The Demon Quake is an event that takes place in the prologue of the Japanese animated film, Demon City Shinjuku.
The maniacal devil-worshipper Rebi Ra has allowed himself to be possessed by demons. Seeking ultimate power over the world of man, Rebi Ra initiates a ritual that will eventually open the gates of Hell and allow demons to rise, laying waste to the Earth. Genechirou Izayoi, a practitioner of the ancient art of Nempo, confronts Rebi Ra atop a skyscraper in Tokyo in order to thwart his evil scheme. The battle is short; Rebi Ra cannot be harmed by Genechirou and simply heals himself instantly. Genechirou is defeated and dies watching as Rebi Ra's scheme is put into action. A gigantic pentagram appears across the city, with the Shinjuku district at its centre. Shinjuku is devastated by an earthquake with the majority of the local population being killed in the disaster. Those who survive the quake are forced to fend for themselves in a literal Hell on Earth as demonic entities appear in Shinjuku and run rampant. The district is cut off and the government declares the area a no-man's land.
Following the Demon Quake, Shinjuku is reduced to a ghost town. Rebi Ra still operates inside the town and spends the next ten years preparing for the demonic ressurrection that will allow the fiends of Hell to conquer the Earth. He is protected at all times by a triage of demons that patrol the desolate streets and kill anyone that tries to interfere with his ritual. Meanwhile, the few people left in Shinjuku descend into chaos and anarchy; criminal gangs scour the streets committing wanton acts of destruction, murder and rape. The main events of Demon City Shinjuku take place ten years after the Demon Quake, with Rebi Ra eventually being killed by Kyoya Izayoi, son of Genichirou. With Rebi Ra's death, the rise of the demons is prevented and the world is saved. It is unknown if Shinjuku is ever fully rebuilt.