The was a natural disaster caused by the eruption of Mount Unzen in Nagasaki Prefecture, Japan.
Volcanic activity began in November 1990, and a large-scale pyroclastic flow occurred on June 3, 1991, causing extensive damage. In total, 43 people were killed or went missing. The volcanic activity finally subsided in 1995.
The event was the deadliest volcanic disaster in post-war Japan until the 2014 eruption of Mount Ontake.
The event is known in Japanese as the .雲仙普賢岳平成大噴火
November 1990 to May 1991 Following the 1792 eruption, Mt. Unzen remained dormant until it erupted again in November 1990.
It started with a slow building active phase in 1968, with the first earthquake swarms continuing until 1975. Large amounts of volcanic gas were ejected by Fugen-dake, which affected about thirty cedar trees. In 1975, bird and animal carcasses were found in the surrounding area, and high concentrations of carbon dioxide were detected emitting from rock fractures. This area was also reported to have erupted volcanic gases during the 1792 eruption. Earthquakes occurred sporadically from 1975, but there were 89 sensitive earthquakes, with strong shaking equivalent to a maximum seismic intensity of 5 from June to September 1979. At Shimabara Onsen, there was a place where the dissolved carbon dioxide concentration rose sharply from 1975 and increased by 30%. From April 1984, swarms occurred in Tachibana Bay. A magnitude 5.7 earthquake with a seismic intensity of 5 occurred in August near the south side of Hayama. As a result of this earthquake, uplift of the Shimabara Peninsula was observed, and magma supply from Tachibana Bay began.
In November 1989, an earthquake swarm began about underneath and west of Fugen-dake. Over the following year, earthquakes continued, and their hypocentres gradually migrated towards the summit. Finally on November 17, 1990 (Heisei 2), the first phreatic, or steam-blast, eruptions began ejecting from two places near a shrine at the summit. These eruptions only blew vaporized hot water and ash up from the two craters. In December of the same year, the volcano's state lulled, seeming like it would end, but it erupted again on February 12, 1991, and on April 3 and 4. The eruption intensified on May 9.
On May 15, the first debris flow caused by accumulated volcanic ash at the summit occurred, and a number of cracks were formed on the west side of the crater, extending in an east–west direction. After this inflation of the summit area, fresh lava was confirmed to be emerging on May 20, 1991. The lava was so viscous that it did not flow out, but instead became a dome which formed in the crater. The lava dome grew like a peach, eventually splitting and collapsing into four pieces under its own weight. After that, the dome continued to grow, continuously supplied with fresh lava from the crater underneath, creating new lobes that hung down from the summit. The 1st to 13th lobes were named in the order they were formed. Collapsing of the lava dome began to occur when the lobes, pushed out by the newly supplied magma, started to give way on the slope, causing pyroclastic flows (in which debris and volcanic gases flow down the mountainside at speeds of 100 km/h).
June 3, 1991 eruption
The threat of further disastrous events prompted authorities to evacuate 12,000 residents from their homes. On June 3, 1991, the volcano erupted violently, possibly as a result of depressurisation of the magma column after a landslide in the crater. A pyroclastic flow triggered by the collapse of a lava dome reached from the crater and killed 43 scientists and journalists, including volcanologists Katia and Maurice Krafft, and Harry Glicken.S Connor, The men and women in wooly hats who risk their lives to study volcanoes, The Independent, April 22, 2010.
Before the eruption The first debris flow in the Mizunashi River (located at the foothills of Unzendake) occurred on May 15, with further flows occurring on the 19th, 20th, and 21st. Shimabara City made an evacuation recommendation to the towns in the Mizunashi River basin each time, resulting in an uneventful evacuation with no human fatalities.
However, on May 20, when the lava dome appeared on Fugen-dake, the lava dome grew in size, and on May 24 part of the dome collapsed, and the first pyroclastic flow occurred. After that, smaller pyroclastic flows occurred frequently, reaching a distance of east of the lava dome on May 26, and on May 29. On May 26, when the tip of a pyroclastic flow approached from the nearest private houses, Shimabara City issued an evacuation recommendation for Kita-Kamikoba town, Minami-Kamikoba town, Shirataki town, Tenjin Motomachi, and Fudan Motomachi – all located in the Mizunashi River basin. The evacuation recommendation was carried out without trouble.
To capture the pyroclastic flow activity on film, media organizations chose a select spot within the evacuation advisory area. This spot was away from the lava dome, away from the Mizunashi River (where debris flows were frequent), and high. This shooting spot was located on the prefectural road in the village of Kita-Kamikoba, and was favored by the media because they were able to view Fugen-dake directly in front of them. The nickname "fixed point" was established. After the first pyroclastic flow on May 24, more than a dozen media members were lined up at the "fixed point". In 1991, the media had started to use color photographs in the paper, and with the Fugen-dake disaster becoming an ongoing topic of coverage, the companies started competing for color photographs. The competition became even more intense as the Mainichi Shimbun succeeded in shooting nightly pyroclastic flows on May 28.
The fact that pyroclastic flows were being recorded in clear images for the first time attracted a great deal of attention from all over the world, and many volcanologists and government officials entered the evacuation advisory area for interviews and photography. On May 28, the Ministry of Construction (at that time officials of the Public Works Research Institute) released a picture taken when entering the path of previous pyroclastic flows below the lava dome, and another group on the afternoon of June 1 or 2. After entering the tip of the deposit path, they surveyed the site for about an hour, photographed the pattern and released it.
In addition, many visitors had begun to gather around Unzendake to see the plumes. In particular, since June 2 was a Sunday, many visitors from other prefectures gathered around the Mizunashi River and observed the pyroclastic flows with binoculars and photographed them with video cameras. Traffic became heavy as a result.
However, on May 26, reporters from the Asahi Shimbun were warned that it was likely for them to be engulfed by the clouds, and in an evacuation recommendation, they were asked to leave the "fixed point" and find another area. According to a photographer who continued aerial photography of the lava dome from the helicopter, "The bottom fans out from the Sabo dam of the Mizunashi River, and if a large pyroclastic flow occurs, there is a strong possibility of it striking the 'fixed point'." The Asahi Shimbun abandoned their position and retreated past the Tsutsuno bus stop, to instead set up a point in Fukae Town outside the evacuation zone on May 28, and switched to 24-hour shooting from there. At the "fixed point", the coverage in the vicinity was limited to patrols.
Similarly, Japan's public broadcaster NHK stopped shooting from within the evacuated area from the end of May and arranged for an unmanned camera in the Kamikoba area. But until the unmanned camera was ready, the photographer who had been lowered backward was advanced to the Kamikoba area once again on June 1. The evacuation area then became smaller, since the commercial broadcasters had shot footage of bright red lava on May 30 and 31; the person in charge of the news program (who ordered the initial retreat of the front-line shooting staff) ordered more images shot like it. Kamikoba soon emerged again as a filming location of the press.
From May 15, when the debris flow evacuation advisory was issued, the fire brigade in charge of the Kamikoba district stayed either at the Minami-Kamikoba town fire brigade, or the Kita-Kamikoba town agricultural training center (which was initially an evacuation residence for locals). On May 29, due to the frequent occurrences of pyroclastic flows, they were evacuated from the firefighting station in Minami-Kamikoba town to the Shiratani Public Hall in the lower Mizunashi River, but on June 2, they returned to the Agricultural Training Center in Kita-Kamikoba town. This was because of the following reasons:
Due to the temporary lull in rainfall, the occurrence of debris flow became more sparse, and it was judged that the area where the pyroclastic flows came out was a steep area and did not reach a gentle slope downward. The evacuation for Tenjin Motomachi and Fudano Motomachi was lifted on the 1st, as well as Shiratani. Therefore, the fire brigade were no longer allowed to stay in the towns downstream from the Kamikoba area, as the evacuation advisories were now canceled. On May 29, preparations for the rainy season were resumed, and so was the removal of sediment from the Mizunashi River. But wire sensors that were installed to detect occurrences of debris flows were cut off on May 26 in a series of pyroclastic flows, and their recovery was still expected. As a result, it was decided to manually monitor the upstream part of the Mizunashi River. The agricultural training center in Kita-Kamikoba was favored for this, as it was located higher than the Minami-Kamikoba fire station. On June 2, several reports from a press conference by the Director General of Shimabara, detailed several TV station crews breaking into homes without permission, borrowing telephones and power outlets, and littering in the area. Also, after May 26, residents would often return to recover personal belongings from their homes, or enter the Kamikoba area to do housework or farm work, and it was considered necessary to guide their evacuation in the event of an emergency. As a result, it was convenient for the agricultural training school (which was close to the "fixed point") to monitor the behavior of the press and warn any nearby locals of danger.
On June 2, all 20 members of the Kamikoba area fire brigade slept at the agricultural training institute. They remained at the training center and were on alert.
Eruption and pyroclastic flows After 3:30 pm on June 3, small and medium-sized pyroclastic flows occurred frequently, and the first large-scale pyroclastic flow occurred at 3:57. Although this pyroclastic flow (and its resulting pyroclastic surge) did not reach the "fixed point", visibility in the vicinity deteriorated significantly as volcanic ash from the dispersing cloud covered the surroundings, in addition to the rainfall that had continued from the morning.
At 4:08 pm, the second large pyroclastic flow occurred, descending the Mizunashi River Valley and reaching to the east from the lava dome. Although the main body of the flow continued to follow the river, the resulting pyroclastic surge spread out in a fan shape once it had exited the valley mouth and continued further, hitting Kita-Kamikoba town ( away from the lava dome) and finally stopping near the Tsutsuno bus stop ( away). The pyroclastic flow also flowed down the Akamatsu-dani River, but changed direction due to a gust of wind from the south, allowing the residents, firefighters, and the shooting staff (who left their camera behind to capture world-famous footage of the eruption) to immediately escape the eruption.