Mount Unzen
Mount Unzen is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island.
In 1792, the collapse of one of its several lava domes triggered a megatsunami that killed 14,524 people in Japan's worst volcanic-related disaster. The volcano was most recently active from 1990 to 1995, and a large eruption in 1991 generated a pyroclastic flow that killed 43 people, including three volcanologists.
Its highest peaks are at and at. The latter emerged during the eruptions of the early, eponymous Heisei era.
Overview
Mount Unzen rises in the central part of the Shimabara Peninsula, Nagasaki Prefecture. It is located on the outer ring of the Chijiwa Caldera centering on Tachibana Bay in the west of the peninsula. It consists of a total of more than 20 mountains, however, the complexity in the shape of was expressed by various numbers. As a result, the usage of only eight mountains was coined in a narrow sense, but historically it is a name that refers to the entire mountain range towering over the sea. It is often confused with the name of the oldest peak,.Surrounding the highest peak of , are , , , , , and . and derive magma from the Chijiwa Caldera. In other words, it is supplied from a magma chamber beneath Tachibana Bay off Obama Onsen. The main peak is, but the volcanic activity from 1990 to 1995 made, which was higher in elevation. is also the highest peak in Nagasaki Prefecture.
Unzen originally read "onsen" in the notation of a hot spring, but it was changed to the current notation when it was designated as a national park. An army radar base was built near the summit of Fugen-dake during the Pacific War, where about 100 people were stationed.
Eruptive history
Prehistoric to October 1990
Mount Unzen is part of the Shimabara Peninsula, which has seen extensive volcanism over millions of years. The oldest volcanic deposits in the region date from over 6 million years ago, and extensive eruptions occurred over the whole peninsula between 2.5 and 0.5 million years ago.The origins of the Unzen complex are traced to the formation of a graben through crustal faulting. This caused parts of the peninsula to subside by up to below sea level and may have caused eruptive activity to localize at one site inside the graben. Eruptions of dacitic lava began from a site slightly to the south of today's Mount Unzen and migrated north over time.
The volcano rapidly grew during its first 200,000 years, forming a very large cone. Later eruptions over the following 150,000 years filled in much of the graben. Initially, activity was dominated by blocky andesitic lava and ash flows, changing to dacitic pumice flows and airfall deposits from 500,000 to 400,000 years ago. The period from 400,000 to 300,000 years ago saw the emplacement of large areas of pyroclastic flow and lahar deposits; these form the major part of the volcanic fan surrounding the volcano. Beginning 300,000 to 150,000 years ago, thick phreatomagmatic deposits were laid down, suggesting the subsidence of the volcano into its graben was rapid during this period.
Activity from 150,000 years ago to the present has occurred at a number of sites around the volcanic complex, building four main domes at different times: the No-dake, Myōken-dake, Fugen-dake and Mayu-yama volcanic peaks. Fugen-dake has been the site of most eruptions during the past 20,000 years and lies about from the center of Shimabara.
In December 1663, Fugen-dake erupted, producing lava flows that covered forest for over an extension of. In the spring of the following year, there was a flood from the Kujuku Island crater, located on the southeastern flank of Fugen-dake at an altitude of. More than 30 were killed.
Unzen's deadliest eruption occurred in 1792, beginning with an earthquake in November of the previous year. On February 10, 1792, eruptions from Fugen-dake's Jigokuato crater began. Lava flows began on March 1 and continued for nearly two months. On March 22, volcanic plumes were produced, and lava also flowed out of the crater. On March 25, fumes rose and lava flowed down the northeastern part of Fugen-dake at a total length of. Eventually, the east flank of the Mayu-yama dome collapsed unexpectedly following a post-eruption earthquake, creating a landslide into Ariake Bay. This caused a tsunami that killed an estimated 15,000 people. it is the worst volcanic related eruption in Japan.
November 1990 to May 1991
After 1792, the volcano remained dormant until it erupted again in November 1990.It started with a slowly building active phase in 1968, with first earthquake swarms continuing until 1975. Large amounts of volcanic gas were ejected by Fugen, 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 gasses 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, the first phreatic, or steam-blast, eruptions began ejecting from two places near a shrine at the summit. These eruptions only allowed vaporized hot water and ash to blow up from the two craters. In December of the same year, the volcano's state lulled and it seemed that 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 a phenomenon known as pyroclastic flows.
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.Before the eruption
The first debris flow in the Mizunashi River occurred on May 15, and others continued to occur in succession on the 19th, 20th, and 21st. Shimabara City made an evacuation recommendation to the towns in the Mizunashi River basin each time, resulting in smooth evacuation of residents and no human fatalities.However, on May 20, when the lava dome appeared on Fugen-dake, it continued to grow day by day, and on May 24 part of the dome collapsed and the first pyroclastic flow occurred. After that, small 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.
In order to capture the pyroclastic flow activity on film, the mass media chose a select spot within the evacuation advisory area. This spot was away from the lava dome, away from the Mizunashi River, 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 on 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 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 backwards 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 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. 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 downwards. 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 recovery of them was still expected to occur. 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 to monitor the behavior of the press and warn any nearby locals of danger.