A city built on a giant vehicle. The frequent and destructive Catastrophes have forced all nations to move their cities regularly to avert massive losses. The birth of the Originium engine made it possible to industrialize Originium, and that was when people started to build larger and larger vehicles. The concept of nomadic cities was realized after vehicles became large enough to move entire cities. With the help of the Multimethod Catastrophe Prediction system, it only takes 2-8 weeks to completely evacuate from the Catastrophe area depending on the situation.
Basically an entire city that can be moved as if it was a colossal vehicle, nomadic cities are used by the Terrans to lessen, if not avoid, the destruction caused by Catastrophes, which could be devastating if one is to occur on sedentary settlements where the inhabitants may not have enough time to evacuate; by using Catastrophe prediction systems, a more-or-less accurate estimate of when a Catastrophe would occur will allow nomadic cities to relocate into a safe area in time.
Nomadic cities utilize Originium reactors to both power its engines and providing electricity to the city itself.
Just how nomadic cities can remain above ground and not affected by Terra's gravity while still capable of moving (albeit at a very slow pace) with several billion, or even trillion tons of structures over it without any sort of inertia that can be felt remains unexplained, though it can be implied that nomadic cities' Originium reactors also provides artificial gravity or some sort of anti-gravity mechanism to counteract its ungodly weight.