A city built on the floor of the Atlantic Ocean is bound to face a multitude of problems--especially one as close in proximity to the Mid-Atlantic Ridge as Rapture. You see, that particular ridge is a divergent boundary between two Earth's plates. Meaning, new crustal material is constantly produced (albeit at a pretty slow rate) at the point where the ridge is formed. Basically, Rapture was built upon a slow-moving conveyer belt of underwater land, which is probably prone to earthquakes and the like thanks to its ever-shifting nature. Of course, all land is essentially a slow-moving conveyor belt (that's why we have multiple continents instead of Pangaea in the first place), but unless you live in along a fault line, you probably won't notice the effects of plate tectonics on a day-to-day basis.