If I’m not mistaken, Castel Gandalfo sits on the rim of a large crater, created when a volcano erupted, and the top of it collapsed many years ago. There is now a lake in that crater, and it’s not surprising there are earthquakes, since I think there is a magma chamber still under that lake.
You are not mistaken! Joan Lewis, EWTN’s Rome News Bureau Chief, did an outstanding job providing facts and information during the pope’s short helicopter flight from the Vatican to Castel Gandalfo. From the news helicopter, you could see precisely that rim which surrounds Lake Albano. Earthquakes frequently occur in Italy. The 500+ y/o home of my in-laws was so severely damaged in 2002 earthquake centered in the Province of Molise, that the house had to be condemned.