Earth is a setting in the Interference series. The world is the home of Alexander Karsath, as well as his family members, and the world in which every video game Alex interferes with is produced.
Unlike the other worlds, Earth never appears as the main setting of a story. It is only ever shown briefly, for about a chapter's length at most, with the main action of the series taking place inside the game worlds.
The only location ever shown in this world is the Karsath family apartment, located in Portland, Oregon in the USA. It is a small second-story, two-bedroom apartment, though one that is big enough for its three occupants Alex, Erica and their mother Wendi.
Alex possesses his own room while Erica and Wendi share one (until Erica is killed). Alex has a shelf dedicated to some of his games as well as older consoles; Erica's side of her room was apparently filled with 'manga and merchandise' (presumably Fullmetal Alchemist merchandise). The TV is in the living room, as is the PS2 and some games for that console, and so this is the room where Alex and Erica make their Interferences.
Other locations mentioned are the apartment where Peter Karsath lives, the bank where Wendi works, unnamed schools that Alex and Erica have attended, and the house in which they used to live before the divorce, all of which are in Portland.