT (0.2 kilotons) that registers as M3.9 quake - what kind of explosive payload is it simulating?
the only thing that comes to mind, is the smallest yield settings of a modern tactical nuke B61-12