The Great Hinamizawa Disaster (雛見沢大災害, Hinamizawa Daisaigai), or simply shortened to GHD, refers to a tragic event in Higurashi no Naku Koro ni that ultimately destroys all of Hinamizawa in most of the arcs. This disaster always occurs if Rika Furude has been killed by Miyo Takano's hands, which is first encountered in Tatarigoroshi-hen. Of course, it can be noted that the disaster does not occur in some arcs if Rika has been killed by someone else (such as Shion Sonozaki in Watanagashi-hen).
The Official Story
The official version of the events are as follows, as per the reports of the Japan Special Defense Forces (JSDF); In the early morning following the hostage situation in Tsumihoroboshi-hen, volcanic gas composed of hydrogen sulfide and carbon dioxide was released from the Onigafuchi Swamp. This gas spread through the village, killing everyone while they slept, after which the JSDF removed the dead bodies from the village, quarantined the area for almost 20 years, and covered Onigafuchi Swamp with thick concrete. Kyosuke Irie committed suicide by poison while Rika was found to have been a murder victim prior to the disaster.
A number of popular theories began popping up online in the aftermath of the Great Hinamizawa Disaster. The popular theory is one of a government conspiracy that covered up an alien bio-terrorist attack. They cite multiple points to support this theory: First of all, experts say that filling the swamp with concrete was a pointless act, as it did no good against the gas. Additionally, hydrogen sulfide should have corroded metal and caused significant environmental damage within the town, though there was actually no proof of this. The SDF soldiers who worked in the area after the disaster received frequent blood tests, which the conspiracy theorists think give evidence that they were testing some kind of bacteria on humans. The primary support for the alien theory comes from Document No. 34 and the words of Rena Ryuuguu from the night prior to the incident.
In truth, the deaths of Jirou Tomitake, Kyosuke Irie, Rika Furude and the entire village were all as part of a plot by Takano to validate the theories of her adopted grandfather, Hifumi Takano, as well as Nomura's own plot to have the Koizumi faction in Tokyo fall. Takano arranged for the deaths of Tomitake, Irie, and Rika; Tomitake was injected with H173 and dumped on the side of the road to meet his end, Irie was poisoned with narcotics, and Rika was disemboweled at the Furude Shrine. By citing her grandfather's theories after Rika's death, she was able to oversee the extermination of the entire town.
- It can be suspected that the gas used in the GHD was a VX nerve agent. As suggested by the condition of the bodies seen in Yakusamashi-hen. All of the bodies shown had trademark symptoms of nerve agent poisoning, i.e., constriction of the pupils, involuntarily salivation and lacrimation (seen as dried drool marks and tear streaks running down their faces).